Lauric Cécillon
email: cecillon@cetiom.fr

Nilvania A De Mello

Sébastien De Danieli

Jean-Jacques Brun

Soil macroaggregate dynamics in a mountain spatial climate gradient

Keywords: Mountain soils, Climate change, Soil aggregation, Soil organic matter

We investigated the response of soil macroaggregate dynamics to soil temperature modification along a spatial gradient located on a forested north-facing slope in the southern French Alps, simulating long-term adjustment of soil-plant interactions to absence or occurrence of soil frost. Soil macroaggregate (> 250 µm) content of Ah horizons was strongly depleted (72%) in colder plots affected by freeze-thaw events, compared to 96% in warmer and frost-free plots (p < 0.05). A visual assessment of soil macroaggregation showed that physical processes were the main drivers of soil macroaggregation in colder plots, with 66% of the 5-12.5 mm fraction and the whole 3.15-5 mm fraction. Conversely, we found a balanced contribution of biological and physical aggregation pathways in warmer plots. All identified macroaggregate types could be classified, depending on their organic matter (OM) quality, using principal component analyses of their near infrared spectra. Such spectral classifications indicated temporal changes in OM quality of macroaggregates, from formation to colonization by fine roots, suggesting ecosystem-specific ontogenic trajectories for soil macroaggregation. Further physico-chemical characterizations of soil macroaggregates and Ah horizons showed that soil organic carbon content in the Ah horizon was constant along the gradient, whereas soil erodibility was reduced in warmer soils, which prevented the occurrence of fragile macroaggregates formed by freeze-thaw events. Our study thus suggests changes in the erodibility of mountain forest soils under changing climate. Soil erodibility could be affected either positively under warmer conditions, or negatively, under increased soil frost.

Introduction

Mountain regions occupy about a quarter of the global land surface and provide goods and services to more than half of humanity (Huber et al. 2005). High-elevation soils are key resources which provide most of these beneficial services, supporting agro-sylvo-pastoral production systems through nutrient cycling, biodiversity, primary production and water supply. In mountain areas, rising or cooling temperatures and extreme weather events are expected to alter spatial and temporal patterns of snow cover, leading to increases in soil freezing [START_REF] Groffman | Colder soils in a warmer world: A snow manipulation study in a northern hardwood forest ecosystem[END_REF][START_REF] Edwards | Changes in the seasonal snow cover of alpine regions and its effect on soil processes: a review[END_REF]. Such changes in soil frost are likely to modify greenhouse gas emissions and exacerbate threats such as organic matter decline, soil erodibility, and decline of soil biodiversity (European Commission 2006;[START_REF] Groffman | Snow depth, soil freezing, and fluxes of carbon dioxide, nitrous oxide and methane in a northern hardwood forest[END_REF][START_REF] Cassagne | The impact of soil temperature increase on organic matter and faunal properties in a frozen calcareous scree in the French Alps[END_REF][START_REF] Matzner | Do freeze-thaw events enhance C and N losses from soils of different ecosystems? A review[END_REF]. These soil threats are closely associated with possible changes in certain soil processes under a changing climate. Although the fate of soil organic matter decomposition in cold areas and its interaction with the living soil has received much scientific attention in the last two decades [START_REF] Heimann | Terrestrial ecosystem carbon dynamics and climate feedbacks[END_REF], changes in soil aggregation process could also have a dramatic influence on each above-mentioned threat to mountain soils involving feedback to climate change [START_REF] Young | The interaction of soil biota and soil structure under global change[END_REF][START_REF] Bronick | Soil structure and management: a review[END_REF][START_REF] Matzner | Do freeze-thaw events enhance C and N losses from soils of different ecosystems? A review[END_REF].

Soil aggregation processes comprise all mechanisms involved in the binding of soil particles into stable aggregates, which are the basic unit of structure in soils [START_REF] Lavelle | Soil ecology[END_REF]. Soil aggregates mediate soil functioning in various ways which have profound effects on soil organic matter (SOM) dynamics, nutrient cycling and soil biodiversity. Aggregates physically protect soil organic matter and soil aggregation is considered the nucleus of all mechanisms of carbon sequestration [START_REF] Blanco-Canqui | Mechanisms of carbon sequestration in soil aggregates[END_REF]. Soil aggregates also regulate water flow, reduce run-off and erosion through their stability and comprises habitat for soil organisms [START_REF] Barthès | Aggregate stability as an indicator of soil susceptibility to runoff and erosion; validation at several levels[END_REF]. The complex dynamics of aggregation are the result of the interaction of many driving factors including soil management, inorganic binding agents, soil fauna, roots, microorganisms and environmental variables [START_REF] Six | A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics[END_REF]). For instance, increases in soil freezing can induce disruption of soil aggregates [START_REF] Groffman | Snow depth, soil freezing, and fluxes of carbon dioxide, nitrous oxide and methane in a northern hardwood forest[END_REF]Kvaernø and Øygarden Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com" 2006). Two main size classes of soil aggregates have been delineated [START_REF] Bronick | Soil structure and management: a review[END_REF])microaggregates (< 250 µm) and macroaggregates (> 250 µm). These size classes match the widespread aggregate hierarchy theory in which microaggregates are bound together to form large compound aggregates [START_REF] Tisdall | Organic matter and water-stable aggregates in soils[END_REF]. Different macroaggregate types can be discriminated using a non-destructive visual approach, depending on their origin (biological or physical) and their age [START_REF] Topoliantz | Earthworm and enchytraeid activity under different arable farming systems, as exemplified by biogenic structures[END_REF]Pulleman et al. 2005). Each macroaggregate type is characterized by a specific physical, chemical and biological functioning, with associated microbial activity and SOM dynamics which can be assessed using near-infrared spectroscopy (NIRS; [START_REF] Hedde | Specific functional signature in soil macro-invertebrates biostructures[END_REF][START_REF] Velasquez | This ped is my ped: visual separation and near infrared spectra allow determination of the origins of soil macroaggregates[END_REF][START_REF] Cécillon | Predicting soil quality indices with near infrared analysis in a wildfire chronosequence[END_REF]).

Hitherto, very few studies have addressed soil macroaggregation dynamics in mountain forest ecosystems. In the present study, we use a spatial climate gradient in the southern French Alps on soil macroaggregates of Ah horizons to simulate long-term effects of soil temperature modification under natural conditions. We propose a framework based on NIRS to assess the temporal dynamics of soil aggregation. Our framework is developed from the ecological concept of ontogeny of detritus described by [START_REF] Moore | Detritus, trophic dynamics and biodiversity[END_REF]. This concept incorporates the heterogeneous nature of detritus and the changes it undergoes over time during decomposition or aggregation processes, which are explicitly represented by ontogenic trajectories [START_REF] Moore | Detritus, trophic dynamics and biodiversity[END_REF]. Near infrared spectroscopy is used as an operational tool for the assessment of the heterogeneity and ontogeny of soil macroaggregates. The objectives of this study were to: (i) identify specific pathways (i.e. trajectories) for soil macroaggregation under different soil microclimate conditions using visual identification and NIRS; and (ii) investigate the relationships of each identified aggregation pathway to soil threats such as declines in SOM and soil erosion.

Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com"

Materials and methods

Site description

We performed our experiment in the southern French Alps (Dévoluy Mountains). We selected a unique study site (~ 5 ha), well protected for decades and minimally affected by human activities.

This site was chosen in order to obtain a spatial climate gradient in which most state factors were kept constant (Amundson and Jenny 1997, see below), which helps to minimize sources of error when assessing forest soil processes [START_REF] Lal | Forest soils and carbon sequestration[END_REF]. Our site was located at Pellafol (44°46' N latitude, 5°52' E longitude), at an elevation of 1250 m a.s.l. in the Sapey forest (Pellafol, Isère, France; [START_REF] Asta | Micromorphological and ultrastructural investigations of the lichen-soil interface[END_REF], on a montane limestone scree. Mean annual air temperature and mean annual precipitation at the two nearest Météo France weather stations (Pellafol -Les Payas, 930 m a.s.l. and Saint Etienne en Dévoluy -Gendarmerie, 1300 m a.s.l.) are 8.6°C, 978 mm and 6.6°C, 1116 mm, respectively.

The Pellafol site comprises (Garti 1980): (c) A sharp (30 m width on average) ecotone (EC) between PF and BF, dominated by birches (Betula pubescens), with an understory mainly composed of Vaccinium vitis-idaea and Rhododendron ferrugineum on Leptosols Eutric with a thick raw humus, similar to the soil of PF.

Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com" Previous studies of the Pellafol site have described soil microclimate as the main factor controlling the distribution of plant communities [START_REF] Lambert | La formation de glace dans les conduits karstiques, les roches fissurées et les pierriers[END_REF]Garti 1980). Air circulations between the limestone blocks create a paraglacial pedoclimate with seasonal soil frost in PF and EC [START_REF] Lambert | La formation de glace dans les conduits karstiques, les roches fissurées et les pierriers[END_REF][START_REF] Asta | Micromorphological and ultrastructural investigations of the lichen-soil interface[END_REF]. Respective mean annual soil temperature in Ah horizon at 30 cm depth (Ts) of PF and EC are -0.2°C and +0.5°C (Figure 1), as measured by data loggers (Hobo, Onset Corporation). Conversely, in BF, soil microclimate is not affected by air circulations and Ts rises to +4.8°C with no soil freezing during winter (Figure 1). Ground thermal properties of such scree slopes have been extensively studied [START_REF] Delaloye | Contribution à l'étude du pergélisol de montagne en zone marginale[END_REF]. Even though snow is deeper and persists for a longer time (from November till April-May) in PF and EC, than in BF, soil frost occurs at 30 cm depth only in PF and EC, because of air circulations, and independently of snowpack depth [START_REF] Delaloye | Thermal anomaly in a cold scree slope (Creux du Van, Switzerland)[END_REF]. Such changes in frost intensity as well as the duration of soil frost correspond to expected climate change effects in most cold regions [START_REF] Groffman | Colder soils in a warmer world: A snow manipulation study in a northern hardwood forest ecosystem[END_REF][START_REF] Matzner | Do freeze-thaw events enhance C and N losses from soils of different ecosystems? A review[END_REF]. The Pellafol site could thus be used to study the effects of soil frost on soil aggregation process under natural conditions.

We selected nine research plots (each 20 x 20 m) within the Pellafol site: three plots in PF (each one in a separate forest patch), three plots in BF and three EC plots. All plots had a similar elevation with a slope of about 30% on similar north-facing limestone scree. In agreement with [START_REF] Amundson | On a state factor model of ecosystems[END_REF], our design allowed us to hold three state factors invariant (climate, topography and parent material), two state factors nearly constant (humans and time if we assume a steady state for forest ecosystems) and two state factors which differ between PF, BF and EC: soil microclimate and organisms (vegetation and soil food web). This carefully selected spatial climate gradient reflects long-term adjustments of ecosystems to local climate conditions, thereby simulating long-term effects of soil temperature modification on their processes like soil aggregation [START_REF] Kueppers | Subalpine forest carbon cycling: short-and long-term influence of climate and species[END_REF][START_REF] Becker | Ecological and land use studies along elevational gradients[END_REF].

Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com" Sampling, pre-treatments and basic characteristics of Ah horizons We studied soil macroaggregate dynamics in Ah horizons, which is the most similar soil horizon across the gradient. In October 2006, we dug five soil pits in each of the nine plots (soil profiles of about 0.7 m width and 0.5 m depth). We carefully collected intact soil aggregates from Ah horizons, between 25 cm and 35 cm depth in each profile, with a knife and a trowel. Most gravels or stones were removed by hand. This strategy allowed for the sampling of Ah horizons with neither contamination from above organic horizons nor compaction that usually occur with core sampling. Materials sampled in the five soil profiles were further merged into a unique composite sample representative of the heterogeneity of the plot. The nine composite samples from Ah horizons were immediately stored in rigid plastic boxes so as to preserve the integrity of soil aggregates. In one of the five soil pits of each plot, we collected one undisturbed volume of soil ( 8x 5 x 3 cm 3 ) from the Ah horizon with a Kubiena box. The nine blocks of undisturbed soil were stored at 5°C before further processing.

Composite soil samples from Ah horizons were air-dried at room temperature. When necessary, clods were gently broken up along natural planes of weakness into aggregates (< 25 mm) and remaining stones, gravel and roots were removed as much as possible. Few aggregates that were disturbed due to sampling were discarded (Pulleman et al. 2005). Soil organic carbon (SOC) and total nitrogen (Nt) content of Ah horizons were determined by dry combustion (NF ISO 10694 1995;NF ISO 13878 1998). Although carbonate content of all soil samples was low as revealed by treatment with acid, complete removal of carbonates was ensured through acid washing prior to all SOC measurements. Mineral nitrogen content (N-NO3 and N-NH4) was determined following the method of [START_REF] Krom | Spectrophotometric determination of ammonia: a study of a modified Berthelot reaction using salicylate and dichloroisocyanurate[END_REF]. pH in H2O was measured according to NF ISO 10390 (2005). Cation exchange capacity (CEC) was determined by the Metson method (NF X 31-130 1999). Exchangeable cations (Ca, Mg, K, Na) were measured by atomic absorption spectroscopy after ammonium acetate extraction (NF X 31-108 2002). Base saturation (BS) was then computed as BS = 100 * [ (Ca + Mg + K + Na) / CEC ]. Soil texture was determined according to NF X 31-107 (2003). Aggregate size distribution was established using a dry sieving Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com" apparatus as described in Pulleman et al. (2005), which is a preferable technique over wet sieving for preserving the integrity of soil aggregates [START_REF] Blanco-Canqui | Mechanisms of carbon sequestration in soil aggregates[END_REF]. Aggregates were sieved for 5 minutes over a stack of sieves with mesh sizes of 12.5 mm, 5 mm, 3.15 mm and 250 µm, respectively. Macroaggregation was quantified by adding soil fractions above 250 µm.

Soil thin sections were made from the nine undisturbed blocks of Ah horizons which had been impregnated with synthetic resins [START_REF] Fitzpatrick | Forest soil carbon inventories and dynamics along an elevation gradient in the southern Appalachian Mountains[END_REF]. For each thin section, image acquisition was carried out in five windows of 20 x 20 mm with a Sony Camera DXC-930P (spatial resolution of 18 µm/pixel, spectral resolution of 256 grey levels) under a stereomicroscope using plain-and cross-polarized light. Soil porosity and stone content of Ah horizons were quantified by image analysis using ImageJ 1.38x software. Soil porosity corresponded to pixels which were white under plain-polarized light and black under cross-polarized light. Stones corresponded to white pixels under cross-polarized light.

Identification of aggregate fractions and macroaggregation pathways

We analysed macroaggregation pathways for two different size classes: 3.15-5 mm and 5-12.5 mm. We separated macroaggregates by hand under a stereomicroscope, according to the methodology described by Pulleman et al. (2005). We discriminated various morphological fractions from two main origins: a biological and a physical pathway. The biological pathway was characterized by biogenic aggregates with rounded shapes produced by soil fauna, often derived from earthworm casts [START_REF] Rusek | Soil microstructures -contributions on specific soil organisms[END_REF]. Physical pathways were characterized by physicogenic aggregates with angular shapes, produced by physical processes like wet-dry or freeze-thaw cycles [START_REF] Lavelle | Soil ecology[END_REF]. Biogenic and physicogenic aggregates were further discriminated into three classes depending on their age: young aggregates (i.e., freshly formed aggregates with clear structures and no visible macroporosity under stereomicroscope); aged aggregates (i.e., older aggregates with smoothed structures and a macroporosity visible under stereomicroscope); colonized aggregates (i.e. older aggregates strongly colonized by fine roots). Both biological and physical macroaggregation pathways were thus arranged in a chronosequence starting from Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com" freshly formed aggregates through to aged macroaggregates, and ultimately their colonization by fine roots. We further identified one category of aggregates associated with plant debris (APD).

We performed an additional assessment of aggregate fractions with NIRS so as to detect possible temporal changes in organic matter (OM) quality within each soil macroaggregation pathway [START_REF] Joffre | Organic matter quality in ecological studies: theory meets experiment[END_REF]. Soil aggregates representative of all macroaggregates samples and identified under stereomicroscope were ground (0.25 mm) to obtain homogeneous powders and scanned (1000-2500 nm) with a near infrared (NIR) spectrophotometer Antaris II (Thermo electron). Each spectrum comprised 64 averaged scans of the sample. Data were collected at a 2 nm resolution resulting in 1557 absorbance values per spectrum.

Basic characteristics of soil macroaggregates

We measured various attributes of soil macroaggregates in order to quantify the specific contribution of each aggregation pathway to soil physical and chemical functioning along our gradient. As for Ah horizons samples, we measured SOC and Nt content of all identified macroaggregate types. We also measured some mechanical properties of soil macroaggregates, which can control the mechanical behaviour of whole soil in relation to SOC sequestration [START_REF] Blanco-Canqui | Mechanisms of carbon sequestration in soil aggregates[END_REF]. Bulk density and porosity of soil macroaggregates were respectively determined by dry weight of aggregates divided by aggregate volume and by water retention weight divided by aggregate volume (n = 3 for each aggregate type). For both calculations, aggregate volume was determined by displacement of water from aggregates which were previously water-proofed with a thin layer of cyanoacrylate [START_REF] Kuczak | Inorganic and organic phosphorus pools in earthworm casts (Glossoscolecidae) and a Brazilian rainforest Oxisol[END_REF]). The susceptibility of soil aggregates to erosion (erodibility) was assessed using the method of [START_REF] Seybold | Aggregate stability kit for soil quality assessments[END_REF], which quantifies the proportion of water stable aggregates (WSA) after a correction for sand content (remaining fraction > 250 µm after wet-sieving in a solution of 10 g l -1 Nahexametaphospate). For each aggregate type, particulate organic matter (POM) content, was also quantified using a wet-sieving technique derived from [START_REF] Cambardella | Particulate soil organic-matter changes across a grassland cultivation sequence[END_REF]. This labile pool of SOM consists of partially decomposed organic debris (Garten et al. 1999) and has been Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com"

shown to be very sensitive to modifications of soil microclimate [START_REF] Link | A reciprocal transplant experiment within a climatic gradient in a semiarid shrub-steppe ecosystem: effects on bunchgrass growth and reproduction, soil carbon, and soil nitrogen[END_REF]. POM was obtained after dispersion of 5 to 10 soil aggregates in a 30 ml solution of 5 g l -1 Nahexametaphosphate, through shaking overnight. The mixture was wet-sieved through a 53 µm mesh sieve and weighed after oven drying (55°C). Sand content previously determined for aggregate stability was removed in order to obtain the final POM fraction (53 µm < POM < 250 µm).

Statistical analyses

The effects of ecosystem type and aggregation pathway on soil physico-chemical properties were assessed by analyses of variance (ANOVA) followed by Tukey's honestly significant difference (HSD) post-hoc test. Each null hypothesis was rejected at p < 0.05. Heterogeneity and temporal changes in OM quality (i.e., ontogeny) of soil macroaggregates within each aggregation pathway were assessed with principal component analysis (PCA) of NIR spectra, using first derivatives as spectral preprocessing. The unconstrained ordination of samples obtained with PCA illustrates the power of NIR data for a blind discrimination of samples based on global OM quality, without any a priori assumption on sample classes [START_REF] Cécillon | Predicting soil quality indices with near infrared analysis in a wildfire chronosequence[END_REF]. All statistical treatments were conducted using R software version 2.7 (R Development Core Team 2008) with the ade4 package for PCA [START_REF] Chessel | The ade4 package -I: one-table methods[END_REF]).

Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com"

Results

Basic characteristics of Ah horizons along the spatial climate gradient

Despite the shift in vegetation and soil microclimate from PF to BF (Figure 1), Ah horizons were similar for most soil properties assessed along the spatial climate gradient (Table 1). Heterogeneity within all ecosystem types was non-negligible for each soil attribute, especially in EC plots, as revealed by the strong standard deviation of reference data (Table 1). No significant differences could be detected for SOC, Nt, NH4, CEC, Ca, Mg, K, Na, stone, clay, silt, sand content and porosity between ecosystem types. Soil texture varied slightly from silt loam to silty clay loam across plots (Table 1). However, ANOVA tests revealed significant differences for base saturation, NO3 and pH (Table 1; p < 0.05). Base saturation was 25% higher in Ah horizon of BF; and Ah horizon of PF was characterized by a higher pH (0.5 unit increase) and N-NO3 content (2.5 mg kg -1 increase). Furthermore, strong changes in aggregate size distribution were detected along the gradient (Table 1; Figure 2). Macroaggregate content (> 250 µm) was reduced from 96% and 90% in Ah horizon of BF and EC, respectively, to 72% in Ah horizon of PF (p < 0.05). This reduction was even more striking when considering the proportion of macroaggregates above 3.15 mm, with 53% in BF, 30% in EC plots and only 9% in PF (Figure 2). Biological activity of Ah horizons also strongly varied along the spatial climate gradient. During the sampling period, Ah horizons showed an important earthworm activity in BF plots, whereas neither individuals nor indices of earthworm bioturbation could be observed in Ah horizons of PF. Earthworm activity was intermediate in EC plots.

Visual assessment of soil macroaggregation pathways in contrasted ecosystems

Within the nine research plots that included both aggregate size fractions of Ah horizons (3.15-5 mm and 5-12.5 mm), a total of 79 samples could be isolated under the stereomicroscope.

In the 5-12.5 mm fraction, we identified eight different macroaggregate types across all research plots (Figure 3), with seven, five and eight categories in BF, PF and EC, respectively. These included young, aged, colonized biogenic (BY, BA, BC) and physicogenic aggregates (PY, PA, PC) as well as APD macroaggregates. One category of macroaggregates, never described in cold mountain soils (fragile rounded aggregates; FRA; Figure 3) was found in Ah horizons of PF and EC. Even though the round shape of FRA resembles biogenic aggregates, we hypothesized that due to weak stability, they were generated by physical processes like freeze-thaw cycles which characterize the soil microclimate of PF and EC (Figure 1). Biological and physical pathways were well-balanced in BF (35% biogenic and 54% physicogenic aggregates) and EC (49% of biogenic and 43% physicogenic aggregates), the remaining being APD in BF, APD and FRA aggregates in EC. Conversely, physical processes overcharacterized soil aggregation in Ah horizons of PF with 61% of FRA and 5% of physicogenic aggregates.

The 3.15-5 mm fraction of Ah horizons was characterized by a smaller diversity of soil aggregates (young and aged biogenic, physicogenic and FRA). Again, biological and physical pathways were equally represented in BF (53% biogenic and 47% physicogenic aggregates) and EC (49% biogenic; and 48% physicogenic aggregates) with the remaining being FRA aggregates in EC. Conversely, soil aggregates from Ah horizons of PF were entirely formed by physical processes (i.e., 87% of physicogenic and 13% of FRA aggregates).

Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com" NIRS assessment of the heterogeneity and the ontogeny of soil macroaggregates Only samples with a minimum dry weight of 1 g could be analysed with NIRS. As a consequence, NIRS measurements were performed for 74 out of the 79 macroaggregate samples visually identified. PCA of NIR spectra were carried out by ecosystem type in order to find specific spectral fingerprints for each aggregate categories, and ontogenic trajectories of soil macroaggregation.

Factorial projections of NIR spectra of soil aggregates were strongly plot-dependent within each ecosystem type, as depicted in Figure 4a representing spectral fingerprints of soil macroaggregates and Ah horizons samples from BF plots. In order to tackle this analytical problem already pointed out by [START_REF] Velasquez | This ped is my ped: visual separation and near infrared spectra allow determination of the origins of soil macroaggregates[END_REF], we tested a new strategy of wavelength selection within NIR spectra inspired from a recent work of [START_REF] Cécillon | Variable selection in near infrared spectra for the biological characterization of soil and earthworm casts[END_REF], so as to remove the plot effect from the PCA. Variable selection was based on the loadings matrix of the PCA. The loadings can be understood as the weights for each original variable (i.e., wavelengths) when calculating the principal components. We removed NIR wavelengths with loadings higher than 0.3 and 0.5, respectively for the first two principal components which were associated with plot effect. Then, a new PCA was performed with the selected wavelengths. Figure 4 illustrates the usefulness of this strategy on the NIR spectral data of BF. NIRS was found suitable for discriminating soil macroaggregates of biogenic and physical pathways from APD aggregates and whole Ah horizon samples (Figure 4b). Eliminating those spectral outliers from the analysis provided a principal component space in which aggregates from biological and physical pathway could be reasonably separated (Figure 4c). Negative values on the first axis (76% of total variance) of the PCA corresponded to BY, BC and PC aggregates, whereas BA, PY and PA aggregates were characterized by positive values on this axis, illustrating the heterogeneity of OM quality of the identified macroaggregate types in a given ecosystem (reasonable results were also obtained for ecotone and pine forests; data not shown).

We also performed PCA of NIR spectra of aggregates from single BF and EC plots (not from single PF plots because too few samples were available), so as to obtain a more detailed representation of the ontogeny of aggregation (Figure 5). Tentative ontogenic trajectories for soil Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com" macroaggregation based on NIRS are proposed in Figure 5. In BF plots, similar trajectories are observed for biological and physical aggregation pathways (except at the formation of biogenic and physicogenic aggregates) with strong differences between the spectral fingerprints of BY and PY aggregates (Figure 5a). Biological and physical trajectories of macroaggregation were much more distinct in EC plots (Figure 5b). The spectral fingerprint of FRA aggregates was found similar to that of physicogenic aggregates, supporting a physical origin for this macroaggregate type (Figure 5b). These proposed ontogenic trajectories for soil aggregation suggest strong temporal changes in OM quality within both biological and physical pathways of soil macroaggregation, from young aggregates through to aggregates colonized by fine roots.

Basic characteristics of soil macroaggregates along the gradient

We measured six attributes relevant to soil physico-chemical functioning on 70 aggregate samples out of the 79 samples visually identified in Ah horizons of our spatial climate gradient (Table 2).

FRA aggregates had very original physical properties compared to all other ones as revealed by ANOVA tests (Table 2). They were characterized by a lower WSA (p < 0.001) and a higher porosity (p < 0.01) than all other macroaggregate types. FRA and APD aggregates also showed a lower bulk density than biogenic and physicogenic aggregates (p < 0.001) but this characteristic was in fact associated with macroaggregates from PF plots. No differences for SOC, Nt and POM content could be found between aggregation pathways, because of strong variations within each pathway and between ecosystem types. POM content was higher in soil macroaggregates of PF plots than in those of EC and BF plots (p < 0.05). Nt content was lowest in the macroaggregates of EC plots (p < 0.05). No effect of ecosystem type was observed on SOC content of soil macroaggregates. Physical and biological pathways showed similar characteristics for all physico-chemical properties assessed (Table 2). When examining more precisely physical and biological macroaggregation pathways in BF plots, some differences for POM content appeared. BY and BC aggregates were enriched in POM, whereas BA aggregates had a weak POM content (p < 0.05). Conversely, POM content of all physicogenic aggregates was almost constant. These results could show a preferential way of utilization of BY aggregates by soil microflora, meaning that the stocked POM is cycled more rapidly within these aggregates.

However, this trend was not observed in EC plots, under colder conditions.

Regarding soil threats, all macroaggregate types showed similar response for organic matter decline with high SOC content (92 g kg -1 on average) and similar Nt and POM content (Table 2), suggesting no long-term effect of soil microclimate and vegetation on soil organic matter content of macroaggregates. FRA aggregates, which dominated the aggregate size fraction above 3.15 mm of Ah horizon of PF plots, were characterized by a very low aggregate stability compared all other macroaggregate types, suggesting a stronger erodibility of Ah horizon in PF plots probably induced by freeze-thaw cycles.

Discussion

Changes in properties of Ah horizons along the spatial climate gradient Holding most state factors constant in our design provided a sound basis for assessing the impact of long-term changes in soil microclimate and vegetation, on soil properties and processes.

Despite the long-term and strong increase in Ts (5.0°C) from PF to BF, SOC content was constant along the gradient as well as many other properties of Ah horizons, like soil texture. While a constant soil texture might be explained by the homogeneous parent material of the Pellafol site (limestone), a constant SOC content in the Ah horizon contrasts with the much thicker organic layer observed in colder plots which might reflect a greater accumulation of labile SOC in PF and EC, in agreement with [START_REF] Kammer | Treeline shifts in the Ural mountains affect soil organic matter dynamics[END_REF], who reported such SOC accumulation in the organic layer of colder soils, and [START_REF] Hagedorn | Short-term responses of ecosystem carbon fluxes to experimental soil warming at the Swiss alpine treeline[END_REF], who showed increased SOC losses in experimentally warmed soils. But our results support those of [START_REF] Leifeld | Storage and turnover of carbon in grassland soils along an elevation gradient in the Swiss Alps[END_REF], who observed no significant trend for SOC concentration in soil horizons deeper than 5 cm along an elevation gradient.

Conversely, some properties of Ah horizons were strongly modified from PF to BF, like N-NO3 or macroaggregate content. Regarding N-NO3 content of Ah horizons, our results support previously published studies, which have described higher N-NO3 content and leaching from plots affected by freeze-thaw cycles [START_REF] Groffman | Colder soils in a warmer world: A snow manipulation study in a northern hardwood forest ecosystem[END_REF]. This increase is thought to be generated by a higher mortality of fine roots under soil frost, which induces a reduction in plant uptake and a strong input of labile OM in soil, thereby increasing nitrification. Denitrification is also usually enhanced, so that freeze-thaw events may turn cold soils into sources of N2O, a strong greenhouse gas [START_REF] Groffman | Snow depth, soil freezing, and fluxes of carbon dioxide, nitrous oxide and methane in a northern hardwood forest[END_REF][START_REF] Repo | Large N2O emissions from cryoturbated peat soil in tundra[END_REF]. Our results suggest that frost intensity may control this process, as it only occurs in PF, under the harshest soil frost (Figure 1), and not in EC plots.

Regarding macroaggregate content, [START_REF] Boix-Fayos | Spatial and short-term temporal variations in runoff, soil aggregation and other soil properties along a Mediterranean climatological gradient[END_REF] showed an increased aggregation on north-facing slopes compared to south-facing slopes in Mediterranean soils due to vegetative differences caused by a colder soil microclimate. Conversely, macroaggregate content was enhanced in warmer plots of our gradient, with a spectacular increase from PF to EC, associated with the small but long-term increase in Ts of EC plots (+0.7°C). These results suggest a strong temperature sensitivity of soil aggregation process at low temperatures. The increased macroaggregation under milder soil microclimate along our gradient was linked to higher earthworm activity, which underlines the role of earthworm bioturbation in the formation of soil structure [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF][START_REF] Lavelle | Soil ecology[END_REF]. The occurrence of bioturbation in warmer plots could reflect either a direct or an indirect effect of soil temperature on earthworms (soil microclimate may indirectly control earthworm activity through changes in the quality of above and below-ground OM inputs, leading to contrasted humus forms under different vegetation cover; [START_REF] Aerts | The freezer defrosting: Global warming and litter decomposition rates in cold biomes[END_REF]). Therefore, our results and those of [START_REF] Boix-Fayos | Spatial and short-term temporal variations in runoff, soil aggregation and other soil properties along a Mediterranean climatological gradient[END_REF], suggest the existence of optimal temperature and vegetation conditions for soil aggregation in mountain ecosystems.

NIRS as an operational tool for assessing soil macroaggregation dynamics A wide diversity of macroaggregate types, differing in their origin and their age could be visually identified on the Pellafol site. NIR spectra of aggregates can be considered as global fingerprints of their OM quality [START_REF] Joffre | Organic matter quality in ecological studies: theory meets experiment[END_REF][START_REF] Hedde | Specific functional signature in soil macro-invertebrates biostructures[END_REF]. Using a specific method for processing NIR spectral fingerprints (wavelength selection based on the loadings matrix of the PCA), we could overcome the plot effect and point out heterogeneities in the OM quality of various macroaggregates types in each ecosystem, as previously shown by [START_REF] Hedde | Specific functional signature in soil macro-invertebrates biostructures[END_REF] and [START_REF] Velasquez | This ped is my ped: visual separation and near infrared spectra allow determination of the origins of soil macroaggregates[END_REF]. In addition, ecosystem-specific ontogenic trajectories for soil macroaggregation could be proposed for the first time, based on this spectral classification of aggregates. Such trajectories illustrate temporal changes in OM quality of soil macroaggregates from their formation through to their colonization by fine roots. These results with NIRS may open the way for further investigations of detritus dynamics in soils, as claimed by [START_REF] Moore | Detritus, trophic dynamics and biodiversity[END_REF].

However, as our technique failed to classify similar soil macroaggregate types from different ecosystems, further developments of NIR spectral processing remain.

Functional implications of soil macroaggregation along the spatial climate gradient

The proportion of biological and physical pathway of aggregation varied significantly along our spatial climate gradient. The slight increase in Ts (and decrease in frost intensity) from PF to EC increased soil macroaggregate stability and strongly enhanced the contribution of the biological pathway to soil macroaggregation in both aggregate size fractions we investigated. Long-term warming of seasonally frozen soils it seems could thus improve their resistance to erosion through enhancing biological macroaggregation. This result corroborates previous studies which reported an increased erodibility of soils subjected to freeze-thaw cycles [START_REF] Groffman | Snow depth, soil freezing, and fluxes of carbon dioxide, nitrous oxide and methane in a northern hardwood forest[END_REF][START_REF] Kvaernø | The influence of freeze-thaw cycles and soil moisture on aggregate stability of three soils in Norway[END_REF].

Regarding POM content of soil macroaggregates, the depletion observed in BA aggregates from BF plots may indicate a priming effect of earthworms on microbial decomposition during casts formation [START_REF] Lavelle | Soil ecology[END_REF]. BY aggregates (young earthworm casts) are enriched in POM and intestinal mucus, which stimulate microbial respiration and may lead to depletion in POM content of BA aggregates. New inputs of fresh organic matter are further provided by fine roots in BC aggregates, enhancing their POM content. Such priming effects are not observed in soil biogenic macroaggregates of EC plots because of their colder microclimate.

We observed a decrease in POM content of macroaggregates from warmer plots (BF and EC, compared to PF), similar to reports by [START_REF] Link | A reciprocal transplant experiment within a climatic gradient in a semiarid shrub-steppe ecosystem: effects on bunchgrass growth and reproduction, soil carbon, and soil nitrogen[END_REF] and [START_REF] Leifeld | Storage and turnover of carbon in grassland soils along an elevation gradient in the Swiss Alps[END_REF], albeit no depletion in their SOC content could be detected along the gradient. A possible explanation for this result might be that increased temperature in Ah horizons may transfer increased amounts of SOC to more stable soil C pools, probably because of accelerated physico-chemical stabilization reactions [START_REF] Jones | Carbon sequestration in temperate grassland ecosystems and the influence of management, climate and elevated CO2[END_REF]. We suggest that enhanced biological process like microbial respiration in warmer plots may counterbalance this phenomenon, thereby stabilizing SOC content.

Conclusion

Climate change in mountain regions could increase the intensity, frequency and duration of soil frost through the alteration of snowpack depth. Using an original spatial climate gradient on limestone scree slopes, which simulated various soil microclimate conditions, we showed a strong response of soil aggregation processes to the experimental design. We observed profound changes in the heterogeneity and the temporal dynamics of soil macroaggregates, reflecting modification of their OM quality. These changes in soil macroaggregation dynamics were associated with modifications of certain soil attributes of Ah horizons related to soil threats. Our results did not provide evidence for SOC content decline under warmer conditions, albeit a decrease in labile SOM pool was observed. Conversely, we identified a potentially harmful effect of changing climate on mountain soils: increased soil frost may lead to strong decreases in soil biological macroaggregation and soil structural stability, thereby enhancing soil erodibility. 12) 91 ( 13) 93 ( 7) 58 ( 17) POM (%) 18 ( 7) 18 ( 6) 24 ( 8) 22 (10)

Abbreviations: SD = standard deviation; WSA = water stable aggregates; POM = particulate organic matter Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com"

  (a) Three open forest patches (0.5 ha each) of dwarf mountain pines (PF; Pinus uncinata). The understory of PF is dense and typical of cold areas with Empetrum nigrum, Vaccinium vitis-idaea, Dryas octopetala and a dense cover of lichens on Leptosols Eutric (IUSS Working Group WRB 2006; Legros 2007), with a very thick raw humus (Humus form = Tangel-Mor; Thickness of OF+OH > 20 cm; Brêthes et al. 1995; Cassagne et al. 2008). (b) A beech-fir forest (BF; Fagus sylvatica and Abies alba), surrounding PF patches. The understory of BF is sparse and typical of a mountane forest, mainly composed of Vaccinium myrtillus and mosses on Leptosols Eutric (IUSS Working Group WRB 2006; Legros 2007), with a shallow humus layer (Humus form = Moder; Thickness of OF+OH < 5 cm; Brêthes et al. 1995).

FiguresFig. 1

 1 Figures

Fig. 2

 2 Fig. 2 Mean aggregate size distribution (%) after dry sieving of Ah horizons along the spatial climate gradient (n = 3 per ecosystem type). Segmented lines represent standard deviations Abbreviations: PF = pine forest; BF = beech-fir forest; EC = ecotone

Fig. 3

 3 Fig. 3 Examples of the different macroaggregate types separated from the 5-12.5 mm size fraction: (BY) young biogenic; (BA) aged biogenic; (BC) colonized biogenic; (APD) associated with plant debris; (PY) young physicogenic; (PA) aged physicogenic; (PC) colonized physicogenic; (FRA) fragile rounded aggregates. All aggregates shown here are from BF plots, except FRA which is from PF Abbreviations: BF = beech-fir forest; PF = pine forest

Fig

  Fig. 4 (a & b)

FigFig. 4

 4 Fig. 4 (c)

Fig. 5

 5 Fig.5PCA of NIR spectra of soil macroaggregates from one research plot of BF and EC. Arrows indicate proposed ontogenic trajectories for biological (grey) and physical (black) pathways of soil macroaggregation (similar results were obtained within plots of each ecosystem type). The meaning of symbols is detailed in Figure4Abbreviations: BF = beech-fir forest; EC = ecotone

Table 2

 2 Effect of aggregation pathway on physico-chemical properties of soil macroaggregates

			Soil macroaggregation pathway
		Biological (n = 29) Physical (n = 27)	APD (n = 8) FRA (n = 6)
	Soil Property	mean (SD)	mean (SD)	mean (SD) mean (SD)
	Organic carbon (g kg -1 )	87 (25)	90 (37)	111 (30)	92 (43)
	Total nitrogen (g kg -1 )	4.1 (1.2)	4.2 (1.5)	5.0 (1.4)	4.7 (2.0)
	Porosity (%)	38 (7)	36 (4)	39 (6)	46 (3)
	Bulk density	0.65 (0.14)	0.61 (0.13)	0.47 (0.10) 0.45 (0.16)
	WSA (%)	94 (

Biogeochemistry 97: 31-43 (2010) http://dx.doi.org/10.1007/s10533-009-9341-9 -"The original publication is available at www.springerlink.com"

Acknowledgements

This work was supported by the French Agency for Environment and Energy Management (ADEME) and Cemagref. Authors are grateful to participants of the conference "Mountain soils under a changing climate and land-use" (WSL, Birmensdorf, Switzerland, 6-8 March 2008) organized within the framework of the COST Action 639 (http://www.cost639.net), and to Frank Hagedorn (WSL, Birmensdorf, Switzerland) and two anonymous reviewers, for their valuable comments. Roy V. Rea (University of Northern British Columbia, Canada), is thanked for improving the English of the manuscript.

Tables

 Table 1 Summary


Aline Durand
email: adurand@mmsh.univ-aix.fr

ELEMENTS FOR A CULTURAL HISTORY OF WOOD IN SOUTHERN FRANCE (X TH -XVI TH CENTURIES)

Up to now, the study of the role of wood in rural architecture from southern France, during the Middle Ages, has remained a marginal investigation subject, as stone has always been considered as omnipresent. Charcoal analysis of building remains allows us to prepare the ground for an history of wood during the Middle Ages. The spectrum of woods used in southern rural buildings is quite large as shown by taxa recognized: Castanea sativa, deciduous mediterranean Quercus, Fraxinus sp., Fagus sylvatica, Ulmus minor, Salix-Populus sp., Pinus type sylvestris, Abies alba. However, and according to our study, from the year 1000 until the beginning of modern times, only one species is mainly used, for building purposes, as a result of technical, economical, and biogeographical reasons -deciduous oak. The other species are used very occasionally, to answer specific needs, or as a result of their abundance and proximity. In fact, all species used were growing in the vicinity of the settlements: the use of timber from further afield has not been documented. Villagers, builders or other specialist workers adapt their needs to the local possibilities: they choose the most suitable woods for construction but also the cheapest ones. Certain observations of wood transverse sections and measurements of wood diameter prove that specimens have also been carefully chosen. Sometimes, wood management strategies seem to have been developed. Further charcoal analyses will hopefully support and enrich these first data and a better understanding of wood uses during the medieval period will be achieved.

In memory of Jean-Marie Pesez

. In this perspective, it appears natural to the supporters of material culture, to attract interest to the building materials of the peasant's house. In France, this subject has been gradually integrated into archeological medieval investigations. Important changes have taken place between the first meeting of Besançon in 1972 (Besançon, 1973) and the recent synthesis One hundred medieval houses (Esquieu et Pesez dir. 1998), as the scientific community lost interest in the monumental construction and recognized the importance of simple soil remains. The woody remains of rural architecture are often found charcoalified during excavation work: laths, beams, battens, frames, fences, even agricultural tools and so on. Charcoal analysis of such artefacts contributes to a better understanding of how plant species were used, of forest management and, occasionally, of conditions of transport.

At first sight, the role of wood in rural architecture of the southern regions seems very marginal, as stone has been considered as an omnipresent building material. In fact, this is not true. First of all, written documents from the 12 th century mention wooden houses, i. e. completely built with wood, both in Camargue, and in the hills of the 'Biterrois' (area around Béziers) [START_REF]Les paysages médiévaux du Languedoc (Xe-XIIe siècles)[END_REF]. Also, during the last decade, excavation work carried out in southern France uncovered hut floors dating back to the VIIIth-XIIth century entirely built with perishable materials both in rural contexts and urban contexts (Ginouvez 1995[START_REF] Garnier | -De la ferme au village : Dassargues du Ve au XIIe siècle (Lunel, Hérault)[END_REF], Mercier et Raynaud 1995[START_REF] Schneider | -Saint-Sébastien-de-Maroiol (34) et l'histoire de la proche campagne du monastère d'Aniane (V-XIIIe s[END_REF]. The use of wood as building material seems to have been more important than previously thought by historians.

I.-SOUTHERN CHRONO-TYPOLOGY (X TH -XVI TH CENTURIES) (TABL.1, FIG 1-2)

1.-The first feudal fortified sites (X th -XII th centuries)

Up to now, fortified sites from the first feudal age provided very few samples of this kind. As a consequence their study appears particularly interesting.

A study of this kind has been carried out at Le Mourrel, a feudal moat and bailey castle at Molleville, in the 'Lauraguais' (Aude,11). On the summital platform, 19 pieces of charcoal were found inside a post hole. All of them were identified as deciduous oak.

East of Forcalquier (Basses-Alpes, 04) in the French Prealps, the moated site of Niozelles was also recently excavated. It may be included in the generation of the first feudal fortifications as one of the occupation phases has been dated X th -XI th century. Several phases of occupation have been distinguished by the excavation work. The charrred woods are issued from layers concerning the transformation of the site : they belonged to the wooden outhouse near one of the habitation structures situated on the top of the moated site. Charcoal analysis results are based on the identification of 303 charcoal specimens from this outhouse. 298 of them were identified as deciduous mediterranean oak (Quercus type pubescens). The specific discrimination of deciduous oaks is very difficult, often quite impossible, based on wood anatomy alone. However, and based on site ecology and biogeography, the samples recognized here as deciduous oak are refered to as mediterranean deciduous oak (Quercus type pubescens).

In the Buech valley in the 'Gapençais' (Hautes-Alpes, 05), the rural dwelling of Oze provided charred elements of a wooden construction. These are considered to be remains from beams or battens found on the occupation soil dated Xth-XIIth. Charcoal fragments have been photographied and drawn in the site. The study of 482 fragments allowed us to identify 10 different taxa. Most of the fragments have been assigned to four species: Pinus type sylvestris, Salix sp., Fagus sylvatica and Quercus type pubescens. Two major results can be pointed out and corroborate archeological observations. First, charcoal analysis has identified only very few taxa (10). This relative specific poverty and the dominance of only four taxa (beech, oak, pine and willow) is characteristic of an ethnobotanical spectrum, not of a paleoenvironmental one. Anthracological data agree with the archeological data in considering these remains as vestiges of a building, not of domestic fires. Secondly, clear traces of wood work have been noticed : cut in edge, quadrangular cut with characteristic decentring of wood medulla. The gymnosperms (pine and fir) present anatomical deformations of ligneous tissues. Close to the growth ring boundary, ligneous rays are deviated obliquely in earlywood and tracheids compressed but without rupture. Latewood appears often unchanged; however, in some cases cellular bands of latewood are also deviated obliquely. According to I. Théry (Théry 1983), this kind of deformation is classified "type 1b" and is not a result of carbonization nor specific growing conditions. At Oze, these features are very rare and observed on soft wood only : maybe the wood had supported a heavy load. These data may provide evidence that we are dealing with artifacts, not with scattered charcoal of domestic fires.

So, in these three southern sites dating back to c. 1000 AD, deciduous oak is the major species used as timber. Tabl. 2 Observation of charcoal transverse sections at Niozelles (X-XI th )

Fig 1 Location of sites in Languedoc

2.-The 'castral' villages (XIII th -XIV th )

Villages are now well established and developing ; no changes in the choice of building material is noticed. Previous anthracological spectra concerning timbers of sites occupied during the middle feudal period are not different from spectra based on early feudal sites.

At the moated village of Laval-Basse (Aude, 11), destruction levels, composed of yellow clayey soil, stones, tiles, charcoal and broken artifacts, have been identified. The study of 45 charcoal specimens allowed us to identify Quercus type pubescens, Quercus type sessilifora-pedunculata, Salix sp., Sorbus sp., Ulmus minor and Vitis vinifera. In one structure (n° 371) big pieces of deciduous oak may be considered as remains of carpentry work. In this site, and based on wood anatomy features, two different types of deciduous oak may be distinguished : one, mediterranean, with earlywood vessels arranged in only one layer and rounded cross section up to 200-300 µm, the other, atlantic, with earlywood vessels arranged in 2 or three layers and elliptic cross section up to 300-400 µm.

In Cabaret, one of the four castles of Lastours (Aude, 11), excavations have taken place in the lower part of the village, near the river. One of the buildings, dated XII th century, provided a concentration of large pieces of charcoal. At first, these remains were believed to come from a beam. However, further research showed that they belong to a single ligneous artifact. This object had a round or oblong shape with a concave side which provide evidence of a carpenter or joiner's work. The study of 582 charcoal fragments testifies to the great importance of the deciduous mediterranean oak.

In the most westerly point of the Black Mountain, at Castlar of Durfort (Tarn, 81), archeologists have uncovered a store-house, dated end of XIII th century / beginning of the XIV th century. All the harvested grain was in place and stocked by heaps (Ruas, sous presse). The charcoal fragments analysed are issued from the timbering of the structure which had fallen down as a result of a fire. This charcoal concentration was covered by a layer of tiles. One of the beams had nearly ten centimeters of diameter. Again, deciduous oak (Quercus type pubescens or Quercus type sessiliflora-pedunculata) is the major taxon among the 1497 charred fragments identified. Fagus sylvatica, Corylus avellana and Salix sp. are also present. The discrete, but clear presence of these taxa is believed to result from repairs or minor additions. They are considered marginal in the whole of the timber structure.

3.-Modern Times

Data from modern times confirm the predominance of deciduous oak in the southern peasant architecture. At Cabrières (Hérault, 34), an ancient castral site, new dwellings are built at the bottom of the hill. In the sixteenth century, several houses are aligned along a little street. Remains of wooden beams or battens have been discovered in one of them, under a layer of tiles. Some of them present very clear traces of nailing. Nails have actually been found in place. Among the 481 charcoal pieces analyzed, 395 have been identified as deciduous mediterranean oak. We must underline, however, that batten were also made out of Quercus ilexcoccifera, Ulmus campestris, Fraxinus ornus-oxyphylla and Castanea sp. This last species had never been found before: it is the first sure attestation of medieval chestnut used as timber in France. Castanea sativa has not been identified as timber wood neither in the Bassin Parisien (Dietrich 1989) nor in Roman time in southern France (Chabal 1997).

Conclusion

Anthracological studies concerning wood at its uses in medieval rural construction have provided the first significant results that must be enriched and refined by further work. It is clear that between the beginning of the 10 th century and the end of the sixteenth century, deciduous oak, and not chestnut, as usually refered to in the literature, is chosen as building material, in the southern medieval villages. Our results clearly show that, in spite of its mechanical and physical properties, Castanea sativa is not used as timber during the Middle Ages. Timbers in medieval buildings, believed to belong to Chestnut wood are, in fact, an illusion. O. Rakham (Rakham 1980) provides a very simple explanation : the two genera have very similar wood anatomical features (just medullary rays discriminates Castanea sativa from deciduous Quercus) and may be easily mistaken. The following question has to be considered now : why is deciduous oak dominant ?

II.-THE MEDIEVAL TIMBERS IN SOUTHERN AREAS : FROM FELLING TO USE (X TH -XVI TH CENTURIES)

Some anatomical characters observed during identification allow us to understand the uses of medieval timber and the 'forest culture' of the peasant population.

1.-The dominance of deciduous oak

During medieval times, the constant use of deciduous oak in rural buildings from southern areas may be easily explained. Its technical and physical qualities are well known: the very hard wood of deciduous oak does not warp, can bear pressures, flexions, crushings and offers very long monoxylem bearings; it also resists well to bad weather, humidity and time; last, but not least, its duramen, very rich in tannins, protects the wood against fongic and insect attacks. Because of these properties, this tree provides an excellent wood for a carpenter's and joiner's work. But these qualities alone are not sufficient to explain the systematic use of oak. The possibilities offered by environmental conditions must also be considered. In fact, deciduous oak is the key-taxon for the different vegetation levels (mesomediterranean, supramediterranean) of all the sites studied. It is in the vicinity of their villages that peasants and craftsmen select the most suitable species for construction. This is also the cheapest choice: gathering of wood stocks from neighbouring areas avoids costly and long transport. For castral sites isolated and perched, it was an indispensable condition. In the plains also, where villages are situated near markets and integrated in commercial and economical relationships, the same choice has been effectuated (even in the XIV th -XVI th centuries).

Anthracological diagnosis also reveals that builders did not use any kind of oak tree. The meticulous observation of charcoal transverse section proves it. At Le Mourrel, the average width of growth rings is 3 mm which results from a normal growing rate. In this case, wood may have little value due to lower technological qualities (cutting up, work), but, on the other hand, it has good mechanic properties : growth rings widths did not present significant variations, which is an important quality standard [START_REF] Venet | Identification et classement des bois français[END_REF].

At Oze, traces of squaring had been clearly observed. However the tool used to carry out this work left no characteristic traces on remains. The same happens at Niozelles (tabl.2), but further information is obtained. Usually, almost all vessels of the deciduous oak are obstructed with tyloses which, when not traumatic, are characteristic of old woods. Growth ring arches are not very marked which may indicate the presence of trunks rather than branches. It is possible that wood was collected in forest stands and mature formations. Some charred fragments were very well preserved and allowed further conclusions. At Niozelles, the orientation of ligneous fibers and growth rings boundaries towards the orientation axis of artifacts are characteristic of a specific way of carving wood known as " cutting up on slabbing off-cut ". This kind of wood work consists in cutting the wood along the trunk's length instead of following the rays. For J.-Y. Hunot [START_REF]Charbons de bois recueillis à l'intérieur de vases funéraires. Approche ethnobotanique. Les charbons de bois, les anciens écosystèmes et le rôle de l'homme[END_REF], this procedure is characteristic of sawing with parallel traces of a saw. This agrees with the interpretation put forward by the archeologist, D. Mouton, according to whom these are remains of timber (beams, butten....). At Cabrières, in the XVI th century, the quality of charcolified pieces is such that an estimation of wood sections and sizes has been effectuated. The observation and measure of growth rings curvature make it possible to calculate minimal diameters (Fig. 3). This is obviously an estimation because the charcoal pieces may come from a bigger trunk or be warped while on fire. Whenever diameter estimations were made possible, the size of oak pieces was always the most important. In the same way, construction remains made out of deciduous oak have bigger dimensions. In the Mediterranean area, this tree can easily grow up to twenty meters high providing that the formation is not treated as coppice or coppice under forest : in this last case, the shoots are small (under 10 m high) and have a middling section. At Cabrières, bark came either from "spring from coppiced stools" with 15 or 20 years between each cutting or from a protected vegetal formation treated as fully-grown trees.

All these observations emphasize the idea that timber had been carefully chosen and seriously examined. Therefore, peasants adapted their needs to their environmental possibilities: either they had preserved the forest or they developed strategies of forest management to obtain long bearings.

2.-The other timbers

Anthracological analysis of timber remains show that the remaining species used for building appear only occasionally. They are not used in the majority of artifacts ; they are not recognized in all archeological sites analysed. In sites close to the biogeographic mountainous level, conifers are present: at Oze, Abies alba and Pinus sp. are identified while at Mézenc (Ardèche, 07), only Abies alba has been found for the moment. Compression wood has been observed in nearly all the samples : the alpine conditions may explain the formation of these tissues. In southern sites, the presence of Fagus sylvatica, Ulmus minor, Salix sp. and Castanea sativa is attested. Important frequencies of riverside taxa are noted. These species might have been carefully selected. They provide good wood and very long monospecific bearings. Ulmus minor and Fagus sylvatica are suitable for the heavy structure of buildings, but not for the roof-piece : at Cabrières, trees assigned to these species seem to have reached smaller average dimensions than deciduous oak. Under Mediterranean climatic conditions, beech wood becomes heavy and hard (classes 7,8,9), and can be treated as a timber [START_REF] Venet | Identification et classement des bois français[END_REF]. On the other hand, willow and poplar are very soft woods, inadequate to the mechanical needs of heavy building. Their selection might have answered specific and punctual needs, such as minor armature, pegs, repairs, penury, mortise-andtenon.. The flexible wood of ash, with fine texture, is usually chosen for this last coupling. The wood of Mediterranean ash is dense (classes 7 and 8), and therefore hard and may be used in carpentry [START_REF] Venet | Identification et classement des bois français[END_REF].

At Niozelles, three other species were identified: Buxus sempervirens and Sorbu sp.-Crataegus sp. These woods have a short durability but are hard enough to be used in joiner's work or cabinetwork. However, hawthorn is marginal in this use. The three samples identified as Rosaceae are well cut: the section (2 cm) is quadrangular, 2-3 cm long, in the shape of a peg. Buxus sempervirens has a good durability and is a very hard wood. These remains are smaller than those of deciduous oak. The use seems specific and not usual, probably to join together other different pieces or to complete a structure.

Conclusion

The spectrum of woods used in southern rural buildings is quite large as shown by taxa recognized: Castanea sativa, Quercus deciduous, Fraxinus sp., Fagus sylvatica, Ulmus minor, Salix sp.-Populus sp., Pinus type sylvestris, Abies alba. However, despite this relative diversity, one species is mainly used as a result of

Fig 2

 2 Fig 2 Location of sites in Provence

Fig. 3

 3 Fig. 3 Minimal diameter of timbers at Cabrières (XVI th ).

Xth-Xth c. XIth-XIIth c. XIth-XIIth c. XIIth c. XIIIth-XIVth c. XIIIth-XIVth c. XIVth c. XVIth c. TAXA Niozelles Oze Le Mourrel Cabaret Laval-Basse Laval-Basse Le Castlar Cabrières

		Remains of	Remains of	Post	Ligneous	Levels of	Remains of	Timbering	Remains of
		construction	construction	hole	artefact	destruction	structure	of store house	construction
	Abies alba			11
	Acer sp.					4
	Acer monspessulanum			4
	Angiospermae	1
	Buxus sempervirens	1
	Castanea sativa-Quercus pubescens								9
	Castanea sativa									52
	Corylus avellana					2				2 4
	Crataegus sp.-Sorbus sp.	3
	Erica sp.								1
	Fagus sylvatica			130			1			19
	Ficus carica						1
	Fraxinus sp.			2
	Fraxinus oxyphylla.									5
	cf. Juniperus sp.			1
	Juniperus sp.			2
	Leguminosae			6		1
	Ligustrum vulgare									5
	Pinus type sylvestris			53
	cf. Populus sp.						1
	Populus sp.						2
	Prunus sp.						1
	Prunoïdeae type Prunus avium			18
	Prunus avium						1
	Prunus amygdalus						1
	Prunus cf. amygdalus						1
	Prunus spinosa						1
	Quercus ilex-coccifera					14				16
	Quercus deciduous				19		15			1497
	Quercus sp.-Castanea sativa			1
	Quercus type pubescens	297		174		534			15	395
	Quercus type sessiliflora-pedunculata					2
	Salix sp.-Populus sp.			3
	Salix sp			72						9
	Sorbus sp.						1			4
	Tilia sp.									1
	Ulmus minor						12			4
	cf. Viburnum sp.					3
	Vitis vinifera sp.								1	4
	Bark or knot	1		3		11	1
	Undeterminate			1
	Undeterminable			1		13	3
	TOTAL	303		482	19	582	45		17	1563	481
		Datation	Angiospermae	Buxus	Quercus	Crataegus sp.	Knot	TOTAL
			undeterminable sempervirens type pubescens	Sorbus sp.
	SE1
	layer 138	begining of XIth c.				11		1		1	13
	layer 139	late Xth c.				32				32
	E1 layer 224 late Xth c.		1	1	16		2		20
	SE2 layer 141 begining of XIth c.				216				216
	SO1
	layer 215	begining of XIth c.				22				22

Tab. 1 Absolute frequencies of taxa (remains of construction)