A ! Lavergne

V ! Daux

R ! Villalba

J ! Barichivich

Temporal! changes! in! climatic! limitation! of! tree-growth! at! upper treeline! forests:! Contrasted! responses! along! the! west-to-east humidity! gradient! in! Northern! Patagonia

Keywords: tree-ring, paleoclimate, northern Patagonia, growth and stable isotopes of cellulose

The Southern Hemisphere, and particularly southern South America, are very underrepresented in global climate reconstructions due to a lack of robust paleoclimatic data. My thesis was therefore focused on the evaluation of the tree rings archive, which is widespread along the Andes, as a paleoclimatic tracer. I studied the regional response to external climate forcing of two native species (Fitzroya cupressoides and Nothofagus pumilio) in northern Patagonia (41°S, Argentina) by analysing their di erent physical tree-ring parameters. In this perspective, I analysed the temporal variations of the width and of the oxygen and carbon isotopic compositions of the cellulose of their rings and related them to local and large-scale climate variations. Two major results of my thesis have emerged: I highlighted (1) non-linear growth responses over time related to shifts in climate regimes and, (2) the potential of cellulose " 18 O and " 13 C to record summer-autumn temperature variations over a large area in the mid latitudes of South America (35°-55°S). As their variations are strongly linked to temperature, the isotopic tools can be used to reconstruct chronologies of temperature but also of climate modes such as the Southern Annular Mode. 5.1 Annual (April to March), summer, autumn, winter, and spring variations of: normalized temperature over 1901-2011 using REG and CRUTS 3.2 gridded data and, cumulative precipitation using Bariloche local station (1931-2009) and CRUTS 3.2 gridded data (1901-2011). A 10-yr spline is applied on each series to highlight the low frequency component. . . . . . . . . . . . . . . . . . 5.2 Correlation coe cients of monthly temperature and precipitation with monthly values of (a) SAM and (b) MEI (ENSO index) over the whole period 1957-2008 (top), and two subperiods of equal length 1957-1982 and 1983-2008. Black lines indicate precipitation and grey lines indicate temperature. Dotted horizontal lines indicate confidence intervals and statistical significance (p < 0.05). . . . . 5.3 Tree-ring NEGEX and SPL10 chronologies at each site. . . . . . . . . . . . . . 5.4 Continuous Wavelet Transform (WT) for detection of spatial and temporal cycles in the F. cupressoides and N. pumilio NEGEX chronologies over the common period 1785-2011. Red tone indicates higher power spectrum, black thick contours denote 95% significance level using a red noise background, and the white line delimits the cone of influence where edge e ects may be important. 5.5 Correlation between individual tree-ring width chronologies and: (a) monthly temperature and (b) precipitation from Bariloche station over 1933-2009, and (c) monthly temperature and (d) precipitation from CRUTS over 1903-2011. Correlations are computed for NEGEX and SPL10 chronologies. Correlations are given for a 18-month window from October of the previous year (*) to March of the following year (°) corresponding to the current growing season. The stippling points indicate months with significant correlations (p<0.05). Previous and current growing season (GS) are shown with a thick black line. . 5.6 (a) Visual comparison and (b) XWT analysis of the F. cupressoides NEGEX chronologies and previous December-March temperature from REG. The double asterisks denote significance at p < 0.01. Red tone indicates higher power spectrum, black thick contours denote 95% significance level using a red noise background, and the white line delimits the cone of influence. (c) Running Pearson correlation coe cients for successive 30-year windows of F. cupressoides NEGEX chronologies with temperature and precipitation over December-March of the previous growing season. The stippling crosses indicate months with significant correlations (p < 0.05). . . . . . . . . . . . . . . . . . 5.7 (a) XWT analysis of the N. pumilio NEGEX chronologies with November (December) temperature from REG. Red tone indicates higher power spectrum, black thick contours denote 95% significance level using a red noise background, and the white line delimits the cone of influence. (b) Running Pearson correlation coe cients for successive 30-year windows of N.pumilio NEGEX chronologies with temperature and precipitation over September-December. The stippling crosses indicate months with significant correlations (p < 0.05). . . . 5.8 Correlation coe cients between monthly indices of (a) SAM, and (b) MEI, and the NEGEX chronologies for the period 1959-2011. Correlations are given for a 18-month window from October of the previous year (*) to March of the following year (°). The shaded area represents current growing season (October through March). The darker bars indicate a correlation coe cient significant at p < 0.05; the vertical lines represent 95% confidence intervals. The

Résumé

Mots clés: cernes d'arbre, paléoclimat, nord de la Patagonie, croissance, isotopes stables L'Hémisphère Sud, et plus précisément le sud de l'Amérique du Sud, sont très sous-représentés dans les reconstitutions climatiques globales, du fait d'un manque de données paléoclimatiques robustes. Mes travaux de thèse ont donc porté sur l'évaluation de l'archive naturelle cerne d'arbre, abondante le long de la Cordillère des Andes, comme traceur paléoclimatique. Je me suis ainsi appliquée à étudier la réponse régionale de deux espèces d'arbre (Fitzroya cupressoides et Nothofagus pumilio) natives du nord de la Patagonie (41°S, Argentine) aux forcages climatiques externes en analysant leurs di érents paramètres physiques. Pour cela, j'ai commencé par déterminer l'influence du climat local et de grande échelle sur leur croissance en étudiant les variations temporelles de leurs largeurs de cerne, puis j'ai essayé d'extraire le signal climatique enregistré dans la composition isotopique de l'oxygène et du carbone de la cellulose de ces cernes. Deux résultats majeurs de ma thèse ont émergé: j'ai mis en évidence (1) des réponses non-linéaires de la croissance au cours du temps liées aux changements de régimes climatiques et, (2) le fort potentiel du " 18 O et " 13 C de la cellulose pour enregistrer les variations de température d'été-automne sur une large région aux latitudes moyennes de l'Amérique du Sud (35°-55°S). Du fait des relations fortes qui les lient aux températures, les outils isotopiques peuvent être utilisés pour reconstituer des chronologies de température mais aussi des modes climatiques telle que l'Oscillation Antarctique.

List of Figures

1. 1 (a) Positive phase of SAM characterized by an increase in atmospheric pressure over the mid-latitudes (pink) and a decline in pressure at high latitudes (blue). This pressure di erence leads to a strengthening of the circumpolar westerly winds (white arrow) and a southward movement of the Southern Hemisphere storm track. From Jones 1.5 Tree-ring chronologies along the Andes Cordillera in South America. Major tree taxa used for developing the chronologies and their geographical distributions. Geographical domain of the dominant tropical and extratropical forcings of regional climate variability across the Andes, as represented by ENSO and SAM. Modified from Villalba [2007] [START_REF] Ferrio | Carbon and oxygen isotope ratios in wood constituents of Pinus halepensis as indicators of precipitation, temperature and vapour pressure deficit[END_REF]. Original data from (IAEA/WMO 2001) and [START_REF] Saurer | Stable carbon isotopes in tree rings of beech: Climatic versus site-related influences[END_REF] and the temperature anomalies from meterological data relative to the historical mean (wrt 1961-1990), all averaged over 12 months, and the reconstructed Multivariate ENSO Index (MEI) and Southern Annular Mode (SAM) index developed by [START_REF] Li | El Niño modulations over the past seven centuries[END_REF] and Villalba et al. [2012], respectively. . . . . . . .

Geographical distribution of

Introduction

Anthropogenic greenhouse gas emissions of carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O) or halocarbons (e.g., have increased since the pre-industrial era (ca. 1750), largely driven by economic and population growth. Their accumulation in the atmosphere, mainly released by human activities related to agriculture or industry, has contributed to alter Earth's energy balance. These gases are now widely acknowledged by the scientific community as a major cause of recent increases in global mean temperature (about 0.5°C since 1970), changes in the world's hydrological cycle [Intergovernmental Panel on Climate Change (IPCC) 2013], and substantial readjustments of global and hemispherical patterns of atmospheric circulation (e.g., [START_REF] Vecchi | Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing[END_REF], [START_REF] Vecchi | Global warming and the weakening of the tropical circulation[END_REF], [START_REF] Horton | Contribution of changes in atmospheric circulation patterns to extreme temperature trends[END_REF]).

While climate variability and forcing mechanisms in the Northern Hemisphere are quite well understood (e.g., Selten et al. [2004], [START_REF] Andersen | High-resolution record of Northern Hemisphere climate extending into the last interglacial period[END_REF], Mann et al. [2008], Frank et al. [2010], Allen et al. [2012], [START_REF] Ljungqvist | Northern Hemisphere temperature patterns in the last 12 centuries[END_REF], [START_REF] Li | Tree-ring reconstruction of summer temperature for A.D. 1475-2003 in the central Hengduan Mountains, Northwestern Yunnan, China[END_REF], Hoell et al. [2014], [START_REF] Khodri | Le climat du dernier millénaire[END_REF]), the Southern Hemisphere (SH) is poorly documented and under-represented in past global climate reconstructions [Ahmed et al. 2013, Intergovernmental Panel on Climate Change (IPCC) 2013]. Nevertheless, the two hemispheres strongly interact through atmospheric teleconnection patterns [START_REF] He | Northern Hemisphere forcing of Southern Hemisphere climate during the last deglaciation[END_REF][START_REF] Kawamura | Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years[END_REF]], thus if we cannot draw a complete picture of the forcings that modulate the climate in both hemispheres, we cannot fully predict future climate changes. Therefore, understanding the climate variability mechanisms over the last millennium in the SH is crucial to estimate global anthropogenic climate changes besides natural climate variability.

South America is a key region for examining the timing and nature of past climate changes in the SH. The Andes are the most important mountain ranges in the SH, they cross the South American continent from the subtropics in Colombia (10°N) to the southern tip of the continent (55°S). There is a significant topographical constraint establishing sharp climate and vegetation gradients from west to east. The present-day climate variability along the Andes mainly results from the superposition of large-scale ocean-atmosphere processes: the Southern Annular Mode (SAM) that originates in the SH high latitudes and the El Niño-Southern Oscillation (ENSO) rooted in the tropics. Both climatic modes are not only influencing seasonal to interannual local temperatures, but also precipitations amount and distribution through atmospheric teleconnections [Archer and Caldeira 2008, Villalba 2007, Garreaud et al. 2008]. Tropical and subtropical regions of southern South America are mainly influenced by ENSO, while climatic variations in the temperate and sub-Antarctic regions are closely related to SAM (Garreaud et al. [2008]; Fig. 1.2). However, the interannual-to-decadal climate variability associated with these atmospheric circulation modes exhibit large spatial and temporal variance that is not well documented [Villalba 2007].

Instrumental records of temperature and precipitation in South America are short (typically less than 70 years), discontinuous and spatially limited. The limited extent and quality of the available records are a constraint for robust evaluation of past climate variations before the 1930s. Therefore, long climate proxies across the Andes are needed to: 1) improve the representation of the SH regions, 2) document the changes of these large-scale climate forcings beyond instrumental records, and 3) help understanding the impact of both climate forcings on the local-to-regional climate of South America.

1.1 Present-day climate forcings in the Southern Hemisphere

Southern Annular Mode (SAM)

The most important driver of the atmospheric circulation of the SH is the sea level zonal mean pressure di erence between mid (40°-50°S) and high (65°S) latitudes [Thompson et al. 2000, Marshall 2003[START_REF] Thompson | Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change[END_REF], commonly referred as the SAM (also known as Antarctic Oscillation AAO). The SAM is controlling the strength and the position of the westerly winds that oscillate between mid and high latitudes around the South Pole. In a positive phase of SAM, the belt of strong westerly winds (or low pressure) contracts towards Antarctica, leading to weaker than normal westerly winds and higher pressures over southern South America (Fig. 1.1;Gillett et al. [2006], Archer and Caldeira [2008], [START_REF] Thompson | Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change[END_REF]). Conversely, a negative SAM phase reflects an expansion of the belt of strong westerlies towards the equator, with stronger storms and low pressure systems over southern South America. system [START_REF] Jones | Atmospheric science: early peak in Antarctic oscillation index[END_REF]. The positive phase has been associated to significant warming and drying over southern South America (Fig. 1.2), and cooling and wetting over Antarctica and most of Australia [START_REF] Carrasco | Secular trend of the equilibrium-line altitude on the western side of the southern Andes, derived from radiosonde and surface observations[END_REF], Gillett et al. 2006[START_REF] Thompson | Interpretation of recent Southern Hemisphere climate change[END_REF]. The warming over southern South America has been larger during austral summer [Gillett et al. 2006]. The trend in SAM towards positive index values has significant implications for the global carbon cycle, in particular by enhancing the CO 2 outgassing of the Southern Ocean [START_REF] Canadell | Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks[END_REF][START_REF] Le Quere | Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change[END_REF][START_REF] Lovenduski | Enhanced CO2 outgassing in the Southern Ocean from a positive phase of the Southern Annular Mode[END_REF]]. From Garreaud et al. [2008] 1

.1.2 El Niño-Southern Oscillation (ENSO)

Large-scale oscillations rooted in the tropical Pacific Ocean are also a ecting the SH atmospheric circulation. The coupled ocean-atmosphere phenomenon ENSO is driving interannual changes in the zonal winds, surface pressure, surface and sub-surface ocean temperatures, precipitation, cloudiness and convection across the tropical Pacific. While El Niño is a large-scale oceanic warming in the tropical Pacific Ocean, the Southern Oscillation is characterized by an interannual seesaw in tropical sea level pressure (SLP) between the western (Darwin) and tropical eastern (Tahiti) Pacific. The Southern Oscillation is an indicator of the strength of the equatorial Walker circulation and the easterly Pacific trade winds [START_REF] Power | What Caused the Observed Twentieth-Century Weakening of the Walker Circulation[END_REF]. The close connection between El Niño and the Southern Oscillation (ENSO) is a ecting much of the interannual (2 to 7 years) tropical and sub-tropical climate [Karoly et al. 1996, Markgraf and[START_REF] Markgraf | The Past ENSO Record: A synthesis[END_REF]. During El Niño episodes (warm phase of ENSO), the equatorward migration of the mid-latitude storm tracks results in a warming of the central and eastern Pacific Ocean and a cooling of the western Pacific Ocean, extending to the north-east and south-west (Fig. 1.3). The reduction of the east to west sea-surface temperature (SST) gradient, which drives the Walker circulation, leads to the weakening of the surface trade winds forming the lower branch of the circulation cell. This phenomenon results in a decrease of rainfall over tropical South America, and an increase of precipitation over the southeastern portion of the continent and central Chile [Garreaud et al. 2008]. Conversely, during La Niña events (cold phase; Fig. 1.3), the easterly trade winds strengthen and the cold upwelling along the equator and the west coast of South America intensifies. Temperature and rainfall anomalies tend to be opposite of those of El Niño episodes. Numerous measures are used to monitor the status of ENSO, such as the Southern Oscillation Index (SOI), the El Niño3. Since the late 1970s, some studies have reported an increase of frequency and persistence of El Niño events and decreasing number of La Niña events [START_REF] Trenberth | The 1990-1995 El Niño-Southern Oscillation event: Longest on record[END_REF][START_REF] Fedorov | Is El Niño Changing?[END_REF][START_REF] Power | What Caused the Observed Twentieth-Century Weakening of the Walker Circulation[END_REF]. These changes in ENSO cycle were linked to the warming of the tropical Pacific over the recent decades and associated with the weakening of the Walker circulation [START_REF] Tanaka | Trend and interannual variability of Walker, monsoon and Hadley circulations defined by velocity potential in the upper troposphere[END_REF][START_REF] Power | Weakening of the Walker Circulation and apparent dominance of El Niño both reach record levels, but has ENSO really changed[END_REF][START_REF] Power | What Caused the Observed Twentieth-Century Weakening of the Walker Circulation[END_REF]. However, it is still unclear whether these readjustments are driven by anthropogenic global warming [START_REF] Cai | Increasing frequency of extreme El Niño events due to greenhouse warming[END_REF], part of the natural climate variability, or by both. [START_REF] Power | Inter-decadal modulation of the impact of ENSO on Australia[END_REF], [START_REF] Liu | Trend-preserving blending of passive and active microwave soil moisture retrievals[END_REF], [START_REF] Dai | The influence of the inter-decadal Pacific oscillation on US precipitation during 1923-2010[END_REF]). The PDO is the leading mode of multidecadal (20-30 years) variability in SST in the extratropical North Pacific [START_REF] Mantua | A Pacific Interdecadal Climate Oscillation with Impacts on Salmon Production[END_REF][START_REF] Minobe | Interdecadal modulation of interannual atmospheric and oceanic variability over the North Pacific[END_REF][START_REF] Mantua | The Pacific Decadal Oscillation[END_REF][START_REF] Trenberth | An apparent hiatus in global warming ?[END_REF]. The IPO is the Pacific-wide manifestation of the PDO and influences multidecadal scales (40-60 years) SST variance in the SH Pacific down to at least 55°S, as in the Northern Hemisphere subtropics [START_REF] Mantua | A Pacific Interdecadal Climate Oscillation with Impacts on Salmon Production[END_REF][START_REF] Power | Inter-decadal modulation of the impact of ENSO on Australia[END_REF][START_REF] Folland | Relative influences of the Interdecadal Pacific Oscillation and ENSO on the South Pacific Convergence Zone[END_REF][START_REF] Parker | Decadal to multidecadal variability and the climate change background[END_REF][START_REF] Dong | The influence of the Interdecadal Pacific Oscillation on Temperature and Precipitation over the Globe[END_REF].

Much of the decadal and interdecadal variability recorded by these climate modes has been related to natural ENSO-like patterns of variability [Gershunov andBarnett 1998, Brown and[START_REF] Brown | A winter precipitation 'dipole' in the western United States associated with multidecadal ENSO variability[END_REF]. During the warm phase of both interannual and decadal-scale oscillations (positive IPO and PDO, and negative SOI-El Niño event), warmer SSTs are observed in the northeastern Pacific along the west coast of the Americas while cooler SSTs prevail in the central North Pacific. Conversely, during the cold phase of both oscillations (negative IPO and PDO, and positive SOI-La Niña event), the north and south Pacific central gyres and equatorial western Pacific warm while the eastern equatorial Pacific cools down [START_REF] Mantua | A Pacific Interdecadal Climate Oscillation with Impacts on Salmon Production[END_REF]]. The amplitudes of the PDO-related anomalies of temperature and precipitation over South America are about half of their ENSO counterparts (Fig. 1.2;Garreaud et al. [2008]). The well-known regime shift of PDO in the 1976-1977 from negative (cold) to positive (warm) phase, has significantly increased global mean temperatures and SSTs in the tropical Pacific Ocean [START_REF] Trenberth | Decadal atmosphere-ocean variations in the Pacific[END_REF][START_REF] Mantua | A Pacific Interdecadal Climate Oscillation with Impacts on Salmon Production[END_REF][START_REF] Minobe | Interdecadal modulation of interannual atmospheric and oceanic variability over the North Pacific[END_REF][START_REF] Carrasco | Secular trend of the equilibrium-line altitude on the western side of the southern Andes, derived from radiosonde and surface observations[END_REF]. Indeed, global warming first became evident beyond the bounds of natural variability since the 1970s during the positive phase of PDO (Fig. 1.3;[START_REF] Trenberth | El Niño and climate change[END_REF], [START_REF] Trenberth | An apparent hiatus in global warming ?[END_REF]). Coincident with the shift of PDO, SAM has become an important modulator of the strength of ENSO [Fogt andBromwich 2006, Terray 2011], suggesting a strong influence of global warming or decadal climate fluctuations on atmospheric teleconnections [START_REF] Stevenson | Will there be a significant change to El Niño in the 21st century?[END_REF], Zhang et al. 2008[START_REF] Terray | Southern Hemisphere extra-tropical forcing: A new paradigm for El Niño-Southern Oscillation[END_REF]. Recently, since about the year 2000 after the well-known El Niño warm year of 1998, the warming trend has slowed down, mainly in the tropical Pacific (Fig. 1.3;[START_REF] Trenberth | An apparent hiatus in global warming ?[END_REF]). This hiatus or plateau in Earth surface warming has been attributed to decadal cooling in the tropical Pacific related to a change from positive to negative phase of PDO [START_REF] Kosaka | Recent global-warming hiatus tied to equatorial Pacific surface cooling[END_REF][START_REF] Trenberth | An apparent hiatus in global warming ?[END_REF], England et al. 2014], intensifying trade winds [START_REF] England | Recent intensification of winddriven circulation in the Pacific and the ongoing warming hiatus[END_REF], changes in El Niño activity [START_REF] Kaufmann | Reconciling anthropogenic climate change with observed temperature 1998-2008[END_REF], Schmidt et al. 2014], increasing volcanic activity [START_REF] Solomon | The persistently variable "background" stratospheric aerosol layer and global climate change[END_REF][START_REF] Fyfe | Human influence on extratropical Southern Hemisphere summer precipitation[END_REF][START_REF] Santer | Volcanic contribution to decadal changes in tropospheric temperature[END_REF], decreasing solar irradiance [START_REF] Schmidt | Non-stationary seabird responses reveal shifting ENSO dynamics in the northeast Pacific[END_REF]] and internal climate variability mainly through the IPO [START_REF] Dai | Decadal modulation of global surface temperature by internal climate variability[END_REF]].

Tree-rings as robust paleoclimate proxies

Tree-ring records provide accurate multi-centuries climate proxies. In southern South America, they represent the most broadly distributed and annually resolved paleoclimate archives throughout the Andes Cordillera (Fig. 1.5). Over 300 tree-ring chronologies have been developed along the Andes from the subtropical inter-mountain valleys to the cold environments at the southern tip of the continent [START_REF] Boninsegna | Dendroclimatological reconstructions in South America: a review[END_REF], Villalba 2007]. Indeed, tree-ring networks derived from sensitive locations can be used to evaluate natural climate variability and to reconstruct large-scale atmospheric circulation patterns at interannual to multi-decadal timescales [Aravena et al. 2002, Lara et al. 2001, Lara and Villalba 1993, Lara et al. 2008, Villalba et al. 2003, Villalba 2007, Villalba et al. 2012]. Several parameters of tree-rings can be used including tree-ring widths, the stable isotope composition of cellulose (" 18 O, " 13 C and " 15 N ), and the density or the width of the vessels.

Tree-ring widths

To date, tree-ring width chronologies were developed to reconstruct past climate variations in southern South America. For instance, previous studies have shown that tree-ring growth of long-lived or high-elevation temperate trees of Patagonia (e.g. Fitzroya cupressoides, Araucaria araucana, Nothofagus pumilio) is largely controlled by atmospheric temperature (e.g., [START_REF] Boninsegna | Dendroclimatological reconstructions in South America: a review[END_REF]). Tree-ring width (TRW) chronologies based on these species were thus developed for reconstructing temperature variations. Two species growing in the dry forest-ecotone of northern Patagonia (Austrocedrus chilensis and N. pumilio) and primarily controlled by precipitation were also used to reconstruct past variations of seasonal and annual precipitation [Villalba andVeblen 1998, Lara et al. 2001]. Snow cover indices (SCI) were developed with N. pumilio in northern Patagonia [Villalba et al. 1997] and streamflow reconstructions were conducted using A. chilensis, A. Araucaria and Pilgerodendron uviferum [Lara et al. 2005;2008, Mundo et al. 2012, Muñoz et al. 2014].

Tree-ring series were also compared to large-scale atmospheric forcings predominantly influencing climatic fluctuations in the region. In the mid-latitudes of the Patagonian Andes (39-43°S), summer temperatures of the past millennium were reconstructed using chronologies of F. cupressoides. Then, those temperature reconstructions were related to SOI summer variations to trace the development and intensity of ENSO [Villalba 2007]. Furthermore, reconstructed annual temperature variations in southern Patagonia from a network of upper-treeline N. pumilio chronologies showed significant correlations with annual SAM [Villalba et al. 2003].

Recently, a TRW-based reconstruction of austral summer SAM using native species of the SH (e.g. for southern South America A. chilensis, A. Araucana, Nothofagus betuloides) indicated that the positive trend observed in the SAM over the late 20 th century has been unprecedented within the last 600 years [Villalba et al. 2012] and may reflect the beginning of a major reorganization of the atmospheric circulation in the high southern latitudes. Therefore the use of TRW as paleoclimate tracers has significantly expand our understanding of climate variability.

While standard dendrochronology focus on radial growth, additional information on the physiological responses of trees to climate can be inferred from stable isotopes in tree-rings and be associated with long-term trends in tree-ring growth and forest productivity.

Stable isotopic composition of tree-rings

The oxygen isotope ratios (" 18 O) in tree-rings as a tracer of climate variability has been largely demonstrated (e.g. among recent studies: [Gessler et al. 2014[START_REF] Haupt | A 400-year reconstruction of July relative air humidity for the Vienna region (eastern Austria) based on carbon and oxygen stable isotope ratios in tree-ring latewood cellulose of oaks (Quercus petraea Matt. Liebl.)[END_REF][START_REF] Hilasvuori | Dependence of tree ring stable isotope abundances and ring width on climate in Finnish oak[END_REF], Labuhn et al. 2014[START_REF] Li | Hydroclimate variability in the North China Plain and its link with El Niño-Southern Oscillation since 1784 A.D.: Insights from tree-ring cellulose "18O[END_REF][START_REF] Rinne | 400-year May-August precipitation reconstruction for Southern England using oxygen isotopes in tree rings[END_REF], Shi et al. 2011]). " 18 O in cellulose has been used to reconstruct past atmospheric conditions such as air temperature [START_REF] Edwards | Climatic and hydrologic variability during the past millennium in the eastern Rocky Mountains and northern Great Plains of western Canada[END_REF], Daux et al. 2011, Labuhn et al. 2014], precipitation amount [START_REF] Masson-Delmotte | Changes in European precipitation seasonality and in drought frequencies revealed by a four-centurylong tree-ring isotopic record from Brittany, western France[END_REF][START_REF] Treydte | The twentieth century was the wettest period in northern Pakistan over the past millennium[END_REF]2007[START_REF] Reynolds-Henne | Temporal stability of climate-isotope relationships in tree rings of oak and pine (Ticino, Switzerland)[END_REF][START_REF] Saurer | An investigation of the common signal in tree ring stable isotope chronologies at temperate sites[END_REF], drought stress [START_REF] Masson-Delmotte | Changes in European precipitation seasonality and in drought frequencies revealed by a four-centurylong tree-ring isotopic record from Brittany, western France[END_REF], " 18 O of precipitation [START_REF] Saurer | Stable carbon isotopes in tree rings of beech: Climatic versus site-related influences[END_REF], Robertson et al. 2001[START_REF] Danis | Reconstruction of past precipitation "18O using treering cellulose "18O and "13C: A calibration study near Lac d'Annecy, France[END_REF]], relative air humidity [START_REF] Saurer | Stable carbon isotopes in tree rings of beech: Climatic versus site-related influences[END_REF], Robertson et al. 2001[START_REF] Edwards | Climatic and hydrologic variability during the past millennium in the eastern Rocky Mountains and northern Great Plains of western Canada[END_REF], cloud cover [START_REF] Shi | Reconstruction of southeast Tibetan Plateau summer climate using tree ring "18O: Moisture variability over the past two centuries[END_REF], and even atmospheric circulation patterns [START_REF] Miller | Forced annular variations in the 20th century Intergovernmental Panel on Climate Change Fourth Assessment Report models[END_REF][START_REF] Roden | Summer precipitation influences the stable oxygen and carbon isotopic composition of tree-ring cellulose in Pinus ponderosa[END_REF][START_REF] Brienen | Detecting evidence for CO 2 fertilization from tree ring studies: The potential role of sampling biases[END_REF], Liu et al. 2012[START_REF] Roden | Is the dual-isotope conceptual model fully operational?[END_REF]. The " 13 C variations in tree-rings have also been widely used to assess past climate variability (e.g., [START_REF] Hemming | Climate variation and the stable carbon isotope composition of tree ring cellulose: An intercomparison of Quercus robur, Fagus sylvatica and Pinus silvestris[END_REF], [START_REF] Mccarroll | Stable carbon isotope ratios of Pinus Sylvestris from northern Finland and the potential for extracting a climate signal from long Fennoscandian chronologies[END_REF], [START_REF] Helle | Climatic history of the Tibetan Plateau for the last 1500 years as inferred from stable carbon isotopes in tree rings[END_REF]) and soil water content [START_REF] Panek | Response of stomatal conductance to drought in ponderosa pine: implications for carbon and ozone uptake[END_REF], as well as to establish changes in the intrinsic water use e ciency through time (iWUE) linking leaf-level CO 2 assimilation rate (A) to stomatal conductance (g) (e.g., [START_REF] Ponton | Carbon isotope discrimination and wood anatomy variations in mixed stands of Quercus robur and Quercus petraea[END_REF], Keenan et al. [2013], [START_REF] Saurer | Spatial variability and temporal trends in water-use e ciency of European forests[END_REF], [START_REF] Frank | Water-use e ciency and transpiration across European forests during the Anthropocene[END_REF]), to study the leaf gas-exchange strategy of the plants under changing ambient CO 2 [START_REF] Marshall | Homeostatic gas-exchange parameters inferred from 13C/12C in tree rings of conifers[END_REF], Ainsworth and Rogers 2007[START_REF] Voelker | A dynamic leaf gas-exchange strategy is conserved in woody plants REFERENCES under changing ambient CO2 : evidence from carbon isotope discrimination in paleo and CO 2 enrichment studies[END_REF][START_REF] Frank | Water-use e ciency and transpiration across European forests during the Anthropocene[END_REF] or to evaluate fluctuations in photosynthetic 13 C discrimination [START_REF] Wingate | Photosynthetic carbon isotope discrimination and its relationship to the carbon isotope signals of stem, soil and ecosystem respiration[END_REF]].

Compared to TRW, there is a great lack of " 18 O and " 13 C tree-ring paleoclimate studies in South America. Only three studies have been performed south of 30°S. Consistent with the decrease in atmospheric " 13 C, [START_REF] Leavitt | South American tree rings show declining "13C trend[END_REF] have detected gradual decline in the " 13 C of F. cupressoides in Chile over the 1800-1991 period but no change in the net influence of atmospheric CO 2 concentrations and environmental factors. The tree-ring " 13 C of N. pumilio from southern Patagonia (El Chaltén; 49°S) has been shown to be a good indicator of temperature variations and water deficit along the altitudinal gradient [START_REF] Srur | Influencias de las variaciones en el clima y en la concentración de CO2 sobre el crecimiento de Nothofagus pumilio en la Patagonia[END_REF]].

Only one annually resolved chronology of tree-ring " 18 O, developed from the whole wood of A. chilensis, has been carried out in the Patagonian Andes [Roig et al. 2006]. This study has shown strong correlation between " 18 O and summer SOI.

These results are encouraging and promising for characterizing climate variability in southern South America and understanding its past interaction with tropical and extra-tropical circulation patterns using tree-rings parameters.

Issues with tree-ring studies: the divergence phenomenon

Temporal stability of the relationship between a climate proxy such as tree-rings and a climate target is crucial for reconstructing climatic variations faithfully. However, over the past 20 years, the uniformitarian assumption underlying paleoclimatic reconstruction has been challenged by the dendrochronological community. Indeed, a number of tree-ring studies have reported an apparent decoupling between relative tree growth and concurrent temperature trends in recent decades at several locations of the boreal forest where temperature has historically controlled tree growth. This temporarily unstable relationship has been defined as the divergence (e.g., Jacoby and D'Arrigo [1995], Bri a et al. [1998; 2004], [START_REF] Wilson | Tree-ring reconstruction of maximum and minimum temperatures and the diurnal temperature range in British Columbia, Canada[END_REF], [START_REF] Frank | Warmer early instrumental measurements versus colder reconstructed temperatures: shooting at a moving target[END_REF], [START_REF] Wilson | A matter of divergence: Tracking recent warming at hemispheric scales using tree ring data[END_REF], D 'Arrigo et al. [2004;2008], [START_REF] Esper | Divergence pitfalls in tree-ring research[END_REF], [START_REF] Büntgen | Fading temperature sensitivity of Alpine tree growth at its Mediterranean margin and associated e ects on large-scale climate reconstructions[END_REF], [START_REF] Esper | Trends and uncertainties in Siberian indicators of 20th century warming[END_REF], Allen et al. [2014]).

The divergence suggests that the dominant environmental drivers of tree growth have changed in recent decades [START_REF] D'arrigo | On the 'Divergence Problem' in Northern Forests: A review of the tree-ring evidence and possible causes[END_REF]]. However, this is still a matter of debate since much of this divergence could arise from end e ects introduced by the statistical techniques used to produce standardized tree-ring width chronologies [Melvin and Bri a 2014a;2008]. Several causes have been proposed for the divergence, including warming-induced drought [Jacoby and D'Arrigo 1995], long-term changes in snow cover [START_REF] Vaganov | Influence of snowfall and melt timing on tree growth in subarctic Eurasia[END_REF], global dimming [START_REF] D'arrigo | On the 'Divergence Problem' in Northern Forests: A review of the tree-ring evidence and possible causes[END_REF], stratospheric ozone depletion [START_REF] Bri A | Large-scale temperature inferences from tree rings: A review[END_REF]] and changes in carbon allocation patterns [START_REF] Lapenis | Climate warming shifts carbon allocation from stemwood to roots in calcium-depleted spruce forests[END_REF]]. In contrast with the numerous studies in the boreal forests, the divergence (or time-dependence) in tree-ring chronologies has been poorly documented in austral forests. Some exceptions are the studies conducted by Allen et al. [2014] who have highlighted general time-dependence between Lagarostrobos franklinii growth and temperature in Tasmania over the last 100 years at interannual to interdecadal timescales, and by [START_REF] Suarez | Sensitivity of Nothofagus dombeyi tree growth to climate changes along a precipitation gradient in northern Patagonia, Argentina[END_REF] who identified an increase in the vulnerability of N. dombeyi to droughts with recent warmer conditions across its entire range of distribution east of the Andes. There-fore, the divergence issue still needs to be evaluated in dendrochronological records from South America.

Currently, few studies in the Northern Hemisphere have documented the divergence phenomenon when using stable isotopes in tree-rings [Daux et al. 2011[START_REF] Seftigen | The influence of climate on 13C/12C and 18O/16O ratios in tree ring cellulose of Pinus sylvestris L. growing in the central Scandinavian Mountains[END_REF][START_REF] Hilasvuori | Stability of climate signal in carbon and oxygen isotope records and ring width from Scots pine (Pinus sylvestris L.) in Finland[END_REF][START_REF] Reynolds-Henne | Temporal stability of climate-isotope relationships in tree rings of oak and pine (Ticino, Switzerland)[END_REF][START_REF] Young | A 500-year record of summer near-ground solar radiation from tree-ring stable carbon isotopes[END_REF][START_REF] Naulier | Temporal instability of isotopes-climate statistical relationships -A study of black spruce trees in northeastern Canada[END_REF]].

Most of them highlighted divergence or time-dependency of " 13 C and to a lesser extent of " 18 O series with summer temperature over the last decades due to: 1) an earlier start of the growing season and an increase in moisture stress with increasing temperature [START_REF] Seftigen | The influence of climate on 13C/12C and 18O/16O ratios in tree ring cellulose of Pinus sylvestris L. growing in the central Scandinavian Mountains[END_REF][START_REF] Hilasvuori | Stability of climate signal in carbon and oxygen isotope records and ring width from Scots pine (Pinus sylvestris L.) in Finland[END_REF][START_REF] Reynolds-Henne | Temporal stability of climate-isotope relationships in tree rings of oak and pine (Ticino, Switzerland)[END_REF]], 2) an uptake of deeper source water in soils during summer growth [Daux et al. 2011] or even 3) an increase of water uptake from snow melt with longer growing seasons [START_REF] Naulier | Temporal instability of isotopes-climate statistical relationships -A study of black spruce trees in northeastern Canada[END_REF]]. However, no study has been focussing on the stability of the climate-" 13 C and " 18 O relationships over the last decades in SH, specifically in southern South America.

Aim and objective

The main goal of this thesis is to investigate the responses of upper treeline forests of southern South America to local and large-scale climate forcings in order to improve our understanding of the tree-ring climate proxy and to assess its potential for paleoclimate reconstructions.

The sensitivity of a given tree species to climate variability depends primarily on its own physiological functioning. However, we still lack knowledge on how climatic signals may vary among species growing in similar temperate climatic conditions, especially in southern South America. Therefore, tree-ring parameters of two native species from northern Patagonia growing at ca. 41°S and previously shown to be good proxies of temperature and/or precipitation are studied here (Fitzroya cupressoides and Nothofagus pumilio).

The principal research questions addressed in this manuscript are:

• Which climatic factors influence the variability of tree-ring parameters (width and carbon and oxygen isotopic composition)?

• What are the best proxies and the best climate targets for paleoclimate reconstructions?

• Are the relations between the chronologies of tree-ring parameters and the local to large-scale climate forcings stable over time?

The results presented in this manuscript will give a better understanding of the complex factors influencing tree-ring proxies. Based on these analyses, we will be able to determine which of the two species is the best recorder of climate variability through time, and which climate target can be reconstructed using either tree-ring growth, stable isotopes, or both.

Outline of the research

The thesis is organized in ten sequential chapters and is divided into four parts. Part I reviews environmental information recorded in the tree-rings, introduces the study area, and presents the methods used to produce the proxy records of the thesis. Part II investigates forest responses to recent climate changes in terms of tree-ring growth. Part III attempts to extract the climatic signal recorded in the stable isotope composition of cellulose as well as to understand physiological responses of trees to climate forcings. Finally, Part IV concludes with a synthesis of the result and an outlook on future research perspectives.

An outline of each chapter is presented below.

Chapter 2. Tree rings as palaeoclimate archives

This chapter introduces the tree-rings as palaeoclimate archives and summarizes the mechanisms that influence tree-ring growth and the stable isotopic signal recorded in cellulose.

Chapter 3. Study area and species

An overview of the local climate variability as well as the two species studied are presented here.

Chapter 4. Tree-ring data and methods

The di erent analytical methods used to develop TRW and stable isotopic chronologies are stated.

Chapter 5. The influence of non-stationary teleconnections in southern South America on tree-ring growth

In this chapter, the responses of tree-ring growth to local climate in F. cupressoides and N. pumilio are examined and are related to large-scale climate forcings (SAM and ENSO). The temporal stability of SAM and ENSO impacts on the climate of the region during austral spring is further analyzed. A temporal decoupling between F. cupressoides tree-ring growth and temperature is observed since the late 1970s, coincident with the well-known step increase in temperature in 1976. No step increase of N. pumilio growth is observed over time but only time-dependency to local climate.

Chapter 6. Temporal changes in climatic limitation of N. pumilio growth

This chapter evaluates changes in climatic limitation of the interannual tree-ring growth of N. pumilio along the precipitation gradient in northern Patagonia by using correlation analysis and a forward process-based model (Vaganov-Shashkin-Lite) of tree-ring formation. The results provide evidence for a non-stationarity in the growth response to recent climate regime shifts.

The main findings of this chapter are published in Dendrochronologia as:

Lavergne A., Daux V., Villalba R. and [START_REF] Lavergne | Temporal changes in climatic limitation of tree growth at upper treeline forests: Contrasted responses along the west-to-east humidity gradient in Northern Patagonia[END_REF], Temporal changes in climatic limitation of tree-growth at upper treeline forests: Contrasted responses along the west-to-east humidity gradient in Northern Patagonia. Dendrochronologia, 36, pp. 49-59, doi:10.1016Dendrochronologia, 36, pp. 49-59, doi:10. /j.dendro.2015.09.09.001.

Here the potential of the oxygen isotopic composition of cellulose of F. cupressoides and N. pumilio is investigated for reconstructing past climate variability. The analysis highlights a strong common climatic signal recorded in tree-ring " 18 O of the two species but an higher ability of N. pumilio to record summer temperature variations representative of a large area south of 38°S, under the influence of SAM. Forests are covering up 30% of the total surface of the Earth [START_REF] Bonan | Forests and climate change: forcings, feedbacks, and the climate benefits of forests[END_REF][START_REF] Fao | State of the world's forests[END_REF]] and contain about 861± 66 PgC [START_REF] Pan | A large and persistent carbon sink in the world's forests[END_REF]. They are very valuable biological systems as they influence climate through exchanges of energy, water, carbon dioxide and other chemical species with the atmosphere (Fig. 2.1).

Trees internal functioning

Trees are predominantly living in close interrelation with the soil and atmosphere compartments. To maintain their metabolism, growth and development, they collect water (H 2 O) and nutrients in the soil and carbon dioxide (CO 2 ) in the atmosphere. The carbon, oxygen and hydrogen that they incorporate have essentially a structural role since they are the skeleton of all organic molecules.

The cambium produces conductive tissues, such as xylem and phloem, that provide stems support and conduction of water and nutrients to the di erent organs. Xylem ensures the vertical transport of water and dissolved minerals from the roots to the leaf system. It consists of dead cells and is impregnated with lignin which sti en the cell walls and enhance both their structural support and the transfer of water. Phloem cells actively transport the sap bringing organic materials from photosynthetic sites (leaves) to storage or high metabolic activity sites.

The hydraulic system: ascent of the sap

Water is the most abundant component of most living organisms. Its thermal properties contribute to regulate the temperature of the plant, helping not to cool or warm up too Small pores on the surface of the leaves, the stomata, regulate the distribution of CO 2 and H 2 O between the leaves and the atmosphere. H 2 O molecules are smaller than those of O 2 or CO 2 , so that the membranes which allow gas exchange are also permeable to water. As the leaves open their stomata to absorb CO 2 for photosynthesis, leaf water escapes into the atmosphere by evaporation. To compensate for the loss of water from the leaves and to keep them hydrated and photosynthetically active, leaves should be irrigated continuously with water [START_REF] Delzon | Recent advances in tree hydraulics highlight the ecological significance of the hydraulic safety margin[END_REF].

The water extracted from the roots is a dilute solution named sap. There is a hydraulic continuity throughout the plant, from the cell walls of the leave where evaporation occurs to the soil particles surrounding the roots. Three determining physical factors cause the rise of the xylem sap and put the water column on pressure: the transpiration, the water potential di erences and the capillary tension forces (Fig. 2.2). The curvatures of meniscus, which are small curved surfaces that form on the evaporation surface of the leaves at the air-water interface, put the water column on pressure. Pressure gradients are thus established from the surface of the meniscus to the ground water in the vicinity of the roots. Water potential quantifies the propensity of water to flow from one area to another due to osmosis, gravity, mechanical pressure or by capillary action caused by the surface tension. Under natural conditions, the water potential of the various parts of a plant is negative (between -0.1 and -3 MPa). In the soil, water is not retained by hydrophilic molecules in the aqueous solution (water potential of nearly 0), while in the leaves, it is strongly linked (very negative potential). The more important the potential di erence of the water between the ground and the leaf, the stronger the water flow, although the water flow can be hampered by the hydraulic resistance of the plant [START_REF] Cruiziat | La cavitation : un mécanisme perturbant la circulation de l'eau chez les végétaux[END_REF].

The continuity of the water column is maintained by the internal pressure forces in the water. As the water molecules are strongly linked, water columns have a very strong cohesion. During the evaporation, the whole column of sap is moving and soil water is absorbed by the roots at the other end. This mechanism of "tension-cohesion" was explained by [START_REF] Dixon | On the ascent of sap[END_REF]. When transpiration increases in the leaves, leaf water potential decreases. If water is available in the soil, water loss through transpiration is compensated by an increase of soil water inside the water column. But if the amount of water in the soil decreases (more negative water potential), leaf water potential must be more negative to cause water upward movement. In order to maintain the potential di erence between the ground and the leaves, other substances must be stored in the leaf cells. The osmotic potential determines the water level in the cell in order to restore the necessary balance of ion or hydrophilic molecules concentrations [START_REF] Cruiziat | La cavitation : un mécanisme perturbant la circulation de l'eau chez les végétaux[END_REF]].

Soils, roots and absorption of nutrients and minerals

The soil is a complex environment composed of mineral rock particles, organic materials in various stages of decomposition, a solution containing solutes of soil, gases, and a variety of microorganisms. The soil is mainly constituate of sand, silt and clay. Soil structure determines the porosity of the soil and influences water retention and drainage. The water from the precipitation infiltrates into the soil, fills the pores and percolates under the e ect of gravity. The size and nature of the pores depend on the type of the soil. Thus, a soil having rather large particles will tend to have a macroporosity and therefore a fast drainage, while a soil with thinner and more compact grains will be less easily drained. The balance between water retention and drainage allows optimal growth. The absorption of minerals by trees involves complex interactions between soil and tree roots. The e ciency of the roots depends on the extension of the rooting zone. Indeed, the root systems prospect the ground area to extract water and nutrients they need for growth. The solid particles resulting from the decay of existing rocks constitute the major source of nutrients that will be absorbed by the tree. Thus, the composition, structure and nutrient supply vary greatly depending on soil type.

Tree growth and wood formation

Wood formation is the result of several processes: cell division of the cambium, cell growth (radial expansion and longitudinal enlargement of the cells), deposition and thickening of the cell wall, and finally, lignification of the cell wall. During the formation of wood, many interdependent processes occur in the whole tree, tissues and cells level. These processes are regulated by several intrinsic factors, such as gene expression and hormonal signals, as well as environmental factors such as temperature and precipitation [START_REF] Schrader | Polar auxin transport in the wood-forming tissues of hybrid aspen is under simultaneous control of developmental and environmental signals[END_REF]2004[START_REF] Gri Ar | Plastic and locally adapted phenology in cambial seasonality and production of xylem and phloem cells in Picea abies from temperate environments[END_REF]]. The combined action of intrinsic and environmental factors is responsible for the unique structure of tree-ring formed at a specific time in a specific position in the architecture of the plant [Vaganov et al. 2006].

Photosynthesis

Trees capture the sun's energy to carry out the chemical reactions for synthesizing complex organic molecules or sugars (carbohydrates) from simple inorganic elements. At the level of the leaf chloroplasts, the green pigment named chlorophyll captures light energy, uses CO 2 from the atmosphere and the water in the sap to synthesize carbohydrates and rejects the O 2 in the atmosphere.

Carbohydrates produced during photosynthesis are potential sources of immediate energy for respiration, reserves of stored energy (starch) and supply for plant tissue construction.

The annual tree-ring

The tree-ring formation duration depends on the species, the geographic location (latitude, altitude) and climate. In temperate regions, the activity of the cambium follows a seasonal cycle between spring and summer. The cambium becomes active with the seasonal increase in temperatures in spring but, with decreasing temperature in autumn, activity slows down to enter in the so-called period of winter dormancy. Tree-rings are formed from soluble sugars, or carbohydrates, that are transported by the phloem. These sugars are a mixture of carbon reserves mobilized for growth and of newly synthesized leaf assimilates through photosynthesis.

During the period of growth, the cells of the cambium form xylem vessels (wood) toward the center while they produce phloem tissues to the outside (trunk; Fig. 2.3). At the beginning of the growing season, the cells are broad and have a thin cell wall (earlywood). Advancing in the growing season, trees are growing more slowly, cells become increasingly small and the cell wall is thickened (latewood).

The structure of the wood may di er between gymnosperms (conifers) and angiosperms (deciduous trees). Conifers are mainly formed of fibers while deciduous have also large vessels which optimize the transport of water. For deciduous, fiber cells of earlywood and latewood does not change significantly, only the distribution of vessels varies; vessels are more numerous in earlywood than in latewood. The tree produces earlywood vessels to restore the hydraulic conductivity before forming new leaves. 30-40% of the annual growth occurs before new leaves are formed and the CO 2 assimilation begins [START_REF] Michelot | Di ering growth responses to climatic variations and soil water deficits of Fagus sylvatica, Quercus petraea and Pinus sylvestris in a temperate forest[END_REF]]. The timber is formed of carbohydrates made during the previous growing season and kept in reserve. The timber consists of 30 to 50% of cellulose (C 6 H 10 O 5 ) n , 15 to 35% of hemicellulose and 15 to 30% of lignin with less than 5% of solvent-extractable substances (mainly sugars and resins).

Stable isotopes in tree-rings

Stable isotopes ratios in tree-ring cellulose or whole wood are natural tracers of climate controls on the water and carbon cycle. They are increasingly used as tools to obtain insight into plant physiological responses to climate [START_REF] Treydte | Climate signals in the European isotope network ISONET[END_REF][START_REF] He Ernan | Pacific puzzle[END_REF], Gessler et al. 2013;;[START_REF] He Ernan | Pacific puzzle[END_REF], Hartl-Meier et al. 2014].

Stable isotopes variability

The natural stable isotopes have identical chemical properties but di erent masses which makes them independently sensitive to physical and chemical biological processes.

Oxygen has 3 natural stable isotopes, each having 8 protons but 8, 9 or 10 neutrons. The natural relative abundances of these 3 isotopes are: 99.76% for 16 O, 0.2% for 18 O and 0.004% for 17 O. In contrast, carbon has 2 stable isotopes, 12 C and 13 C, each with 6 protons and 6 or 7 neutrons, respectively. The natural relative abundances of these 2 isotopes are 98.99% for 12 C and 1.11% for 13 C.

By convention, the stable isotopes ratios are expressed using the delta notation (") in reference to a known isotope ratio standard, i.e. the Vienna-SMOW (Standard Mean Ocean Water) for oxygen and the Vienna-PDB (or VPDB; [START_REF] Coplen | Reporting of stable hydrogen, carbon, and oxygen isotopic abundances (Technical Report)[END_REF]) for carbon. The oxygen and carbon isotope ratios (" 18 O and " 13 C) are expressed in parts per thousand (% ):

" = 1 R sample R standard ≠1 2 1000
where R sample et R standard are respectively isotopic ratio of sample and standard.

Variability in the isotopic composition of H 2 O and CO 2

Global changes in " 18 O precipitation are intricately related to climate [Dansgaard 1964]. Within the global hydrological cycle, the " 18 O of water is subject to several changes of isotopic ratio between a source and its product, known as the isotopic fractionation (Fig. 2.4).

Equilibrium and kinetic isotope fractionation of " 18 O

The isotopic fractionation can be thermodynamic or kinetic (e.g., [START_REF] Criss | Principles of stable isotope distribution[END_REF]). On the one hand, the mass di erences induce a thermodynamic or equilibrium fractionation, which depends on temperature. The heavier water molecules containing the heavier isotopes ( 18 O) condensate preferentially and accumulate into the condensed phase due to the lower saturation vapor pressure of the heavy isotopes. On the other hand, the di erence between the molecular di usivity of these isotopic species (related to the mass and to the symmetry of the molecules) induced a kinetic fractionation. The heavier molecules have a lower di usivity. Thus, during evaporation, the isotopic composition of water evaporated (enriched in 16 O) tend to be poorer in 18 O at non-equilibrium than at thermodynamic equilibrium. The kinetic fractionation depends in particular on the relative air humidity as well as the turbulent and molecular di usivity of water isotopes in the air.

Spatial distribution of " 18 O

The kinetic and equilibrium isotope fractionation associated with the initial evaporation of water vapor from its source (typically ocean water) generate atmospheric moisture depleted in 18 O relative to water source [START_REF] Craig | Deuterium and oxygen 18 variations in the ocean and the marine atmosphere[END_REF]. The cooling of a moist air mass, mixing with a cooler air mass or even radiative energy loss, lead to the condensation of the water in the atmosphere (liquid or solid). The equilibrium fractionation results in a precipitation isotopically enriched in heavy isotope 18 O compared to the water vapor in the atmosphere. Isotope exchange of liquid water drops with water vapor within clouds or below clouds base can also occur during the fractionation processes.

The amount of fractionation varies according to temperature, altitude, distance from moisture sources and local precipitation amount [Dansgaard 1964[START_REF] Aggarwal | Stable isotopes in global precipitation: A unified interpretation based on atmospheric moisture residence time[END_REF]. Condensation and precipitation are controlled by temperature. High temperature favors the evaporation of the lighter water molecules leading to an increase in 18 O/ 16 O ratio in the water vapor that condensed (temperature e ect). When temperatures rise above approximately 20°C (the amount e ect threshold) and there is significant precipitation and/or high humidity, the abundance of 18 O in the meteoric water decreases [START_REF] Rozanski | Isotopic Patterns in Modern Global Precipitation[END_REF]]. Both the temperature and amount dependencies of isotopes are influenced by moisture source and transport patterns. Therefore the " 18 O of precipitation records information about regional weather patterns or large-scale atmospheric circulation dynamics [START_REF] Baldini | Spatial variability in the European winter precipitation "18O-NAO relationship: Implications for reconstructing NAO-mode climate variability in the Holocene[END_REF][START_REF] Field | Observed and modeled controls on precipitation "18O over Europe: from local temperature to the Northern Annular Mode[END_REF].

Variability in " 13 C

Atmospheric CO 2 is the principal carbon source for terrestrial plants. Currently the mean " 13 C of atmospheric CO 2 is about -8% [START_REF] Keeling | Atmospheric CO2 and 13CO2 exchange with the terrestrial biosphere and oceans from 1978 to 2000: Observations and carbon cycle implications[END_REF] but is changing seasonally with relative 13 C depletion during winter and 13 C enrichment during summer. The increase in the emissions of 13 C-depleted CO 2 due to fossil fuel combustion over the last decades has induced a progressive decrease in " 13 C. [START_REF] Epstein | Oxygen and Hydrogen Isotopic Ratios in Plant Cellulose[END_REF] recommended to use a single chemical component (usually the cellulose) for paleoclimate studies since each component shows di erences in their oxygen and carbon isotope composition [START_REF] Leavitt | Tree-ring C-H-O isotope variability and sampling[END_REF]Danzer 1993, Barbour et al. 2002]. On the one hand, mobile [START_REF] Ferrio | Carbon and oxygen isotope ratios in wood constituents of Pinus halepensis as indicators of precipitation, temperature and vapour pressure deficit[END_REF]. Original data from (IAEA/WMO 2001) and [START_REF] Saurer | Stable carbon isotopes in tree rings of beech: Climatic versus site-related influences[END_REF] compounds such as resins and sugars might move along the parenchyma cells between several tree-rings [START_REF] Leavitt | Tree-ring C-H-O isotope variability and sampling[END_REF]Danzer 1993, McCarroll andLoader 2004]. On the other hand, the distribution of the di erent components may vary from year-to-year and inside a same ring [START_REF] Epstein | Oxygen and Hydrogen Isotopic Ratios in Plant Cellulose[END_REF][START_REF] Borella | C analysis of tree rings: Pooling, milling, and cellulose extraction[END_REF]]. The time of deposition of cellulose and lignin into the whole wood is also delayed [START_REF] Boerjan | Lignin biosynthesis[END_REF]]. These processes potentially alter past climate signal recorded in the whole wood. Part of the oxygen in lignin precursors is derived from molecular oxygen and not exclusively from water [START_REF] Boerjan | Lignin biosynthesis[END_REF][START_REF] Vanholme | Lignin biosynthesis and structure[END_REF], Gessler et al. 2014], as it is the case for sugars and cellulose. Therefore, for the present study, we decided to use only cellulose.

Use of cellulose for paleoclimate studies

Isotopic fractionation in trees

The carbon used for the annual tree-ring synthesis is derived from atmospheric CO 2 molecules absorbed by the leaf stomata and used during photosynthesis, whereas oxygen is derived from soil or ground water and thus indirectly from precipitation. Isotope ratios in the wood are very di erent from those measured in the air or in the source water. Indeed, fractionations occur prior to storage. The degree of fractionation is controlled to some extent by the response of the trees to their environment (e.g., McCarroll and Loader [2004]).

Oxygen isotope in cellulose

The oxygen isotopic composition of cellulose depends on : (i) the oxygen isotopic composition of the source water, (ii) the evaporative enrichment of leaf water during transpiration, (iii) the biochemical fractionation during photosynthesis in the leaf, and (iv) the isotopic exchange of oxygen between water and photosynthetic products (Fig. 2.5 and 2.6).

(i) Source water oxygen isotopic composition

The isotopic composition of the source water depends primarily on the isotopic composition of local rainfall (" 18 O p ). However, " 18 O of water taken up by the roots can be modified from that of " 18 O p by: 1) evaporation from the soil surface, 2) temporal separation of precipitation and uptake by roots and 3) uptake of groundwater that is isotopically distinct from recent precipitation [START_REF] Adams | Stable isotopes at natural abundance in terrestrial plant ecology and ecophysiology: An updata[END_REF].

After precipitation water has reached the soil, evaporation from top soil layers may induce an enrichment of the original soil water isotopic composition. This may lead to depth enrichment gradients with highest 18 O enrichment in the upper soil layers than in deeper soil layers.

Depending on soils and environments, the degree of enrichment will vary. For instance, under a vegetation cover where the sunlight absorption by the soil may be weak, the energy and di usion gradients necessary for soil evaporation are weak, thus soil evaporation is limited [START_REF] Polissar | E ects of aridity and vegetation on plant-wax dD in modern lake sediments[END_REF]. The rooting depth of the trees is also important for the source water. Indeed, the isotopic composition of water in shallow soils may reflect the isotopic signal of recent precipitation (modulated by evaporation), while water from deeper soils has a longer residence time and its isotopic composition potentially represents a long-term mean of accumulated precipitations (and which have been more or less enriched by evaporation).

There is no fractionation during water uptake by roots [START_REF] Wershaw | Hydrogen isotope fractionation in water passing through trees[END_REF], neither during the transport of water from the roots to the leaves. Thus, the isotopic signature of xylem water can be used as a proxy for the signature of the water extracted from the roots.

(ii) Evaporative enrichment of leaf water

The critical site of fractionation is at the leaf level where evaporation due to leaves transpiration leads to a loss in light isotopes 16 O and enrichment of heavy isotopes 18 O of the leaf water [START_REF] Farquhar | Vegetation e ects on the isotope composition of oxygen in atmospheric CO2[END_REF] as much as 20% [START_REF] Saurer | Oxygen Isotope Analysis of Cellulose: An Interlaboratory Comparison[END_REF]]. The evaporative enrichment of the leaf water is influenced by temperature and light energy, by stomatal conductance and vapor pressure deficit (VPD), both of which are related to the relative humidity (RH) [START_REF] Farquhar | Heavy water fractionation during transpiration[END_REF]], and the resulting fractionation is modulated by the " 18 O of water vapor outside of the leaf [START_REF] Roden | Hydrogen and oxygen isotope ratios of tree ring cellulose for field-grown riparian trees[END_REF]; Fig. 2.6). The enrichment of the leaf water 18 O e is given by the Craig-Gordon model:

18 O e = ' ú + ' k + ( 18 O v -' k ) e a /e i
where ' ú is the liquid-water equilibrium fractionation associated with the depletion of water vapor in H 18 2 O dependent on temperature [START_REF] Bottinga | Calculated fractionation factors for carbon and hydrogen isotope exchange in the system[END_REF]], e k is the kinetic fractionation factor during the di usion of water vapour from the leaf to the atmosphere (26.5 % ), 18 O v is the isotopic di erence of atmospheric water vapour compared with source water, and e a and e i are the ambient and intercellular vapor pressures [START_REF] Craig | Deuterium and oxygen 18 variations in the ocean and the marine atmosphere[END_REF][START_REF] Dongmann | On the enrichment of H2 18O in the leaves of transpiring plants[END_REF][START_REF] Barbour | Correlations between oxygen isotope ratios of wood constituents of Quercus and Pinus samples from around the world[END_REF]2002;[START_REF] Dobbertin | Tree growth as indicator of tree vitality and of tree reaction to environmental stress: A review[END_REF].

The Craig-Gordon model only explains the isotopic enrichment of water at the site of evaporation. However, the water in the leaf is not well mixed. Indeed, di usion of enriched water from the sites of evaporation to the rest of the leaf is counteracted by the input of unenriched xylem water though the transpiration flow: this is the Péclet e ect [START_REF] Barbour | Oxygen isotope ratio of leaf and grain material correlates with stomatal conductance and grain yield in irrigated wheat[END_REF]2001]. This process varies according to the intensity of the transpiration, the leaf water being less enriched when stomatal conductance and transpiration are important; then the 18 O e overestimates the net e ect of enrichment due to transpiration.

The evaporative enrichment is expanded to the whole leaf lamina as:

18 . According to the model, the original e ect of transpiration and stomatal conductance would be reinforced through its cooling e ect [Farquhar et al. 1993, Wang and[START_REF] Wang | Using stable isotopes of water in evapotranspiration studies[END_REF].

O L = 18 O e 1 1≠

(iii) Biochemical fractionation during photosynthesis

During the first step of carbon fixation and synthesis of sucrose (or assimilates) within the leaf, two oxygen atoms from the leaf water and a third one derived from CO 2 is incorporated into organic matter [START_REF] Farquhar | Interpretation of oxygen isotope composition of leaf material[END_REF][START_REF] Schmidt | 18O pattern and biosynthesis of natural plant products[END_REF]. The biochemical fractionation due to oxygen exchange between carbonyl groups (C = O) in the organic molecules and water [DeNiro and [START_REF] Deniro | Relationship Between the Oxygen Isotope Ratios of Terrestrial[END_REF][START_REF] Sternberg | Biogeochemical implications of the isotopic equilibrium fractionation factor between the oxygen atoms of acetone and water[END_REF][START_REF] Farquhar | Interpretation of oxygen isotope composition of leaf material[END_REF]] leads to an enrichment of approximately 27% of the source water signature [Sternberg andDeNiro 1983, Cernusak et al. 2003]. Recently, the equilibrium fractionation factor (' wc ) has been shown to be temperature dependent below 20°C [Sternberg and Ellsworth 2011], therefore temperature might a ect the transfer of the leaf water isotope signal to organic matter via its e ects on ' wc [START_REF] Clinton | Light, temperature, and soil moisture responses to elevation, evergreen understory, and small canopy gaps in the southern Appalachians[END_REF][START_REF] Lemenih | E ects of canopy cover and understory environment of tree plantations on richness, density and size of colonizing woody species in southern Ethiopia[END_REF]].

(iv) Isotopic exchange between water and organic molecules

Sucrose molecules are then transported to plant sinks from the leaf to the stem through the phloem. During phloem loading and transport, more oxygen exchange between xylem water and organic compounds occurs [START_REF] Sternberg | Variation in oxygen isotope fractionation during cellulose synthesis: Intramolecular and biosynthetic e ects[END_REF]]. To form cellulose, the sucrose molecules cleave into hexose phosphates. One of the five oxygen atoms in the repeating unit of cellulose becomes part of a carbonyl group and is exchangeable with local water [START_REF] Barbour | Stable oxygen isotope composition of plant tissue: a review[END_REF]]. A proportion of the hexose phosphate also go through a futile cycle via triose phosphates before cellulose synthesis [START_REF] Hill | Rapid recycling of triose phosphates in oak stem tissue[END_REF]. Such cycle allows a further exchange of 3 out of 5 oxygen atoms in the cellulose unit with local water. The total proportion of oxygen available for the exchange (p e ) depends on the number of cycles through the triose phosphate [Farquhar et al. 1998, McCarroll andLoader 2004] and was evaluated to be on an average 42% for a wide range of species and environmental conditions [START_REF] Cernusak | Environmental and physiological controls over oxygen and carbon isotope composition of Tasmanian blue gum, Eucalyptus globulus[END_REF]]. However, some observations have indicated that this value is not constant during the entire growing season [START_REF] Gessler | Tracing carbon and oxygen isotope signals from newly assimilated sugars in the leaves to the tree-ring archive[END_REF][START_REF] O Ermann | The long way down-are carbon and oxygen isotope signals in the tree ring uncoupled from canopy physiological processes?[END_REF]. For instance, in spring during the beginning of the growing season when most assimilates are likely to derive from remobilized organic matter [START_REF] Helle | Beyond CO2-fixation by Rubisco -An interpretation of 13C/12C variations in tree rings from novel intra-seasonal studies on broad-leaf trees[END_REF], oxygen atoms from the xylem tend to exchange more with organic matters flowing through the phloem than in summer and autumn [O ermann et al. 2011]. Gessler et al. [2013] have shown that the extent of oxygen atom exchange with non-enriched water is specific.

Finally, given the di erent fractionation processes described above, the isotopic composition of the cellulose " 18 O can be related to the isotopic composition of the source water " 18 O sw and the leaf water " 18 O lw [Yakir andDeNiro 1990, Sternberg et al. 1986] as:

" 18 O cell = 0.42 " 18 O sw + 0.58 " 18 O lw + 27%
Therefore, the dominant environmental controls on " 18 O of cellulose are those controlling the source water isotopic composition and the leaf enrichment, which are related to precipitation " 18 O, temperature and relative humidity.

Carbon isotopes in cellulose

The present day " 13 C of atmospheric CO 2 is about -8% [START_REF] Keeling | Atmospheric CO2 and 13CO2 exchange with the terrestrial biosphere and oceans from 1978 to 2000: Observations and carbon cycle implications[END_REF] and Scripps CO 2 project, http://scrippsco2.ucsd.edu/). Trees are depleted in 13 C compared to atmospheric CO 2 with values around -20% for leaves and -30% for cellulose [McCarroll and Loader 2004].

As we have seen before, the stomata on the leaf surface allow gas exchange between the atmosphere and the leaf (Fig. 2.5 and 2.7). Their opening and closing regulate transpiration and carbon dioxide uptake, which is then dissolved in leaf water. Prior to be synthesized as sugars, carbon dioxide is subject to two isotopic fractionation (Fig. 2.7).

First, during the di usion of air through the stomata, CO 2 molecules containing lighter 12 C di use more easily than those containing heavier 13 C. This induces a depletion in 13 C in the leaf air relative to the ambient air; this fractionation due to di usion is estimated to be of about -4.4% [START_REF] Farquhar | Carbon Isotope Discrimination and Photosynthesis[END_REF]].

The second isotopic fractionation occurs during carboxylation by the photosynthetic enzyme RuBisCO, which sets the internal CO 2 . The lighter isotope 12 C is preferentially used leading to a fractionation of about -27% , the net fractionation due to carboxylation. Thus, " 13 C value of leaf sugars is controlled by the ratio of the concentrations of intercellular (c i ) to ambient (c a ) air CO 2 (c i /c a ) [START_REF] Farquhar | Discrimination and the Intercellular[END_REF], and by CO 2 assimilation rate [START_REF] Helle | Beyond CO2-fixation by Rubisco -An interpretation of 13C/12C variations in tree rings from novel intra-seasonal studies on broad-leaf trees[END_REF]. If stomata are opening and c i is high, then stomatal conductance is much higher than the rate of photosynthesis and there will be a strong carboxylation discrimination against 13 C, yielding low " 13 C. Conversely, if stomata are closing and c i is small, the stomatal conductance is low compared to the rate of photosynthesis, then c i tends to increase and carboxylation discrimination against 13 C decreases, leading to higher " 13 C [Helle andSchleser 2004, McCarroll andLoader 2004].

Therefore, the dominant environmental controls on " 13 C in cellulose are those controlling the rates of stomatal conductance and photosynthesis. At dry sites where trees tend to be moisture-stressed, stomatal conductance dominates, which is controlled by air relative humidity and soil moisture. Conversely, at humid sites, the rate of photosynthesis, which is primarily controlled by irradiance and temperature, is dominating [McCarroll and Loader 2004]. In warm and dry areas, the " 13 C provides a good indicator of the severity of the climatic conditions.

Toward long-term tree-ring archives

Relative strengths of photosynthate supply and demand by multiple sinks can create competition for the substrate. Under stress conditions, carbon allocation to stem growth might be reduced since the use of available carbohydrates for leaf, bud and root growth is more vital [START_REF] Dobbertin | Tree growth as indicator of tree vitality and of tree reaction to environmental stress: A review[END_REF]]. Therefore tree-ring might be incompletely supplied in organic matter. Conversely, during periods of high assimilation rates, the supply will be higher and then oxygen and carbon isotope signatures from these organic matter will dominate the tree-ring isotope signal [Gessler et al. 2014].

Age-related variations in oxygen and carbon isotope compositions of tree-rings can be observed due to changes in sources and physiological ageing e ects [START_REF] Mcdowell | An investigation of hydraulic limitation and compensation in large, old Douglas-fir trees[END_REF][START_REF] Marshall | Co-occurring species di er in tree-ring "18O trends[END_REF], Treydte et al. 2006a, Labuhn et al. 2014]. The development of the root system, the increase of hydraulic resistances of taller trees and concomitant lower stomatal conductance leads to significant di erences between young and old trees. Young trees may have access of water from the upper soil layers with relatively more evaporative 18 O-enriched water than in deeper layers [START_REF] Drake | Water resource partitioning, stem xylem hydraulic properties, and plant water use strategies in a seasonally dry riparian tropical rainforest[END_REF] and may have slightly lower water supply leading to higher 18 O enrichment at the leaf level.

While " 13 C in cellulose is related to the rate of stomatal conductance and photosynthesis, " 18 O in cellulose is controlled by evaporative processes and stomatal conductance but is independent of variations in photosynthetic assimilation rates. Therefore, the combine use of oxygen and carbon isotopic composition of cellulose allows to disentangle the di erent types of gas-exchange responses to environmental pressure and to interpret " 13 C variations as reflecting variation in photosynthetic capacity [START_REF] Scheidegger | Linking stable oxygen and carbon isotopes with stomatal conductance and photosynthetic capacity: a conceptual model[END_REF][START_REF] Grams | Combining "13C and "18O analyses to unravel competition, CO2 and O3 e ects on the physiological performance of di erent-aged trees[END_REF][START_REF] Silva | Probing for the influence of atmospheric CO2 and climate change on forest ecosystems across biomes[END_REF][START_REF] Gómez-Guerrero | Growth decline and divergent tree ring isotopic composition ("13C and "18O) contradict predictions of CO2 stimulation in high altitudinal forests[END_REF]].

3 Study area and species

Geographical and climatic settings

The Patagonian Andes

Patagonia is a large region extending from about 37°S down to the southern tip of South America (55°S). It is a key region to examine the influences of the ocean-atmosphere processes on climate and water availability, and to investigate the response of highly sensitive ecosystems to climate variability. South of 40°S, the N-S Andes act as a topographic barrier to the persistent westerlies bringing moisture from the South Pacific Ocean, leading to one of the most dramatic west-to-east precipitation gradient in Earth [Smith and Evans 2007].

The study area is located in northern Patagonia (37°S-46°S; Argentina) at around 41°S in the east-leeward side of the Andes (Fig. 3.1). In this region, the mountain range induces a sharp decline in mean annual precipitation from up to 4000 mm/year at Cerro Tronador, near the continental divide, to ca. 500 mm/year toward the xeric Patagonian steppe, ca. 50 km to the east of the mountains (Fig. 3.1;Jobbagy et al. [1995], Veblen [1979]). Gross Primary productivity (GPP) follows the eastward decline in precipitation and ranges from around 2000 gC. m 2 . year ≠1 in the most productive forests to about 500 gC. m 2 . year ≠1 in the Patagonian steppe (Fig. 3.1;Muñoz et al. [2014], Zhao et al. [2005]).

Variability and trends in climate records

At Bariloche in the eastern side of the gradient (41°12'S -71°12'W, 840 m asl; Fig. 3.1), the average annual temperature is 8.5°C, with relatively cold winters (average JJA temperature 3°C) and mild summers (average DJF temperature 14.2°C). Monthly precipitation is higher than two times the temperature (Gausen diagram; Fig. 3.2). The climate is therefore not arid in the region. The annual migratory cycle of the Pacific anticyclone induces strong precipitation seasonality (Fig. 3.2). The average annual precipitation is 875 mm but precipitation is concentrated during early fall (27%) and winter (48%), followed by a drier period during spring (15%) and summer (9%; [START_REF] López | ! Sustainable management! of! Lenga! (Nothofagus! pumilio)! forests! through! group! selection system[END_REF]). The cloud cover is mainly concentrated in winter months, while the photosynthetically active radiation is higher in summer (Fig. 3.2). The regional climate does not show dramatic trends over the last decades. However, changepoint analysis [Beaulieu et al. 2012] indicates that long-term precipitation exhibits a shift towards lower mean values from ca. 1952 onwards, which coincides with the start of the positive trend in the Southern Annular Mode (SAM; Fig. 3.3). Temperature anomalies in the region show a significant negative trend of 1.3°C (p < 0.01) between 1945 and 1975 followed by the well-documented step increase around 1976, coincident with the increase in sea surface temperature (SST) in the Tropical Pacific Ocean, but with no overall significant warming trend afterwards (0.03°C for 1977-2008, p > 0.1). These changes in mean temperature and precipitation resulted in a warm and wet period before ca. 1952, a period of significant cooling between 1952 and 1975 and a warm and dry period since around 1976.

Sampled species

The vegetation is characterized by high diversity of endemic species. Four representative floristic ecosystem of the Patagonian Andes are identified in the Nahuel Huapi Argentinian National park, which areas are defined by the combination of the wide variation of precipitation and the marked di erences in topography. The Valdivian temperate rain forest, a temperate broadleaf and mixed-forest ecoregion, is located in the west part of the park, close to Cerro Tronador from 3200 m at the peaks of the mountains to about 650 m on the Nahuel Huapi Lake. On a narrow band of high precipitation over a discontinuous distribution from 39°50'S to 43°30'S, grows the long-lived conifer Fitzroya cupressoides (blue in Fig. 3.4). Given its longevity (above 3600 years) [Lara and Villalba 1993;1994], this slow-growing endemic species is one of the most interesting species for long-term climate reconstruction in the region. Locally known as Alerce, it is growing at middle elevations, on shallow and poorly drained, peaty or sandy soils, rich in organic matter, with low pH and high C/N ratio [START_REF] Peralta | Suelos forestales representativos de la Cordillera de los Andes[END_REF]].

In 1973, Alerce has been classified as Endangered under Appendix I of CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora). In 1976, it has been declared a National Monument and the exploitation of live trees been prohibited. In 1979 it has been included as a threatened species under the Endangered Species Act in the United States, forbidding its import to that country [START_REF] Premoli | Fitzroya cupressoides[END_REF]. F. cupressoides species is highly sensitive to climate change, therefore it is even more endangered.

Another species of interest growing in the Southern Chilean and Argentinean Patagonia and dominating the subantarctic forests is the deciduous Nothofagus pumilio. Locally known as Lenga in Mapuche language, this beech is well distributed along the north-south temperature gradient from about 35°S to 56°S [Donoso 1981] and the strong west-to-east precipitation gradient, and ranges from sea-level to the Andean treeline (green in Fig. 3.4). It grows in areas with low temperature and abundant snow, and tolerates temperatures below -30°C. In dry locations, N. pumilio forests develop mainly on rich soils formed on recent volcanic ashes [Rusch 1993] and their roots can reach ground water down to 2-3 m depth [Schulze et al. 1996].

The vegetative growth period begins in September when air and soil temperatures are higher than a threshold value and ends in May but is shorted with elevation due to the delayed leaf budding and the acceleration of leaf fall [Rusch 1993]. Potential productivity decreases with altitude due to smaller leaf size [Rusch 1993].

Its spatial distribution provides a unique opportunity to compare the influence of contrasting cool-wet vs warm-dry regional climates on the trees at altitudinal treelines. The climatic parameters recorded in its tree-rings are powerful tools to evaluate the impact of these climatic gradients in a climate change context.

Sample collection in the field

The sampling campaign was conducted in March 2013 by the end of the growing season. Six sites (2 for F. cupressoides and 4 for N. pumilio) previously sampled during the 1990s [Lara andVillalba 1993, Villalba et al. 2003;1997] were re-visited to update the network of tree-ring chronologies. The sites are located in tree-line environments on moderate to steep slopes (approximately 15°to 40°) along the regional precipitation gradient (Fig. 4.1). The soils are therefore very likely well drained. From west to east, the sampling sites are Glaciar Frías (GF) and Río Alerce (RA) for F. cupressoides and, Paso de la Nubes (NUB), La Almohadilla (ALM), Diego de León (DL) and Challhuaco (CHA) for N. pumilio (Tab. 4.1). At each site, at least 30 trees were cored at about 1.3 m height above ground from two directions at 90°or 180°of each other using a Pressler borer with a diameter of 5 mm.

Standard dendrochronological techniques

Previous collections from the selected sites available in IANIGLA were processed in a first step, and then, increment-borer samples from F. cupressoides and N. pumilio collected during the field campaign were analyzed. In the dendrochronology laboratory, the samples were dried and mounted in wood pre-grooved sticks with their fiber direction perpendicular to the mount, to be fixed in place and then finely sanded. The cores were manually polished with aluminium oxide sandpaper grit of di erent sizes. More than 500 cores were examined and cross-dated at IANIGLA laboratory following the technique described by Stokes and Smiley [1968].

Sample datation

Since the biological growth season in the Southern Hemisphere does not coincide with the calendar year, the date of the annual ring was assigned to the year when ring growth began [Schulman 1956]. The ring 2012 was still not complete therefore the last ring dated was the 2011 one.

Tree rings were visually dated using a binocular microscope starting from outer parts (younger) to the centre (older). With a pencil, decades were marked with one dot, half-century with two dots and a century, with three dots in order to be easily identifiable. This method simplified the measuring process as it was possible to identify exactly missing or false rings and to correct easily errors.

Tree-ring width measurements and cross-dating

Absolute tree-ring widths (TRW) were measured to the nearest 0.001 mm using a measurement station connected to recorder data files software. By manually steered movement of the measuring table underneath a microscope observation, the distance from one ring boundary to another was recorded by marking the ring end with a mouse-click. The absolute TRW measurements were possible to adjust to account for errors in the cross-dating process.

Chronology standardization

Tree-ring growth is influenced by natural and anthropogenic environmental factors [Fritts 1976[START_REF] Schweingruber | Tree rings and environment[END_REF]]. The TRW series thus record environmental information and can be decomposed following the principle of global growth [START_REF] Cook | The Decomposition of Tree-Ring Series for Environmental Studies[END_REF][START_REF] Cook | Methods of dendrochronology -Applications in the environmental sciences[END_REF]:

R t = A t + C t + "D 1t + "D 2t + E CO2t + e t
in which: R t is the observed tree-ring width in the year T, A t is the age/size related growth trend due to normal physiological aging processes, C t characterized the TRW climate influence in the year t, D 1t corresponds to the occurrence of disturbance factors within the forest stand (endogenous) in the year t, D 2t is the occurrence of disturbance factors from outside the forest stand (exogenous) in the year t, E CO2t reflect the CO 2 fertilization e ect, e t are random processes not accounted by those above processes, and " is either a '0' for absence or '1' for presence of the disturbance signal. Cross-dated series, with high sensitivity and high correlation with the adjusted master series, were detrended and indexed to remove biological age trend. Indeed, as trees grow, their ring widths tend to decline [START_REF] Cook | The 'segment length curse' in long tree-ring chronology development for palaeoclimatic studies[END_REF]. Once the tree has reached maturity, the photosynthates used to produce wood cells have to be spread around on an ever-increasing surface area. To overcome this geometric e ect and to identify the most likely method to preserve the multi-decadal variability in the records, we tested di erent standardization methods with the CRUST (Climatic Research Unit Standardisation of Tree-ring data) software, which allows computing "signal-free" (SF) corrections [START_REF] Melvin | A "signal-free" approach to dendroclimatic standardisation[END_REF]2014a;b]. In a SF standardization, the detrending curve is estimated using measurement data from which the common growth-forcing signal has been removed, i.e. where the detrending curve is not distorted by the e ects of climate forcing [START_REF] Melvin | A "signal-free" approach to dendroclimatic standardisation[END_REF]. The curve-fitting standardization process is applied iteratively until the SF residual-chronology has near-zero variance [Melvin and Bri a 2014a].

The first method (NEGEX) uses either a modified negative exponential curve or a simple negative line to detrend individual signal-free series (Fig. 4.4). The resulting mean chronologies preserve high to medium-frequency common growth signal. The second method (SPL10) involves detrending measured ring-width series by removing the variance represented by a (signal-free) 10-year fixed-length cubic smoothing spline. This generated 'high-pass filtered chronologies' containing little variability at time scales longer than 10 years. The third method (RCS or Regional Chronology Standardization) allows overcoming the frequency limitation of curve-fitting standardization and preserving the medium-frequency variance (Fig. 4.4). It does so by using the average of the measurements of the rings of all trees at each ring age as the expected growth value for each individual tree [START_REF] Bri A | Tree-Ring Density Reconstructions of Summer Temperature Patterns across Western North America since 1600[END_REF][START_REF] Esper | Low-frequency signals in long treering chronologies for reconstructing past temperature variability[END_REF][START_REF] Bri A | A Closer Look at Regional Curve Standardization of Tree -Ring Records: Justification of the Need, a Warning of Some Pitfalls, and Suggested Improvements in Its Application[END_REF], Melvin and Bri a 2014a]. The TRW series are aligned by cambial age and the arithmetic average of measurements is used to produce the RCS detrending curve.

The multiple RCS method fits di erent detrending curves representing a di erent growth-rate class after sorting the trees on fast-to slow-growing trees. The averaged tree-index series of the multiple RCS is then used for SF correction.

For the RCS detrending, the lack of pith o set files may be problematic. Here, the NEGEX and RCS detrending resulted in similar resultant chronologies. However, relatively low sample depth of the N. pumilio ring width chronology at NUB makes RCS a suboptimal choice for detrending.

Chronology statistics

For dendroecological investigations, a range of raw data statistical indices can be used to evaluate not only the sensitivity of tree radial growth to limiting factors, but also lag-e ects due to tree internal resources (i.e. carbon reserves stored in the previous year).

Rbar is the mean correlation coe cient for all possible pairings among TRW series from individuals cores, computed for a specified common time interval [START_REF] Bri A | Interpreting high-resolution proxy climate data: the example of dendroclimatology[END_REF]. We used a 50-years window with an overlap of 25 years between adjacent window for N. pumilio cores and a 100-years window with an overlap of 50 years for F. cupressoides cores (Fig. 4.5).

The signal strength in a mean chronology may be gauged by calculating the degree to which it represents the hypothetical (noise-free) chronology, or a chronology that has been infinitely replicated [START_REF] Bri A | Interpreting high-resolution proxy climate data: the example of dendroclimatology[END_REF]. This signal strength is measured in the individual TRW chronology by the Expressed Population Signal statistic (EPS). TRW chronologies were considered reliable when their EPS was superior to 0.85 indicating that 85% of the chronology variance is common signal while 15% is residual noise ( e.g., [START_REF] Cook | Methods of dendrochronology -Applications in the environmental sciences[END_REF]) and their samples depth larger than 10 trees. For the F. cupressoides chronologies, these conditions are reached from 1234 and 1110 onwards at GF and RA respectively; for the N. pumilio chronologies from 1750, 1595, 1634 and 1785 at NUB, ALM, DL and CHA respectively (Fig. 4.3 and 4.5).

Stable isotopes in tree-rings: analytical procedure 4.3.1 Sample selection for stable isotopic analysis

Five (2 for F. cupressoides and 3 for N. pumilio) out of the 6 sites were selected for the isotopic study over the 1952-2011 period. This choice represented a compromise between sampling representativeness and workload. These sites are Glaciar Frías (GF), Río Alerce (RA), Paso de las nubes (NUB), La Almohadilla (ALM) and Challhuaco (CHA). The isotopic analyses were performed only on the last 60 tree-rings in order to: 1) understand the physiological processes occurring over the last years, and 2) calibrate the oxygen and carbon isotopic signal on climate variations for future climate reconstructions.

Analysis of at least four trees per site is usually considered a minimum for representative reconstruction of environmental variability from stable isotopes in tree rings [Leavitt and Long 1984, Robertson et al. 1997, Treydte et al. 2001;2006a[START_REF] Leavitt | Tree-ring C-H-O isotope variability and sampling[END_REF], Daux et al. 2011]. We therefore selected six most representative trees from the dated samples for the isotopic analyses, based on the following criteria: 1) ring widths were wide enough to produce samples for isotopic measurement; 2) the trees sampled were older than 150 years to avoid any possible juvenile e ect, and 3) cores were highly correlated with the site master TRW chronologies. In order to determine the adequate sample depth to obtain a representative isotope signal, we analyzed individually six trees at two sites (one site per species; GF for F. cupressoides, NUB for N. pumilio). The other samples were pooled to reduce analysis time and costs (one pool per site). Pooled isotope values are often similar to the mean of individually analyzed trees (e.g., [START_REF] Shi | Sampling strategy and climatic implications of tree-ring stable isotopes in Southeast Tibetan Plateau[END_REF]).

Sample preparation

For ring-porous tree species, earlywood-vessel area is related to environmental factors that prevailed during the time of vessel formation [START_REF] Sass-Klaassen | Vessel formation in relation to leaf phenology in pedunculate oak and European ash[END_REF]], therefore earlywood is formed with carbohydrates reserves accumulated during previous growing season. Thus, to retain the climatic signal of the current year, only the latewood portion of the tree-ring should be used for isotope analysis. However, the native beech N. pumilio has di usive pores di cult to unambiguously distinguish and separate properly. Therefore earlywood was not removed and the whole rings were cut.

For coniferous, the separation of earlywood from latewood has been shown to be unnecessary to make a reliable climate calibration as photosynthates from the current year are predominantly used [START_REF] Kress | Stable isotope coherence in the earlywood and latewood of tree-line conifers[END_REF]]. Moreover, F. cupressoides samples displayed narrow tree-rings making accurate earlywood-latewood separation di cult and yielding insu cient material to replicate isotopic analysis. Therefore, the whole ring was sampled in this case too.

Annual rings were separated with a scalpel under binocular magnifier (Fig. 4.6). The wood samples were chipped then grounded in a ball mill to ensure their homogeneity as the isotopic composition of cellulose varies within a single tree-ring and between early and latewood. --cellulose was extracted from the wood according to the SOXHLET chemical method elaborated by [START_REF] Green | Carboxymethyl cellulose[END_REF] and modified by Leavitt and Danzer [1993] (Fig. 4.6). Each milled sample was put in small pouch formed from a Teflon filter with 10 µm pores, which are solvent-resistant and closed with a Teflon thread. A teflon tag was attached to each pouch with the sample number. Batches of 30 samples were processed together.

The chemical treatment to extract --cellulose lasted one week.

• Extractives such as resins were removed in a Soxhlet extractor with a mixture of 140 ml of toluene and 70 ml of ethanol heated at 70°C for 10 h (Fig. 4.6). The repetition of this last step for 10 h more using only 200 ml of ethanol allowed to remove greases.

• Water-soluble constituents were eliminated by boiling the samples in an Erlenmeyer flask with deionized water for 6 h.

• The lignin was eliminated by oxidation in a solution of acidified sodium chlorite. The samples were heated overnight to 70°C in 700 ml of deionized water with 7 g of sodium chlorite (NaClO 2 ) and 1 ml of glacial acetic acid (CH 3 COOH). The following day, three successive additions of sodium chlorite and, glacial acetic acid were made at an interval of 2 h. Samples were heated for 3 -4 h after the latest addition. Samples were transferred in a beaker of deionized water and placed on a sonicator during 30 minutes, then the water was emptied. 7 to 8 rising steps were realized to remove the oxydation products, which are water-soluble.

• Samples were then transferred in a 17.57% solution of sodium hydroxyde (NaOH) in a sealed vial for 1h to remove hemicellulose. 6 rising steps of 30 minutes with deionized water were done in the sonicator. Finally, samples were placed in a 10% solution of acetic acid (CH 3 COOH) for 10 min to neutralize all hemicellulose molecules and then, were rinced with dionized water in the sonicator for 8 times 10 minutes.

• At the end, the samples were dried in an oven at 70°C over night.

The --cellulose extracted was homogenised ultrasonically with a sonotrode apparatus, and freeze-dried by lyophilisation. This assured the strong representativeness of " 18 O measured for the whole sample.

Mass spectrometric analysis of cellulose

The oxygen and carbon isotopic compositions of cellulose were determined with high temperature conversion elemental analysers (TC-EA, Thermo Scientific for oxygen and EA NC2500, Carlo Erba for carbon) coupled with a Finnigan MAT252 mass spectrometer (at LSCE, Gifsur-Yvette, France). Samples and standard were inserted into the autosampler (Zeroblank, Costec) purged with helium and were automatically dropped into the elemental analyser one by one. The pyrolysis (for measuring oxygen) or combustion (for measuring carbon) of samples under a continuous flow of helium carrier gas allowed decomposing the cellulose into simple compounds, mainly CO for oxygen and CO 2 for carbon. The pyrolysis or combustion products passed through gas chromatographic columns. The molecules of the vaporized samples were then carried by helium gas to the ion source of the mass spectrometer where they were ionized by an electron beam to form positive ions (Fig. 4.7). The electrons were emitted by a metallic filament. The ion beam was accelerated by a potential di erence imposed between the filament and the ionization chamber. The electrons were confined in their path by a magnetic field directed parallel to the direction of the filament-trap. The ions that exit the chamber through a slit were focused by a series of plates. In the magnetic field, the ion beam was deflected due to the Lorentz force to allow the separation of ions of di erent masses. Each beam was directed to three di erent Faraday collectors. When one ion impacted the collector, 1.6 10 ≠19 C was released and a potential di erence was observed across the resistance. Ions beams separated in the three collectors and measured through three amplifiers, were then used for the determination of the ratio in oxygen or carbon of each sample.

An internal laboratory reference of cellulose (Whatmann CC31) was used to correct for instrument drifts and to normalize the data into internationally accepted standards. Along the sequence of analysis (21 samples analysed per plate), the isotopic composition of CC31 was measured every 3 samples. The peak areas of a sample were compared to a standard reference gas (CO or CO 2 ). The main interest of the passage of the pyrolysis or combustion products through gas chromatographic columns was to spread the peak areas measured by the amplifiers to reduce the ion beam intensity. Then the measurements were calibrated using the cellulose reference CC31. The stable isotope analysis of each sample was repeated at least once and up to three times. This methodology allows rejecting outlier measurements, which do not fit with the maximum standard deviation of the isotopic measurement

Oxygen isotopic analyses

The " 18 O cell was determined according to the procedure described in [START_REF] Shi | Sampling strategy and climatic implications of tree-ring stable isotopes in Southeast Tibetan Plateau[END_REF]. Cellulose samples of around 0.20 mg were loaded in silver-foil capsules and weighted with a precision balance. The pyrolysis was done at 1350°C. The pyrolysis products passed through a gas chromatographic column containing a molecular sieve of 5A, heated at 60°C, in order to separate N 2 from CO. CO gas was carried from the elemental analyser to the ion source of the mass spectrometer where it was ionized. The masses 28 ( 12 C 16 O), 29 ( 13 C 16 O or 12 C 17 O) and 30 ( 12 C 18 O or 13 C 17 O 2 ) were measured and the ratios 29/28 and 30/28 were determined from the time integrals of the peak areas of the ion intensities. The ratio 30/28 of interest was corrected from the e ect of 17 O contained in the mass 30 of CO 2 using the ratio 29/28. The standard deviation (SD) obtained from the measurement of the isotopic composition of 10 consecutive CC31 standards was typically ± 0.20 % (analytical error: ± 0.20 % ).

Carbon isotopic analyses

For " 13 C measurements, cellulose samples of around 0.10 mg were loaded in tin-foil capsules. The gas chromatographic column was filled with chromium oxide and heated at 950°C. When the tin capsules containing the samples were in contact with the oxygen of the column, a combustion flash (1s at 1800°C) induced the vaporization of the sample molecules. The tin redeposited at the top of the column and accumulated as the sample flew. The other molecules migrated in the column until they reached its center where the temperature was higher, leading to a complete combustion of CO 2 . The excess oxygen was then trapped by a second gas chromatographic column containing copper, heated at 60°C. The stream output of this second column was composed of helium, CO 2 and water, which was trapped before introducing the flow in the ion source of the mass spectrometer via a capillary. Finally, the masses 44 (

Part II

RESPONSE OF TREE-RING GROWTH TO CLIMATE

5 The influence of non-stationary teleconnections in southern South America on tree-ring growth

Abstract

Over the last decades, tree-ring growth has responded non-linearly to broad-scale warming in the Patagonian Andes. Here we develop well-replicated tree-ring width chronologies of F. cupressoides and N. pumilio at high-elevation forests of northern Patagonia in order to: 1) examine the responses of tree-ring growth to local climate, 2) detect any decoupling in the selected tree-ring chronologies to local climate and, 3) investigate any association of radial growth to variability in Southern Annular Mode (SAM) and El Niño-Southern Oscillation (ENSO). Interannual variability in both seasonal climate and in tree-ring growth is strongly teleconnected to SAM and ENSO variability. Therefore, the temporal stability of SAM and ENSO impacts on the climate of the region during austral spring was further analyzed. F. cupressoides tree-ring growth is negatively correlated to previous summer (December-March) temperature. N. pumilio growth is positively (negatively) correlated to current summer temperature (precipitation) at the humid-to-mesic sites, while opposite relationships are observed at the xeric site. SAM and ENSO are influencing tree-ring growth of both species through their e ects on local temperature and precipitation. However, whereas in recent decades the positive phase of SAM has led to warmer and drier springs, the readjustment of atmospheric teleconnections has not induce a significant change of the influence of ENSO on the local climate. A low-frequency decoupling was detected between F. cupressoides tree-ring growth and previous February temperature since the late 1970s onwards, which may be partly related to the well-known step increase in temperature in 1976, coincident with the shift of the Pacific Decadal Oscillation (PDO, or the ENSO-like multidecadal climate variability in the Pacific Ocean). The current study suggests a time-dependency of N. pumilio growth to local climate probably linked to SAM or ENSO variability.

Résumé

Introduction

The present-day climate variability in southern South America mainly results from the superposition of large-scale ocean-atmosphere climate forcings such as the Southern Annular Mode (SAM), the Pacific Decadal Oscillation (PDO) and the El Niño-Southern Oscillation (ENSO). All climatic modes are not only influencing seasonal to interannual local temperatures, but also precipitations amount and distribution through atmospheric teleconnections [Villalba 2007, Archer and Caldeira 2008, Garreaud et al. 2008]. While ENSO (and PDO) warm events have been associated to significant warming and wetting of the Patagonian Andes, the positive phase of the SAM has been related to significant warming and drying over southern South America [Garreaud et al. 2008;2013].

However, the impacts of the atmospheric teleconnections on Southern Hemisphere climate have been shown to be not stationary over time (e.g. [START_REF] Silvestri | Nonstationary impacts of the southern annular mode on Southern Hemisphere climate[END_REF], Lopez-Parages and Rodriguez-Fonseca [2012], [START_REF] Gallant | Nonstationary Australasian teleconnections and implications for paleoclimate reconstructions[END_REF]). Indeed, [START_REF] Silvestri | Nonstationary impacts of the southern annular mode on Southern Hemisphere climate[END_REF] have depicted a poleward migration of the spatial hemispheric circulation anomaly pattern typically associated with SAM, between the 1960s-70s and 1980s-90s, leading to the weakening of the eastward flow and moisture convergence in the second subperiod, and therefore the inhibition of precipitation as well as the advection of the positive temperature anomaly into the Patagonia region. This pattern has led to changes in the association between SAM and local climate. Moreover, in the late 1970s, a regime shift of PDO from negative (cold) to positive (warm) phase has also been observed leading to the increase of SSTs in the tropical Pacific Ocean and to global warming [START_REF] Trenberth | Decadal atmosphere-ocean variations in the Pacific[END_REF][START_REF] Minobe | Interdecadal modulation of interannual atmospheric and oceanic variability over the North Pacific[END_REF][START_REF] Carrasco | Secular trend of the equilibrium-line altitude on the western side of the southern Andes, derived from radiosonde and surface observations[END_REF]. Over the past 100 years, there have also been important changes in ENSO behavior [START_REF] Power | The predictability of interdecadal changes in ENSO activity and ENSO teleconnections[END_REF], Cai et al. 2014]. Since the late 1970s, some studies have reported an increase of frequency and persistence of El Niño events and decreasing number of La Niña events [START_REF] Trenberth | The 1990-1995 El Niño-Southern Oscillation event: Longest on record[END_REF][START_REF] Fedorov | Is El Niño Changing?[END_REF][START_REF] Power | What Caused the Observed Twentieth-Century Weakening of the Walker Circulation[END_REF]. These changes in ENSO cycle were linked to the warming of the tropical Pacific over the recent decades and associated with the weakening of the Walker circulation [START_REF] Tanaka | Trend and interannual variability of Walker, monsoon and Hadley circulations defined by velocity potential in the upper troposphere[END_REF][START_REF] Power | Weakening of the Walker Circulation and apparent dominance of El Niño both reach record levels, but has ENSO really changed[END_REF][START_REF] Power | What Caused the Observed Twentieth-Century Weakening of the Walker Circulation[END_REF]. As a result, the impact of ENSO on southern South America climate has probably changed over the last decades (personal communication, M. Khodri). In this context, a better interpretation of the relationship between atmospheric teleconnections and local climate is needed.

Low temperature is usually a key factor limiting tree-ring growth at high-elevation and highlatitude treelines, either directly through its influence on heat-dependent ecophysiological processes (by regulating rates of photosynthesis and carbon accumulation [Fritts 1976]), or indirectly through its influence on growing season length [START_REF] Tranquillini | The Physiological Ecology of the Alpine Timberline[END_REF]]. Accordingly, recent global warming is expected to favor upward advance of treeline, to enhance radial tree growth near altitudinal treelines and to produce an earlier start of the growing season [START_REF] Korner | A re-assessment of high elevation treeline positions and their explanation[END_REF][START_REF] Körner | A world-wide study of high altitude treeline temperatures[END_REF][START_REF] Hoch | Growth, demography and carbon relations of Polylepis trees at the world's highest treeline[END_REF]. However, while the radial growth of N. pumilio has been demonstrated to be temperature limited in cold and wet environments [Villalba et al. 1997], Daniels et al. [2004] and Álvarez et al. [2015] have highlighted that this species responds non-linearly to broad-scale warming in the Patagonian Andes. Daniels et al. [2004] observed that, between 1957 and 1976, while the climate was rather cool in northern Patagonia, radial growth was promoted during the warmest growing seasons, whereas during the warm and dry 1977-1996 period, coincident with the warm phase of PDO, cool and wet summers promoted growth. Álvarez et al. [2015] related the interannual variability in both seasonal climate and N. pumilio growth to ENSO and SAM variability at 40°S in Chile, and concluded that SAM and ENSO are key climatic forcings of tree growth. Villalba et al. [2003] also observed that N. pumilio growth is modulated by annual variations in SAM.

Likewise, F. cupressoides, known to grow in wet and cool areas and to be sensitive to previous summer temperature [Lara and Villalba 1993;1994, Urrutia-Jalabert et al. 2015], started to show a decoupling between its radial growth and temperature since the late 1970s (personal communication, R. Villalba). Villalba [2007] showed that F. cupressoides growth is also related to summer variations of SOI, a tracer of ENSO status. Warm and dry (cool and wet) summer conditions coincident with El Niño (La Niña) episodes tended to have negative (positive) e ect on tree-growth the following year. This last finding suggests that ENSO (and perhaps SAM) may modulate F. cupressoides growth through its influence on temperature.

There is a need to assess whether the decoupling or time-dependence observed in the growth of these two species to climate variations is occurring at our sampling sites and, if it is the case, whether these changes can be related to changes in climate regimes. Therefore, here we assess temporal changes in the impacts of atmospheric teleconnections on the southern South America climate, examine the responses of tree-ring growth to local temperature and precipitation in N. pumilio and F. cupressoides forests in northern Patagonia, identify any decoupling in the selected tree-ring chronologies with local climate and investigate the possible associations of variability of tree radial growth to variability in ENSO and SAM at interannual and decadal timescales.

Material and methods

Tree-ring data

Mean TRW chronologies for each of the six sites of the current thesis (at ca. 41.10°S-71.40°W) were developed following standard dendrochronological procedures (see Chapter 4). Tab. 5.1 shows the descriptive statistics of each TRW site chronology. Two di erent standardization methods were used in the CRUST software: the NEGEX and the SPL10 tree-ring chronologies.

Climate data and analyses of teleconnections with climate forcings

We assembled the available monthly mean series of surface temperature and precipitation from several stations located in the 40°-43°S and 71°-73°W area. The records were sorted according to their quality and length. We used the precipitation data from the longest instrumental station in the study area ; Bariloche at 41.09°S-71.10°W derived from the Servicio Meteorológico Nacional or SMN) and the temperature datasets from Bariloche (1914-2009, SMN) and Esquel (1901Esquel ( -2012;;42.56°S-71.10°W extracted from the Global Historical Climate Network or GHCN) stations. The climatic-series selected were homogenized and gap-filled using HOMER (HOMogenization softwarE in R) [Mestre et al. 2013, Venema et al. 2013]. A regional temperature record (REG) was created by averaging normalized homogenized monthly temperature series (spanning 1901-2012).

Uncertainties in the climate data plausibly a ect the stability of the relationships with treering chronologies. Therefore, gridded datasets of global historical climatic anomalies were also tested for temperature and precipitation. The CRU TS3.2 land-surface air temperature data Table 5.1: Descriptive statistics for the tree-ring width and NEGEX/SPL10 chronologies. EPS: the Expressed Population Signal statistic assessing the strength of the common growth signal over time. A chronology with an EPS higher than 0.85 is considered to be robust [START_REF] Bri A | Tree climate relationships and dendroclimatological reconstructions in the British Isles[END_REF], Wigley et al. 1984]; Mean sens.: the mean sensitivity of tree growth, a measure of the relative changes in width in successive rings [Fritts 1976]; SD: the standard deviation of the mean; Rbar: the mean correlation of all inter-tree index series [Wigley et al. 1984]; Mean cor.: the average value of the Pearson's coe cients of the correlations between each individual series and the master series (average of all the other series of the site); Autocor.: the lag-one autocorrelation, a measure of the average dependence of the tree-ring chronology in one year on the same tree-ring chronology of the previous year, calculated over 1785-2011 and 1901-2011 (in parenthesis).

Species The temporal stabilities of such relationships were further explored here using Pearson correlation values between SAM and ENSO with temperature and precipitation, computed over two subperiods of equal length (1957-1982and 1983-2008;Fig. 5.2 and Tab. 5.2).

Tree-ring growth analyses

To determine the dominant oscillatory modes and cycles of F. cupressoides and N. pumilio growth, we performed a Continuous Wavelet Transform (WT) analysis [START_REF] Torrence | A Practical Guide to Wavelet Analysis[END_REF].

Tree growth related to local and large-scale climate

To identify the climatic forcing of mid-(high-) frequency controlling tree growth, the NEGEX (SPL10) tree-ring chronologies were compared with unfiltered (10-year high-pass filtered) climatic series using simple and partial correlations [Meko et al. 2011]. The use of partial correlation allows assessing the relationship between two variables while controlling for the e ects of a third variable. In the study area, the mean temperature is negatively correlated with total precipitation in summer therefore partial correlations between mean NEGEX (SPL10) tree-ring chronologies and precipitation while controlling for temperature, were calculated. The correlation analyses were computed from October of the year prior to ring formation to March of the year of formation. The significance of each correlation was evaluated using bootstrapping with 1000 Monte Carlo simulations [Ebisuzaki 1997]. To evaluate causal relationships in the time-frequency space between the F. cupressoides and N. pumilio growths and the most limiting factor, we conducted a Cross Wavelet Transform (XWT) analysis [START_REF] Grinsted | Application of the cross wavelet transform and wavelet coherence to geophysical time series[END_REF]] between the NEGEX chronologies and the climate target. Wavelet coherence is also a useful complement of the visual evaluation of decoupling because the significance of changes in coherence at di erent frequencies over time is calculated (e.g. [START_REF] Pisaric | Anomalous 20th century tree growth, Mackenzie Delta, Northwest Territories, Canada[END_REF]).

We also computed correlation functions between the NEGEX chronologies and monthly indices of SAM (SAM-Marshall) and ENSO (MEI), and conducted XWT analysis between the NEGEX chronologies and the most recent SAM and ENSO reconstructions. For this purpose we used the tree-ring SAM and ENSO reconstructions recently developed by Villalba et al. [2012] and [START_REF] Li | El Niño modulations over the past seven centuries[END_REF], respectively.

Results

Variability and trends in climate records

Instrumental and gridded climate records show similar variations after ca. 1930. Over the oldest part of the record the two temperature series vary in opposite directions (Fig. 5.1). As a matter of fact, the oldest part of the instrumental chronologies, during the first decades of the 20 th century is known to be of poor quality.

Teleconnections of local climate variability to climate forcings

Over the whole period 1957-2008, the regional temperature tends to be positively correlated to SAM all along the year but correlation coe cients are only statistically significant in October, December and February (r from 0.35 to 0.37, p < 0.05; Fig. 5.2). The best correlation is found in spring-summer over the period September-February (r = 0.49, p < 0.05, Tab. 5.2). Local Table 5.2: Seasonal (September to February) correlations of SAM-T and SAM-P over the 1957-2008, 1957-1982 and 1983-2008 Correlations between MEI and regional temperature are significant and positive in March (r = 0.35, p < 0.05; Fig. 5.2). Monthly precipitation is significantly correlated with MEI index in late spring (November; r = 0.31, p < 0.05) but is negatively correlated with MEI in summer (January, r = -0.44, p < 0.05). Thus, El Niño-like conditions are associated with wet and warm springs and dry summers.

Over the two sub-periods of equal length (25 years), the relationships between both climate forcings and local climate are changing (Fig. 5.2). Over the first sub-period , temperature is negatively correlated with SAM in November (r = -0.38, p < 0.05) and positively in February (r = 0.43, p < 0.05), while precipitation is only significantly and negatively correlated with SAM in February (r = -0.39, p < 0.05). Combining the months from September to February as done above, we found significant positive correlation between SAM and temperature (r = 0.38, p < 0.05; Tab. 5.2) and negative one although only significant at 94% with precipitation (r = -0.22, p < 0.06). Over the second sub-period , temperature is significantly and positively correlated with SAM in July (r = 0.34, p < 0.05), from October to December (r = 0.33 to r = 0.48, p < 0.05) and in March (r = 0.40, p < 0.05). Correlations are opposite with precipitation and significant from October to December (r = -0.33 to r = -0.49, p < 0.05). The combination of September to February months shows strong positive correlation between SAM and temperature (r = 0.48, p < 0.05; Tab. 5.2) and negative with precipitation (r = -0.51, p < 0.05). Thus, the association of positive SAM conditions and warmer and drier spring-summers has increased over the second sub-period.

Over the first sub-period , MEI index is significantly and negatively correlated with October temperature (r = -0.34, p < 0.05) and positively with December, January and March temperature (r = 0.33 to r = 0.47, p < 0.05). In contrast, precipitation is significantly correlated with MEI in January (r = -0.58, p < 0.05) and May (r = 0.54, p < 0.05). Over the second sub-period , temperature and MEI are only significantly correlated in December (r = -0.36, p < 0.05), while precipitation is significantly and positively correlated to MEI in July (r = 0.32, p < 0.05) and November (r = 0.46, p < 0.05) and negatively in January and February (r = -0.31 to r = -0.35, p < 0.05). Thus, there is no significant changes of the impacts of ENSO on local climate between the two sub-periods. El Niño-like conditions are associated with wet and warm springs and dry summers.

Overview on growth variations

The autocorrelations of the NEGEX chronologies are quite high over the 1785-2011 period (common to all the series; AR1 = 0.48 to 0.74) but are lower when only considering the last century from 1901 to 2011 (AR1 = 0.30 to 0.51; Tab. 5.1). They may be partly attributable to multi-decadal trends. When these trends are removed with a 10-year low-pass spline (SPL10 chronologies), the autocorrelations are lower (AR1 = 0.02 to 0.20 over 1785-2011 and 0.02 to 0.33 over 1901-2011). The two F. cupressoides chronologies, which have almost 8 centuries in common (1234-2011), are strongly correlated to each other. Over 1785-2011, their correlation coe cient is 0.71 for the NEGEX chronologies and 0.88 for the SPL10 chronologies (Tab. 5.3). At the two sites, tree-ring growth increases from ca. 1900 up to now (Fig. 5.3) but this pattern of growth is more pronounced at RA. In the 1785-2011 interval, the pairwise correlations of the N. pumilio NEGEX chronologies are low and even not significant (e.g. r = 0.23 between CHA and NUB) to relatively high (r = 0.62, p < 0.05 between ALM and DL; Tab. 5.3). The SPL10 chronologies are rather well correlated to one another (r up to 0.72, p < 0.05 between ALM and DL). Over the last century, the overall trends of the NEGEX chronologies are: increase at ALM, slight decrease at DL and CHA and quasi-constancy at NUB.

The continuous WT spectral analysis of the F. cupressoides and N. pumilio NEGEX chronologies shows significant oscillatory modes in tree-growth at sub-decadal (< 8 years) frequencies but these spectral modes are intermittent and non-stationary (Fig 5.4). Tree growth exhibits high spectral power: 1) in the low-frequency domain with multi-decadal periodicities (> 16 years) before 1840 at GF and NUB, 2) in the mid-frequency domain (between 8 and 16 years) over 1850-1900 at the N. pumilio sites and, 3) in the high-frequency domain (< 8 years) after 1850 at the two F. cupressoides sites and, before 1850 and after around 1900 at the N. pumilio sites. Red tone indicates higher power spectrum, black thick contours denote 95% significance level using a red noise background, and the white line delimits the cone of influence where edge e ects may be important.

Tree-growth related to local climate

The NEGEX chronologies of F. cupressoides are negatively correlated to summer temperature during the previous growing season (December to March; r from -0.23 to -0.35, p < 0.05 at GF), and positively with current winter precipitation (June at GF; r = 0.33, p < 0.05; Fig 9 .2). Correlations are stronger using local meteorological stations than using the CRUTS gridded data. The SPL10 chronologies are more strongly related to temperature and precipitation than the NEGEX ones.

Temperature has the primary influence on F. cupressoides growth during the previous summer (combination of December to March months; r = -0.39, p < 0.05 at GF). While the two NEGEX chronologies are strongly related to averaged December-March temperature over the period 1901-1976 (r = -0.48, p < 0.01; season. At the two sites, a significant low frequency modulation of the correlations occurs for February temperature from the late 1970s onwards. The decoupling observed by visual comparison between mean previous December-March and tree-ring growth is not statistically significant using the Gershunov test.

N. pumilio growth is positively related to temperature and negatively to precipitation in latespring of the current growing season at the humid-to-mesic sites (NUB, ALM and DL; r = 0.34 with temperature at DL and r = -0.28 with precipitation at NUB in November, p < 0.05; Fig 9 .2). In contrast, at the xeric-eastern most site (CHA), N. pumilio growth is negatively related to temperature and positively to precipitation over the same months (r = -0.38 with temperature and r = 0.20 with precipitation in December, p < 0.05). Correlations are similar using local meteorological stations and the CRUTS gridded data. The best correlations between SPL10 chronologies and monthly 10-year high-pass filtered temperature and precipitation data are also obtained in spring-summer months. No net decoupling is observed between November (December) temperature and N. pumilio NEGEX chronologies over time (not shown).

The XWT spectral analysis shows that at the two most western humid-to-mesic sites, tree-ring growth is related to November temperature at high (< 4 years) and low frequency (between 8 and 16 years) before the late 1950s. Afterwards, the coherence has decreased in the two bands (Fig 5.7). At the most xeric eastern site, the XWT shows an intermittent association between tree growth and temperature at high to mid frequency (between 4 and 8 years) from ca. 1940s onwards.

The 30-year running correlations highlight a time-dependence of tree growth to temperature and precipitation (Fig 5 .7). On the one hand, over the period ranging from early 1970s to late 1980s, November-December temperature tend to have a significant and positive influence on tree growth at ALM (and at the humid site NUB, not shown). On the other hand, high temperature and low precipitation in December are significantly limiting tree-growth until late 1980s at the more xeric site CHA. Since around 1970s, September temperature and precipitation start to influence tree-growth at this site but significant changes at p < 0.05 are only observed for the precipitations (Gershunov test).

Tree growth related to large-scale climate forcings

While monthly SAM indices are not significantly correlated to F. cupressoides growth (Fig 5 .8), monthly MEI tends to be negatively correlated to F. cupressoides growth in December of the previous growing season (r = -0.29, p < 0.05 at GF) and positively in July of the current winter (r = 0.27 to 0.33, p < 0.05). Thus, consistent with the net temperature e ect on tree-ring growth, El Niño-like warm conditions in the eastern tropical Pacific during the previous summer are associated to a decrease of tree growth the following year.

The XWT spectral analysis between the F. cupressoides NEGEX chronologies and the largescale climate forcings shows that tree growth has an intermittent physical association with the SAM reconstruction ( SAM is positively correlated with N. pumilio growth at ALM in January (r = 0.24, p < 0.05; Fig 5.8), and negatively at the most xeric site CHA in current November (r = -0.37, p < 0.05; Fig 5.8). Thus, consistent with the net temperature e ect on tree-ring growth, a positive SAM is mostly associated to a decrease of growth at the most xeric site and, to a lesser extent, to an increase of growth at the mesic sites. Furthermore, monthly MEI tends to be positively correlated with tree growth in summer months of the current growing season at the three most mesic sites (r = 0.30, p < 0.05 in January at DL) and negatively at the xeric site (r = -0.28, p < 0.05 in November). Thus, El Niño events are associated with above average tree growth at the mesic sites and below average growth at the xeric site.

The XWT spectral analysis of the N. pumilio NEGEX chronologies reveals intermittent high frequency periodicities (< 8 years) with the SAM reconstruction that are stronger between 1900 and1980 (Fig 5.9). The NEGEX chronologies are also associated to ENSO at wave forms of high frequency (< 8 years) from 1870 onwards and of mid-to-low frequency (between 8 and 16 years) mainly in the 1850-1900 period (Fig 5.9).

Discussion

Non-stationary atmospheric teleconnections

Relevant previous studies of SAM and ENSO have examined the nonstationary nature of their impacts on Southern Hemishere climate during austral spring-summer [START_REF] Fogt | Decadal variability of the ENSO teleconnection to the high-latitude south pacific governed by coupling with the Southern Annular mode[END_REF][START_REF] Silvestri | Nonstationary impacts of the southern annular mode on Southern Hemisphere climate[END_REF][START_REF] Fogt | Understanding the SAM influence on the South Pacific ENSO teleconnection[END_REF]]. The correlation analysis done here on SAM variability over two (1957-1982 and 1983-2008) sub-periods is consistent with these previous studies. Our results find a significant increase of the imprint of the spatial circulation anomaly pattern typically associated with SAM between the two sub-periods. However, although directional changes of ENSO influence on local climate was expected with atmospheric teleconnections' readjustment, the local climate impact of ENSO at 41°S was temporally stable between the two sub-periods. [START_REF] Fogt | Decadal variability of the ENSO teleconnection to the high-latitude south pacific governed by coupling with the Southern Annular mode[END_REF] found that the decadal variability of the ENSO teleconnection to the high-latitude South Pacific is related to the coupling with the SAM. They showed a weak teleconnection during the 1980s due to the interference between the South Pacific pattern and the SAM, while teleconnections were stronger over the 1990s due to strong SAM positive phase. Here, the analyses were made in larger timescales in periods preceding and following the early 1980s therefore this ENSO response to SAM forcing was not possible to detect. An interesting result from the analysis is that at multi-(25 years) decadal scale, no significant change is observed in the influence of ENSO events on local climate, at least in the area of study. Further insight on the ENSO-SAM relationships and the nonstationary nature of ENSO impacts on north Patagonian climate is paramount.

Temporal changes in growth patterns related to local-to-large-scale climate changes

In this section, we discuss the time-dependence (or divergence) observed in tree-ring growth of F. cupressoides and N. pumilio to local climate variability and the influence of large-scale climate forcings on these decouplings.

F. cupressoides growth patterns

F. cupressoides growth shows strong coherence between the two sites. The Continuous WT analysis of the NEGEX chronologies reveals that tree growth variability occurs at wave forms of high frequency over the 1850-2011 period with sub-and multi-decadal cycles (Fig 5.4). At both sites, tree-ring growth is controlled at first order by previous summer (December-March) temperature but is also showing some dependence to winter (June) precipitation. The negative relationship with temperature is consistent with the documented preferences of this species for cool growing season [Lara and Villalba 1993;1994]. According to Urrutia-Jalabert et al.

[2015], the carbon assimilation of F. cupressoides may decrease with stomata closure during dry conditions leading to low radial increment. High temperature during the previous growing season combined with higher respiration rates may reduce starch reserves that can be used the following growth period [Deslauriers et al. 2014]. Therefore, vapor pressure deficit is likely to be the driver of the response of tree-ring growth to temperature [Urrutia-Jalabert et al. 2015].

In the present-day warmer and drier summer conditions, the growth of this species should be penalized. However, since the late 1970s, coincident with the well-known step increase of temperature in 1976 associated with shift in PDO, at the sites studied, tree-ring growth has shown a positive trend with increasing temperature (Fig 5 .6), suggesting a low-frequency decoupling (decoupling in trend) between tree growth and temperature. Consistently, the running correlation study showed weaker relationships between tree growth and previous summer temperature since the late 1970s. Significant coherence between the F. cupressoides chronologies and previous December-March temperature was observed before the 1980s in the ca. 6-8 year band but this coherence decreased since then. However, the low frequency modulation test of significance between the two time-series (Gershunov test) highlighted that a significant change in relationships only occurs in February at the two sites while in other summer months, changes were not significant at p < 0.05. Thus, the low-frequency decoupling between tree growth and temperature was more pronounced when temperatures reached their maximum, suggesting the emergence of another limiting factor of tree growth than temperature mostly in the warmest summer month.

Our findings (Fig. 5.2) as well as previous studies [Miller et al. 2006, Montecinos andAceituno 2003] have shown that positive phase of SAM is associated with warmer and drier spring conditions in the study region, mostly over the second sub-period , while warm phase of ENSO lead to warm and dry conditions in summer, mostly in the first sub-period . The negative relationship between previous summer MEI and F. cupressoides growth (Fig. 5.8) suggests that El Niño-like conditions in the tropical Pacific (i.e. warmer SSTs) are associated to a decrease of tree growth the following year through its e ect on summer local temperature. This is consistent with Villalba [2007] that shows negative relationships between F. cupressoides growth and previous summer SOI. Furthermore, El Niño-like conditions in current winter tend to favor tree growth, through its e ect on precipitation. However, we did not find significant relationships between tree growth and SAM. These results suggest that the ENSO mode induce local inter-annual (2 to 7 years) climate variations with consequences on F. cupressoides growth, while multi-decadal (20-30 years) variability of SAM do not a ect the growth of this species significantly in this short 50 years length of common records.

The results of the multi-centennial spectral analysis demonstrate that F. cupressoides growth contains oscillatory modes at inter-annual to multi-decadal scales related to SAM and ENSO.

XWT analyses between the NEGEX chronologies and SAM reconstruction show that tree-ring growth is associated to SAM at cycles of less than 8 years over the 1409-2006 period (Fig 5.9) period (Fig 5.9). This is in disagreement with the correlation study which do not show significant relationships between both time-series over this time interval. Nevertheless, the correlations are computed over a smaller period where the wavelet coherence is not so strong (Fig 5 .9) therefore both results are not inconsistent. Furthermore, the NEGEX chronologies exhibit common oscillation modes with ENSO at mid-to-high frequencies over 1409-2006, which are less intermittent from 1870 onwards. These findings suggest that both climate modes have indirectly influenced F. cupressoides growth over time through their e ect on temperature and precipitation, and that over the past 140 years, ENSO has increased its influence on local climate and therefore on tree growth compared to previous periods.

N. pumilio growth patterns

Depending on the sampling site characteristics (soil, exposure, altitude, position in the westto-east gradient of precipitation), we found outstanding di erences in the responsiveness of N. pumilio tree-ring growth to climate. At humid-to-mesic treelines (NUB, ALM and DL), above average growth is associated with above average temperature and below average precipitation in late spring-early summer (November-January). In contrast, the xeric high-elevation treeline forest (CHA) appears limited by high temperature and low water availability in December, and low water input in July. Thus, climatic variations which favor tree growth at wet sites are disadvantageous at the driest site, and vice versa. These observations were also supported by Daniels et al. [2004] that highlighted site-dependent relationships of N. pumilio growth to interannual and decadal scale climate variations near 40°S in Chile and Argentina. We suggest that in the eastern locations such as CHA where the moisture supply from the westerlies is considerably reduced, high temperature in summer may exacerbate drought stress.

No net decoupling with local climate was observed in the tree-ring chronologies, but we found strong time-dependence of the tree-ring series to local climate variability. Indeed, the running correlation study showed unstable relationships between temperature and precipitation through time, with a period of high November-December temperature control on tree-growth centered in the early 1970s to late 1980s at the humid-to-mesic sites, and an emergent influence of November precipitation since then at these sites. In contrast, local temperature and precipitation in December were strongly related to tree-ring growth at the xeric site over a larger period (from early 1950s to late 1980s), while an emerging tree-ring growth response to September temperature and precipitation was observed since the early 1970s. Consistently, the results of the XWT analysis showed significant but intermittent coherence between the N. pumilio chronologies and November (December) temperature. At the humid-to-mesic sites, coherence with November temperature in the ca. 4-6 year band was observed before the early 1960s and disappeared afterwards, while at the xeric site, mid-to-high frequency coherence with December temperature was observed from late 1950s onwards.

The positive relationships of summer MEI (and to a lesser extent of SAM index) with N. pumilio growth at NUB, ALM and DL indicate that El Niño-like conditions in summer are associated with above average N. pumilio growth at the humid-to-mesic sites. These findings are consistent with Álvarez et al. [2015] that show strong response of N. pumilio growth to ENSO en SAM in Choshuenco Volcano in Chile. Conversely, the negative relationships between tree growth at CHA and MEI and SAM index in November suggests that these conditions are unfavorable for tree-ring growth at the xeric site. Therefore, positive interannual departures of both SAM and MEI lead to an increase of tree-ring growth at the humid-to-mesic locations but a decrease of tree-ring growth at the xeric site mainly due to their e ects on growing season temperature and precipitation.

XWT analyses between tree growth and SAM and ENSO reconstructions reveal intermittent frequencies coherence between the NEGEX chronologies and SAM and ENSO over the 1758-2011 period but with a general increase of coherence in the high-to-mid frequencies since the 1870 onwards. These findings suggest that during the past 140 years, the two modes of climatic variability have induced strong local climate variations which have influenced N. pumilio growth. Further study of the time-dependent response of this species to local and large-scale climate is presented in Chapter 6 that will allow better understanding of the spatial di erences between sampling sites.

Conclusions

In this study we have explored the tree-ring growth response of F. cupressoides and N. pumilio forests to local and large-scale climate variability over the last decades to centuries in northern Patagonia.

We have seen that large-scale climate forcings such as ENSO and SAM are influencing longterm variations of tree-ring growth through their e ects on local temperature and precipitation. However, at 41°S, the impacts of atmospheric teleconnections on local climate were not stationary during austral spring. Whereas in recent decades the dominance of the positive phase of SAM has led to warmer and drier springs, the readjustment of atmospheric teleconnections has not induce a significant change of the influence of ENSO on the local climate but an increasing intensity of ENSO events.

The two species were responding di erently to such climatic forcings. While F. cupressoides tree growth was sensitive to both SAM and ENSO but mostly to inter-annual variability of ENSO over the last decades, the patterns of N. pumilio growth to climate variability were time-dependent and only partially associated to SAM or ENSO. Notably, the growth of the two species showed an increasing influence of ENSO over the past 140 years.

Published paper in Dendrochronologia

This chapter has been published as: Lavergne A., Daux V., Villalba R. and [START_REF] Lavergne | Temporal changes in climatic limitation of tree growth at upper treeline forests: Contrasted responses along the west-to-east humidity gradient in Northern Patagonia[END_REF], Temporal changes in climatic limitation of tree-growth at upper treeline forests: Contrasted responses along the west-to-east humidity gradient in Northern Patagonia. Dendrochronologia, 36, pp. 49-59, doi:10.1016/j.dendro.2015.09.001.

1.! Introduction

Changes! in! summer! climate! regimes! associated! with! the! anomalous! positive! trend! in! the! Southern! Annular! Mode! (SAM;! Archer and! Caldeira,! 2008)! have! influenced! the! patterns! of! forest! growth across! much! of! the! extra-tropical! Southern! Hemisphere! [START_REF] Villalba | Unusual southern hemisphere tree growth patterns induced by changes in the southern annular mode[END_REF].! Growing! seasons! in! the! temperate! belt! of! the! Southern! Hemisphere! have! become! warmer! and! drier! as! the! mid-latitude westerly! flow! has! weakened! during! recent! decades! (Gillett! et! al., 2006;[START_REF] Garreaud | The Andes climate and weather[END_REF].! Warming! has! stimulated! tree! growth in! cool! and! moist! subalpine! forests! of! Tasmania! and! New! Zealand (Allen! et! al.,! 2014[START_REF] Allen | ! The! climatic! response! of Phyllocladus! aspleniifolius! (Labill.)! Hook.! f! in! Tasmania[END_REF][START_REF] Cook | ! Warm-season temperatures! since! 1600! BC! reconstructed! from! Tasmanian! tree! rings! and! their relationship! to! large-scale! sea! surface! temperature! anomalies[END_REF][START_REF] Villalba | Unusual southern hemisphere tree growth patterns induced by changes in the southern annular mode[END_REF] whilst! drying! has! reduced! growth! rates! in! mesic! and! dry! forests of! southern! South! America! [START_REF] Christie | ! Aridity! changes! in! the temperate-Mediterranean! transition! of! the! Andes! since! AD! 1346! reconstructed from! tree-rings[END_REF]! Mundo! et! al., 2012;[START_REF] Villalba | Unusual southern hemisphere tree growth patterns induced by changes in the southern annular mode[END_REF].! Temporal! stability! of! significant! cor-relations! between! tree-ring! chronologies! and! climatic! variables suggest! that! forests! growing! at! marginal! locations,! where! growth is! dominantly! limited! by! either! temperature! or! moisture,! have responded! consistently! to! this! observed! shift! in! growing! season climate! [START_REF] Villalba | Unusual southern hemisphere tree growth patterns induced by changes in the southern annular mode[END_REF][START_REF] Mu Ñoz | ! Patterns! and! drivers of! Araucaria! araucana! forest! growth! along! a! biophysical! gradient! in! the northern! Patagonian! Andes:! linking! tree! rings! with! satellite! observations! of soil! moisture[END_REF].! However,! growth responses! of! mesic! north! Patagonian! forests! and! at! some! treeline locations! where! temperature! and! moisture! simultaneously! co-limit tree-ring! growth,! appear! to! be! more! complex! and! time-dependent (Daniels! and! Veblen,! 2004;! Álvarez! et! al.,! 2015;[START_REF] Suarez | Sensitivity of Nothofagus dombeyi tree growth to climate changes along a precipitation gradient in northern Patagonia, Argentina[END_REF][START_REF] Suarez | Sensitivity of Nothofagus dombeyi tree growth to climate changes along a precipitation gradient in northern Patagonia, Argentina[END_REF].! Warmer! conditions! and! large-scale! changes in! atmospheric! circulation! during! the! late! twentieth! century! have reduced! temperature! limitation! but! might! have! increased! moisture! limitation,! especially! for! species! growing! at! mesic! and! humid environments! along! the! north! Patagonian! precipitation! gradient [START_REF] Suarez | Sensitivity of Nothofagus dombeyi tree growth to climate changes along a precipitation gradient in northern Patagonia, Argentina[END_REF].

The! beech! Nothofagus! pumilio! is! the! dominant! subalpine! tree species! in! northern! Patagonia.! The! radial! growth! of! the! species! at the! upper! treeline! is! typically! controlled! by! spring-summer! climate! and! has! been! used! to! reconstruct! past! temperatures! and snow! cover! duration! (Villalba! et! al.,! 1997;! Lara! et! al.,! 2001; Aravena! et! al.,! 2002).! Yet,! correlations! between! climate! and! treering! growth! at! some! treeline! locations! in! northern! Patagonia! have been! shown! to! vary! substantially! between! early-20th! century cool-wet! and! current! warm-dry! climate! regimes! [START_REF] Raffaele | Comparative! morphology! of! annual! shoots! in! seedlings! of! five! Nothofagus species! from! Argentinian! Patagonia.![END_REF]! Daniels! and! Veblen,! 2004).! Furthermore,! contrary! to! the expectation! of! enhanced! growth! rates! with! recent! warming,! tree growth! at! an! adjacent! high-elevation! site! in! the! western! side! of the! Andes! appears! to! have! declined! during! recent! decades! (Álvarez et! al.,! 2015).! These! observations! could! suggest! a! non-linear! growth response! to! temperature! or! the! emerging! influence! of! other! environmental! controls! at! some! high-elevation! forests! in! the! region. Similar! site-level! temporal! instabilities! in! historical! patterns! of climate! limitation! of! tree! growth! have! been! observed! at! treeline forests! in! North! America! [START_REF] Ohse | ! Do! limiting! factors! at! Alaskan! treelines shift! with! climatic! regimes?![END_REF][START_REF] Chavardès | Unstable! climate-growth! relations! for! white! spruce! in! southwest! Yukon[END_REF], where! the! so-called! tree-ring! divergence! phenomenon! has! been described! (Jacoby! and! D'Arrigo,! 1995;! D'Arrigo! et! al.,! 2004[START_REF] D'arrigo | On the 'Divergence Problem' in Northern Forests: A review of the tree-ring evidence and possible causes[END_REF].

The! recent! development! of! generic! forward! models! of! tree-ring formation! offer! a! new! approach! for! studying! the! nature! of! growth responses! to! climate,! overcoming! some! limitations! of! traditional linear! correlation! analyses! (Anchukaitis! et! al.,! 2006;[START_REF] Boucher | ! An inverse! modeling! approach! for! tree-ring-based! climate! reconstructions! under changing! atmospheric! CO2 concentrations[END_REF]! Evans! et! al.,! 2013! Evans! et! al.,! ,! 2006;;[START_REF] Vaganov | ! How! well! understood! are! the processes! that! create! dendroclimatic! records?! A! mechanistic! model! of! the climatic! control! on! conifer! tree-ring! growth! dynamics[END_REF] (Fritts,! 1976).! VS-Lite! has! been! used! to! simulate! and! evaluate regional! patterns! of! climate! limitation! of! tree! growth! in! a! range of! environments! from! semi-arid! to! temperate! and! boreal! regions [START_REF] Breitenmoser | ! Forward! modelling! of! tree-ring width! and! comparison! with! a! global! network! of! tree-ring! chronologies[END_REF][START_REF] Evans | ! Climate! field! reconstruction! uncertainty! arising! from! multivariate! and nonlinear! properties! of! predictors[END_REF]! Tolwinski-Ward! et! al., 2011).

In

2.! Material! and! methods

2.1.! Regional! setting! and! study! species Patagonia! extends! from! about! 37 • S! to! 55 • S! and! represents the! southernmost! portion! of! the! South! American! continent.! The regional! climate! is! mainly! driven! by! the! interactions! between! the circum-Antarctic! cyclonic! belt! to! the! south! and! the! subtropical Pacific! high-pressure! cell! to! the! northwest.! The! strong! westerlies! resulting! from! the! large! pressure! differences! between! these semi-permanent! circulation! features! in! the! atmosphere! permanently! interact! with! the! north-south! mountain! range! of! the! Andes [START_REF] Aceituno | ! On! the! functioning! of! the! Southern! Oscillation! in! the! South American! sector.! Part! I:! surface! climate[END_REF]! Villalba! et! al.,! 2003;[START_REF] Garreaud | The Andes climate and weather[END_REF].! The Andes! Cordillera! acts! as! a! topographic! barrier! to! the! persistent westerlies! bringing! moisture! from! the! South! Pacific! Ocean.! The! air masses! discharge! most! of! the! humidity! in! the! way! up! to! the! mountains! on! the! western! slopes! of! the! Andes! to! descend! drier! on! the eastern! slopes.! Therefore,! the! mountain! range! induces! a! dramatic decline! in! mean! annual! precipitation! from! 4000! mm! at! Lago! Frías, near! the! continental! divide,! to! about! 500! mm! toward! the! Patagonian! steppe! (Jobbagy! et! al.,! 1995;! Veblen,! 1979).! Gross! Primary productivity! (GPP)! follows! the! eastward! decline! in! precipitation and! ranges! from! around! 2000! g! C! m 2 year -1 in! the! most! productive! forests! to! about! 500! g! C! m 2 year -1 in! the! Patagonian! steppe (Fig. ! 1;[START_REF] Mu Ñoz | ! Patterns! and! drivers of! Araucaria! araucana! forest! growth! along! a! biophysical! gradient! in! the northern! Patagonian! Andes:! linking! tree! rings! with! satellite! observations! of soil! moisture[END_REF]! Zhao! et! al.,! 2005).! In! northern! Patagonia, the! annual! migratory! cycle! of! the! Pacific! anticyclone! induces! strong precipitation! seasonality.! Precipitation! is! largely! concentrated! from late! fall! to! early! spring! followed! by! a! drier! and! mild! period! during summer! and! early! fall! (López! Bernal! et! al.,! 2012).

N.! pumilio,! locally! known! as! Lenga,! is! a! light-demanding! deciduous! tree! widely! distributed! along! the! Andes! from! 35 • 36 ′ S! to! 56 • S! in Argentina! and! Chile! (Donoso,! 1981).! This! species! usually! dominates the! upper! portion! of! the! altitudinal! limit! of! the! woody! vegetation. In! northern! Patagonia,! N.! pumilio! grows! at! elevations! ranging! from 1100! to! 1600! m! under! a! great! variety! of! soils,! environmental! conditions,! and! disturbance! regimes! (Schlatter,! 1994).! The! growing season! of! the! species! in! the! region! extends! between! September and! May! but! its! duration! decreases! with! elevation! due! to! the! lower temperatures! (Rusch,! 1993).! N.! pumilio! forests! develop! mainly! on fertile! soils! derived! from! recent! volcanic! ashes! (Rusch,! 1993).! In drier! sites,! roots! can! reach! water-saturated! soils! or! ground! water up! to! 2-3! m! depth! (Schulze! et! al.,! 1996).

2.2.! Sampling! and! tree-ring! processing

During! the! austral! summer! of! 2013,! four! treeline! sites,! previously! sampled! during! the! 1990s! (Villalba! et! al.,! 1997(Villalba! et! al.,! ,! 2003)),! were re-visited! to! update! the! network! of! tree-ring! chronologies! in! northern! Patagonia.! Our! sampling! sites! are! located! along! the! regional precipitation! gradient! from! Cerro! Tronador! (41

• 09 ′ S,! 71 • 53 ′ W)! to the! drier! Challhuaco! valley! near! Bariloche! (41 • 09 ′ S,! 71 • 18 ′ W).! From west! to! east,! the! sampling! sites! are! Paso! de! las! Nubes! (NUB), La! Almohadilla! (ALM),! Diego! de! León! (DL)! and! Challhuaco! (CHA) (Fig.! 1;! Table! 1).! The! drier,! easternmost! part! of! the! gradient! rep- resents! the! eastern! edge! of! N.! pumilio! distribution! on! the! leeward flank! of! the! Andes.! At! each! site,! at! least! 30! trees! were! double-cored at! about! 1.3! m! above! ground! using! a! Pressler! borer! with! a! diameter of! 5! mm.
Since! the! growing! season! in! the! Southern! Hemisphere! overlaps! two! calendar! years,! the! date! of! each! ring! was! assigned! to the! year! when! ring! growth! began! (Schulman,! 1956).! Thus,! the last! complete! tree! ring! in! the! cores! sampled! in! the! summer! of 2013! was! dated! to! the! year! 2011! because! the! 2012! tree! ring! was still! incomplete.! Overall,! more! than! 500! cores! were! examined! and cross-dated! following! standard! dendrochronological! procedures (Stokes! and! Smiley,! 1968).! Quality-control! and! accuracy! of! measurements! and! cross-dating! were! conducted! with! the! COFECHA program! (Holmes,! 1983).! Cross-dated! tree-ring! width! series! were detrended! and! indexed! using! a! 10-year! fixed-length! cubic! smoothing! spline! implemented! in! the! CRUST! routine! (Melvin! and! Briffa, 2014a[START_REF] Bonan | Forests and climate change: forcings, feedbacks, and the climate benefits of forests[END_REF]! in! order! to! retain! only! inter-annual! variability! in! the chronologies! and! filter! out! multi-decadal! trends.! The! standardized series! at! each! site! were! averaged! to! produce! a! mean! site! chronology. The! descriptive! statistics! of! the! individual! tree-ring! chronologies are! shown! in! Table ! 1. The! Expressed! Population! Signal! statistic! (EPS; [START_REF] Briffa | ! Tree-! Climate! Relationships! and! Dendroclimatological Reconstructions! in! the! British! Isles.! Dissertation[END_REF]Wigley! et! al.,! 1984)! was! calculated! to! assess! in! the! chronology! the strength! of! the! common! growth! signal! over! time.! Based! on! EPS (>0.85)! and! sample! depth! (10! has! been! arbitrarily! considered! as! a minimum! number! of! trees),! the! optimal! common! period! between chronologies! was! 1785-2011! (Table ! 1).! The! first! Empirical! Orthogonal! Function! (EOF),! referred! as! to! the! leading! pattern! of! growth variability,! was! computed! from! the! correlation! matrix! of! the! four tree-ring! chronologies! to! evaluate! the! spatiotemporal! patterns! of tree! growth! across! the! transect! over! the! period! 1785-2011.

2.3.! Climatic! limitation! of! tree! growth

Temperature! and! moisture! controls! of! tree-growth! variability were! first! empirically! identified! using! simple! and! partial! correlations! (Meko! et! al.,! 2011) The! station! records! were! homogenized! and! gap-filled! using! the R! routine! HOMER! (HOMogenization! softwarE! in! R;! Mestre! et! al., 2013;[START_REF] Venema | Benchmarking homogenization algorithms for monthly data[END_REF]! and! climatic! series! from! additional! stations! located! in! the! area! (40-43 • S! and! 71-73 • W).! The! use! of! partial correlation! allows! assessing! the! relationship! between! two! variables while! controlling! for! the! effects! of! a! third! variable.! In! our! region,! the mean! temperature! is! negatively! correlated! with! total! precipitation in! summer! (DJFM;! r! =! -0.57,! p! <! 0.01)! therefore! partial! correlations between! tree-ring! growth! and! precipitation! controlling! by! temperature! were! computed.! Correlations! were! computed! for! 18! months from! October! of! the! year! prior! to! ring! formation! to! March! of! the current! year.! The! significance! of! each! correlation! was! evaluated using! bootstrapping! with! 1000! Monte! Carlo! simulations! (Ebisuzaki (1997)).

We! also! used! the! VS-Lite! forward! model! version! 2.3! (Tolwinski-Ward! et! al.,! 2011;! available! at! http://www.ncdc.noaa.gov/paleo/ softlib/softlib.html)! to! mechanistically! identify! the! climate! controls of! tree-growth! and! simulate! tree-ring! chronologies! as! a! function! of climate.! VS-Lite! is! a! simplified! version! of! the! full! Vaganov-Shashkin forward! model! [START_REF] Vaganov | ! How! well! understood! are! the processes! that! create! dendroclimatic! records?! A! mechanistic! model! of! the climatic! control! on! conifer! tree-ring! growth! dynamics[END_REF]! 2006)! implemented! in! Matlab. It! simulates! annual! tree-ring! width! using! the! principle! of! limiting factors! and! requires! for! input! only! latitude,! monthly! mean! temperature,! and! monthly! total! precipitation.! The! model! estimates monthly! soil! moisture! from! temperature! and! total! precipitation data! via! the! empirical! one-layer! Leaky! Bucket! model! of! hydrology! [START_REF] Huang | ! Analysis! of model-calculated! soil! moisture! over! the! United! States! (1931-1993)! and applications! to! long-range! temperature! forecasts[END_REF].! Snow! dynamics! are! not! considered! in! the model! and! thus! all! precipitation! is! assumed! to! be! liquid.! Seasonal insolation! or! day! length! is! determined! from! site! latitude! and! does not! vary! from! year-to-year.! For! each! year,! the! model! simulates! standardized! (ageless)! tree-ring! width! anomalies! from! the! minimum! of the! monthly! growth! responses! to! temperature! (gT)! and! moisture (gM),! modulated! by! insolation! (gE).

In! this! study,! most! of! the! tuneable! model! parameters! (e.g., soil! moisture,! runoff,! root! depth)! were! set! to! the! default! values proposed! in! Tolwinski- Ward! et! al.! (2011) . The! growth! response functions! for! temperature! (gT)! and! moisture! (gM)! in! VS-Lite! involve only! two! parameters.! The! first! parameter! represents! the! temperature! (T 1 )! or! moisture! (M 1 )! threshold! below! which! growth! will! not occur,! while! the! second! is! the! optimal! temperature! (T 2 )! or! moisture (M 2 )! above! which! growth! is! not! limited! by! climate.! Unlike! the! full version! of! the! VS! model,! VS-Lite! does! not! have! an! upper! threshold! in! the! growth! response! functions! above! which! the! influence of! temperature! and! moisture! turns! negative.! The! growth! function parameters! were! estimated! for! each! site! via! Bayesian! calibration over! the! entire! period! with! climatic! data! between! 1931! and! 2008. For! this,! the! model! was! evaluated! 15,000! times! for! each! site! using three! parallel! Markov! Chain! Monte! Carlo! (MCMC)! chains! with! uniform! prior! distribution! for! each! parameter! and! a! white! Gaussian noise! model! error! (Tolwinski-Ward! et! al.,! 2013).! The! posterior median! for! each! parameter! was! used! to! obtain! the! "calibrated" growth! response! for! a! given! site.! Finally,! the! model! was! run! over the! entire! period! 1931-2008! using! the! calibrated! parameters! for each! site! to! produce! a! simulated! tree-ring! chronology! that! represents! an! estimate! of! the! climate! signal! of! tree! growth.! To! compute annual! ring! widths,! we! integrated! the! overall! simulated! growth rates! (i.e.,! the! point-wise! minimum! of! monthly! gT,! gM! and! gE) over! the! period! between! previous-year! March! and! current-year June.

To! evaluate! the! temporal! stability! of! the! calibrated! growth response! functions,! we! divided! the! period! 1931-2008! into! two equal! 39-year! intervals! and! withhold! the! second! half! for! validation of! the! parameters! estimated! in! the! fist! half.! In! addition,! we! conducted! series! of! calibrations! over! a! 21-year! sliding! window! from 1931! to! 2008! to! detect! the! timing! of! changes! in! growth! parameters observed! at! some! sites! with! complex! climate! responses.

3.! Results

3.1.! Tree-ring! chronologies! and! their! correlation! with! climate

Tree-growth! patterns! of! N.! pumilio! show! common! inter-annual variations! at! the! three! most! mesic! sites! during! the! last! century (Fig. ! 1b).! The! mean! series! intercorrelation! of! 0.46! indicates! some degree! of! common! interannual! variability! in! tree! growth! across the! gradient.! The! leading! EOF! of! the! four! chronologies! accounts for! 58%! of! the! total! variance! over! the! common! period! showing positive! loadings! for! all! chronologies.! The! two! chronologies! in! the middle! part! of! the! gradient! (ALM! and! DL)! have! the! largest! loadings (Table ! 1 Overall,! the! empirical! correlation! analyses! reveal! that! the! climatic! response! of! tree! growth! at! our! high-elevation! sites! varies along! the! precipitation! gradient.! The! mean! patterns! of! climate response! at! the! sites! have! also! changed! significantly! through! time during! the! study! period,! with! an! emerging! positive! response! to summer! precipitation! at! the! wetter! sites! and! a! strong! increase! in moisture! limitation! at! the! driest! site.! Next! we! attempt! to! mechanistically! model! tree! growth! variability! at! these! sites! as! a! function of! climate.

3.2.! Forward! modeling! of! tree-ring! growth

3.2.1.! Model! calibration! and! verification

The! Bayesian! calibration! approach! used! here! provides! optimal! estimates! for! the! parameters! of! the! growth! response! function (T 1 ,! T 2 ,! M 1 and! M 2 )! given! the! model! structure,! climate! data! and actual! tree! ring! data! for! each! site.! When! the! model! is! calibrated over! the! first! half! of! the! study! period! )! all! the! correlations! between! the! resulting! simulated! and! actual! tree-ring chronologies! are! statistically! insignificant! during! the! validation period! (1970-2008;! Table! 3).! So! the! model! is! not! able! to! simulate! the! observed! tree-ring! growth! variability! at! any! of! the! sites in! spite! of! a! good! skill! in! all! the! sites! during! the! calibration! period, with! a! percentage! of! explained! variance! of! actual! growth! variability between! 14! and! 17%.

The! lack! of! validation! of! the! model! suggests! that! the parametrization! might! be! unstable! over! time.! To! further! investigate this! issue! and! better! define! the! calibration! period,! the! calibration at! each! site! was! performed! over! a! sliding! window! of! 21! years As! a! result,! moisture! dynamics! during! the! growing! season! of! the model! cannot! fully! capture! the! variability! of! actual! tree-ring! growth and! the! optimization! tends! to! adjust! for! it! by! reducing! the! moisture! parameter! for! optimal! growth.! Indeed,! the! lagged! correlation between! the! simulated! and! actual! tree-ring! chronologies! at! this! site is! significant! during! the! second! half! of! the! study! period! regardless of! the! calibration! period! used! (Table ! 3).

For! all! further! analyses,! we! chose! to! calibrate! the! model! over! the entire! period! of! study! to! obtain! a! parameter! set! more! representative! of! the! periods! of! changing! growth! responses! observed! in! the actual! tree-ring! chronologies.! The! values! of! the! calibrated! parameters! over! this! long! period! are! consistent! with! local! estimates! on! the sliding! 21-year! window,! except! during! the! recent! periods! described above! (Fig. ! 4).! The! estimated! minimum! and! optimal! thresholds! for growth! averaged! across! the! sites! are! respectively! 6.1! ±! 0.8 • C! and 14.5! ±! 2.1 • C! for! temperature! and! 0.05! ±! 0.005! v/v! and! 0.34! ±! 0.03 v/v! for! moisture! (Table ! 2).! These! climatic! thresholds! define! the Calib.! 1931[START_REF] Bottinga | Calculated fractionation factors for carbon and hydrogen isotope exchange in the system[END_REF]Calib.! 1931-2008Valid.! 1931-2008! 1931[START_REF] Bottinga | Calculated fractionation factors for carbon and hydrogen isotope exchange in the system[END_REF]! 1970-2008! 1931[START_REF] Bottinga | Calculated fractionation factors for carbon and hydrogen isotope exchange in the system[END_REF]! 1970-2008! 1931-1975! 1976

3.2.2.! Simulated! tree-ring! chronologies! and! patterns! of! climate limitation

The! simulated! tree-ring! chronologies! are! all! significantly! correlated! with! the! actual! tree-ring! chronologies! over! the! 77-year! study period! (r! =! 0.21! to! 0.38;! p! <! 0.05)! but! the! explained! variance! over this! period! is! less! than! 15%! (Fig. ! 5a).! The! overall! skill! of! the! model is! higher! at! the! two! most! temperature-limited! sites! in! the! wetter part! of! the! gradient! (NUB! and! ALM;! r! >! 0.37)! and! decreases! towards the! drier! eastern! locations! (DL! and! CHA;! r! <! 0.33).! Despite! using! the whole! study! period! for! calibration,! the! model! is! still! unable! to! correctly! simulate! tree! growth! variability! after! the! 1970s! at! the! three wettest! sites! and! after! around! 1963! at! the! driest! site! (Fig. ! A .3).! Correlations! between! the! simulated! and! actual! tree-ring! chronologies lose! significance! at! all! sites! in! the! 1976-2008! period! (Table ! 3).! As discussed! in! the! previous! section,! this! issue! is! related! to! the! shifts in! climate! response! of! tree! growth! observed! at! the! sites,! which! the model! is! not! able! to! represent! because! of! its! structure.

The! simulated! patterns! of! climate! limitation! vary! consistently along! the! precipitation! gradient,! with! a! pattern! of! increasing! moisture! limitation! of! growth! rates! towards! the! east! (Fig. ! 5b).! Growth rates! tend! to! be! limited! by! low! temperatures! (gT! <! gM)! at! the beginning! and! end! of! the! growing! season! and! by! soil! moisture availability! (gM! <! gT)! during! the! peak! of! the! growing! season! in! the austral! summer.! At! the! wettest! treeline! site! (NUB),! low! temperatures! limit! growth! rates! during! the! entire! growing! season! and summer! soil! moisture! availability! becomes! limiting! only! in! some years! ( In! order! to! determine! whether! the! patterns! of! climatic! limitation! of! simulated! tree-ring! growth! have! changed! between! the wet-warm! climate! regime! prior! to! 1952! and! the! subsequent! dry cooling! period! between! 1952! and! 1975,! the! mean! growth! rates due! to! soil! moisture! and! temperature! were! compared! for! the! two periods! (Fig. ! 5c).! Temperature! limitation! during! the! peak! of! the growing! season! increased! slightly! during! the! cooling! period! across all! sites! but! the! change! is! not! statistically! significant! (p! >! 0.1).! Likewise,! moisture! limitation! across! the! sites! also! increased ! between 1931! and! 1951! and! 1952-1975.! The! change! is! greater! than! that! of temperature! limitation! but! reaches! statistical! significance! (p! <! 0.1) only! during! March! and! April! at! two! sites! (ALM! and! CHA).! Because the! simulated! growth! response! is! controlled! by! the! point-wise! min- imum! of! either! temperature! or! moisture,! the! increasing! moisture limitation! on! tree-growth! is! effectively! felt! at! the! driest! site! (CHA) rather! than! at! the! two! sites! mentioned! above.! These! results! show that! despite! its! limitations,! the! model! can! provide! useful! mechanistic! understanding! of! the! complex! climate-growth! relationships observed! at! these! high-elevation! forests.

4.! Discussion

4.1.! Patterns! of! tree-ring! growth! along! the! moisture! gradient

The! comparison! of! the! updated! tree-ring! chronologies! with climate! variations! along! the! west-to-east! precipitation! gradient suggests! a! significant! climate! control! on! inter-annual! tree-ring growth! during! the! current! growing! season.! However,! we! found important! differences! in! the! responsiveness! of! tree-ring! growth to! climate! between! west! and! east! sides! of! the! precipitation! gradient.! They! are! probably! due! to! the! effects! of! water! availability! on physiology! and! surface! radiation! budget.

At! mesic-to-humid! treelines,! above-average! N.! pumilio! growth is! associated! with! above-average! temperature! and! below-average precipitation! in! late! spring-early! summer! (November-January; Fig. ! 3).! The! link! between! temperature-precipitation! and! growth may! arise! from! the! effect! of! cloudiness.! Indeed,! cloud! cover,! limits! daytime! heating! and! is! often! associated! to! rainfall! in! Patagonia (Fig. ! 4;[START_REF] González | ! Some! indicators! of! interannual! rainfall! variability! in Patagonia! (Argentina).![END_REF],! which! conditions! reduce! the! incoming solar! radiation,! decreasing! the! photosynthetic! activity! of! the! trees and! thus! their! carbohydrates! production! [START_REF] Rozas | Summer! drought! and! ENSO-related! cloudiness! distinctly! drive! Fagus! sylvatica growth! near! the! species! rear-edge! in! northern! Spain[END_REF].! The In! contrast! to! the! three! mesic! populations,! xeric! high-elevation treeline! forests! appear! limited! by! high! temperature! and! low! water availability! in! December! and! July.! Water! falling! in! December! alleviates! drought! stress,! which! is! beneficial! to! trees.! These! results! are! in agreement! with ! Lara! et! al.! (2001;[START_REF] Dobbertin | Tree growth as indicator of tree vitality and of tree reaction to environmental stress: A review[END_REF],! who! showed! similar! relationships! between! N.! pumilio! growth! and! summer! conditions! at xeric! sites! in! Chile! (at! 36-39 • S).! At! high-elevations,! the! precipitation! falling! mainly! as! snow! in! July,! a! thick! snowpack! could! not! only provide! snowmelt! water! but! also! protect! soil! from! frost,! making melt! water! infiltration! into! the! soil! possible.! Therefore,! the! positive correlation! between! ring-width! and! precipitation! in! July! is! likely induced! by! the! positive! effect! of! the! snow! pack! accumulation! and the! consequent! soil! water! recharge! [START_REF] Magnin | ! Interannual! variations! in! primary! and secondary! growth! of! Nothofagus! pumilio! and! their! relationships! with! climate[END_REF]! Villalba et! al.,! 1997).! Consistently,! the! VS-lite! model! simulates! a! dominant control! of! tree-growth! by! moisture! availability.

4.2.! Changes! in! climate! limitation! of! year-to-year! tree-ring growth

According! to! our! data,! local! climate! variations,! driven! by changes! in! climate! regime,! yielded! temporal! changes! of! the year-to-year! growth-limiting! factors.! Notably,! the! positive! relation! between! November-December! temperature! and! growth! at the! mesic-to-humid! sites! is! enhanced! during! the! dry! and! cool 1952-1975! period,! which! may! reveal! an! increase! of! the! snowpack! influence! on! growth.! During! this! time! period,! at! the! two wettest! sites,! the! positive! relation! with! soil! moisture! in! spring (September-November)! also! indicates! that! water! availability, likely! released! by! spring! snowmelt,! promoted! tree-ring! growth. At! the! driest! site,! warm! December! temperatures! apparently! exacerbating! moisture! limitation! had! a! negative! impact! on! growth. According! to! the! model,! the! moisture! limitation! has! increased at! all! sites! between! the! wet-warm! climate! regime! prior! to ! 1952 and! the! subsequent! dry-cool! period! between! 1952! and! 1975,! but the! increase! of! sensitivity! to! moisture! has! been! significant! only at! the! driest! site.! So,! at! all! sites,! the! growth! response! functions have! undergone! some! modifications.! Nevertheless,! from! 1931! to ca.! 1975,! measured! and! modelled! growths! are! coherent.! It! means that! the! effects! of! environmental! parameters! like! snowpack! and cloudiness,! not! explicitly! accounted! for! by! the! model,! do! not! introduce! bias! detrimental! to! the! reconstruction! of! growth! variations through! time.

The! correlation! between! growth! and! December! precipitation has! been! positive! at! the! driest! site! all! over! the! 1931-2008! period but! a! positive! response! of! growth! to! the! current! and! previous! year moisture! has! emerged! since! the! mid-1970s.! At! the! mesic-to-humid sites,! the! relation! between! growth! and! current! year! soil! moisture does! not! show! a! clear! trend! although! the! correlations! with! precipitation! have! turned! to! positive! at! ca.! 1976.! These! findings! suggest that! the! impact! of! humidity! (rain,! moisture)! on! growth! has! not only! increased! at! the! dry! edge! of! the! precipitation! gradient,! but! has been! enhanced! along! the! whole! transect.! These! results! are! consistent! with [START_REF] Suarez | Sensitivity of Nothofagus dombeyi tree growth to climate changes along a precipitation gradient in northern Patagonia, Argentina[END_REF]! who! have! observed! an! increase of! the! similarity! of! Nothofagus! dombeyi! growth! patterns! along! the gradient! during! the! last! decades.

Whatever! the! calibration! period! chosen,! the! VS-Lite! model! cannot! simulate! post-1970s! growth! variations! consistent! with! the actual! tree-ring! growth! chronologies.! As! the! model! proved! efficient! over! the! rest! of! the! time! period,! such! a! discrepancy! suggests that! some! environmental! parameters,! which! are! not! taken! into account! in! the! model,! have! reached! critical! thresholds! beyond which! growth! is! significantly! impacted.! We! have! proposed! above that! cloudiness! and! snowpack! can! be! responsible! for! changes! in tree-ring! growth! through! their! effects! on! radiation,! temperature and! moisture! availability.! To! our! knowledge! the! temporal! changes in! cloudiness! is! not! well! documented! in! the! region.! Data! retrieved from! the! NASA/GEWEX! Surface! Radiation! Budget! show! that! over 1984-2007,! the! cloud! fraction! has! slightly! decreased! in! summer (-4%)! with! a! concomitant! increase! of! the! photosynthetically! active radiations! (+3.5%;! Fig. ! A .4).! These! trends! are! consistent! with! an increasing! effect! of! cloud! cover/radiation! on! growth.! However,! they are! small! and! we! wonder! if! such! subtle! changes! can! be! responsible for! the! deterioration! of! the! growth-climate! relation.! Moreover,! the records! do! not! go! far! enough! back! in! time! to! investigate! possible effects! in! the! 1970s.! The! dependence! of! growth! on! snowpack! may also! be! stronger! over! the! recent! decades! than! previously.! Indeed, the! substantial! change! of! the! amount! and! duration! of! the! snowpack! in! northern! Patagonia! in! the! last! decades! may! have! altered the! water! dynamics! in! the! soil! and! consequently! the! tree! growth! in a! sizable! way.! A! well-documented! interval! of! glacier! advance! culminating! in! 1976-1977! reported! in! the! north! Patagonian! Andes, followed! by! a! period! of! glacier! retreat,! has! taken! place! in! response to! changes! in! climate! conditions! [START_REF] Masiokas | ! 20th-century! glacier! recession! and! regional! hydroclimatic! changes! in northwestern! Patagonia[END_REF].! Consistently,! the! snow! cover! duration! has! decreased! from! the! late! 1970s (at! least! to! the! late-1990s,! end! of! the! studied! period! in ! Villalba! et! al., 1997).! These! findings! advocates! for! stronger! impact! of! snowpack on! year-to-year! tree-ring! growth! over! the! recent! decades.

Other! controlling! processes! may! also! explain! the! apparent! lack of! the! VS-Lite! model! skill! in! simulating! ring-width! over! the! recent period.! For! instance,! the! upward! trend! of! global! concentration! of atmospheric! CO 2 has! been! increasing! at! an! accelerating! rate! since the! mid-1960s! (Robertson! et! al.,! 2001).! The! increase! of! atmospheric CO 2 concentration! may! promote! tree! growth! by! enhancing! the photosynthetic! activity! and/or! reducing! the! stomatal! conductance, leading! to! the! increase! of! the! intrinsic! water-use! efficiency! of! trees (e.g., ! Ainsworth! and! Rogers,! 2007;! Keenan! et! al.,! 2013).! A! combination! of! factors,! such! as! those! mentioned! above,! may! be! responsible for! the! decoupling! between! modelled! and! measured! growth! after the! 1970s.

4.3.! Limitation! of! the! model! and! perspectives

The! moisture! in! the! one-layer! Leaky! Bucket! hydrological! model is! an! estimation! of! the! soil! content! in! the! upper! 1-2! m! [START_REF] Huang | ! Analysis! of model-calculated! soil! moisture! over! the! United! States! (1931-1993)! and applications! to! long-range! temperature! forecasts[END_REF]! and! integrates! the! land! surface! water! balance! over! several months. ! Schulze! et! al.! (1996)! have! shown! that! although! N.! pumilio roots! reach! ground! water! at! 2-3! m! depth,! its! major! water! supply comes! from! the! upper! soil! horizons! (at! ca.! 4! cm! depth).! So,! the actual! hydrological! module! may! not! be! representative! of! the! source water! of! the! trees.! However,! the! overall! good! agreement! we! obtain between! simulated! and! measured! ring-width! before! the! late! 1970s is! not! consistent! with! a! poor! adequacy! of! the! hydrological! module. A! possible! explanation,! which! would! deserve! additional! investigations,! could! be! that! the! depth! of! the! water! reservoir! impacts! the tree! growth! in! a! sizable! way! only! when! the! water! supply! is! below! a certain! threshold.! If! so,! using! a! multiple! layer! hydrological! module may! alleviate! the! post-late! 1970s! discrepancy! between! observed and! simulated! ring-width! chronologies.

The! difficulty! to! reproduce! tree-ring! growth! using! the! VS-Lite model! may! also! be! inherent! to! its! structure! and! parametrization. For! instance,! the! model! lacks! upper! limits! for! optimal! temperature and! moisture.! As! a! result,! an! optimal! growth! is! simulated! even! in extremely! warm! or! humid! years,! which! may! not! reflect! the! reality.! Another! structural! weakness! is! the! failure! to! take! into! account lagged! effects! while,! in! the! Patagonian! context,! the! previous! year conditions! have! proved! to! influence! growth! (this! study! and! e.g., Aravena! et! al.,! 2002;! Villalba! et! al.,! 2003).! The! full! process-based Vaganov-Shashkin! model! (Vaganov! et! al.,! 2006),! which! simulates! daily! cambial! dynamics! and! xylem! growth,! or! even! more complex! eco-physiological! models,! such! as! TREERING2000! (Fritts et! al.,! 2000)! or! MAIDEN! (Misson,! 2004),! are! able! to! link! photosynthesis! and! carbohydrate! allocation! to! stem! growth! and! integrate a! higher! number! of! processes! and! controlling! factors.! They! are certainly! more! efficient! for! analyzing! tree! growth-climate! relationships.! However,! some! of! them! are! designed! for! coniferous! trees (Vaganov-Shashkin! and! TREERING2000),! which! prevent! running them! for! N.! pumilio,! and! all! of! them! operate! with! daily! data,! which preclude! their! use! in! poorly! documented! remote! regions! such! as Patagonia.! Ongoing! monitoring! of! tree! physiology,! environmental conditions,! and! wood! cell! formation! will! provide! a! more! detailed representation! of! the! complex! biological! and! ecological! processes operating! in! the! Patagonian! sites.

5.! Conclusions

We! have! assessed! the! year-to-year! responses! of! N.! pumilio growth! to! recent! changes! in! climate! regimes! ! along the! steep! precipitation! gradient! of! northern! Patagonia.! The! combine! use! of! statistical! and! forward! modelling! analyses! has! allowed detecting! substantial! changes! in! the! sensitivity! of! tree-ring! growth through! time,! coincident! with! shifts! in! climate! regimes.! Humidity control! on! growth! has! increased,! and! this! limitation! has! been! more significant! at! the! xeric! than! at! the! mesic! sites.

Over! the! 1931-! mid-1970s! period,! the! VS-Lite! model! has! skills! to reproduce! the! tree-ring! variations.! From! the! mid-1970s! onwards, simulated! and! measured! tree-ring! growths! are! not! consistent. We! hypothesize! that! some! environmental! parameters! (cloudiness, snowpack,! atmospheric! CO 2 ),! whose! variations! are! not! explicitly taken! into! account! in! the! model,! have! reached! critical! thresholds impacting! tree-growth.! Beside,! by! design,! the! model! itself! may! not be! able! to! generate! realistic! tree-ring! width! over! a! certain! temperature! threshold.

Ongoing! monitoring! of! tree-growth! and! climate! will! provide! the input! and! control! data! necessary! to! run! more! sophisticated! models in! order! to! quantify! the! impact! of! various! environmental! parameters! on! growth! and! simulate! their! effects! in! projected! climatic conditions. Across the globe, the spatial variations of stable isotopes in the water cycle are due to movements of air masses as well as to isotopic fractionations associated to changes in water phases, which create directional isotopic spatial patterns across regions and continents [START_REF] Bowen | Isoscapes: Spatial Pattern in Isotopic Biogeochemistry[END_REF][START_REF] West | Isoscapes: Understanding movement, pattern, and process on earth through isotope mapping[END_REF]. At present, more than 60 stations are providing information on stable isotope composition of precipitation over the South American continent.

Part III

STABLE ISOTOPES IN TREE RINGS

Spatial and temporal distributions of oxygen-18 content in precipitation over this continent are mainly influenced by (i) the atmospheric moisture masses coming from the Atlantic and Pacific oceans and the Caribbean sea; (ii) the presence of the world largest continental evaporative basin (Amazon Basin), (iii) the seasonal movement of the Inter-Tropical Convergence Zone (ITCZ) over the central and northern part of the continent and, (iv) the mountain ranges of the Andes (Fig. 7. 1;Rozanski et al. [1995]).

Spatial variability of " 18 O p in southern South America

The spatial distribution of the oxygen isotope ratios of precipitation " 18 O p in the southern Andean region (south of 30°S) is dominated by the altitude e ect and mainly controlled by seasonal changes in temperature [Rozanski et al. 1995]. Indeed, in addition to their e ect on the spatial distribution of the amount of precipitation, the Andean mountain ranges have a significant influence on " 18 O p . The orographic uplift of the air masses over the Andes causes pseudoadiabatic cooling and orographic precipitation on the western side. The decrease of isotope ratios from west to east in this windward side is of about 7.3% /100 km at 47°S -48°S [START_REF] Stern | Stable isotope composition of precipitation across the southern Patagonian Andes[END_REF].

After the air masses have passed over the mountain range of the Andes to the leeward side, their moisture content and dew point temperature decrease, and they warm adiabatically upon descent. This commonly results in an 'orographic rain shadow' on the eastern side of the mountains with an average slope at ca. 33°S from 0.6 % /100 m (up to the elevation of 1500 m) to 0.2 % /100 m (at higher altitudes; Rozanski et al. [1995]). While on the windward side of the Andes at 47°S -48°S oxygen compositions of precipitation " 18 O p and surface waters " 18 O w depend primarily on the distance from the main source of moisture (Pacific Ocean) and weakly on the site elevation, isotope ratios of waters on the leeward side may only depend on the processes occurring in the windward side and on the height of the orographic barrier [START_REF] Stern | Stable isotope composition of precipitation across the southern Patagonian Andes[END_REF]. Therefore, at 47°S -48°S, " 18 O p and " 18 O w mainly reflect depletion of the heavy isotopes resulting from progressive rainout during orographic uplift of an air mass as it passes over the mountain belt [START_REF] Stern | Stable isotope composition of precipitation across the southern Patagonian Andes[END_REF]Blisniuk 2002, Lenters andCook 1995].

To my knowledge, further north at ca. 41°S, the spatial variation of " 18 O p and " 18 O w has not been documented. This information is important for understanding the causes of the variation of the oxygen isotopic composition in tree-rings. However, this is an important issue to understand the oxygen isotope ratio observed in the cellulose of our tree-rings that we wanted to use as a paleoclimate tracer. Therefore, in this chapter we describe and compare the " 18 O p and " 18 O w of surface (or spring) waters sampled at our tree-ring sites.

Oxygen isotopes in spring waters related to "

18 O p at 41°S

Spring and precipitation waters

Locally derived spring waters were collected at the tree sampling sites in March 2013 and May 2014 and analyzed for " 18 O w . Aliquots of three milliliters of spring water were equilibrated with CO 2 at 20°C for about 8 hours. CO 2 was analyzed with a Finnigan MAT 252 at Laboratoire des Sciences du Climat et de l'Environnement (LSCE). Reproducibility of " 18 O w measurements was of ± 0.05% .

In order to situate the sampled spring water in the regional isotopic context, monthly measurements of " 18 O p from the neighboring local stations in the windward and leeward sides of the Andes were obtained from the Global Network for Isotopes in Precipitation (GNIP) database.

We chose the stations that have the longest records in the region (Fig. 7.

Orographic rain shadow

The isotopic values of the spring waters, ranging from -13.55% to -8.66 % , are ca. 4% higher in March than in May, which is in agreement with the seasonal variations of " 18 O p observed in the neighboring isotopic stations (Fig. 7.3). The dominant trend in the data is a strong decrease of " 18 O w with increasing distance from the Pacific Ocean, the principal water vapor source [START_REF] Garreaud | The Andes climate and weather[END_REF], Garreaud et al. 2013]. Variations of " 18 O w with longitude W show a steep eastward drop of -1% in March and -3% in May over ca. 40 km (equivalent to ca. -2.5 to -7.5% /100 km).

The west-to-east decrease of " 18 O w matches the trend in " 18 O p : the annual (April-March) " 18 O p decreases indeed by ca. 5 ± 0.9% over ca. 150 km, from Puerto Mont at the windward side (72°56'W) to Bariloche at the leeward side (71°12'W) of the Cordillera, during their 5year long common period of record (Fig. 7.3). The orographic rain shadow generated by the Andes at 41°S is similar to the 4-5% di erence measured at both sides of the Andes at 47°S -48°S [START_REF] Stern | Stable isotope composition of precipitation across the southern Patagonian Andes[END_REF].

At the GNIP stations closest to the study sites (Puerto Mont and Bariloche), monthly values of " 18 O p are significantly and positively correlated with mean temperatures (Fig. 7.3). Furthermore, over the continuous " 18 O p record from 1989 to 2009 at Puerto Mont, the increasing trend of 0.8% is concomitant with a 0.7°C increase in temperature (Fig. 7.3). These findings suggest a decrease of " 18 O p of the condensate precipitation (rain or snow) with decreasing atmospheric temperature, consistent with the simple Rayleigh distillation model [Dansgaard 1964]. Therefore, at 41°S, similar physical processes than those occurring at 47°S -48°S [START_REF] Stern | Stable isotope composition of precipitation across the southern Patagonian Andes[END_REF] should drive the variability of the oxygen isotopic compositions of waters.

The longitudinal trend shown here relies mainly on waters collected on the field during two campaigns (March 2013 and May 2014). These water samples are likely isotopically di erent from the precipitation averaged over several years and may not be fully comparable to the GNIP mean precipitation. Therefore, the trends shown here should be considered with caution and additional measurements are clearly necessary to strengthen these observations. Nevertheless, the agreement between the isotopic composition of the sampled spring water and the precipitation advocates for a rather superficial origin of spring waters.

Abstract

Tree-ring " 18 O chronologies from two native species (Fitzroya cupressoides and Nothofagus pumilio) in northern Patagonia were developed to assess their potential for paleoclimate reconstructions. The five annually resolved cellulose " 18 O chronologies (2 for F. cupressoides and 3 for N. pumilio) are located on the Andes along the steep west-to-east precipitation gradient. Over the common 60-years long interval, the five site-" 18 O cell chronologies exhibit a strong common signal as indicated by the significant mean inter-correlation between the two species (r = 0.61, p < 0.05) and the high percentage (65%) of total variance explained by the first Empirical Orthogonal Function. Although correlation analyses reveal that the two mean species-" 18 O cell chronologies are mainly modulated by December-May temperature, the N. pumilio chronology shows a greater sensitivity to record temperature variations (r = 0.57, p < 0.05). The " 18 O cell of N. pumilio contains a regional temperature signal representative of a large area in southern South America under the influence of the Southern Annular Mode (SAM). This study indicates that " 18 O cell in N. pumilio is a promising proxy to reconstruct past variations in temperature in South America south of 38°S.

Résumé

Des chronologies du " 18 O des cernes d'arbre provenant de deux espèces natives du nord de la Patagonie (Fitzroya cupressoides et Nothofagus pumilio) ont été développées afin d'évaluer leur potentiel pour de futures reconstructions paléoclimatiques. Les cinq chronologies annuelles du " 18 O de la cellulose (2 pour F. cupressoides et 3 pour N. pumilio) proviennent de sites situés sur les Andes, le long du gradient régional de précipitation ouest-est. Au cours de leurs 60 années communes, les cinq chronologies isotopiques montrent un fort signal commun comme indiqué par l'inter-corrélation significative entre les deux espèces (r = 0.61, p < 0.05) et le fort pourcentage (65%) de la variance totale expliqué par le premier Empirical Orthogonal Function. Bien que les analyses de corrélation montrent que les deux chronologies moyennes spécifiques de " 18 O cell sont principalement controlées par les températures de Décembre à Mai, la chronologie isotopique de N. pumilio a une plus grande sensibilité pour enregistrer les variations de température (r = 0.57, p < 0.05). Le " 18 O cell de N. pumilio possède un signal régional de température représentatif d'une grande region du sud de l'Amérique du Sud sous l'influence du Southern Annular Mode (SAM). Cette étude indique que le " 18 O cell de N. pumilio est un proxy prometteur pour reconstruire les variations passées de température en Amérique du Sud au sud de 38°S.

Published paper in Journal of Geophysical Research -Biogeosciences

This chapter has been published as: Lavergne A., Daux V., Villalba R., Pierre M., Stievenard M., Vimeux F. and Srur A.M. ( 2016), Are the oxygen isotopic compositions of Fitzroya cupressoides and Nothofagus pumilio cellulose promising proxies for climate reconstructions in northern Patagonia?, Journal of Geophysical Research -Biogeosciences, 121, doi:10.1002/2015JG003260.

Introduction

Recent increase in global mean temperature concurrent with major changes in the world's hydrological cycle [Intergovernmental Panel on Climate Change (IPCC) et al., 2013] strengthen the need to estimate anthropogenic climate changes in addition to natural climate variability. While climate variability and forcing mechanisms in the Northern Hemisphere are quite well understood [e.g., [START_REF] Andersen | High-resolution record of Northern Hemisphere climate extending into the last interglacial period[END_REF]Selten et al., 2004;Mann et al., 2008;Frank et al., 2010;Allen et al., 2012;Hoell et al., 2014], the Southern Hemisphere (SH) is poorly documented and underrepresented in global climate reconstructions [START_REF] Neukom | Multi-centennial summer and winter precipitation variability in southern South America[END_REF][START_REF] Ahmed | Continental-scale temperature variability during the past two millennia[END_REF][START_REF] Ipcc) | IPCC fifth assessment report, climate change 2013: The physical science basis[END_REF]. In this context, South America is a key region for examining past climate changes in the SH.

The Andes Cordillera is the most important mountain range in the SH and crosses the South American continent from the northern tropics in Colombia (10°N) to the southern tip of the continent (55°S). The Patagonian Andes act as an impressive topographic barrier to the persistent Westerlies winds, which brings to the continent moisture from the South Pacific Ocean, leading to one of the most dramatic west-to-east precipitation gradient on Earth [Smith and Evans, 2007]. The variability of the extratropical western South America climate (i.e., interannual and interdecadal changes) is mostly driven by the interactions, through atmospheric teleconnections, between the circum-Antarctic cyclonic belt (reflected by changes in the Southern Annular Mode, SAM) and the subtropical Pacific high-pressure cell, which in turn is affected by changes in the El Niño-Southern Oscillation. The interactions between these large-scale tropical and high-latitude climate forcings influence the latitudinal distribution of rainfall and the temperature patterns from the subtropics to subantarctica latitudes [Gillett et al., 2006;Archer and Caldeira, 2008;Garreaud et al., 2008]. Their large spatial and long-term temporal variability are poorly documented over the last millennia [Villalba, 2007]. Therefore, the acquisition of paleoclimate data in Patagonia may improve the representation of the SH in global climatic reconstructions.

Paleoenvironmental information recorded in natural archives can be extracted through a combination of traditional and more advanced techniques [START_REF] Bradley | Dendroclimatology[END_REF]. Paleoproxies allow reconstruction of past climates beyond the limit of instrumental records and thus help the interpretation of recent and ongoing climate LAVERGNE ET AL. change. Tree rings provide continuous and annually resolved paleoclimatic records that may extend from several centuries to millennia [e.g., [START_REF] Jones | High-resolution palaeoclimatology of the last millennium: A review of current status and future prospects[END_REF][START_REF] Corona | Millennium-long summer temperature variations in the European Alps as reconstructed from tree rings[END_REF][START_REF] Yang | A 3,500-year tree-ring record of annual precipitation on the northeastern Tibetan Plateau[END_REF]. Variations in the width and density of annual rings have been widely applied as proxy estimates of past climatic conditions, such as air temperature and precipitation [e.g., Fritts, 1976;Lara et al., 2008;[START_REF] Barichivich | Climate signals in high elevation tree-rings from the semiarid Andes of north-central Chile: Responses to regional and large-scale variability[END_REF][START_REF] D'arrigo | Regional climatic and North Atlantic Oscillation signatures in West Virginia red cedar over the past millennium[END_REF]. In the Andes Mountain of southern Chile and Argentina, tree rings have been used to reconstruct a wide variety of climatic variables at different spatial scales [see Villalba et al., 1997;Lara et al., 2005;[START_REF] Boninsegna | Dendroclimatological reconstructions in South America: a review[END_REF]Muñoz et al., 2014]. In particular, the long-lived and slow-growing endemic Fitzroya cupressoides and the dominant deciduous Nothofagus pumilio in subantarctic forests have been used for temperature reconstructions [START_REF] Villalba | Climatic fluctuations in northern Patagonia years as inferred from tree-ring during the last 1000 records[END_REF]Lara andVillalba, 1993, 1994;Lara et al., 2001Lara et al., , 2005;;Aravena et al., 2002;[START_REF] Villalba | Long-term multi-proxy climate reconstructions and dynamics in South America (LOTRED-SA): State of the art and perspectives[END_REF]. The temporal longevity of F. cupressoides [Lara and Villalba, 1993] and the vast geographical distribution of N. pumilio [Veblen, 1979;Schlatter, 1994], not only along the north-south temperature but also the west-to-east precipitation gradients, make these species valuable archives of past climate variations. However, due to the environmental complexity introduced by the Andes, tree growth may be influenced by different physical, biological, and climatic factors making tree ring width (TRW) and density difficult to be interpreted. The potential of tree ring oxygen isotopes for high-resolution climatic reconstruction has largely been demonstrated [e.g., Gessler et al., 2014;Hartl-Meier et al., 2014;Labuhn et al., 2014;[START_REF] Porter | Spring-summer temperatures since AD 1780 reconstructed from stable oxygen isotope ratios in white spruce tree-rings from the Mackenzie Delta, northwestern Canada[END_REF]. Climatic responses indicate, in some cases, a stronger sensitivity to climate of the isotopic signatures than the TRW [McCarroll and Loader, 2004;Daux et al., 2011;Gessler et al., 2014]. The oxygen isotopic composition of cellulose (δ 18 O cell ) records the δ 18 O of the source water derived from the precipitation, which itself is related to the temperature variations at middle and high latitudes. It is modulated by the evaporation in the ground and evaporative/diffusion processes in the leaf, which are largely controlled by vapor pressure deficit [e.g., Barbour et al., 2005;Gessler et al., 2013].

There is a lack of tree ring δ 18 O studies in South America; only one annually resolved chronology of tree ring δ 18 O from Austrocedrus chilensis has been carried out in the northern Patagonian Andes [Roig et al., 2006]. The strong correlation between this 100 year long series and the Southern Oscillation Index (SOI), a measure of the development and intensity of El Niño/La Niña events in the Pacific Ocean, is promising and encourages the continuation of δ 18 O-based climate reconstructions in Patagonia. Here we tested the performance of the oxygen isotopic composition of the cellulose of F. cupressoides and N. pumilio for recording climate variability. For this purpose, we developed annually resolved δ 18 O cell chronologies from five high-elevation forest sites in northern Patagonia (41°S) for the last 60 years (1952-2011), conduct interspecies comparisons of the oxygen isotope signals, and explore the links between these isotopic records and the climate of the region.

Materials and Methods

Sample Preparation and Isotopic Analyses of Cellulose

Tree ring samples were collected during the austral summer of 2013 at five sites in northern Patagonia, Argentina (Figure 1). Two sites of F. cupressoides and three sites of N. pumilio, growing at elevations of 950-1050 and 1270-1610 m above sea level, were selected along the regional precipitation gradient from Cerro Tronador (41°09′S, 71°53′W) to the drier Challhuaco Valley near Bariloche (41°05′S, 71°21′W, Figure 1 and Table 1). The sampling sites were located on moderate to steep slopes (approximately 15°to 40°) for both species. The soils were therefore very likely well drained. Paired increment cores were extracted from more than 30 trees growing in similar growth conditions (competition and microsite conditions), at breast height (1.3 m above the ground), using a Pressler borer with a diameter of 5 mm. The core samples were dated to the calendar year of their formation and crossdated following the techniques described by Stokes and Smiley [1968]. Since the biological growth season in the Southern Hemisphere overlaps two calendar years (from September to May), rings were assigned to the year when ring growth began [Schulman, 1956]; therefore, the last complete ring in the cores sampled in summer 2013 was 2011 (the 2012 ring was still incomplete at the time of the sampling). The N. pumilio trees analyzed here are part of a ring width data set which has been used by [START_REF] Lavergne | Temporal changes in climatic limitation of tree growth at upper treeline forests: Contrasted responses along the west-to-east humidity gradient in Northern Patagonia[END_REF] to evidence the change with time of the effect of climate on tree ring growth.

At least four trees per site are usually considered a minimum sample for completing a representative reconstruction of environmental variability from the isotopic composition of tree ring cellulose [Leavitt and Long, 1984;Robertson et al., 1997;Treydte et al., 2001Treydte et al., , 2006;;[START_REF] Leavitt | Tree-ring C-H-O isotope variability and sampling[END_REF]]. Here we selected six trees from each of the five sites, according to the following criteria: (1) their ring widths were wide enough to conduct isotopic measurements;

(2) the trees sampled were older than 150 years so that their 60 outlying rings were not affected by juvenile effects, and (3) their ring widths chronology was highly correlated with the site chronology. Since N. pumilio has diffuse pores and F. cupressoides very narrow rings, early and late woods could not be separated and whole rings were cut instead. For each species, we selected 12 years distributed throughout the last 60 year period for examining the intertree δ 18 O variability. The years were selected so as to cover a variety of ring widths. The δ 18 O cell from the six individual trees of each test set shows good reproducibility (average pairwise correlation coefficient of 0.7 and 0.53 for N. pumilio and F. cupressoides, respectively; Table 2). Reflecting this coherence, an Expressed Population Signal [START_REF] Briffa | ! Tree-! Climate! Relationships! and! Dendroclimatological Reconstructions! in! the! British! Isles.! Dissertation[END_REF]Wigley et al., 1984] superior to 0.85 and a confidence interval around the mean inferior to 0.5% were obtained for four and six trees, respectively for N. pumilio and F. cupressoides. Except for the two test sets, at each site, 60 year long , annually resolved isotope chronologies (site-δ 18 O cell hereafter) were produced by pooling dated growth rings from trees prior to the isotopic analyses. a Site: Site abbreviation used in the study: GF: Glaciar Frías, RA: Río Alerce, NUB: Paso de las nubes, ALM: La Almohadilla, and CHA: Challhuaco; Lat.: latitude, Lon.: longitude, Alt.: altitude, Mean: mean value, SD: standard deviation, and AC1: lag 1 autocorrelation of the site-δ 18 O cell chronologies.

Journal of Geophysical Research: Biogeosciences

10.1002/2015JG003260

The wood samples were chipped and then grounded in a ball mill for homogenization. α-cellulose was extracted from the wood according to the SOXHLET chemical method derived from Leavitt and Danzer [1993]. α-cellulose was homogenized ultrasonically with a sonotrode apparatus and freezedried. Cellulose samples of around 0.20 mg were loaded in silver foil capsules. The δ 18 O cell was determined with a high-temperature conversion elemental analyzer (Thermo Scientific) coupled with a Finnigan MAT252 mass spectrometer (at Laboratoire des Sceinces du Climat et de l'Environnement) according to the procedure described in [START_REF] Shi | Sampling strategy and climatic implications of tree-ring stable isotopes in Southeast Tibetan Plateau[END_REF]. An internal laboratory reference of cellulose (Whatmann® CC31) was used to correct for instrument drift and to normalize the data to internationally accepted standards. Along the sequence analyses, the isotopic composition of CC31 was measured every three samples. The standard deviation (SD) obtained from the measurement of the isotopic composition of 10 consecutive CC31 standards was typically ± 0.25‰. The analysis of oxygen isotopes of each sample was repeated at least once and up to three times. This methodology allows rejecting outlier measurements, which do not fit with the maximum accepted range.

Correlations and Calibrations

The first empirical orthogonal functions (EOF1) were extracted from the five site-δ 18 O cell chronologies. For each species, the site-δ 18 O cell chronologies are well intercorrelated (0.52 < r < 0.72, p < 0.05; Table 2); therefore, we constructed two species-δ 18 O cell chronologies of 12 trees each by averaging the specific site chronologies. Temperature and precipitation controls on δ 18 O cell variability were empirically identified using simple and partial correlations (Seascorr MATLAB routine [Meko et al., 2011]) of the species-δ 18 O cell chronologies and leading EOF with monthly temperature and precipitation from Bariloche (nearest meteorological station at 41°12′S-71°12′W, 840 m above sea level; Servicio Meteorológico Nacional, Figure 1). The station records were homogenized and gap filled using the R routine HOMER (HOMogenization softwarE in R [Mestre et al., 2013;Venema et al., 2013]) and climatic series from additional stations located in the area (40°-43°S and 71°-73°W).

The use of partial correlation allows assessing the relationship between two variables while controlling for the effects of a third variable. In northern Patagonia, temperature is negatively correlated with precipitation in summer (December-March; r = À0.57, p < 0.01 ; 1931-2009). Therefore, we performed partial correlations between species-δ 18 O cell chronologies and precipitation while controlling for temperature. Calculations were conducted over the whole growing season from September to May. The significance of each correlation was evaluated using bootstrapping with 1000 Monte Carlo simulations [Ebisuzaki, 1997].

On the basis of these correlations, we conducted split periods of calibration-verification tests with half of the climatic time series (identified as potential climatic target) for calibration and the other half for assessing the quality of the reconstruction (verification). To assess the predictive skill of the model, the coefficient of determination (r 2 ) of the regression models, reduction of error statistic (RE) [Fritts, 1976], coefficient of efficiency (CE) [Cook et al., 1994], and the Durbin-Watson statistic (DW) [Durbin and Watson, 1971] were systematically compared for independent calibration

Results

Interspecies Coherence of δ 18 O cell

The means (and standard deviations) of the species-δ 18 O cell chronologies are 29.14 ± 0.57‰ and 30.06 ± 0.61‰ for F. cupressoides and N. pumilio, respectively (N = 60 in both cases; Table 1). The serial correlations range from 0.04 to 0.29, indicating almost no lag effects in these records. The difference of 0.92‰ between the two speciesδ 18 O cell chronologies is not statistically significant (Student's t test: t = 8.64, p < 0.01).

The species-δ 18 O cell time series are highly correlated with one another (r = 0.61, p < 0.01; Table 2). This is in accordance with the leading EOF of the five site-δ 18 O cell chronologies, accounting for 65% of the total variance, and thus indicating strong spatial interspecies coherency. The species-δ 18 O cell time series both show a progressive increase of approximately 0.9‰ from the 1960s to the mid-1990s and a relative leveling off during the last decades (since 1990-2000; Figure 2). Remarkably, a feature shared by the two series is the approximately À2‰ drop in 1991.

Climatic Signals Recorded in Tree Ring δ 18 O

The correlations of the two species-δ 18 O cell chronologies tend to be positive with temperature and negative with precipitation. For F. cupressoides, only the correlation with April temperature is significant (r = 0.26, p < 0.05; Figure 3). For N. pumilio, the correlations with temperature over December-May are significant (r = 0.32 to 0.43, p < 0.05). Correlations of the two species-δ 18 O cell chronologies with monthly precipitation are not significant at 95% (Figure 3). The leading EOF is significantly and positively correlated to temperature (r = 0.29 to 0.40; p < 0.05) over the same months as N. pumilio, and thus, it mainly captures the δ 18 O cell signal of the N. pumilio trees.

Significant relationships between the isotopic chronologies and the mean of December to May temperatures confirm the primary influence of temperature on δ 18 O cell variations in the two species (r = 0.30 and r = 0.57 for F. cupressoides and N. pumilio, respectively, p < 0.05; Figure 3). The δ 18 O cell in N. pumilio captures more strongly the mean temperature variations. Consistently, the leading EOF of the site-δ 18 O chronologies significantly correlates with the mean temperature over the period December-May (r = 0.50, p < 0.05).

To assess the spatial representativeness of the temperature signal recorded in the species-δ 18 O cell chronologies, we conducted a spatial correlation analysis between these time series and gridded temperature records from the CRUTS 3.2 data set [Harris et al., 2014] averaged over December-May in the area 30°S-56°S and 55°W-80°W. The highest correlations for N. pumilio appear near the sampling sites, but significant correlations are also observed all across southern South America from 35°S to 55°S (Figure 4). Similar, but weaker, spatial correlations are observed for F. cupressoides (Figure S1 in the supporting information). These results indicate that the regional N. pumilio δ 18 O cell chronology (and to a lesser extent the F. cupressoides chronology) accurately records mean temperature variations over a large area across Patagonia.

Calibration of Tree Ring δ 18 O on Temperature

The final goal of this study is the reconstruction of climate in southern South America during the past centuries. Here we explore the robustness of the δ 18 O cell -based reconstruction of climate using the oxygen isotopic composition of the cellulose in N. pumilio tree rings as a proxy for December-May temperature. Split-period 3) indicate that the linear regression model (over 1954-2010) is reliable and has high predictive skills (RE > 0 and CE > 0). A DW statistic of around 1.58 for the linear model, whatever the calibration period considered, suggests only minimal residual autocorrelation. We therefore calibrated the mean December-May temperature over the whole period. Our linear regression model accounts for 35% of the actual temperature variance for the period 1954-2010, with a RMSE of 0.58°C, representing 11% of its variability range (Figure 5). For comparison, the predictive skills of the F. cupressoides calibration model were lower than for N. pumilio (r 2 = 0.09, DW = 1.71, and RMSE = 0.69°C; Table S1 in the supporting information).

Discussion

In the following paragraphs, we address the performance of the δ 18 O cell of F. cupressoides and N. pumilio as climatic proxies for long-term paleoclimate reconstructions. Variations in regional temperature appear to modulate the δ 18 O cell fluctuations, particularly in N. pumilio. At midlatitudes, atmospheric temperature determines to a certain extent the atmospheric vapor condensation and the associated oxygen isotopic fractionation in the meteoric water. As a result, the δ 18 O of the condensate precipitation (rain and snow) (hereafter δ 18 O P ) decrease with decreasing atmospheric temperature [Dansgaard, 1964]. There are few GNIP (Global Network of Isotopes in Precipitation) stations in southern South America (Figure 1), and most records are discontinuous and short. At the GNIP stations closest to the study sites (Puerto Mont and Bariloche), monthly values of δ 18 O P and mean temperatures are significantly correlated (Figure S2b in the supporting information), in line with earlier findings relating temperature to δ 18 O P in South America south of 40°S [Rozanski and Araguás-Araguás, 1995]. Moreover, over the continuous δ 18 O P record from 1989 to 2009 at Puerto Mont, the increasing trend of 0.8‰ is concomitant with a 0.7°C increase in temperature (Figure S2c in the supporting information). Considering that rain is the main source of water for trees, the positive trend of about 0.9‰ (from 1960-1970 to 1990-2000) in the two speciesδ 18 O cell chronologies may indirectly reflect the increasing trend in temperature via the δ 18 O of the source water (hereafter δ 18 O SW ), which itself is derived from δ 18 O P .

In addition, soil and plant evaporation can profoundly modify the isotopic precipitation signal. Warm temperatures can indeed generate higher evaporation leading to enriched superficial soil water and/or enriched leaf water, possibly inducing enriched δ 18 O cell in tree rings [e.g., Gessler et al., 2014]. Moreover, the equilibrium biochemical fractionation (ε bio ) associated with the imprinting of oxygen isotope signal in cellulose is dependent on temperature below 20°C [Sternberg and Ellsworth, 2011]. At Bariloche, the temperature of the growing season (December-May) averaged over 1952-2011 is 11.25 ± 0.75°C. Applying the equation derived by Sternberg and Ellsworth [2011] yields ε bio = 28.52 ± 0.26‰. As shown by the low value of the standard deviation, ε bio is very stable over this 60 year long period. Therefore, the positive trend observed in the δ 18 O cell chronologies cannot be ascribed to the variations of this parameter. In other words, the relationship between temperature and δ 18 O cell may be inferred from the dependence on temperature (1) of δ 18 O P and ( 2) of the evaporative enrichment, but information is lacking to specify the relative contributions of these possible causes.

The abrupt decrease of approximately 2‰ in 1991 observed in the cellulose of the two species is very likely related to the temperature drop and the precipitation increase, which were reported in the southern sector of South America following the Pinatubo eruption of 15 June 1991 in Luzon (Philippines) [START_REF] Rosenbluth | Recent temperature variations in southern South America[END_REF][START_REF] Trenberth | Effects of Mount Pinatubo volcanic eruption on the hydrological cycle as an analog of geoengineering[END_REF]. At Bariloche, the December-May drop in temperature associated with the Pinatubo eruption was about 1°C (compared to previous and following years). The second lowest δ 18 O cell in the F. cupressoides chronology is also coincident with the Agung eruption in 1963-1964 in Bali (Indonesia).

Which Species Is the Most Appropriate for Climate Reconstruction?

Despite the overall consistency between the two species, the climate signal recorded in F. cupressoides δ 18 O cell is weaker than in N. pumilio. Dendroclimatological studies conducted in Patagonia have reported negative relationships between the TRW of F. cupressoides and temperatures during the previous growing season [START_REF] Villalba | Climatic fluctuations in northern Patagonia years as inferred from tree-ring during the last 1000 records[END_REF]Lara andVillalba, 1993, 1994;[START_REF] Neira | Desarrollo de cronologia de ancho de anillos para alerce[END_REF], and a high degree of temporal autocorrelation in the TRW chronologies [START_REF] Villalba | Climatic fluctuations in northern Patagonia years as inferred from tree-ring during the last 1000 records[END_REF]. This growth dependence on previous year climatic Journal of Geophysical Research: Biogeosciences 10.1002/2015JG003260 conditions is related to starch reserves developed in previous summer. Starch reserves are less abundant if warm and dry conditions during the growing season limit carbon assimilation through the reduction of stomatal aperture [Deslauriers et al., 2014;Urrutia-Jalabert et al., 2015]. The simultaneous use of starch reserve and current photoassimilates, in different proportions from one year to the next, may lead to an ambiguous δ 18 O cell signal in F. cupressoides, poorly related to current year temperature. However, the lack of autocorrelation in the δ 18 O cell series from F. cupressoides indicates no substantial carryover effects due to remobilized reserves from previous summer. Further work is needed to understand F. cupressoides physiology and to examine the long-term carbon turnover of stored carbohydrates and its use for growth. F. cupressoides is one of the longest-lived tree in the world with a maximum life span over 3600 years [Lara and Villalba, 1993]. Therefore, these records could be very valuable archives if we were able to better understand the variations of the isotopic composition of its cellulose.

Is There Strong

Local-to Large-Scale Climate Variability Recorded in δ 18 O cell ? Spatial correlation fields between gridded December-May temperature and N. pumilio-δ 18 O cell shows a homogenous pattern over a large area south of ca. 38°S. Temperature in this extended region is mainly influenced by variations in the SAM. Indeed, in Patagonia, the strength of the Westerlies winds, which controls the variations of precipitation [START_REF] Garreaud | Atmospheric circulation and climatic variability, The Physical Geography of South America[END_REF][Garreaud et al., , 2008[Garreaud et al., , 2013] ] and influences atmospheric temperature, particularly in summer [START_REF] Gupta | Coupled ocean-atmosphere-ice response to variations in the Southern Annular Mode[END_REF]Garreaud et al., 2013], is mainly modulated by the SAM [Thompson et al., 2000;Marshall, 2003;Garreaud et al., 2008]. The N. pumilio-δ 18 O cell chronology exhibits a significant positive correlation with the December-May SAM index (r = 0.43, p < 0.05; 1957-2010 [Marshall, 2003]). This correlation is quite strong, but weaker than the relation reported with temperature (r = 0.57, p < 0.05). We have shown that temperature controls directly and/or indirectly the oxygen isotopic composition of N. pumilio cellulose. However, as temperature is likely not the only controlling factor on δ 18 O cell , and conditioned by different regional (in particular cloud cover) and local effects in addition to SAM, the link between δ 18 O cell and SAM is expected to be weaker. As a result the performance of the δ 18 O cell -based reconstruction of SAM is poorer than the temperature reconstruction (RE = 0.186, DW = 1.652, and RMSE = 0.92, representing 18% of the SAM variability from December to May).

The dominant climatic signal recorded in the δ 18 O cell of N. pumilio and F. cupressoides differs from the A. chilensis δ 18 O, located 5°north of our study area [Roig et al., 2006]. Indeed, A. chilensis δ 18 O was shown to record past variations in the summer SOI, which is the main mode of climate variability at the northwest corner of Patagonia (approximately 36°S [Montecinos and Aceituno, 2003]). Even if the links between the mode of climate variability and the isotopic compositions of tree rings are not straightforward, the latter contain relevant information on the former. Thus, our results encourage the development of multicentennial δ 18 O cell chronologies.

Conclusions

A reliable paleoclimate reconstruction based on tree ring proxies requires a comprehensive understanding of the factors influencing their variability. This study represents the first to investigate the potential of the oxygen isotopic composition of the cellulose in F. cupressoides and N. pumilio in northern Patagonia. Given its longevity (above 3600 years), the long-lived F. cupressoides is comparatively the most interesting species for long-term climate reconstructions in this area. Presently, our poor understanding of its complex physiology makes it difficult to determine the climatic signal recorded in its δ 18 O cell . Our results indicate that N. pumilio represents a more robust paleoclimate archive. We show that δ 18 O cell of N. pumilio is consistent between sites, strongly related to variations in December-May temperature, and to a lesser extent to hemispheric forcings such as SAM. δ 18 O cell -based reconstructions would be representative of temperature variations over large areas of southern South America influenced by SAM. South of 38°S, N. pumilio is shown to be a good candidate for reconstructing not only local and regional temperature but also SAM variations. Analyses of a 200 year long δ 18 O cell chronology are currently in progress.

Ocell data used in this paper will be provided on request to A. Lavergne or V. Daux.

Abstract

We use annually resolved tree-ring width (TRW), " 18 O and " 13 C chronologies to investigate tree growth and physiological response of five N. pumilio and F. cupressoides forests in northern Patagonia to changes in atmospheric CO 2 concentrations (c a ) and climate over the last 60 years. The basal area increment (BAI) was calculated using TRW series and, 13 C discrimination ( 13 C), leaf intercellular CO 2 concentrations (c i ) and intrinsic water-use e ciency (iWUE) were inferred from " 13 C values. Over the last 60 years, we found significant increase of c i and reduction of 13 C (by ca. 1%), resulting from increasing iWUE (by ca. 30%) and coinciding with rising c a (80 ppm) at four of five sites. Trees at the fifth site exhibited the strongest increase of c i but the lowest increase of iWUE of all sites. Direct CO 2 e ects on stomatal conductance were explored using " 18 O as a proxy. We tested the climate signal recorded in the " 13 C series after removing the low-frequency trends due to physiological responses to increasing c a . We found strong relationships between " 13 C and summer temperature at all species and sites, but higher sensitivity of F. cupressoides to record summer temperature variations representative of a large area south of 35°S.

Résumé

Nous utilisons des chronologies de largeurs de cernes (TRW), de " 18 O et de " 13 C pour étudier la croissance des arbres et la réponse physiologique de cinq forêts de N. pumilio and F. cupressoides dans le nord de la Patagonie à l'augmentation des concentrations en CO 2 atmosphérique (c a ) et au changement climatique pendant les 60 dernières années. L'augmentation de la surface basale (BAI) a été calculée en utilisant les séries de TRW et la discrimination du 13 C ( 13 C), la concentration en CO 2 à l'intérieur de la feuille (c i ) et l'e cacité d'utilisation en eau intrinsèque (iWUE) ont été déduites des valeurs de " 13 C. Au cours des derniers 60 ans, nous avons observé dans quatre des cinq sites, une augmentation significative de c i , une réduction de 13 C (de l'ordre de 1%), ce qui a provoqué une augmentation de iWUE (de l'ordre de 30%) en parallèle de l'augmentation de c a (de l'ordre de 80 ppm). Les arbres dans le 5 e site montrent une augmentation beaucoup plus importante de c i mais la plus faible augmentation de iWUE de tous les sites. Les e ets directs du CO 2 sur la conductance stomatique ont été explorés à l'aide du " 18 O comme traceur. Nous avons testé le signal climatique enregistré dans le " 13 C après avoir soustrait la tendance de basse-fréquence due aux réponses physiologiques des arbres à l'augmentation de c a . Nous avons trouvé de fortes relations entre le " 13 C et les températures d'été quelle que soit l'espèce ou le site considéré, mais une plus grande sensibilité du F. cupressoides pour enregistrer les variations de température d'été représentatives de la région au sud de 35°S.

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Introduction

The global concentration of atmospheric CO 2 has increased from approximately 277 parts per million (ppm) in 1750 [Joos andSpahni 2008, Ciais et al. 2013] to almost 400 ppm in 2014 (Scripps CO 2 Program; http://scrippsco2.ucsd.edu). The increase has primarily been caused by the release of carbon to the atmosphere from fossil fuel burning and other land-use change activities [START_REF] Ciais | Carbon and Other Biogeochemical Cycles. Climate Change 2013: The Physical Science Basis[END_REF]]. Rising atmospheric CO 2 concentrations (c a ) should enhance tree growth by improving the photosynthetic activity (fertilization e ect) or by increasing intrinsic water-use e ciency (iWUE) [Ainsworth and Rogers 2007, Keenan et al. 2013, Camarero et al. 2015], the ratio between photosynthetic assimilation rates (A) and stomatal conductance (g). The iWUE is a key measure of potential water costs to maintain a given rate of carbon assimilation per unit leaf area [START_REF] Frank | Water-use e ciency and transpiration across European forests during the Anthropocene[END_REF]. The synergistic e ects of elevated c a and temperature are expected to further stimulate forest productivity in temperature-limited systems [START_REF] Huang | Response of Forest Trees to Increased Atmospheric CO2[END_REF][START_REF] Bonan | Forests and climate change: forcings, feedbacks, and the climate benefits of forests[END_REF][START_REF] Salzer | Recent unprecedented tree-ring growth in bristlecone pine at the highest elevations and possible causes[END_REF]]. However, warmer and drier climatic conditions can modulate the responses of A and g to elevated CO 2 and lead to growth decline in water-limited systems [START_REF] Silva | Recent widespread tree growth decline despite increasing atmospheric CO2[END_REF][START_REF] Allen | A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests[END_REF][START_REF] Linares | Growth patterns and sensitivity to climate predict silver fir decline in the Spanish Pyrenees[END_REF]2012[START_REF] Silva | Probing for the influence of atmospheric CO2 and climate change on forest ecosystems across biomes[END_REF][START_REF] Lévesque | Increased wateruse e ciency does not lead to enhanced tree growth under xeric and mesic conditions[END_REF].

The combined analyses of tree growth and stable isotopes in tree rings allow investigating trends in growth and physiological responses in order to disentangle warming-induced e ects from CO 2 -related e ects on growth, and to detect any shift in plant-atmosphere interactions caused by changes in climate [START_REF] Saurer | Stable carbon isotopes in tree rings of beech: Climatic versus site-related influences[END_REF][START_REF] Ferrio | Carbon and oxygen isotope ratios in wood constituents of Pinus halepensis as indicators of precipitation, temperature and vapour pressure deficit[END_REF][START_REF] Nock | Long-term increases in intrinsic water-use e ciency do not lead to increased stem growth in a tropical monsoon forest in western Thailand[END_REF][START_REF] Gómez-Guerrero | Growth decline and divergent tree ring isotopic composition ("13C and "18O) contradict predictions of CO2 stimulation in high altitudinal forests[END_REF]. Changes in WUE are regulated at the leaf level by variations in g [START_REF] Farquhar | Discrimination and the Intercellular[END_REF]. Since g might be a ected di erently by changes in CO 2 and climate, the impacts of recent changes in forest-atmosphere interactions could be assessed by the investigation of synchronous shifts in WUE and tree growth [START_REF] Silva | Probing for the influence of atmospheric CO2 and climate change on forest ecosystems across biomes[END_REF]. However, while " 13 C ratio is directly related to A, water balance and WUE, the nature of such change (due to changes in water availability, in the e ciency of A or in g cannot be determined using this parameter alone [START_REF] Scheidegger | Linking stable oxygen and carbon isotopes with stomatal conductance and photosynthetic capacity: a conceptual model[END_REF][START_REF] Linares | From pattern to process: Linking intrinsic water-use e ciency to drought-induced forest decline[END_REF][START_REF] Battipaglia | Elevated CO2 increases tree-level intrinsic water use e ciency: Insights from carbon and oxygen isotope analyses in tree rings across three forest FACE sites[END_REF]].

The " 18 O is mainly influenced by the isotopic signature of the source water derived from the precipitation, modulated by the evaporative enrichment in the ground, evaporative/di usion processes in the leaf and biochemical fractionation (e.g. McCarroll and Loader [2004], Gessler et al. [2013]). The degree of leaf water enrichment depends on the ratio of the vapor pressures in the atmosphere and in the intercellular spaces (c a /c i ratio; [START_REF] Dongmann | On the enrichment of H2 18O in the leaves of transpiring plants[END_REF]), whereby reduced relative humidity (RH) causes an increase of " 18 O in the leaf water, and then in the cellulose " 18 O (e.g., DeNiro and [START_REF] Deniro | Relationship Between the Oxygen Isotope Ratios of Terrestrial[END_REF], [START_REF] Farquhar | Interpretation of oxygen isotope composition of leaf material[END_REF]). " 18 O is independent of variations in A, therefore the information on evaporative enrichment recorded in " 18 O is useful for separating the e ects of A and g on iWUE [Scheidegger et al. 2000, Roden and[START_REF] Roden | Is the dual-isotope conceptual model fully operational?[END_REF]. Thus, the coupled analyses of " 13 C and oxygen " 18 O can be used to disentangle the di erent types of leaf gas-exchange with environmental pressure [Silva andMadhur 2013, Gómez-Guerrero et al. 2013].

In this study, we use a dual isotope approach coupled with dendrochronological measurements of N. pumilio and F. cupressoides in northern Patagonia over a 60 years period from 1952 to 2011 to: 1) determine spatio-temporal variations in tree ring width, " 13 C and " 18 O, 2) examine whether CO 2 stimulation or warming-induced stress has been predominant in these forests and, 3) investigate what climatic information can be derived from the carbon isotope ratios. These are preliminary results that still need more improvement.

Materials and methods

Study sites, sampling trees and isotopic analyses are detailed in the previous Chapter 8. The carbon isotopic compositions of cellulose were measured at the five sites of N. pumilio and F. cupressoides.

Basal area increment (BAI) calculation

Due to age-related e ects in tree growth, tree-ring widths decline with age and thus any declining growth may be di cult to detect from changes in tree-ring width alone [START_REF] Silva | Recent widespread tree growth decline despite increasing atmospheric CO2[END_REF]. The conversion of tree-ring width into basal area increment (BAI) may overcome this problem, as unlike width, age-related trends in unstandardized BAI are generally positive. Here, the BAI was calculated for each year of each mean tree-ring width chronology as:

BAI t = fi (R 2 t -R 2 t≠1
) where R is the mean tree radius (cm) and t is the year of tree-ring formation [START_REF] Silva | Recent widespread tree growth decline despite increasing atmospheric CO2[END_REF].

We used linear regression analysis (Mann-Kendall Test of the R platform) to assess temporal (since 1952) trends in BAI, using the corresponding regression slopes as the rates of change through time.

Isotope discrimination ( 13 C), intercellular CO 2 concentrations (c i ) and intrinsic water-use e ciency (iWUE) calculation

To evaluate physiological changes over the 1952-2011 period, di erent parameters were calculated using the measured isotope ratios of the cellulose of the tree-rings (" 13 C raw ). The isotope discrimination resulting from the preferential use of 12 C over 13 C during photosynthesis is defined as [START_REF] Farquhar | Discrimination and the Intercellular[END_REF]]:

Eq. ( 1): = (" 13 C atm -" 13 C raw ) / ( 1+ " 13 C atm / 1000 )

where " 13 C atm is the isotopic value of atmospheric CO 2 and " 13 C raw the measured isotope ratios of the cellulose of the tree-rings.

For C3 plants, [START_REF] Farquhar | Discrimination and the Intercellular[END_REF] described the dependence of 13 C on plant physiological properties by linearly relating the ratio of intercellular (c i ) to ambient (c a ) CO 2 concentrations through:

Eq. ( 2)

: 13 C ¥ " 13 C atm -" 13 C raw = a + (b -a) ci ca ¥ ci ca
where a is the fractionation associated with the di usion of CO 2 through the stomata (4.4% ) and b, the fractionation resulting from the enzymatic carbon fixation (27% ).

Records of " 13 C atm obtained from Antarctic ice core data and directs measurements, summarized by McCarroll and Loader [2004] for the period 1952-1999 and from the South Pole station retrieved in the Scripps CO 2 website (http://scrippsco2.ucsd.edu/) for the period 2000-2011, were used to calculate 13 C using Eq. ( 2).

The intrinsic water-use e ciency (iWUE), which corresponds to the ratio of net photosynthesis A to conductance for water vapor g is defined as [START_REF] Ehleringer | Carbon and water relations in desert plants: An isotopic perspective[END_REF]]:

Eq. ( 3)

: iWUE = A g = (c a -c i ) 1 1.6 ¥ c a -c i
In order to determine if tree-ring growth changes are related to changes in iWUE, we used correlation functions to identify significant relationships between BAI and iWUE.

Using the same approach applied to tree ring growth, we used Mann-Kendall trend test to assess cumulative changes in 13 C, c i and iWUE over the past six decades (since 1952). The corresponding regression slopes were used as rates of change through time.

Correction of non-climatic trends

The " 13 C raw chronologies commonly show a decreasing trend due to the rise of 13 C-depleted atmospheric CO 2 induced by fossil fuel burning and deforestation since industrialization (the Suess e ect). Therefore, the series must be corrected for this anthropogenic influence by removing the preindustrial isotope signature (e.g., -6.4% , AD 1850; [START_REF] Saurer | Stable carbon isotopes in tree rings of beech: Climatic versus site-related influences[END_REF]) to the actual atmospheric " 13 C raw (for each year) [McCarroll and Loader 2004].

Eq. ( 4): " 13 C cor = " 13 C raw -" 13 C atm

For this aim, we used the " 13 C atm data tabulated by McCarroll and Loader [2004] and retrieved from the Scripps CO 2 project described above. Trends in " 13 C cor were evaluated using the Mann-Kendall trend test.

Common variability recorded in the isotopic chronologies

For assessing the common information recorded in all isotopic chronologies, we computed intercorrelations among sites and species, and extracted the first empirical orthogonal function (EOF1) from the five site-" 13 C cor chronologies.

Analyses of " 13 C relationships with local climate

In order to determine any relationship with climate parameters, we computed simple and partial correlation functions [Meko et al. 2011] of " 13 C cor with temperature and precipitation, while controlling for temperature. We used monthly records of temperature and precipitation from Bariloche station (nearest meteorological station at 41°12'S -71°12'W, 840 m asl; Servicio Meteorológico Nacional). Calculations were conducted over the whole current growing season from September to May. The significance of each correlation was evaluated using bootstrapping with 1000 Monte Carlo simulations [Ebisuzaki 1997].

Results

Tree growth and BAI

The F. cupressoides mean radial growth series have no specific trend over time (Fig. 9.1), while the BAI shows a long period of increase at the two sites.

At the humid (NUB) and xeric (CHA) sites of the gradient, N. pumilio mean radial growth increases and then decreases since ca. 1950, while at the mesic site (ALM), the mean radial growth is quite stable over time. The BAI increases at all sites and species. However since the early 1990s, the BAI exhibits a slight decrease at the xeric site (CHA), indicating that the growth decline observed at this site cannot be attributed to aging. Over the whole 1952-2011 period, the BAI shows a significant and positive trend at F. cupressoides and the N. pumilio mesic (ALM) sites (from 0.03 cm 2 per year to 0.09 cm 2 per year, p < 0.05; Tab. 9.1), while trends are not significant at the humid (NUB) and xeric (CHA) sites of N. pumilio. Since the early 1990s, the BAI starts to decrease at the xeric site (CHA) and to stagnate at the humid site (NUB) when the atmospheric CO 2 concentrations reach ca. 350 ppm (Fig. 9.2). The two F. cupressoides " 13 C raw chronologies decrease of around 1.05% in 60 years (0.9% at GF and 1.2% at RA, p < 0.001; Tab. 9.1).

The two isotopic discriminations 13 C (¥ c i /c a ) show similar negative trend over the 1952-2011 period that are only significant at GF (-0.01% per year, p = 0.0001; Tab. 9.1). The intercellular c i and iWUE exhibit similar positive slopes over the whole period (from 0.48 to 0.57 ppm and from 0.47 to 0.52 umol.mol ≠1 per year, p < 0.001, resp.).

The N. pumilio " 13 C raw chronologies decrease of around 0.72% in 60 years at the humid (NUB) and xeric (CHA) sites (-0.66% at NUB and -0.78% at CHA, p < 0.001; Tab. 9.1), while the decrease was much stronger at the mesic (ALM) site (-2.4% in 60 years, p = 3.9.10 ≠9 ; Fig. 9.3).

The 13 C of N. pumilio show negative and significant trends at NUB and CHA (ca. -0.01% per year, p = 0.001; Tab. 9.1), but a positive trend at ALM (0.01% per year, p = 0.037). The negative (positive) trend may reflect a less (more) than proportional decline in cellulose " 13 C in relation to atmospheric values. The intercellular c i and the iWUE exhibit similar positive slopes over the whole period (from 0.50 to 0.91 ppm per year, p < 0.001, and from 0.26 to 0.51 umol.mol ≠1 per year, p < 0.001, resp.). Generally, the F. cupressoides trees exhibit mean values of iWUE between ca. 95 and 130 cm 2 per umol.mol ≠1 , while the N. pumilio trees have lower values of iWUE between ca. 70-80 and 110-120 cm 2 per umol.mol ≠1 .

The BAI and the iWUE are positively correlated for F. cupressoides over the whole 1952-2011 period, while they are no significantly correlated for N. pumilio (Fig. 9.4).

Carbon and oxygen isotopic chronologies

Trends in " 13 C cor of F. cupressoides are positive but only significant at GF (0.011% per year, p = 0.0002; Fig. 9.3). Tree-ring " 13 C cor and " 18 O are not significantly correlated (r = 0.26, p > 0.05; Fig. 9.5). Trends in " 13 C cor of N. pumilio are positive at NUB and at CHA (ca. 0.01% per year, p < 0.005), but are negative at ALM (-0.01% per year, p = 0.033). Tree-ring " 13 C cor and " 18 O for this species are significantly and positively correlated (r = 0.59, p < 0.05; Fig. 9.5).

The F. cupressoides " 13 C cor chronologies are strongly inter-correlated (r = 0.55, p < 0.05; Tab. 9.2), while the N. pumilio are not significantly correlated (p > 0.05). Therefore, there is a high variability among sites of N. pumilio. However, the leading EOF of the five site-" 13 C cor is accounting for 52% of the total variance, thus half of the isotopic signal is common to all sites and species. Given that there is a high coherency between the two specific carbon isotopic chronologies (r = 0.67, p < 0.05), we used the two species " 13 C cor chronologies for the climate patterns analysis recorded therein. The two specific " 13 C cor chronologies show similar patterns related to climate (Fig. 9.6). They are significantly and positively related to monthly temperature from December to February (average DJF, r = 0.78, p < 0.01 for F. cupressoides and r = 0.69, p < 0.05 for N. pumilio). Partial correlations with monthly precipitation are not significant.

In order to assess the spatial representativeness of the temperature signal recorded in the " 13 C cor chronologies, we conducted a spatial correlation analysis between the specific F. cupressoides chronology and gridded temperature records from the CRUTS 3.

Discussion

In this section, we discuss the trends in intrinsic water-use e ciency (iWUE) and basal area increment (BAI) of F. cupressoides and N. pumilio, the physiological responses to environmental pressure and the influence of local climate on the carbon isotopic composition. These are preliminary interpretations that still deserve more reflexion.

Trends in iWUE and BAI

Over the 1952-2011 period, the iWUE has increased by ca. 30% at all sites, except at the mesic site (ALM) of N. pumilio, where it has only increased by ca. 16%. This increase should be caused by either greater photosynthetic rates (carbon uptake), by reductions in stomatal conductance and, consequently, lower transpiration [START_REF] Francey | An explanation of 13C/12C variations in tree rings[END_REF] or a combination of both. Over the same period, the atmospheric CO 2 concentration (c a ) has risen from ca. 312 to 392 ppm (McCarroll and Loader [2004] and Scripps CO 2 project), an increase of ca. 26% (80 ppm/312 ppm), which is similar to the increase of iWUE observed at most of the sites.

Absolute growth rate (measured as average BAI) of the F. cupressoides and N. pumilio forests have increased over the whole period except at the xeric site (CHA) of N. pumilio, which showed a slow decline from early 1990s onwards. The BAI series were positively and significantly related to iWUE only for F. cupressoides, which could reflect CO 2 stimulation.

Despite increasing iWUE, the BAI increase was not significant for most sites of N. pumilio, suggesting that any stimulation conferred by higher CO 2 concentrations might not have been enough to compensate for other stressors [START_REF] Peñuelas | Twentieth century changes of tree-ring "13C at the southern range-edge of Fagus sylvatica: Increasing water-use e ciency does not avoid the growth decline induced by warming at low altitudes[END_REF][START_REF] Silva | Past century changes in Araucaria angustifolia (Bertol.) Kuntze water use e ciency and growth in forest and grassland ecosystems of southern Brazil: Implications for forest expansion[END_REF]]. These changes in BAI should reflect changes in productivity and have also been observed across latitudinal [START_REF] Silva | Recent widespread tree growth decline despite increasing atmospheric CO2[END_REF][START_REF] Andreu-Hayles | Long tree-ring chronologies reveal 20th century increases in wateruse e ciency but no enhancement of tree growth at five Iberian pine forests[END_REF], altitudinal [START_REF] Peñuelas | Twentieth century changes of tree-ring "13C at the southern range-edge of Fagus sylvatica: Increasing water-use e ciency does not avoid the growth decline induced by warming at low altitudes[END_REF]

Isotopic composition and physiological responses

Independently of species and site, tree-ring " 13 C raw decreased over the past 60 years. This 13 C depletion occurred simultaneously with rising c a , which decrease the relative abundance of 13 CO 2 compared to 12 CO 2 in the atmosphere [McCarroll and Loader 2004]. This is consistent with " 13 C raw decrease reported previously in other forest ecosystems (e.g., [START_REF] Andreu-Hayles | Long tree-ring chronologies reveal 20th century increases in wateruse e ciency but no enhancement of tree growth at five Iberian pine forests[END_REF], [START_REF] Silva | Probing for the influence of atmospheric CO2 and climate change on forest ecosystems across biomes[END_REF], [START_REF] Gómez-Guerrero | Growth decline and divergent tree ring isotopic composition ("13C and "18O) contradict predictions of CO2 stimulation in high altitudinal forests[END_REF]).

However, reductions in " 13 C raw over the 60-year period (of ca. 1.05% at F. cupressoides sites and of ca. 0.72% at the humid NUB and xeric CHA sites of N. pumilio) were lower than those observed in the atmosphere for the same period (ca. 1.4% ), resulting in reduced source (atmosphere) to product (biomass) discrimination ( 13 C). Rising CO 2 levels generally reduced stomatal conductance g leading to higher iWUE [START_REF] Tricker | Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO2[END_REF]]. However, the partial closure of the stomata (lower g) is also expected under water stress as a strategy to reduce leaf transpiration and keep water inside the leaf. If so, the decrease in g should lead to greater evaporative enrichment in 18 O, while also decreasing 13 C [START_REF] Scheidegger | Linking stable oxygen and carbon isotopes with stomatal conductance and photosynthetic capacity: a conceptual model[END_REF]]. As a result, both " 13 C cor and " 18 O should increase. Here, we found a significant positive correlation between " 13 C cor and " 18 O only for N. pumilio, while no significant relationships were found with F. cupressoides. Therefore, these findings suggest that changes in iWUE of N. pumilio should be more likely driven by reductions in g rather than increases in the e ciency of CO 2 assimilation (A) [START_REF] Nock | Long-term increases in intrinsic water-use e ciency do not lead to increased stem growth in a tropical monsoon forest in western Thailand[END_REF]]. The lack of significant relationships between " 13 C cor and " 18 O of F. cupressoides might indicate that changes in g are not responsible for the increase of iWUE, rather our results suggest an increase of A.

One exception was the intermediate N. pumilio site (ALM), which exhibited a stronger decrease of " 13 C raw than the one of the atmosphere (of ca. 2.4% compared to ca. 1.4% of " 13 C atm ), and a concurrent increase of 13 C. At this site, although both tree ring " 13 C cor and " 18 O were significantly and positively correlated (r = 0.32, p < 0.05), their long-term trends were opposite. This result could suggest a very weak stomatal response to c a increase, leading to moderate iWUE increase at this site [START_REF] Saurer | Carbon isotope discrimination indicates improving water-use e ciency of trees in northern Eurasia over the last 100 years[END_REF]].

These physiological responses to environmental changes have some implications for climate reconstructions studies. Indeed, temperature appears to be the main climatic factor modulating " 13 C cor at all sites and species. High summer temperature significantly enhanced " 13 C cor , while precipitation had no significant influence. However, since fractionation of carbon by trees of the two species are dominated by di erent processes (stomatal conductance versus photosynthetic assimilation rate for N. pumilio and F. cupressoides, respectively), the primary climatic control on " 13 C cor should be di erent among the two species. Some previous studies have shown that in dry environments, stomatal conductance is likely to dominate and stable isotopes will correlate with air humidity and temperature, while in moist environments, where water stress is rare, rate of photosynthesis is likely to dominate, giving strong correlations with summer temperature [START_REF] Young | A 500-year record of summer near-ground solar radiation from tree-ring stable carbon isotopes[END_REF]]. Temperature, however, does not directly control the rate of photosynthesis, the dominant control being photon flux [START_REF] Mccarroll | Stable carbon isotope ratios of Pinus Sylvestris from northern Finland and the potential for extracting a climate signal from long Fennoscandian chronologies[END_REF], McCarroll and Loader 2004[START_REF] Young | A 500-year record of summer near-ground solar radiation from tree-ring stable carbon isotopes[END_REF]. Therefore, while N. pumilio " 13 C cor should be primary limited by temperature, F. cupressoides " 13 C cor should be primarily controlled by sunlight rather than summer temperature.

In order to test the hypothesis of an indirect e ect of summer temperature on stable isotopes of F. cupressoides through sunlight radiation, we computed Pearson's correlations between stable isotopes ratio of both species with averaged summer (December to February) photosynthetically active radiation (PAR or irradiance) and cloud cover (CDF) extracted from the NASA/GEWEX Surface Radiation Budget (SRG) Release 3.0 dataset over the period 1982-2007 (Fig. 9.8) (Fig. 9.8). The correlation analysis shows that PAR (and indirectly CDF) averaged over summer months are significantly correlated to " 13 C cor and " 18 O of F. cupressoides (r = 0.46 to 0.48, p < 0.05 with PAR and r = -0.44 to -0.50, p < 0.01 with CDF), while correlation coe cients are not significant for N. pumilio. Therefore, at least over the last three decades, sunlight has a stronger controlling e ect on stable isotopes of F. cupressoides in comparison with its influence on N. pumilio. However, over the same period 1982-2007, the relationships between summer temperature and " 13 C cor for both species are stronger than the one with PAR (or CDF) (r = 0.62 and 0.64, p < 0.05, for N. pumilio and F. cupressoides, respectively), therefore no conclusion can be drawn on the primary controlling e ect of sunlight rather than summer temperature on stable isotopes of F. cupressoides.

These findings may be taken with caution since further analysis is needed to better understand the responses of F. cupressoides and N. pumilio to environmental changes.

Performance of " 13 C as climate proxy

The strongest response of carbon isotope to climate was observed between " 13 C cor of F. cupressoides and summer temperature (r = 0.78, p < 0.05). The " 13 C cor for this species recorded a large and homogeneous pattern of summer temperature south of ca. 35°S (Fig. 9.7).

In this extended region, temperature is mainly influenced by variations in Southern Annular Mode (SAM; e.g., Garreaud et al. [2013]). When comparing the specific " 13 C cor chronology of F. cupressoides with the December-February SAM index from Marshall [2003], the correlation coe cient between both series was quite strong (r = 0.40, p < 0.05; 1957-2010 period), although weaker than the one reported with temperature (r = 0.78, p < 0.05). The SAM describes the north-south movement of the westerly wind belt that circles Antarctica and is defined as a complex index involving sea level pressure anomalies between mid and high latitudes. It is not only influencing variations in temperature but also in latitudinal distribution of rainfall from the subtropics to subantarctica latitudes [Gillett et al. 2006, Archer and Caldeira 2008, Garreaud et al. 2008]. Therefore, as precipitation amount has not a signifiant influence on " 13 C and " 18 O (Fig. 9.6) and temperature is mostly modulating both isotopes, the SAM-" 13 C cor is expected to be weaker than the one with temperature.

In chapter 8, " 18 O of N. pumilio was shown to be useful for reconstructing past December-May temperature in a large area south of 40°S. Here, our findings suggest that the " 13 C cor of the low-growing F. cupressoides is even more sensitive to past summer temperature than " 18 O of N. pumilio. Therefore it appears to be a much better climate tracer of past summer temperature than " 18 O. The linear regression model (over 1954-2010) of " 13 C cor on summer temperature accounts for 54% of the actual summer temperature variance (Fig. 9.7). This model is reliable and have strong predictive skills, thus our results encourage the development of multicentennial " 13 C chronology of F. cupressoides for long temperature reconstructions in the Southern Hemisphere.

Conclusion

Despite some leaf gas-exchange di erent strategies among species and sites, which need to be further investigated, our study suggests that the iWUE of N. pumilio and F. cupressoides forests in northern Patagonia has increased in the last six decades, mainly driven by changes in stomatal conductance and carbon uptake. Furthermore, despite di erent controlling factors of " 13 C and " 18 O, temperature variations are modulating both isotopes in similar manner (e.g., high temperature in summer months resulting in increased isotope values). The " 13 C cor chronology of F. cupressoides is comparatively the most interesting climate proxy for reconstructing past summer temperature representative of a large area. Further work is needed to better understand the physiological response of these two species to rising atmospheric CO 2 concentrations.

The aim of this thesis was to assess and determine how two native species (Fitzroya cupressoides and Nothofagus pumilio) growing in northern Patagonia respond to local and large-scale climate forcings using di erent tree-ring parameters (width, " 18 O and " 13 C) in order to improve the understanding of the tree rings as climate proxies and to assess their potential for paleoclimate reconstructions.

Main findings

Respectively, two and five sites of F. cupressoides and N. pumilio were sampled along the west-to-east precipitation gradient in northern Patagonia (41°S) in the 2013 austral summer. Large-scale climate forcings a ecting the southern South America climate (i.e., Southern Annular Mode and El Niño -Southern Oscillation) were shown to influence long-term variations of tree-ring growth of both species, through their e ects on local temperature and precipitation (Chapter 5). However, the climate-growth relationships of both species were not stable through time. While F. cupressoides growth showed a significant low-frequency decoupling with summer (February) temperature from the late 1970s onwards, coincident with the wellknown step increase of temperature in 1976, time-dependent responses of N. pumilio growth to local climate were detected along the west-to-east precipitation gradient.

Further analyses on interannual N. pumilio growth fluctuations combining correlation functions and a forward process-based model (Vaganov-Shashkin-Lite;Tolwinski-Ward et al. [2011]) have allowed detecting substantial changes in the sensitivity of N. pumilio growth to local climate through time, coincident with shifts in climate regimes (Chapter 6).

The decoupling (or time-dependence) observed in the tree-ring growth of these two species with climate variables constitutes an issue for tree-ring studies, since reconstructing past climate variations using these tree-ring width chronologies can result in under-or over-estimating the climate reconstruction outside the modern calibration period.

The oxygen and carbon isotopic compositions of cellulose of both species were then measured over the last 60 years in order to assess whether tree-ring " 18 O and " 13 C chronologies can be used instead and are means for overcoming the lack of stationarity of the tree-ring width chronologies.

While the climate signal recorded in " 18 O of F. cupressoides was quite poor, the " 18 O of N. pumilio was strongly related to December-May temperature variations (Chapter 8). " 18 O -based reconstructions using this species would be representative of temperature variations over a large area south of 38°S, in the area of influence of SAM.

While the physiological responses of both species to increasing CO 2 concentrations (c a ) and climate change are not yet well understood (further analyses are necessary), common patterns of " 13 C variability among species were observed. Notably the intrinsic water-use e ciency has increased at all sites over the last 60 years (Chapter 9). The " 13 C of both species was strongly related to summer temperature. Because it is a very long lived species, F. cupressoides is more interesting than N. pumilio for long-term summer temperature reconstructions using " 13 C.

As with " 18 O of N. pumilio, the " 13 C contained a regional temperature signal representative of a large area south of 35°S, under the influence of SAM.

Based on these findings, important conclusions can be drawn which have significant implications for future works. First, the strongest climatic signal recorded in the di erent tree-ring parameters of both species is related to temperature, therefore the best climate target for paleoclimate reconstructions should be linked to temperature. Second, the lack of stationarity observed in the tree-ring width chronologies of the two species could be overcome to built robust reconstructions by using stable isotopic chronologies, therefore the best climate proxies should be either tree-ring " 18 O or " 13 C. Finally, a more complete understanding of the physiological responses of both species to increasing c a and climate change should help to better define the proxy-target relation.

Further research

This thesis has demonstrated that the responses of F. cupressoides and N. pumilio growth to climate variability can be complex and not straightforward, which prevents using their tree-ring width chronologies for building climate reconstruction on a strong foundation. The apparent non-linear growth response of both species described in Chapters 5 and 6 has important implications for our expectations on how austral trees might respond to continued climate change. Research on this issue is needed to identify the physiological causes and whether it is a specific response restricted to the area of study or is widespread at other latitudes.

The main focus of this thesis was the evaluation of the climate factors controlling the di erent tree-ring parameters at interannual to decadal scales, whose temporal variations could be reconstructed. The tree-ring " 18 O and " 13 C were shown to be the most promising proxies of past temperature variations over a large area under the influence of SAM (Chapters 8 and 9). However, little analysis was conducted to disentangle the influence of non-climatic factors, such as CO 2 and nitrogen fertilization and snow cover changes on tree-ring growth and stable isotopic compositions of cellulose. The primary analysis of " 13 C data presented in Chapter 9 suggests that trees should maintain a dynamic leaf gas-exchange strategy to increasing c a but further analysis is needed to strongly evaluate the physiological responses of the trees.

Performing more simulations with the full process-based Vaganov-Shashkin (VS) model [Vaganov et al. 2006] or the newly VS-oscilloscope model [START_REF] Shishov | VS-oscilloscope: a new tool to parameterize tree radial growth based on climate conditions[END_REF], which simulate daily cambial dynamics and xylem growth, or using more complex process-based biochemical models, such as TREERING2000 [Fritts et al. 2000] or MAIDEN [Misson 2004], which are able to link photosynthesis and carbohydrate allocation to stem growth, would help deciphering the processes responsible for non-stationary growth. The isotopic composition of cellulose could also be simulated for further understanding of its controlling factors using a new version of the MAIDEN model (MAIDENiso) [START_REF] Danis | MAIDENiso: a multiproxy biophysical model of tree-ring width and oxygen and carbon isotopes[END_REF]], which estimates " 18 O and " 13 C associated with the daily amount of carbon allocation to the stem, or using ORCHIDEEiso [START_REF] Krinner | A dynamic global vegetation model for studies of the coupled atmosphere-biosphere system[END_REF]], a component of the IPSL climate model [START_REF] Marti | The new IPSL climate system model: IPSL-CM4. Note du Pôle de Modélisation[END_REF], which represents water and isotopes transfers in the soil-vegetation-atmosphere continuum and estimates " 18 O and carbon isotope discrimination during leaf photosynthesis ( 13 C). However, some of these models are only designed for coniferous trees, which prevent their use for N. pumilio and all of them operate with daily data which preclude their use in poorly documented remote regions such as Patagonia.

Ongoing in-situ monitoring of radial growth, wood cell formation and environmental conditions will provide a more detailed representation of the complex biological and ecological processes operating in Patagonia. The processing of the data recently acquired with dendrometers installed with water content sensors and independent automatic weather stations during the field campaign in summer 2013 at di erent sites mentioned in this thesis (RA, ALM and DL; Australian Research Council project led by P. Baker), should allow determining what biotic and abiotic factors are driving variability in tree-ring formation [START_REF] Downes | Measuring and modelling stem growth and wood formation: An overview[END_REF], Drew and Downes 2009, Gutiérrez et al. 2011].

A better process-based understanding of the climatic controls on tree-ring " 18 O requires documenting the processes at play along the soil-plant-atmosphere continuum. This could be achieved by a continuous monitoring of precipitation, soil water from di erent depths, stem water, vapor and leaf water " 18 O. I am involved in the elaboration of a project on this topic which should take shape in the next months.

Finally, the thesis has shown the potential of tree-ring cellulose " 18 O and " 13 C of F. cupressoides and N. pumilio for reconstructing past summer temperature variations. Therefore, the production of long tree-ring " 18 O and " 13 C series would be very valuable to further understand the spatiotemporal temperature variations in southern South America over the last millennium and to estimate global anthropogenic climate changes besides natural climate variability in the Southern Hemisphere. This will be my main task for the next months...

Introduction

¸Currently, the Patagonian Andes are undergoing unprecedented environmental changes associated with global warming and a readjustment of the atmospheric circulation at the continental and hemispheric scale (AR5, IPCC, Chap. 10). ¸Several tree species (e.g., Fitzroya cupressoides, Araucaria araucana, Nothofagus pumilio) have been used to develop millennial temperature reconstructions from the Patagonian Andes across a range of elevations and latitudes [START_REF] Villalba | Long-term multi-proxy climate reconstructions and dynamics in South America (LOTRED-SA): State of the art and perspectives[END_REF]. ¸Some high-elevation chronologies show a loss of climate sensitivity in recent decades.

We aim at documenting this "divergence phenomenon" by analyzing the functioning of trees (via tree-ring width and isotopic composition).

¸Low inter-tree variability (Fitzroya: EPS* > 0.85 with 6 individus)

4. Cellulose d 18 O: preliminary results

CONCLUSIONS

¸The two tree species responses to climate change are different. ¸Fitzroya cupressoides productivity is increasing with climate change.

¸Nothofagus pumilio growth may be controlled by water availability. ¸δ18 O and δ 13 C analyses of cellulose are currently in progress. They will help understanding if and how tree functioning is affected by climate change (e.g. WUE) and, therefore, assessing the potential of tree-ring widths for past climate reconstructions.

The objective of the study is to quantify regional vegetation response to climate change by combining conventional dendroclimatic and isotopic approaches. As the growth of several species (e.g., Fitzroya cupressoides, Araucaria araucana, Nothofagus pumilio) is largely controlled by temperature, tree-ring width series were used to develop millennial temperature reconstructions across a range of elevations and latitudes in the Patagonian Andes (e.g. [START_REF] Villalba | Long-term multi-proxy climate reconstructions and dynamics in South America (LOTRED-SA): State of the art and perspectives[END_REF]. However, the growth-temperature relationship was shown to be unstable at some patagonian sites over the recent years (e.g. Aravena et al., 2002;Daniels & Veblen, 2004) Lavergne et al., 2014, in prep. Over the last decades, the environment has been warmer and drier.

Increasing

Results (I). Tree-ring width

Changes are more pronounced in spring and summer.

Introduction

Unprecedented changes in the Southern Annular Mode (SAM) have impacted summer climate regimes [1] and forest growth in the extratropical Southern Hemisphere [2]. The current anomalous positive phase of SAM has weakened the westerly flow and promoted an overall poleward expansion of the dry subtropical belts. Thus the ongoing SAM phase might have negative e ect in water availability [3], in particular across the mid-latitudes of southern South America (Fig. 1) and might a ect water-limited patagonian tree-growth.

Here, we examine inter-annual variations in seasonal climate limitation of tree-ring growth in upper treeline forests of southern beech Nothofagus pumilio) along a steep precipitation gradient in northwestern Patagonia.

The species occurs in a wide diversity of environments with contrasting regional climates, thus, we would expect changes in tree growth and forest dynamics along the gradient.

Study sites

Materials & methods

Four upper treeline N. pumilio forests sampled along the precipitation gradient (41S)

• Simple linear correlation analysis with local climate (Bariloche) and satellite-observed surface soil moisture [START_REF] Liu | Trend-preserving blending of passive and active microwave soil moisture retrievals[END_REF] to quantify long-term associations between climate and tree growth along the gradient • Vaganov-Shashkin-lite model of tree-ring growth [START_REF] Tolwinski-Ward | An e cient forward model of the climate controls on interannual variation in tree-ring width[END_REF] to capture potential non-linear growth responses to temporal changes in climate limitation (calibrated on 1931-1969 interval, verified over 1970-2008)

Main result

Gradual changes in climate limiting factors along the steep precipitation gradient of northwestern Patagonia since the mid-1970.

Tree-growth and climate

• Tree growth stimulated :

-by warm and dry springs at mesic treelines -by cool and wet early-summer at xeric treelines • Gradual increase of summer soil moisture control on tree-growth along the gradient. alienor.lavergne@lsce.ipsl.fr Correl. coe : left, 1931Correl. coe : left, -1969Correl. coe : left, and right, 1970Correl. coe : left, -2008Correl. coe : left, , [ ] 1931Correl. coe : left, -1976 and ( ) and ( )

VS-lite model

Introduction

Alors que les forçages climatiques sont assez bien compris dans l'Hémisphère Nord (HN), l'Hémisphère Sud (HS) est peu documenté et sous-représenté dans les reconstitutions climatiques globales [1]. Pourtant les deux hémisphères interagissent fortement par le biais de téléconnections atmosphériques [2]. Donc, si l'on ne connait pas bien les mécanismes de variabilité climatique dans l'HS, on ne peut comprendre le système climatique global et prédire les changements climatiques futurs. First of all, I would like to thank my thesis supervisor, Valérie Daux, without whom all this experience would not have been possible. During my master 2, she has given me the taste of dendroisotopy and, from the beginning, she has been very enthusiastic to build a thesis topic in collaboration with the laboratory IANIGLA of Mendoza (Argentina). During these three years, she has always been an unfailing support, supervising my work and improving its quality with advices and suggestions. We shared many great moments together, either in the field during the sampling campaign in Bariloche (Argentina) in March 2013, or in March 2015 in Nepal during the Asiandendro conference and the conference tour organized afterwards, or even during our long working hours together, analyzing and interpreting my results.

I am also very grateful to Ricardo Villalba, my co-supervisor of thesis, which has opened me the doors of its laboratory IANIGLA in 2012 to do my master 2 internship with Ana Marina Srur.

He has always been very motivated and enthusiastic, whatever my projects were to complete my thesis. The hours spent in the field at El Chaltén in January 2013 were unforgettable. A special memory for the day we have been really scared while crossing a mountain river near the Laguna Toro to have access to a spectacular panoramic view of the Southern Patagonian Icefield at Paso del Viento. As its name suggests (Path of wind), we have been able to feel the strength of the southern Westerlies winds coming from the Pacific and to contemplate an amazing landscape of ice.

I would not have been able to have so much data without the valuable help for sampling trees of the IANIGLA team in March 2013. The Argentinian philosophy is everyone helps the project of everyone, therefore in less than two weeks, all together we have managed to achieve a collection of more than 500 samples! I really want to thank Ana Marina Srur, Mariano Amoroso, Eugenia Marcotti, Vero Chillo, Pierre Pitte, Emilio Bianchi, Matías Fernando, María Laura Suarez, Irene Adriana Garibotti, Ricardo Villalba and other Australian scientists for their help in sampling, but also Mae Corvalán, Claudia Guerrido, Clara Inés Pissolito and Ever Tallei with whom I shared great moments in the Southern Patagonia field campaign in January 2013. I really also want to thank Vale Aschero, Martín Hadad, Mariano Morales, Lidio López, Misael Palacios, Julieta Arco and Ignacio Alberto Mundo for their help during the development of the tree-ring width chronologies at IANIGLA. Thanks also to Lucas Bianchi, Milagros Rodríguez Catón and Marcos Radins for the development of previous versions of these chronologies, and to Fleur Couvreux for helping me with the meteorological data.

After my first year of thesis in Mendoza, I had the chance to be very well welcomed at LSCE, particularly by Monique Pierre and Michel Stievenard who helped me a lot for selecting my tree samples, extracting cellulose from tree rings and analyzing them with the mass spectrometer. Despite several months of problems with the mass spectrometer, they have always been persistent in their job and, at the end of my thesis, have considerably helped me finalize the measurements of the carbon isotopic composition of the cellulose. We have been able to let of stream and run together on Friday noon in the forest of Saint Aubin! Special thanks to Ana Marina Srur and Mariano Amoroso who taught me the basics of dendrochronology and trees sampling at El Bolsón (Argentina) for my master

Science popularization

  [2012]. (b) Annual mean SAM reconstruction based on proxy records that encompass the full mid-latitude to polar domain across the Drake Passage sector. FromAbram et al. [2014] . . . . . . . . . . . . . . . 1.2 Annual mean precipitation and surface air temperature regressed upon Mulivariate ENSO (El Niño-Southern Oscillation) Index (MEI; left column), Pacific Decadal Oscillation (PDO) index (center column) and Southern Annular Mode (SAM) index (right column). Precipitation and surface air temperature from University of Delaware gridded dataset. From Garreaud et al. [2008] . . . 1.3 El Niño and La Niña mechanisms. Global surface temperatures relative to historical mean in 1951-1980. The El Niño index is based on sea surface temperature in the El Niño3.4 area (5°N -5°S, 120°-170°W) in the eastern tropical Pacific for the 1951-1980 period. Green triangles mark times of volcanic eruptions that produced an extensive stratospheric aerosol layer, leading to a decrease of global mean temperature. From He ernan [2014] . . . . . . . . . . . . . . . . . . . . . 1.4 The PDO based on an EOF analysis of SST anomalies with the global mean removed from 1900 to May 2013 in the 20°N-70°N and 110°E-100°W region of the North Pacific, which explains 25% of the variance. The principal component time series, given below in normalized units, is regressed on global SSTs to give the map above. The black curve is a 61 months running average. From Trenberth and Fasullo [2013] . . . . . . . . . . . . . . . . . . . . . . . . . . . .

  ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Overview of the di erent processes influencing the carbon (a) and oxygen (b) isotope signature, from primary sources to tree-ring cellulose. From Gessler et al. [2014] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Main controls on the fractionation of oxygen isotopes in cellulose and the influencing environmental factors. From McCarroll and Loader [2004] . . . . . . . 2.7 Main controls on the fractionation of carbon isotopes in cellulose and the influencing environmental factors. From McCarroll and Loader [2004] . . . . . . . 3.1 (a) Study area along the west-to-east climate and productivity gradient in northern Patagonia. Gross Primary Production (GPP) is based on the MOD17 product [Zhao et al. 2005]. (b) Transect across the southern Andes at approximately 41°S, showing changes in mean January and July temperatures, in seasonal (summer and winter) precipitation distribution, and in total annual precipitation associated with topography and distance from the Pacific Ocean. From Villalba et al. [2003] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 (a) Long-term (1931-2009) mean monthly precipitation (grey bars) and mean temperature (black line) at Bariloche meteorological station (41°12'S -71°12'W, 840 m asl; Servicio Meteorológico Nacional). Error bars are also shown. (b) Annual cycle of photosynthetically active radiation (PAR in W/m2; black line) and cloud fraction (grey line) from the NASA/GEWEX Surface Radiation Budget gridded dataset over the 1984-2007 period. . . . . . . . . . . . . . . . . . . . . . 3.3 Temporal variations of local and large-scale climate forcing showing changes in mean values. From top to bottom: the standardized precipitation index (SPI)

  Fitzroya cupressoides (blue) and Nothofagus pumilio (green) in the study area. . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Samples collection in the field. Location of the tree-ring sites of F. cupressoides (blue) and N. pumilio (black) along the west-to-east precipitation gradient. . . 4.2 Schematic illustration of crossdating. From Schweingruber [1988] . . . . . . . . 4.3 Tree-ring width (TRW) chronologies (mm) from F. cupressoides and N. pumilio. Grey lines indicate the single TRW series. The sample replication is shown in the lowest panel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Tree-ring standardized chronologies with NEGEX and RCS fitting methods. . . 4.5 Rbar of TRW series and Expressed Population Signal (EPS) of the NEGEX chronologies for each site and species. . . . . . . . . . . . . . . . . . . . . . . . 4.6 Sample preparation, cellulose extraction and isotopic composition measurement at LSCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . http://chemistry.tutorvista.com/analytical-chemistry/biomolecules.html . . . .

Figure 1 . 1 :

 11 Figure 1.1: (a) Positive phase of SAM characterized by an increase in atmospheric pressure over the mid-latitudes (pink) and a decline in pressure at high latitudes (blue). This pressure difference leads to a strengthening of the circumpolar westerly winds (white arrow) and a southward movement of the Southern Hemisphere storm track. From Jones [2012]. (b) Annual mean SAM reconstruction based on proxy records that encompass the full mid-latitude to polar domain across the Drake Passage sector. From Abram et al. [2014]

Figure 1 . 2 :

 12 Figure 1.2: Annual mean precipitation and surface air temperature regressed upon Mulivariate ENSO (El Niño-Southern Oscillation) Index (MEI; left column), Pacific Decadal Oscillation (PDO) index (center column) and Southern Annular Mode (SAM) index (right column). Precipitation and surface air temperature from University of Delaware gridded dataset.FromGarreaud et al. [2008]

  4 index measuring the SST anomaly in the central and eastern Pacific to monitor the oceanic component of ENSO (Fig. 1.3, or the Multivariate ENSO Index (MEI) defined as the first seasonally varying principal component of six atmosphere-ocean variable fields in the tropical Pacific basin).

Figure 1 . 3 :

 13 Figure 1.3: El Niño and La Niña mechanisms. Global surface temperatures relative to historical mean in 1951-1980. The El Niño index is based on sea surface temperature in the El Niño3.4 area (5°N -5°S, 120°-170°W) in the eastern tropical Pacific for the 1951-1980 period. Green triangles mark times of volcanic eruptions that produced an extensive stratospheric aerosol layer, leading to a decrease of global mean temperature. From He ernan [2014]

Figure 1 . 4 :

 14 Figure 1.4: The PDO based on an EOF analysis of SST anomalies with the global mean removed from 1900 to May 2013 in the 20°N-70°N and 110°E-100°W region of the North Pacific, which explains 25% of the variance. The principal component time series, given below in normalized units, is regressed on global SSTs to give the map above. The black curve is a 61 months running average. From Trenberth and Fasullo [2013]

Figure 1 . 5 :

 15 Figure 1.5: Tree-ring chronologies along the Andes Cordillera in South America. Major tree taxa usedfor developing the chronologies and their geographical distributions. Geographical domain of the dominant tropical and extratropical forcings of regional climate variability across the Andes, as represented by ENSO and SAM. Modified fromVillalba [2007]

Figure 2 . 1 :

 21 Figure 2.1: Biogeophysical processes of terrestrial ecosystems. From [Bonan 2008]

Figure 2 . 2 :

 22 Figure 2.2: Sap ascent. From https://courses.candelalearning.com/

Figure 2 . 3 :

 23 Figure 2.3: Cross-section of a tree stem.

Figure 2 . 4 :

 24 Figure 2.4: Main fractionation steps and typical values of " 18 O in a temperate climate. " 18 O SMOW , standard mean " 18 O in ocean water; " 18 O V , " 18 O P , " 18 O LW , " 18 O in water vapor, precipitation (either rainfall or snow) and leaf water, respectively; VPD, vapor pressure deficit; gs, stomatal conductance.Redrawn from[START_REF] Ferrio | Carbon and oxygen isotope ratios in wood constituents of Pinus halepensis as indicators of precipitation, temperature and vapour pressure deficit[END_REF]. Original data from (IAEA/WMO 2001) and[START_REF] Saurer | Stable carbon isotopes in tree rings of beech: Climatic versus site-related influences[END_REF]

Figure 2 . 5 :

 25 Figure 2.5: Overview of the di erent processes influencing the carbon (a) and oxygen (b) isotope signature, from primary sources to tree-ring cellulose. From Gessler et al. [2014]

Figure 2 . 6 :

 26 Figure 2.6: Main controls on the fractionation of oxygen isotopes in cellulose and the influencing environmental factors.From McCarroll and Loader [2004]

Figure 2 . 7 :

 27 Figure 2.7: Main controls on the fractionation of carbon isotopes in cellulose and the influencing environmental factors. From McCarroll and Loader [2004]

Figure 3 . 1 :

 31 Figure 3.1: (a) Study area along the west-to-east climate and productivity gradient in northern Patagonia. Gross Primary Production (GPP) is based on the MOD17 product [Zhao et al. 2005]. (b) Transect across the southern Andes at approximately 41°S, showing changes in mean January and July temperatures, in seasonal (summer and winter) precipitation distribution, and in total annual precipitation associated with topography and distance from the Pacific Ocean. From Villalba et al. [2003]

Figure 3 . 2 :

 32 Figure 3.2: (a) Long-term (1931-2009) mean monthly precipitation (grey bars) and mean temperature (black line) at Bariloche meteorological station (41°12'S -71°12'W, 840 m asl; Servicio Meteorológico Nacional). Error bars are also shown. (b) Annual cycle of photosynthetically active radiation (PAR in W/m2; black line) and cloud fraction (grey line) from the NASA/GEWEX Surface Radiation Budget gridded dataset over the 1984-2007 period.

Figure 3 . 3 :

 33 Figure 3.3: Temporal variations of local and large-scale climate forcing showing changes in mean values. From top to bottom: the standardized precipitation index (SPI) and the temperature anomalies from meterological data relative to the historical mean (wrt 1961-1990), all averaged over 12 months, and the reconstructed Multivariate ENSO Index (MEI) and Southern Annular Mode (SAM) index developed by Li et al. [2013] and Villalba et al. [2012], respectively.

Figure 3 . 4 :

 34 Figure 3.4: Geographical distribution of Fitzroya cupressoides (blue) and Nothofagus pumilio (green) in the study area.

Figure 4 . 1 :

 41 Figure 4.1: Samples collection in the field. Location of the tree-ring sites of F. cupressoides (blue) and N. pumilio (black) along the west-to-east precipitation gradient.

Figure 4 . 2 :

 42 Figure 4.2: Schematic illustration of crossdating.From Schweingruber [1988] Quality-control and accuracy of TRW measurement dating was conducted with the visual and statistic co-dating computation program, COFECHA[Holmes 1983]. Trees growing in

Figure 4 . 3 :

 43 Figure 4.3: Tree-ring width (TRW) chronologies (mm) from F. cupressoides and N. pumilio. Grey lines indicate the single TRW series. The sample replication is shown in the lowest panel.

Figure 4 . 4 :

 44 Figure 4.4: Tree-ring standardized chronologies with NEGEX and RCS fitting methods.

Figure 4 . 5 :

 45 Figure 4.5: Rbar of TRW series and Expressed Population Signal (EPS) of the NEGEX chronologies for each site and species.

Figure 4 . 6 :

 46 Figure 4.6: Sample preparation, cellulose extraction and isotopic composition measurement at LSCE

Figure 4 . 7 :

 47 Figure 4.7: Full diagram of a mass spectrometer. From http://chemistry.tutorvista.com/analyticalchemistry/biomolecules.html

2

 2 Material and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5.2.1 Tree-ring data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5.2.2 Climate data and analyses of teleconnections with climate forcings . . . . . 52 5.2.3 Tree-ring growth analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 5.3.1 Variability and trends in climate records . . . . . . . . . . . . . . . . . . . . 54 5.3.2 Teleconnections of local climate variability to climate forcings . . . . . . . . 54 5.3.3 Overview on growth variations . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.3.4 Tree-growth related to local climate . . . . . . . . . . . . . . . . . . . . . . 58 5.3.5 Tree growth related to large-scale climate forcings . . . . . . . . . . . . . . 61 5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 5.4.1 Non-stationary atmospheric teleconnections . . . . . . . . . . . . . . . . . . 62 5.4.2 Temporal changes in growth patterns related to local-to-large-scale climate changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Figure 5 . 1 :

 51 Figure 5.1: Annual (April to March), summer, autumn, winter, and spring variations of: normalized temperature over 1901-2011 using REG and CRUTS 3.2 gridded data and, cumulative precipitation using Bariloche local station (1931-2009) and CRUTS 3.2 gridded data (1901-2011). A 10-yr spline is applied on each series to highlight the low frequency component.

Figure 5 . 2 :

 52 Figure 5.2: Correlation coe cients of monthly temperature and precipitation with monthly values of (a) SAM and (b) MEI (ENSO index) over the whole period 1957-2008 (top), and two subperiods of equal length 1957-1982 and 1983-2008. Black lines indicate precipitation and grey lines indicate temperature. Dotted horizontal lines indicate confidence intervals and statistical significance (p < 0.05).

Figure 5 . 3 :

 53 Figure 5.3: Tree-ring NEGEX and SPL10 chronologies at each site.

Figure 5 . 4 :

 54 Figure 5.4: Continuous Wavelet Transform (WT) for detection of spatial and temporal cycles in the F. cupressoides and N. pumilio NEGEX chronologies over the common period 1785-2011.Red tone indicates higher power spectrum, black thick contours denote 95% significance level using a red noise background, and the white line delimits the cone of influence where edge e ects may be important.

  Fig 5.6.a), they show a strong low-frequency decoupling with previous summer temperature since 1977 (r = -0.28, p > 0.05).Consistently, the XWT spectral analysis shows that tree growth has an intermittent physical association with temperature (Fig 5.6.b) at less than 8 years between 1901 and 2011 and that, there has been a general decrease in coherence in the ca. 8 year band since the 1980s.Running correlation plots(Fig 5.6.c) show significant negative correlation between the F. cupressoides chronologies and temperature over December-March of the previous growing

Figure 5 . 5 :

 55 Figure 5.5: Correlation between individual tree-ring width chronologies and: (a) monthly temperature and (b) precipitation from Bariloche station over 1933-2009, and (c) monthly temperature and (d) precipitation from CRUTS over 1903-2011. Correlations are computed for NEGEX and SPL10 chronologies. Correlations are given for a 18-month window from October of the previous year (*) to March of the following year (°) corresponding to the current growing season. The stippling points indicate months with significant correlations (p<0.05). Previous and current growing season (GS) are shown with a thick black line.

Figure 5 .

 5 Figure 5.6: (a) Visual comparison and (b) XWT analysis of the F. cupressoides NEGEX chronologies and previous December-March temperature from REG. The double asterisks denote significance at p < 0.01. Red tone indicates higher power spectrum, black thick contours denote 95% significance level using a red noise background, and the white line delimits the cone of influence. (c) Running Pearson correlation coe cients for successive 30-year windows of F. cupressoides NEGEX chronologies with temperature and precipitation over December-March of the previous growing season. The stippling crosses indicate months with significant correlations (p < 0.05).

Figure 5 .

 5 Figure 5.7: (a) XWT analysis of the N. pumilio NEGEX chronologies with November (December) temperature from REG. Red tone indicates higher power spectrum, black thick contours denote 95% significance level using a red noise background, and the white line delimits the cone of influence. (b) Running Pearson correlation coe cients for successive 30-year windows of N.pumilio NEGEX chronologies with temperature and precipitation over September-December. The stippling crosses indicate months with significant correlations (p < 0.05).

  Fig 5.9) at less than 8 years between 1409 and 2006. Tree growth has a more constant association with ENSO reconstruction in the high-frequency domain (< 8-10 years) since around 1880 onwards.

Figure 5 . 8 :

 58 Figure 5.8: Correlation coe cients between monthly indices of (a) SAM, and (b) MEI, and the NEGEX chronologies for the period 1959-2011. Correlations are given for a 18-month window from October of the previous year (*) to March of the following year (°). The shaded area represents current growing season (October through March). The darker bars indicate a correlation coe cient significant at p < 0.05; the vertical lines represent 95% confidence intervals. The SAM-Marshall index [Marshall 2003] was extracted from the British Antarctic Survey platform (1957-2015; http://www.nerc-bas.ac.uk/icd/gjma/sam.html). The Multivariate ENSO Index (MEI) was extracted from the NOAA platform (1950-2015; http://www.esrl.noaa.gov/psd/enso/mei/).

Figure 5 . 9 :

 59 Figure 5.9: Crosswavelet analysis (XWT) between the NEGEX chronologies and, SAM index reconstructed [Villalba et al. 2012] and ENSO index reconstructed [Li et al. 2013] for: a) F. cupressoides over the period 1409-2005 and b) Nothofagus pumilio over the period 1785-2005. Red tone indicates higher power spectrum, black thick contours denote 95% significance level using a red noise background, and the white line delimits the cone of influence.

Abstract

  Moreover, from the late 1970s onwards, F. cupressoides radial growth started to diverge from previous February temperatures. This low-frequency trend o set coincide with the welldocumented step increase of temperature around 1976. In contrast, no trend was observed in N. pumilio growth which can be related to environmental factors, but only contrasting timedependent responses to local climate were detected from west to east over the precipitation gradient. Next chapter (Chapter 6) will focus on the time-dependence of N. pumilio growth along the precipitation gradient to better understand the response of this species to local climate.Over the last decades, gradual changes in summer climate in the Southern Hemisphere have a ected forest growth in contrasting ways in moist and dry regions. Here, we use correlation analysis and a forward process-based model(Vaganov-Shashkin-Lite) to investigate spatial and temporal changes in climate limitation of the interannual tree-ring growth of Nothofagus pumilio at the upper treeline along a precipitation gradient in northern Patagonia. Patterns of climate limitation vary consistently along the gradient. At mesic and humid treelines, tree-ring growth is positively related to growing season temperature and negatively to precipitation. At xeric treelines, the opposite is observed. Moreover, the climate-growth relations are not stationary. In particular, according to the model, the step decrease in precipitation in 1952 induced an increase of the moisture limitation at the dry edge of the gradient. Correlation analyses evidence that the dependence of growth on moisture after 1952 has enhanced since 1976. While the model consistently reproduces tree-ring width variations over the 1931-1975 period, it does not capture the growth patterns in the following years. Some environmental parameters (cloudiness, snowpack, atmospheric CO 2 ) a ecting moisture, radiation and stomatal aperture may have reached thresholds beyond which the e ect on tree-growth has become sizable.

Fig.! 1

 1 Fig.! 1.! (a)! Location! of! the! tree-ring! sites! and! Bariloche! meteorological! station! along! the! west-to-east! climate! and! productivity! gradient! in! northern! Patagonia.! Gross! Primary Production! (GPP)! is! based! on! the! MOD17! product! (Zhao! et! al.,! 2005).! (b)! Mean! standardized! tree-ring! width! chronologies! of! Nothofagus! pumilio! from! northern! Patagonia! for each! site! along! the! west-to-east! precipitation! gradient! (NUB,! ALM,! DL! and! CHA)! between! 1900! and! 2011.

  .! These! models! can! represent! mixed! and! non-linear! tree! growth! responses to! changing! climatic! factors! from! daily! to! seasonal! and! decadal time! scales.! The! Vaganov-Shashkin-Lite! model! (VS-Lite;! Tolwinski-Ward! et! al.,! 2011)! is! perhaps! the! best! model! available! for! studying the! nature! of! tree! growth! in! regions! were! detailed! daily! climate! data and! field! observations! are! not! readily! available.! This! model! requires only! monthly! precipitation! and! temperature! data! to! provide! a! representation! of! the! climatic! controls! of! tree-ring! growth! based! on! the principle! of! limiting! factors! and! non-linear! growth! response! functions!

  ! this! study,! we! combine! linear! correlation! analysis! with! the VS-Lite! model! to! investigate! changes! in! the! climate! drivers! of N.! pumilio! growth! in! upper! treeline! forests! along! the! west-toeast! precipitation! gradient! of! northern! Patagonia.! We! address! the following! question:! is! water! limitation! becoming! more! important! than! temperature! limitation! in! recent! decades! with! the! drier and! warmer! SAM-driven! climate! regime,! especially! in! the! more moisture-limited! environments?! In! cold! and! wet! environments,! we expect! the! typical! pattern! of! temperature! limitation! described! in previous! studies! (Villalba! et! al.,! 1997)! whilst! in! cold! and! dry! environments! toward! the! east,! tree! growth! should! become! more! limited by! water! availability.! With! the! persistent! positive! phase! of! the! SAM, and! the! concomitant! warming! and! drying! in! northern! Patagonia,! we also! expect! that! water! limitation! should! have! become! more! important! in! recent! decades!(Daniels! and! Veblen,! 2004;[START_REF] Suarez | Sensitivity of Nothofagus dombeyi tree growth to climate changes along a precipitation gradient in northern Patagonia, Argentina[END_REF].! This! is! the! first! detailed! modeling! study! investigating! the controls! of! tree-ring! growth! conducted! in! the! region! and! thus! provides! novel! mechanistic! insights! into! the! responses! of! these! austral forests! to! recent! climate! change.

  ! between! individual! tree-ring! chronologies and! monthly! temperature! and! precipitation! from! the! nearest meteorological! station! at! Bariloche! (41 • 09 ′ S-71 • 10 ′ W,! 840! m! asl; Servicio! Meteorológico! Nacional,! Fig.! 1)! over! the! 1934-2009! period.

  Lavergne! et! al.! /! Dendrochronologia! 36! (2015)! 49-59Table! 1Location! and! descriptive! statistics! of! the! four! tree-ring! chronologies! of! N.! pumilio! used! in! this! study.! Sampling! sites! are! Paso! de! las! nubes! (NUB),! La! Almohadilla! (ALM),! Diego de! León! (DL)! and! Challhuaco! (CHA).

Fig.! 2 .

 2 Fig.! 2.! Tree-ring! growth! responses! to! climate! over! 1934-2009.! (a)! Simple! correlations! with! temperature! and! (b)! partial! correlations! with! precipitation! controlled! by temperature.! The! correlations! are! calculated! from! October! of! the! antepenultimate year! (*)! to! March! of! the! following! year! ( • ).! Correlations! are! based! on! 10-year! highpass! filtered! series.! The! black! points! indicate! months! with! significant! correlations (p! <! 0.05).

  ),! indicating! that! the! chronologies! at! the! extremes! of! the gradient! share! less! common! variability.! These! results! suggest! that there! are! some! differences! in! growth! variability! along! the! gradient, which! imply! differences! in! climate! limitation.Long-term! correlations! between! the! tree-ring! chronologies! and climate! indicate! that! tree-ring! growth! at! the! mesic! treeline! sites (NUB,! ALM! and! DL)! is! positively! associated! with! temperature! during! late! spring! (November/December;! r! =! 0.31,! p! <! 0.05,! at! NUB over! 1934-2009;! Fig.! 2)! and! negatively! with! precipitation! during the! same! period! (November;! partial! r! =! -0.3,! p! <! 0.05,! at! DL).! This indicates! that! growth! is! stimulated! by! warm! and! dry! late! spring conditions.! In! contrast,! tree! growth! at! the! driest! treeline! (CHA) is! negatively! correlated! with! temperature! (r! =! -0.31,! p! <! 0.05)! and positively! with! precipitation! (partial! r! =! 0.32,! p! <! 0.05)! in! December. A! significant! and! positive! influence! of! precipitation! on! tree! growth controlled! by! temperature! is! also! observed! at! this! site! in! April! and July! (partial! r! =! 0.27! and! 0.32,! p! <! 0.05,! respectively).! Thus,! growth at! this! xeric! treeline! site! is! stimulated! by! cool! and! wet! conditions during! early! summer! and! early! fall.! This! result! again! indicates! that limiting! factors! of! tree! growth! at! the! wet! and! dry! extremes! of! the precipitation! gradient! are! substantially! different.! The! leading! EOF of! the! four! chronologies! is! significantly! and! positively! correlated with! temperature! (r! =! 0.27,! p! <! 0.05)! and! negatively! with! precipitation! (partial! r! =! -0.27,! p! <! 0.05)! in! November,! and! thus! it! only captures! the! growth! signal! of! the! mesic! sites! at! the! middle! and western! sectors! of! the! precipitation! gradient. The! regional! climate! does! not! show! dramatic! trends! during the! most! recent! part! of! the! study! period.! Change-point! analysis (Beaulieu! et! al.,! 2012)! indicates! that! long-term! precipitation! and the! corresponding! soil! moisture! derived! from! the! VS-Lite! model, exhibit! a! shift! towards! lower! mean! values! from! ca.! 1952! onwards, which! coincides! with! the! start! of! the! positive! trend! in! the! Southern Annular! Mode! (SAM;! Fig.! A.1).! Temperature! anomalies! in! the! region show! a! significant! negative! trend! of! 1.3 • C! (p! <! 0.01)! between! 1945 and! 1975! followed! by! the! well-documented! step! increase! around 1976,! coincident! with! the! increase! in! sea! surface! temperature! in the! Tropical! Pacific! Ocean,! but! with! no! overall! significant! warming trend! afterwards! (0.03 • C! for! 1976-2008,! p! >! 0.1).! These! changes! in mean! temperature! and! precipitation! resulted! in! a! warm! and! wet period! before! ca.! 1952,! a! period! of! significant! cooling! between! 1952 and! 1975! and! a! warm! and! dry! period! since! around! 1976. Sliding! correlations! between! the! tree-ring! chronologies! and modelled! moisture! reveal! that! tree! growth! responses! varied! substantially! among! the! three! different! climate! regimes! identified above! (Fig.! 3).! Tree! growth! at! the! two! wettest! treeline! sites! (NUB and! ALM)! shows! a! consistent! shift! to! negative! correlations! (p! <! 0.1) with! moisture! around! 1980! over! an! extended! seasonal! period! that includes! most! of! the! previous! growth! year.! Prior! to! 1980,! correlations! with! moisture! are! weak! or! slightly! positive,! except! in! the wettest! site! where! correlations! with! moisture! prior! to! the! 1960s are! also! negative! and! significant! (p! <! 0.1)! over! most! of! the! previous! growth! year.! Correlations! with! temperature! during! the! critical November! and! December! months! are! also! temporally! instable! at these! sites! (Fig.! A.2),! and! are! more! significant! and! positive! during the! cooling! period! 1952-1975! (Fig.! A.1).! In! turn,! correlations! with precipitation! become! significantly! negative! in! November! but! significantly! positive! in! December! during! the! warm! and! dry! period 1976-2008! (Fig.! A.2). Tree! growth! at! the! site! representing! the! middle! of! the! precipitation! gradient! (DL)! is! also! predominantly! negatively! correlated with! moisture,! but! the! pattern! of! temporal! instability! in! the! correlations! is! less! clear! than! in! the! wettest! sites! (Fig.! 3).! Temporal changes! in! correlation! with! November! and! December! temperature and! precipitation! at! this! site! are! similar! to! those! at! the! wettest sites! (Fig.! A.2).! Growth! at! the! driest! site! (CHA)! shows! a! dramatic change! from! no! moisture! sensitivity! to! strong! sensitivity! around the! early! 1970s,! with! the! emergence! of! significant! (p! <! 0.1)! and! positive! correlations! during! most! of! the! current! and! previous! growth years! (Fig.! 3).! At! this! site,! the! relationship! between! tree-growth and! moisture! and! precipitation! in! December! has! been! consistently positive! since! around! 1950! (Figs.! 3! and! A.2).! In! turn,! the! correlation with! temperature! during! this! month! has! been! consistently! nega-

Fig.! 3

 3 Fig.! 3.! Temporal! patterns! of! correlation! between! N.! pumilio! tree-ring! growth! and! soil! moisture! derived! from! the! VS-Lite! model! from! April! of! the! previous! growing! year! (p) to! December! of! the! current! growing! year.! Correlations! are! shown! for! 21-year! running! windows.! The! stippling! denotes! significant! correlations! at! the! 90%! confidence! level (p! <! 0.1).Table! 2Statistics! of! the! Bayesian! estimation! of! the! site-by-site! tuned! VS-Lite! growth response! parameters! T1,! T2,! M1 and! M2 for! the! calibration! period! 1931[START_REF] Bottinga | Calculated fractionation factors for carbon and hydrogen isotope exchange in the system[END_REF]! and 1931-2008. Calib.! 1931[START_REF] Bottinga | Calculated fractionation factors for carbon and hydrogen isotope exchange in the system[END_REF]! Calib.! 1931-2008 Mean! Min! Max! SD! Mean! Min! Max! SD T1 ( • C)! 5.0! 4.4! 5.5! 0.4! 6.1! 5.0! 6.6! 0.8 T2 ( • C)! 15.4! 14.2! 17.1! 1.1! 14.5! 11.7! 16.4! 2.1 M1 (v/v)! 0.05! 0.049! 0.054! 0.002! 0.05! 0.05! 0.06! 0.005 M2 (v/v)! 0.35! 0.30! 0.36! 0.03! 0.34! 0.30! 0.39! 0.03

  between! 1931! and! 2008.! The! skill! of! the! parameter! set! obtained for! each! window! was! judged! by! the! correlation! between! the! actual and! simulated! tree-ring! chronologies! over! the! period! 1931-2008. The! results! of! this! parameterization! sensitivity! exercise! show! that the! minimum! temperature! (T 1 )! and! moisture! (M 1 )! parameters! for growth! are! stable! over! time! (not! shown).! In! contrast,! the! parameters! for! optimal! growth! (T 2 ,! M 2 )! vary! substantially! during! the! last two! to! three! decades! (Fig.! 4). At! the! wettest! sites,! the! somewhat! unrealistic! decrease! in! calibrated! parameters! over! the! last! decades! (Fig.! 4)! is! related! to! the observed! shift! to! negative! correlations! of! tree! growth! with! moisture! (Fig.! 3).! Since! the! simple! growth! response! function! of! the! model does! not! have! an! upper! threshold! beyond! which! the! effect! of! excessive! moisture! becomes! negative,! the! Bayesian! optimization! reduces the! moisture! threshold! for! optimal! growth! to! diminish! the! overall effect! of! moisture! on! the! simulated! growth! rates.! This! results,! in turn,! on! simulated! growth! being! completely! limited! by! temperature,! and! to! further! achieve! this,! the! optimization! also! reduces the! temperature! threshold! for! optimal! growth.! The! change! in! the calibrated! optimal! moisture! parameter! at! the! driest! site! (Fig.! 4b) appears! to! be! related! to! the! shift! to! increased! moisture! sensitivity! of! tree! growth! at! this! site! in! 1975-1976! (Fig.! 3).! Most! of! this change! in! moisture! sensitivity! in! actual! tree-ring! growth! occurred prior! to! the! start! of! the! seasonal! window! for! integration! of! the! simulated! growth! rates! (previous-year! March! to! current-year! June).

Fig.! 4

 4 Fig.! 4.! Estimated! optimal! temperature! (T2;! a)! and! moisture! (M2;! b)! for! growth! parameters! over! a! 21-year! sliding! window! as! an! indication! of! the! temporal! stability! of! the growth! response! functions! at! each! site.! Horizontal! dashed! lines! indicate! the! median! of! the! T2 and! M2 parameters! calibrated! over! the! entire! period! 1931-2008.! The! red! (a) and! blue! (b)! shading! denote! the! 90%! credible! intervals! for! the! median.! Growing! season! temperature! and! moisture! averaged! over! each! 21-year! sliding! window! are! shown! in the! bottom! panels! to! facilitate! the! interpretation! of! changes! in! parameters! over! time.! The! grey! bars! denote! the! correlations! (1931-2008)! between! the! actual! tree-ring! width chronologies! and! those! simulated! with! the! parameters! calibrated! over! the! corresponding! 21-year! sliding! window.! The! bars! above! the! line! are! significant! at! 95%! confidence level.! (For! interpretation! of! the! references! to! colour! in! this! figure! legend,! the! reader! is! referred! to! the! web! version! of! this! article). Table! 3 Intercomparison! between! actual! and! simulated! tree-ring! chronologies.! Correlation! coefficients! are! given! for! the! entire! period! of! calibration! 1931-2008,! the! two! half! of! the entire! period! 1931-1969! and! 1970-2008! and! for! the! periods! before! and! after! 1976.! The! one! year! lagged! correlations! between! simulated! and! observed! tree-ring! chronologies over! the! periods! 1970-2008! and! 1976-2008! are! significant! for! the! most! xeric! site! CHA! (rlag-1).! The! asterisk! (*)! denotes! significance! at! the! 95%! confidence! level.

  Fig.! 5a-b).! Temperature! limitation! during! the! peak! of! the growing! season! decreases! towards! the! east! whilst! moisture! limitation! increases! resulting! in! a! mixed! pattern! of! climatic! limitation for! the! mesic! and! drier! sites! (Fig.! 5b).! During! drought! years,! the model! tends! to! overestimate! moisture! limitation,! which! results in! much! narrower! simulated! rings! compared! with! the! actual! ring width! anomalies! (see! crosses! in! Fig.! 5a-b).

Fig.! 5

 5 Fig.! 5.! Intercomparison! of! actual! and! simulated! tree-ring! chronologies! together! with! the! associated! seasonal! patterns! of! climate! limitation! simulated! by! the! VS-Lite! forward model! at! each! site! (NUB,! ALM,! DL! and! CHA).! (a)! Comparison! between! actual! (black)! and! simulated! (red)! tree-ring! chronologies.! Correlation! coefficients! are! given! for! the period! 1931-2008.! The! asterisk! denotes! significance! at! the! 95%! confidence! level.! (b)! Annual! and! long-term! mean! monthly! growth! rates! due! to! soil! moisture! (gM,! blue)! and temperature! (gT,! purple)! for! the! period! 1931-2008.! The! black! crosses! in! (a)! and! (b)! denote! the! three! years! with! the! strongest! simulated! moisture! limitation! for! each! site.! (c) Modelled! mean! growth! rates! due! to! soil! moisture! and! temperature! for! the! wet! and! warm! period! 1931-1951! (gM1 and! gT1)! and! the! cooling! period! 1952-1976! (gM2 and! gT2). Shaded! regions! are! 95%! bootstrapped! confidence! intervals! about! the! means,! and! demonstrate! that! average! growth! rates! as! a! function! of! soil! moisture! are! indistinguishable between! the! two! periods! except! during! the! late! summer-spring! at! the! three! easternmost! sites! of! the! gradient! (ALM,! DL! and! CHA).

  climatic! response! of! the! three! mesic! populations,! as! well! as! the leading! EOF! of! all! the! chronologies,! could! thus! be! primarily! influenced! by! dominant! windward! fronts! from! the! Pacific! Ocean! during the! growing! season.! This! could! be! actually! the! prevailing! climatic response! throughout! the! whole! gradient.! Temperature! and! precipitation! may! also! affect! growth! through! snow! cover.! According to! Villalba! et! al.! (1997),! the! snow! persistence! in! spring! near! the Cerro! Tronador,! at! the! western! edge! of! the! gradient,! indeed! depends primarily! on! November! precipitation! and! temperature.! The! combination! of! low! precipitation! and! high! temperature! during! this! spring month! likely! promotes! faster! snowmelt,! which! in! turn! quickens cambial! activity! and! stimulates! tree! growth. The! VS-Lite! model! shows! a! mixed! control! of! growth! by! temperature! and! moisture! at! the! intermediate! sites! (ALM,! DL)! and! an exclusive! control! by! temperature! at! the! humid! one.! These! results do! not! match! strictly! the! observations.! However,! if! the! hypotheses! we! have! put! forward! are! correct,! the! model,! which! does! not account! for! cloudiness! and! snow! dynamics! cannot! fully! simulate the! climate-growth! relations! (see! 4.3).

Figure 7 . 1 :

 71 Figure 7.1: Distribution pattern of weighted seasonal " 18 O p in precipitation over South America. The prevailing directions of surface winds and the position of the Intertropical Convergence Zone (ITCZ) during the austral summer (December) and winter (June) are also marked (after Rozanski et al. [1995]). The black dote denotes the study area. Original map from the Global Network of Isotopes in Precipitation (GNIP, Maps and Animations, International Atomic Energy Agency, Vienna. Accessible at http://isohis.iaea.org).

  2): Temuco (38°26' S, 72°22'W, 114 m elevation), Puerto Mont (41°28'S, 72°56'W, 16 m elevation), Bariloche (41°12 S, 71°12'W, 840 m elevation) and Coyhaique (45°21 S, 72°07, 310 m elevation).

Figure 7 . 2 :

 72 Figure 7.2: Location of the GNIP isotopic stations

Figure 7 . 3 :Contents 8 . 1

 7381 Figure 7.3: (a) Oxygen isotopic composition of spring waters collected in March 2013 and May 2014 and of precipitation from the GNIP database (Puerto Mont at sea level on the western side, Temuco in the north at 38°S, Coyhaique in the south at 45°S and Bariloche on the eastern side) according to the longitude (°W). Black: March, Grey: May and dark blue: annual GNIP averaged. The crosses are the GNIP data (" 18 O p ), the rectangles are the surface water data (" 18 O w ). (b) Comparison between monthly " 18 O p and monthly temperature for Puerto Mont (blue) and Bariloche (grey) GNIP stations. The respective linear regression and r 2 are shown. (c) Evolution of " 18 O p (blue) and monthly temperature (grey) from Puerto Mont GNIP station over the 1989-2009 period.

  species are compared • The isotopic climate signal recorded in F. cupressoides is relatively weak • The δ 18 O in N. pumilio is useful for reconstructing temperature over a large area Supporting Information: • Supporting Information S1 Correspondence to: A. Lavergne, and V. Daux, alienor.lavergne@lsce.ipsl.fr; valerie.daux@lsce.ipsl.fr Citation: Lavergne, A., V. Daux, R. Villalba, M. Pierre, M. Stievenard, F. Vimeux, and A. M. Srur (2016), Are the oxygen isotopic compositions of Fitzroya cupressoides and Nothofagus pumilio cellulose promising proxies for climate reconstructions in northern Patagonia?, J. Geophys. Res. Biogeosci., 121, doi:10.1002/ 2015JG003260.

Figure 1 .

 1 Figure 1. (a) Location map of the δ 18 O P precipitation stations from GNIP network (Temuco, Bariloche, Puerto Mont, and Coyhaique) and the tree ring sampling sites. Long-term mean (over 1979-2000) of summer precipitation (December-January-February) are plotted as contour lines (in mm/d). (b) F. cupressoides and (c) N. pumilio trees. (d) Elevation along a west-east transection at 41°S (adapted from Villalba et al. [2003]) and location of tree ring sites (green circles) and meteorological stations (blue circles). Sampling sites are Glaciar Frías (GF) and Río Alerce (RA) for F. cupressoides (pale green) and Paso de las Nubes (NUB), La Almohadilla (ALM), and Challhuaco (CHA) for N. pumilio (light green). The pale blue arrows indicate the main direction of the Westerlies winds.

Figure 2 .

 2 Figure 2. The two 60 year species-δ 18 O cell time series (in ‰ versus SMOW). The five site-δ 18 O cell chronologies are shown in dashed lines.

Figure 3 .

 3 Figure 3. Correlation of the species-δ 18 O cell chronologies and leading EOF with monthly temperature and precipitation from Bariloche station for the period 1954-2009. Correlations are given for a 9 month window from September, at the start of the growing season, to May of the following year (°) corresponding to the end of the growing season. The correlation coefficients of the δ 18 O cell series with the December-May mean temperature and total precipitation are also shown (D-M°). The stippling points indicate months with significant correlations (p < 0.05).

Figure 4 .

 4 Figure 4. Correlation field across southern South America between December and May temperature (CRUTS 3.2 data set [Harris et al., 2014]) and the N. pumilio δ 18 O cell chronology for the period 1954-2011. The stippling points indicate grid points with significant correlations (p < 0.05). The black points situate the sampling sites.

a r 2 :

 2 Squared correlation coefficient, RE: reduction of error statistic, CE: coefficient of efficiency, DW: Durbin-Watson statistic, and RMSE: root-mean-square error of the estimate for two independent periods. Overall period (1954-2010): r 2 = 0.35; DW = 1.58, and RMSE = 0.58 representing 11% of the range of December-May temperature.

Figure 5 .

 5 Figure 5. Actual and reconstructed December-May mean temperatures with the N. pumilio δ 18 O cell mean chronology for the period 1954-2011. The pink color band around the reconstruction indicates the root-mean-square error (RMSE = ±0.58°C). The explained variance (r 2 ) is indicated.

4. 1 .

 1 What Are the Physical Mechanisms Relating δ 18 O cell and Temperature?

  13 C relationships with local climate . . . . . . . . . . . . . . . 103 9.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 9.3.1 Tree growth and BAI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 9.3.2 Temporal variations in " 13 C raw , 13 C, c i and iWUE . . . . . . . . . . . . . 105 9.3.3 Carbon and oxygen isotopic chronologies . . . . . . . . . . . . . . . . . . . . 106 9.3.4 " 13 C related to climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 9.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 9.4.1 Trends in iWUE and BAI . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 9.4.2 Isotopic composition and physiological responses . . . . . . . . . . . . . . . 110 9.4.3 Performance of " 13 C as climate proxy . . . . . . . . . . . . . . . . . . . . . 111 9.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

Figure 9 . 1 :

 91 Figure 9.1: Temporal evolution of (a) mean radial growth and (b) basal area increment (BAI) of the five sites over the 1110-2011 period.

Figure 9 . 2 : 9 . 3 . 2

 92932 Figure 9.2: Basal area increment (BAI in cm 2 ) versus atmospheric CO 2 concentration (ppm) between 1850 and 2011.

Figure 9 . 3 :

 93 Figure 9.3: (a) Spatio-temporal variations of tree-ring " 13 C raw and " 13 C cor at each site over 1952-2011. (b) Temporal evolution of the isotopic discrimination 13 C, the retio between intercellular to ambient air CO 2 concentrations (c i /c a ) ratio and intrinsic water-use e ciency (iWUE) over 1952-2011. In dashed lines are the trends during the whole period.

Figure 9 . 4 :

 94 Figure 9.4: Basal area increment (BAI) versus intrinsic water-use e ciency (iWUE) for each species.The variance (r 2 ) explained is shown.

Figure 9 . 5 :

 95 Figure 9.5: " 13 C cor variations versus " 18 O for each species. Correlation coe cients between the two series are shown. The asterisk denotes significance at p < 0.05.

Figure 9 . 6 :

 96 Figure 9.6: Correlation between specific " 13 C cor chronologies and: (a) monthly temperature and (b) monthly precipitation over 1954-2011. Correlations are given for a 9-month window from September to May (°). The stippling points indicates months with significant correlations (p < 0.05).

  2 dataset [Harris et al. 2014] averaged over December-February in the area 30°S-56°S and 55°W-80°W (Fig.9.7.a). We found a strong imprint of summer temperature recorded in " 13 C cor chronology south of around 35°S , indicating that the regional F. cupressoides chronology accurately records mean summer temperature variations representative of a large area across southern South America.The linear regression model(over 1954-2010) of " 13 C cor -on summer temperature is reliable and have predictive skills (Residual Error = 0.54, that is RE > 0[Fritts 1976], Durbin-Watson statistic = 1.81, that is DW ¥ 2,[Durbin and Watson 1971] and Root Mean Squared Error = 0.71°C). This model accounts for 54% of the actual summer temperature variance (Fig.9.7.b).

Figure 9 .

 9 Figure 9.7: (a) Correlation field across southern South America between December-February temperature (CRUTS 3.2 dataset [Harris et al. 2014] and " 13 C cor chronology of F. cupressoides for the period 1954-2011. The stippling points indicate grid points with significant correlations (p < 0.05). The black points situate the sampling sites. (b) Actual and reconstructed December-February mean temperature for the period 1954-2011. The pink colour band around the reconstruction indicates the root mean squared error (RMSE ± 0.71°C). The explained variance (r 2 ) is also indicated.

Figure 9 . 8 :

 98 Figure 9.8: Comparison between " 13 C cor and " 18 O chronologies for F. cupressoides (a and c) and N. pumilio (b and d) and averaged summer, (A) photosynthetically active radiation (PAR in W/m 2 ) and (B) cloud fraction (CDF, in %) extracted from the NASA/GEWEX Surface Radiation Budget (SRG) Release 3.0 dataset over the period 1982-2007. Correlations are shown with significance at 99% (**) and 95% (*).

2 .Fitzroya:Figure . 1 :

 21 Figure .1: Poster presented at the European Geosciences Union General Assembly, Vienna, Austria, April 2014

  investigate the changes in the relations between treering parameters (ring-width and 18 O of cellulose) and climate along a precipitation gradient 2. Site and samples Northern Patagonia: ca. 41°10'S -71°30'W Cores from 2 native shallow-rooted species : Fitzroya cupressoides, a long-lived endemic cypress and Nothofagus pumilio, a well-distributed beech 2 cypress sites (GF and RA), 4 beech sites (NUB, ALM, DL and CHA) along a West-East precipitation gradient; 100 to 200 cores/sites ADAPTABILITY OF TREES TO CLIMATE CHANGE IN NORTH PATAGONIA Intra-and inter-sites consistency of the 18 O cellulose Increased 18 O cellulose in pace with regional warming Principal component analysis Only 42% of the common signal may be climate TRI series are recording mainly the year-to-year climate variations PC1: moderate corr. with T° (R=-0.27, pD-M) PC2: Treespecies physiological functioning PC3: Precip. gradient Better climate-18 O correlations for Nothofagus than for Fitzroya Nothofagus: Strong correlations with current spring temperatures (Feb. to May) Decreasing along the precipitation gradient Acknowledgments: This research is being supported by LEFE-PATISO project from CNRS-INSU, Université de Versailles -St Quentin, LSCE and IANIGLA institute.

Fitzroya: o

 o Increased growth over the last 60 years while this species is known to like relatively cool and wet summers

Fitzroya

  Is the West-East humidity gradient responsible for differences in growth pattern over the last years?

D

  tree variability (EPS* > 0.85 with 6 and 4 individus for Fitzroya and Nothofagus, resp.) Strong inter-site correlations Correlations between TRI series and climate Strength of correlations: high frequency > composite Fitzroya well correlated to previous summer temperatures. For Nothofagus, climatic variations that favour tree growth at wet sites are disadvantaging the one at the dry sites, and inversely, Instrumental temperature and precipitation data homogenization with HOMER (Mestre et al, 2013) Standardization of tree-ring width with Signal Free program (Melvin et al., 2007) Year-by-year analyses of the δ 18 O of tree-ring cellulose for the last 60 years (analytical error =0.2‰) Correlation-based calibrations3. Methods1 Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS, Gif-sur-Yvette, France 2 Laboratorio de Dendrocronologia e Historia Ambiental, IANIGLA, CONICET, Mendoza, Argentina alienor.lavergne@lsce.ipsl.fr, ricardo@mendoza-conicet.gob.ar, valerie.daux@lsce.ipsl.fr''Take-home'' messages:The two tree species responses to climate change are different. F. cupressoides productivity has increased over the last years. N. pumilio growth may have leveled off due to water shortage. Oxygen isotopic variations of beech cellulose are better related to temperature than TRI and can be used to reconstruct past temperature variations. 300-year long δ 18 O chronologies at NUB δ 13 C analyses to constrain tree water stress the two species have shallow roots, cellulose 18 O may reflect the 18 O of current-year precipitation modulated by evapotranspiration..

Figure . 2 :

 2 Figure .2: Poster presented at the PAGES meeting -LOTRED-South America 3rd symposium on Climate Change and Human Impact in Central and South America over the last 2000 years -Observations and Models, Medellin, Colombia, July 2014

Figure 1 :

 1 Figure 1: Summer (DJF) Southern Annular Mode (SAM) influence on soil moisture across southern South America

Figure 2 :

 2 Figure 2: Location map of the tree-ring sites

Figure 3 :

 3 Figure 3: Correlation coe cients between tree-ring chronologies and monthly temperature, precipitation and soil moisture. Stipped points are significant. Conclusion At inter-annual time-scale, the shift between actual and simulated growth in 1976-1977 coincides with a well-known shift towards warmer SSTs in the tropical Pacific associated with warmer growing season temperatures. Di erences between wet and dry sites since mid-1970 : do the dry site indicates possible strategies of adaptation ? Deep roots allowing use of previous year water ? Use of stored starch ? ae dendrometric measurements may help to understand the physiological processes (in progress)

Figure 4 :

 4 Figure 4: Comparison between actual (black) and simulated (red) tree-ring chronologies and associated seasonal patterns of climate limitation. Correl. coe : left, 1931-1969 and right,

  lag-1-yr 1977-2008. Asterisk = significant • Growth is temperature-limited (gT<gM) during the beginning and the end of the growing season and water-limited during summer months (gM<gT) • Changes in climate limiting factors since mid-1970 • Lagged response between actual and simulated growths at the dry edge of the biome over 1977-2008

Figure 1 :Figure 2 :

 12 Figure 1: Principaux forçages climatiques influençant le climat de l'Amérique du Sud (ENSO et SAM) et répartition des chronologies de largeurs de cernes le long de la Cordillère des Andes. Modifié de [3] L'Amérique du Sud est une région clé pour étudier les variations climatiques passées dans l'HS. S'étendant des tropiques en Colombie (10°N) jusqu'en Terre de Feu (55°S), elle est traversée du nord au sud par la Cordillère des Andes. L'action conjointe de plusieurs modes de variabilité climatique (Southern Annular Mode ou SAM et El Niño-Southern Oscillation ou ENSO) via de fortes téléconnexions atmosphériques entre basses et moyennes latitudes, induit des variations climatiques spatiales et temporelles complexes.

Figure 3 :

 3 Figure 3: (a) Carte de corrélation entre le " 13 C moyenné des 2 sites de F. cupressoides et la température moyenne d'été (Décembre-Février) ; (b) Température moyenne d'été observée et reconstruite en utilisant le " 13 C de F. cupressoides • Modèle de régression linéaire du " 13 C moyenné des 2 sites de F. cupressoides avec la température d'été : 54% de la variance totale expliquée par la température d'été, réduction de l'erreur (RE) = 0.60 > 0, Durbin-Watson ± 2 (pas d'autocorrélation) et di érence entre observation et reconstitution (RMSE) = 0.71°C Remerciements Cette recherche a été financée par CNRS-INSU et UVSQ (France) et IANIGLA-CONICET (Argentine).

Figure . 4 :

 4 Figure .4: Poster presented at the Colloque Holocène of the Association Francaise pour l'Etude du Quaternaire, Paris, France, December 2015

  Café desSciences LSCE, Gif-sur-Yvette, France, 2014-2015 : Organization and conduction of monthly scientific seminars of the Theme 1 "Archives & Tracers" of LSCE Scientific volunteer facilitator : Forum of Climate 2014 Troyes, France, November 27 : Small experiments (the origin of the ocean circulation, the acidification of the oceans) International forum of Meteorology 2014 City Hall of Paris, France, April 3-6 : Small experiments (influence of the size of particles in the atmosphere on the di usion of sunlight, temperature variations due to evaporation and condensation, the origin of the ocean circulation, the acidification of the oceans) Science Festival 2013 Gif-sur-Yvette, France, October 20 : Dendrochronology overview (functioning of trees, sampling, analytical methods used for measure of tree-ring width and isotopic composition of cellulose, data processing for paleoclimatic recon-Espace Technologique / Immeuble Discovery Route de l'Orme aux Merisiers RD 128 / 91190 Saint-Aubin, France Titre : Evaluation de l'archive naturelle cernes d'arbre comme traceur du climat passé au nord de la Patagonie Mots clés : cernes d'arbre, paléoclimat, Patagonie, croissance et isotopes stables Résumé : L'Hémisphère Sud, et plus précisément le sud de l'Amérique du Sud, sont très sous-représentés dans les reconstitutions climatiques globales, du fait d'un manque de données paléoclimatiques robustes. Mes travaux de thèse ont donc porté sur l'évaluation de l'archive naturelle cernes d'arbre, abondante le long de la Cordillère des Andes, comme traceur paléoclimatique. Je me suis ainsi appliquée à étudier la réponse régionale de deux espèces d'arbre (Fitzroya cupressoides et Nothofagus pumilio) natives du nord de la Patagonie (41°S, Argentine) aux forçages climatiques externes en analysant leurs différents paramètres physiques.Pour cela, j'ai commencé par déterminer l'influence du climat local et de grande échelle sur leur croissance en étudiant les variations temporelles de leurs largeurs de cerne, puis j'ai essayé d'extraire le signal climatique enregistré dans la composition isotopique de l'oxygène et du carbone de la cellulose de ces cernes. Deux résultats majeurs de ma thèse ont émergé: j'ai mis en évidence (1) des réponses non-linéaires de la croissance au cours du temps liées aux changements de régimes climatiques et, (2) le fort potentiel du δ 18 O et δ 13 C de la cellulose pour enregistrer les variations de température d'été-automne sur une large région aux latitudes moyennes de l'Amérique du Sud (35°-55°S). Du fait des relations fortes qui les lient aux températures, les outils isotopiques peuvent être utilisés pour reconstituer des chronologies de température mais aussi des modes climatiques telle que l'Oscillation Antarctique.Title : Evaluation of tree-ring archive as paleoclimatic tracer in northern PatagoniaKeywords : tree-ring, paleoclimate, Patagonia, growth and stable isotopes Abstract: The Southern Hemisphere, and particularly southern South America, are very under-represented in global climate reconstructions due to a lack of robust paleoclimatic data. My thesis was therefore focused on the evaluation of the tree-ring archive, which is widespread along the Andes, as a paleoclimatic tracer. I studied the regional response to external climate forcing of two native species (Fitzroya cupressoides and Nothofagus pumilio) in northern Patagonia (41°S, Argentina), by analysing their different physical tree-ring parameters. In this perspective, I analysed the temporal variations of the width and of the oxygen and carbon isotopic compositions of the cellulose of their rings and related them to local and large-scale climate variations. Two major results of my thesis have emerged: I highlighted (1) nonlinear growth responses over time related to shifts in climate regimes and, (2) the potential of cellulose δ 18 O and δ 13 C to record summerautumn temperature variations over a large area in the mid latitudes of South America (35°-55°S). As their variations are strongly linked to temperature the isotopic tools can be used to reconstruct chronologies of temperature but also of climate modes such as the Southern Annular Mode.

  . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Biogeophysical processes of terrestrial ecosystems. From [Bonan 2008] . . . . . 2.2 Sap ascent. From https://courses.candelalearning.com/ . . . . . . . . . . . . . . 2.3 Cross-section of a tree stem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Main fractionation steps and typical values of " 18 O in a temperate climate. " 18 O SMOW , standard mean " 18 O in ocean water; " 18 O V , " 18 O P , " 18 O LW , " 18 O in water vapor, precipitation (either rainfall or snow) and leaf water, respectively; VPD, vapor pressure deficit; gs, stomatal conductance. Redrawn from

2 Tree-rings as palaeoclimate archives Contents 2.1 Trees internal functioning

  The hydraulic system: ascent of the sap . . . . . . . . . . . . . . . . . . . . 15 2.1.2 Soils, roots and absorption of nutrients and minerals . . . . . . . . . . . . . 17 2.

	The main findings of this chapter are published in Journal of Geophysical Research -
	Biogeosciences as:
	Lavergne A., Daux V., Villalba R., Pierre M., Stievenard M., Vimeux F. and
	Srur A.M. (2016), Are the oxygen isotopic compositions of Fitzroya cupressoides
	and Nothofagus pumilio cellulose promising proxies for climate reconstructions
	in northern Patagonia?, Journal of Geophysical Research -Biogeosciences, 121,
	doi:10.1002/2015JG003260.
	Part I
	BACKGROUND AND METHODS

. . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.1.1

2 Tree growth and wood formation . . . . . . . . . . . . . . . . . . . . . . . 18 2

  .2.1 Photosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2.2 The annual tree-ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.

3 Stable isotopes in tree-rings . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2

  .3.1 Stable isotopes variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.3.2 Variability in the isotopic composition of H 2 O and CO 2 . . . . . . . . . . . 19 2.3.3 Use of cellulose for paleoclimate studies . . . . . . . . . . . . . . . . . . . . 20 2.3.4 Isotopic fractionation in trees . . . . . . . . . . . . . . . . . . . . . . . . . . 21

  18 O in a temperate climate. " 18 O SMOW , standard mean " 18 O in ocean water; " 18 O V , " 18 O P , " 18 O LW , " 18 O in water vapor, precipitation (either rainfall or snow) and leaf water, respectively; VPD, vapor pressure deficit; gs, stomatal conductance. Redrawn from

Contents 3.1 Geographical and climatic settings . . . . . . . . . . . . . . . . . . . . . . 29

	3.1.1 The Patagonian Andes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
	3.1.2 Variability and trends in climate records . . . . . . . . . . . . . . . . . . . . 29
	3

.2 Sampled species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4 Tree-ring data and methods Contents 4.1 Sample collection in the field . . . . . . . . . . . . . . . . . . . . . . . . . 35 4.2 Standard dendrochronological techniques . . . . . . . . . . . . . . . . . . 37

  Chronology statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

	4.2.1 Sample datation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
	4.2.2 Tree-ring width measurements and cross-dating . . . . . . . . . . . . . . . . 37
	4.2.3 Chronology standardization . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
	4.2.4

40 4.3 Stable isotopes in tree-rings: analytical procedure . . . . . . . . . . . . . 40 4

  .3.1 Sample selection for stable isotopic analysis . . . . . . . . . . . . . . . . . . 40 4.3.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 4.3.3 Mass spectrometric analysis of cellulose . . . . . . . . . . . . . . . . . . . . 43

Table 4 . 1 :

 41 Site location and description of the tree-ring chronologies of F. cupressoides and N. pumilio.

	Species/ site	Lat. (S) Lon. (W) Elev. (m) Expos. Period	Cores
	Fitzroya cupressoides
	GF	41°08'	71°48'	950	NW	888-2011	159
	RA	41°11'	71°46'	1060	SW	836-2011	210
	Nothofagus pumilio
	NUB	41°09'	71°48'	1274	NW	1644-2011 110
	ALM	41°11'	71°47'	1425	NE	1539-2011 193
	DL	41°16'	71°43'	1520	NW	1546-2011 166
	CHA	41°15'	71°17'	1612	NE	1670-2011 146

  12 C 16 O 2 ), 45 ( 13 C 16 O 2 or 12 C 17 O 16 O) and 46 ( 13 C 17 O 16 O, 12 C 18 O 16 O or 12 C 17 O 2 )were measured and the ratio 46/44 and 45/44 were determined. As for the oxygen, the ratio 45/44 of interest was corrected for the e ect of 12 C in the mass 45 of CO 2 using the ratio 45/44. The analytical error for carbon measurements was ± 0.1 % .

  The correlations were computed over the period 1933-2009 for comparison with local stations and over 1903-2011 for comparison with the CRU TS3.2 gridded data.

For the detection of decoupling, we used a visual comparison of the tree-ring chronologies and climate target (e.g.

Jacoby and D'Arrigo [1995], D'Arrigo et al. [2009]

) and running Pearson correlation coe cients for successive 30-year long windows (e.g.

[START_REF] Büntgen | Growth/climate response shift in a long subalpine spruce chronology[END_REF]

,

[START_REF] Andreu-Hayles | Long tree-ring chronologies reveal 20th century increases in wateruse e ciency but no enhancement of tree growth at five Iberian pine forests[END_REF]

,

[START_REF] Naulier | Temporal instability of isotopes-climate statistical relationships -A study of black spruce trees in northeastern Canada[END_REF]

). The significance (at p < 0.05) of the low frequency modulation was assessed using the g.test function (package Treeclim) in R. This test is a multivariate extension of the test originally proposed by

[START_REF] Gershunov | Low-Frequency Modulation of the ENSO-Indian Monsoon Rainfall Relationship: Signal or Noise?[END_REF]

.

  periods. One (two) asterisk(s) indicate correlations statistically significant at 95% (99%).

	Period	SAM/T SAM/P
	1957-2008 0.49*	-0.36**
	1957-1982 0.38*	-0.22
	1983-2008 0.48*	-0.51*
	precipitation is negatively correlated to monthly SAM index over most of the months but are
	not significant. However, significant correlations are found when combining spring-summer
	months (SONDJF; r = -0.36, p < 0.01, Tab. 5.2). Thus, positive SAM conditions in spring-
	summer are associated with warmer and drier spring summers.

Table 5 . 3 :

 53 Pearson's coe cients of the pairwise correlations of the NEGEX (left) and SPL10 (right) chronologies over 1785-2011 (p < 0.05). In italic are the non-significant correlations.

	Site	RA NUB ALM DL	CHA	RA NUB ALM DL	CHA
	GF	0.71 0.18 0.27	0.25 0.26	0.88 0.01 0.06	0.17 0.15
	RA	1	0.08 0.39	0.49 0.45	1	0.01 0.05	0.15 0.15
	NUB		1	0.48	0.34 0.23		1	0.71	0.61 0.28
	ALM			1	0.62 0.44		1	0.72 0.36
	DL				1	0.51		1	0.45

Résumé

  Au cours des dernières décennies, des changements progressifs du climat d'été dans l'Hémisphère Sud ont a ecté la croissance des arbres de facon contrastée entre les régions humides et sèches. Ici, nous utilisons des analyses de corrélations et un modèle simple de simulation de la croissance des arbres (Vaganov-Shashkin-Lite) afin détudier les changements spatiaux et temporels des facteurs limitants la croissance interannuelle des cernes d'arbres de N. pumilio le long du gradient de précipitation au nord de la Patagonie. Les patterns de croissance varient de facon importante le long du gradient. Dans les forêts mésiques et humides à la limite des arbres, la croissance des cernes d'arbre est stimulée par une température d'été élevée mais peu de précipitations, tandis que dans la forêt xérique, l'inverse est observé. De plus, les relations climat-croissance ne sont pas stationnaires. En particulier, d'après le modèle, la diminution brusque des précipitations en 1952 a induit une augmentation de la limitation par l'humidité à l'est du gradient. Des analyses de corrélations glissantes mettent en évidence que la dépendance de la croissance à l'humidité après 1952 s'est accentuée depuis 1976. Bien que le modèle reproduit les variations de largeurs de cernes sur la période 1931-1975, il ne capture pas les modèles de croissance dans les années suivantes. Certains paramètres environnementaux (nébulosité, manteau neigeux, CO 2 atmosphérique) a ectant l'humidité, le rayonnement solaire et l'ouverture des stomates ont dû atteindre des seuils au-delà desquels leurs e ets sur la croissance des arbres sont devenus significatifs.

7 Spatial and temporal variations of stable water isotopes in southern South America Contents 7.1 Spatial variability of

" 18 O p in southern South America . . . . . . . . .

. 83 7.2 Oxygen isotopes in spring waters related to

" 18 O p at 41°S . . . . . . . . 84 7.2.1 Spring and precipitation waters . . . . . . . . . . . . . . . . . . . . . . . . . 84 7.2.2 Orographic rain shadow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Table 1 .

 1 Location of the Tree Ring Sites and Statistics on the Respective δ 18 O cell Chronologies

	a

Table 2 .

 2 Pearson's Correlation Coefficients Among the Site-and Speciesδ

	18 O cell Chronologies and the Leading EOF Over the 1952-2011 Period a
	Site	RA	NUB	ALM	CHA	EOF1	F. Cup.
	GF	0.72	0.38	0.48	0.49	0.75
	RA		0.52	0.60	0.45	0.82
	NUB			0.71	0.52	0.78
	ALM				0.70	0.88
	CHA					0.79
	N. pum.						0.61
	a Site abbreviations as in Table 1. N. pum.: N. pumilio species-δ chronology and F cup.: F. cupressoides species-δ 18 O cell chronology. 18 O cell Correlations are significant at p < 0.05.

Table 3 .

 3 Calibration and Verification Statistics of N. pumilio δ 18 O cell Against December-May Temperature Using Simple

	Linear Regression	a
		Calibration					Verification
	Period	r 2	RMSE	DW	Period	RMSE	r 2	RE	CE
	1954-1982	0.26	0.77	2.08	1983-2010	0.42	0.33	0.26	0.67
	1983-2010	0.33	0.50	1.68	1954-1982	0.69	0.26	0.33	0.51

  Basal area increment (BAI) calculation . . . . . . . . . . . . . . . . . . . . 102 9.2.2 Isotope discrimination ( 13 C), intercellular CO 2 concentrations (c i ) and intrinsic water-use e ciency (iWUE) calculation . . . . . . . . . . . . . . . 102 9.2.3 Correction of non-climatic trends . . . . . . . . . . . . . . . . . . . . . . . . 103 9.2.4 Common variability recorded in the isotopic chronologies . . . . . . . . . . 103 9.2.5 Analyses of "

	9 Physiological responses to environmental
	changes and climate variability recorded in " 13 C
	Contents
	9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
	9.2 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
	9.2.1

Table 9 . 1 :

 91 Temporal trends in BAI, 13 C, c i , iWUE, " 13 C raw and " 13 C cor evaluated using Mann-Kendall trend test for F. cupressoides and N. pumilio sites over the 1952-2011 period . BAI in cm 2 per year, in % per year, c i in ppm per year, iWUE in umol.mol ≠1 per year and, " 13 C raw and " 13 C cor in % per year

	Site	BAI	MK	13 C MK	c i	MK	iWUE MK	" 13 C raw MK	" 13 C cor MK
	GF	0.04	0.029	-0.01	0.0001 0.48 6.5.10 ≠13 0.52	1.1.10 ≠14 -0.015	4.2.10 ≠7	0.011	0.0002
	RA	0.03	0.0002	-0.004 0.079	0.57 2.2.10 ≠16 0.47	2.2.10 ≠16 -0.022	1.7.10 ≠13 0.004	0.11
	NUB 0.001 1	-0.015 0.001	0.54 3.1.10 ≠10 0.49	4.5.10 ≠13 -0.011	0.006	0.015	0.001
	ALM 0.09	3.06.10 ≠4 0.01	0.037	0.91 9.3.10 ≠15 0.26	1.9.10 ≠5	-0.036	3.9.10 ≠9	-0.01	0.033
	CHA -0.01 0.60	-0.01	0.0012 0.50 1.0.10 ≠10 0.51	9.3.10 ≠15 -0.013	0.0006	0.012	0.002

Table 9 . 2 :

 92 Pearson's correlation coe cients among the site and species " 13 C cor chronologies over the 1952-2011 period. Correlations are significant at p < 0.05. In italic are the non-significant correlations.

	r	RA NUB ALM CHA F. cup.
	GF	0.55 0.47	0.32	0.43
	RA	0.38	0.33	0.58
	NUB		0.15	0.24
	ALM			0.30
	N. pum.			0.67
	9.3.4 " 13 C related to climate

  2 internship in bases de la dendrochronologie et de l'échantillonnage des arbres pour mon stage de master 2 en Février 2012 à El Bolsón (Argentine). Et aussi à Jonathan Barichivich qui m'a introduit à l'utilisation du modèle VS-Lite et à Matlab, et qui m'a suggéré de suivre une voie nouvelle dans ma recherche. La rédaction de mon premier article a été rendue possible grâce à lui! Merci aussi aux deux membres de mon comité de thèse, Myriam Khodri et Luc Lambs, qui m'ont bien aiguillée au cours de ma thèse et avec qui j'ai participé à deux conférences aux deux coins du monde, avec Myriam à la conférence LOTRED-PAGES organisée à Medellín (Colombie) en Juillet 2014 et avec Luc à la conférence Asiandendro à Katmandou (Népal) en Mars 2015, et au voyage trés dépaysant qui a suivi. Toute mon expérience à Mendoza n'aurait pas été aussi constructive sans la présence de mes colocataires Soledad Rodríguez Catón et Hugo Berón, de tous les amis du IANIGLA et des milongueros de Mendoza (Javier, Oscar, Pilar, Miguel, Nazareño, Lucho, Mariano, Sebastián, Mario, Evangelina, Carlos, Cecilia, Ana, Jorge, Roberto, Romi...). Du côté de Paris, merci à l'équipe GLACCIOS, notamment Inga Labuhn et Jiaoyang Ruan, mes colocataires de bureau d'avoir partagé des moments de dur labeur (!!), Dominique Genty, Edouard Régnier et Ulrich von Grafenstein pour nos déjeuners très sympathiques accompagnés d'Emilie Dassié, Delphine Angst, Anais Orsi, Mathieu Casado, Alexandra Touzeau... Et merci aux amis du LSCE pour nos activités sportives ou ludiques (Nadine Schneider, Martha Vogel, Inga Labuhn, Felix Vogel, Emmanuel Arzoumanian, Jonathan Barichivich, Francesca Guglielmo, Luis Molina...) et aux amis et proches de Mendoza ou Paris (Adèle De Palaminy, Adriana Rolón, Mae Corvalán, Jimmy Parra, Nora Zannoni, les italiens du RER B, et tous ceux que j'aurai oubliés par mégarde, et le beau Davide Komla Ebri).Et pour finir cette longue liste de remerciements, je tenais à remercier ma famille qui m'a toujours soutenue dans mes projets les plus fous (enfin pour eux!), mes parents Viviane Sautereau et Eric Lavergne et mes soeurs Théodora Lavergne et Alexandra Lavergne Lecointre, mais aussi ma famille argentine AFS qui a changé le cours de ma vie (Claudia Paladino et Carlos, Marcos, Denise and Simón Lagier (malgré tout!), mais aussi Benjamin Ybañez, Gabriel Ariza, Gabriel Ybañez, Wanda Skorupski, Marcela Magín...) et qui ont été mes premiers supporters. Merci à l'UVSQ et au CNRS qui m'ont aidé à financer ma recherche.

1.1.3 Pacific Decadal Oscillation (PDO) and Interdecadal Pacific Oscillation (IPO)On longer time scales, shifts in mean surface temperature are closely associated with ENSOlike multidecadal climate variability in the Pacific Ocean, often referred to as the Pacific Decadal Oscillation (PDO;[START_REF] Mantua | A Pacific Interdecadal Climate Oscillation with Impacts on Salmon Production[END_REF]) or the Interdecadal Pacific Oscillation (IPO;

Au cours des dernières décennies, la croissance des cernes d'arbre n'a pas répondu de manière linéaire au réchau ement climatique de grande échelle dans les Andes Patagoniennes. Ici, nous développons des chronologies de largeurs de cernes des espèces F. cupressoides et N. pumilio provenant de forêts de haute altitude du nord de la Patagonie afin : 1) d'examiner la réponse de la croissance des cernes d'arbre au climat local, 2) de détecter un possible découplage des chronologies de largeurs de cernes sélectionnées au climat local et, 3) de rechercher quelques associations entre la croissance radiale et la variabilité dde l'Oscillation Antarctique (SAM) et de l'Oscillation du Sud -El Niño (ENSO). La variabilité interannuelle du climat saisonnier et celle de la croissance des cernes d'arbre est fortement connectée à la variabilité de SAM et ENSO. Par conséquent, la stabilité temporelle de SAM et ENSO sur le climat de la région en printemps austral a également été analysée. La croissance des cernes d'arbre de F. cupressoides est négativement corrélée aux températures d'été (Décembre-Mars) de l'année précédente. La croissance de N. pumilio est positivement (négativement) corrélée aux températures (précipitations) d'été de l'année courante aux sites humides à mésiques, tandis que des relations opposées sont observées au site xérique. SAM et ENSO influencent la croissance des cernes d'arbre des deux espèces par leurs e ets sur les températures et les précipitations locales. Cependant, alors que dans les dernières décennies la phase positive du SAM a conduit à des printemps plus chauds et secs, le réajustement des téléconnexions atmosphériques n'a pas conduit à un changement significatif de l'influence de ENSO sur le climat local. Un découplage de basse fréquence est observé entre la croissance de F. cupressoides et les températures de Février de l'année précédente depuis la fin des années 1970, en partie lié à l'augmentation abrupte des températures en 1976 dû au changement de phase de l'Oscillation décennale du Pacifique (PDO, ou l'expression pluridécennale de variabilité climatique liée à ENSO dans l'océan Pacifique). La présente étude suggère une dépendance temporelle de la croissance de N. pumilio au climat local probablement liée à la variabilité de SAM ou ENSO.---------------Article in preparation

Acknowledgments This! research! has! being! supported! by! LEFE-PATISO! project! from CNRS-INSU! and! by! a! grant! from! University! of! Versailles-St! Quentin (France).! We! thank! the! IANIGLA! team! for! the! assistance! for! fieldwork,! L.! Bianchi,! M.! R.! Radins! and! M.! Rodriguez! Catón! for! providing some! updated! N.! pumilio! chronologies,! A.! Srur! for! her! helpful! attention! during! the! dendrochronological! procedures,! O.! Mestre! and B.! Dubuisson! from! Météo-France! for! assistance! with! the! HOMER software! and! two! anonymous! reviewers! for! their! constructive! suggestions! to! improve! the! manuscript. Acknowledgments This research was supported by LEFE-PATISO project from CNRS-INSU, a PhD grant from University of Versailles-St. Quentin (France), and the Australian Research Council (ARC DP120104320). We thank the IANIGLA team for the assistance during fieldwork, L. Lambs and C. Guerrido for fruitful discussion, and O. Mestre and B. Dubuisson from Météo-France for assistance with the HOMER software. We are very grateful to D. McCarroll and an anonymous reviewer for their rigorous and critical reading of an early version of the paper.

The climatic data of precipitation and temperature were retrieved from stations of the Servicio Meteorológico Nacional network, the monthly SAM data sets from Marshall [2003] (www. nerc-bas.ac.uk/icd/gjma/sam.html), the gridded monthly precipitation and temperature data from NOAA/ESRL Physical Sciences Division (http://www. esrl.noaa.gov/psd/data/gridded/data. cmap.html) and CRUTS 3.2 data set [Harris et al., 2014], and the oxygen isotope composition of precipitation from the GNIP database (http://isohis.iaea. org). The δ

Acknowledgements

This research is supported by CNRS-INSU, UVSQ (France), LSCE (France) and IANIGLA-CONICET (Argentina).

Figure .3: Poster presented at The Fourth International Asian Dendrochronological Conference On Climate Change and Tree Rings, Nepal, Katmandou, March 2015

Acknowledgements

This thesis has been made possible thanks to the collaboration and the energy of a large number of persons. February 2012 . Also to Jonathan Barichivich who introduced me to the use of the VS-Lite model and Matlab, and who suggested me to follow a new orientation for my research. The writing of my first paper has been possible thanks to him! I am also grateful to the two members of my thesis committee, Myriam Khodri and Luc Lambs, who encouraged me during my thesis and with whom I attended two conferences at both corners of the world: with Myriam at the LOTRED-PAGES conference held in Medellin (Colombia) in July 2014, and with Luc at Asiandendro conference in Katmandu (Nepal) in March 2015, and the very exotic journey that followed. All my experience in Mendoza would not has been as constructive as it has been without the presence of my flatmates Soledad Rodríguez Catón and Hugo Berón, of all the friends of IANIGLA and of the Mendoza milongueros (Javier, Oscar, Pilar, Miguel, Nazareño, Lucho, Mariano, Sebastián, Mario, Evangelina, Carlos, Cecilia, Ana, Jorge, Roberto, Romi...). On the Paris side, thank you to the GLACCIOS team, including Inga Labuhn and Jiaoyang Ruan, my o cemates for sharing hard moments (!!), Dominique Genty, Edouard Régnier and Ulrich von Grafenstein for our very friendly lunches with Emilie Dassié, Delphine Angst, Anais Orsi, Mathieu Casado, Alexandra Touzeau ... And thank you to the LSCE friends for our sportive or funny activities (Nadine Schneider, Martha Vogel, Inga Labuhn, Felix Vogel, Emmanuel Arzumanian, Jonathan Barichivich, Francesca Guglielmo, Luis Molina ...) and to the friends and relatives of Mendoza or Paris (Adèle De Palaminy, Adriana Rolón, Mae Corvalán, Jimmy Parra, Nora Zannoni, the Italians from RER B and all those I have forgotten, and the beautiful Davide Komla Ebri). Finally to complete this long list of acknowledgements, I would like to thank my family who always supported me in my wildest dreams (well for them!), my parents Viviane Sautereau and Eric Lavergne, and sisters Theodora Lavergne and Alexandra Lavergne Lecointre, but also my AFS Argentinean family that changed the course of my life (Claudia Paladino, Carlos, Marcos, Denise and Simón Lagier (still!), but also Benjamin Ybañez, Gabriel Ariza, Gabriel Ybañez, Wanda Skorupski, Marcela Magín ... ) and were my first supporters. And thanks to UVSQ and CNRS who helped finance my research. Remerciements Cette thèse a été rendue possible grâce à la collaboration et l'énergie d'un grand nombre de personnes. Tout d'abord, j'aimerai remercier ma directrice de thèse, Valérie Daux, sans qui toute cette aventure n'aurait jamais pu voir le jour. M'ayant donnée le goût de la dendroisotopie pendant mon master 2, elle a, dès le départ, été très enthousiaste pour construire un sujet de thèse en collaboration avec le laboratoire IANIGLA de Mendoza (Argentine). Pendant ces trois années, elle a toujours été d'un soutien exemplaire et indéfectible, elle a supervisé mon travail et l'a considérablement amélioré grâce à ses suggestions et conseils. Nous avons partagé beaucoup de très bons moments ensemble, que ce soit sur le terrain pendant la campagne d'échantillonnage à Bariloche (Argentine) en Mars 2013, ou au Népal pendant la conférence Asiandendro et le voyage organisé par la suite en Mars 2015, ou même pendant nos longues heures de travail pour analyser et interpréter mes résultats. Je suis aussi très reconnaissante à Ricardo Villalba, mon co-encadrant de thèse, qui m'a ouvert les portes de son laboratoire IANIGLA en 2012 pour y réaliser mon stage de master 2 avec Ana Marina Srur. Il a toujours été très motivé et enthousiaste, quels que soient mes projets pour mener à bien ma thèse. Les heures passées sur le terrain à El Chaltén en Janvier 2013 ont été mémorables. Un souvenir spécial pour la fois où nous avons eu très peur lors de la traversée d'un fleuve de montagne près de la Laguna Toro pour pouvoir accéder à une vue panoramique spectaculaire du champs de glace de la Patagonia Australe à Paso del Viento. Comme son nom l'indique, nous avons pu y sentir de front la force des vents Westerlies provenant du Pacifique, et pu contempler un paysage de glace magnifique. Je n'aurai pas pu avoir autant de données sans l'aide précieuse de l'équipe du IANIGLA pour l'échantillonnage des arbres en Mars 2013. La philosophie argentine étant tout le monde aide les projets de tout le monde, à nous tous, en moins de deux semaines, nous avons réussi à réaliser une collection de plus de 500 échantillons! Je remercie donc Ana Marina Srur, Mariano Amoroso, Eugenia Marcotti, Vero Chillo, Pierre Pitte, Emilio Bianchi, Matías Fernando, María Laura Suarez, Irene Adriana Garibotti, Ricardo Villalba et d'autres scientifiques Australiens pour leur aide durant l'échantillonnage, mais aussi Mae Corvalán, Claudia Guerrido, Clara Inés Pissolito et Ever Tallei avec qui j'ai partagé de grands moments sur le terrain en Janvier 2013, et Valeria Aschero, Martín Hadad, Mariano Morales, Lidio López, Misael Palacios, Julieta Arco et Ignacio Alberto Mundo pour leur aide pendant le développement des chronologies de largeurs de cernes au IANIGLA. Merci aussi à Lucas Bianchi, Marcos Radins et Milagros Rodríguez Catón pour le développement de versions précédentes de ces chronologies, et à Fleur Couvreux pour m'avoir bien aidé avec les donnà es météorologiques. Après ma 1ère année de thèse à Mendoza, j'ai eu la chance d'être très bien accueillie au LSCE, particulièrement par Monique Pierre et Michel Stievenard qui m'ont beaucoup aidée pour sélectionner mes échantillons, extraire la cellulose des cernes d'arbre et les analyser au spectromètre de masse. Malgré plusieurs mois de panne du spectromètre de masse, ils ont toujours été persévérants et m'ont bien secourue à la fin de ma thèse pour finaliser certaines chronologies de la composition isotopique du carbone de la cellulose. Et nous avons pu nous défouler et courir ensemble les vendredis midi dans la forêt de Saint Aubin! Un remerciement spécial à Ana Marina Srur et Mariano Amoroso qui m'ont enseignée les

Appendix

 (Beaulieu! et! al.,! 2012)! in! the! mean! of! precipitation! and! temperature! series! is indicated! by! the! vertical! lines! (1952! and! 1976).! From! top! to! bottom:! 12-month! standardized! precipitation! index! (SPI),! 12-month! standardized! soil! moisture! derived from! the! VS-Lite! model,! monthly! temperature! anomalies! relative! to! the! historical mean! (wrt! 1961-1990)! and! the! historical! (Abram! et! al.,! 2014)! and! tree-ring! reconstructed! [START_REF] Villalba | Unusual southern hemisphere tree growth patterns induced by changes in the southern annular mode[END_REF]! SAM! index.! Note! that! the! later! is! plotted! with! reversed y-axis! to! aid! comparison! with! the! local! climate! series.