D R Martin

Gerard De Kauwe

Sönke Dr

Zaehle

Kristina Dr

Anderson-Teixeira

David Dr

Ellsworth

Simone Dr

Fatichi

Dr Lianhong Gu

Colleen M Dr

Iversen

Mingkai Dr

Jiang

Trevor F Dr

Keenan

Prof Christian Koerner

D R Benton

Neil Taylor

César Dr

Terrer

Anthony P Walker
email: walkerap@ornl.gov

Martin G De Kauwe

Ana Bastos

Soumaya Belmecheri

Katerina Georgiou

Ralph Keeling

Sean M Mcmahon

Belinda E Medlyn

David J P Moore

Richard J Norby

Sönke Zaehle

Kristina J Anderson-Teixeira

Giovanna Battipaglia

Roel J W Brienen

Kristine G Cabugao

Maxime Cailleret

Elliott Campbell

Josep Canadell

Philippe Ciais

Matthew E Craig

David Ellsworth

Graham Farquhar

Simone Fatichi

Joshua B Fisher

David Frank

Heather Graven

Lianhong Gu

Vanessa Haverd

Kelly Heilman

Martin Heimann

Bruce A Hungate

Colleen M Iversen

Fortunat Joos

Mingkai Jiang

Trevor F Keenan

Jürgen Knauer

Christian Körner

Victor O Leshyk

Sebastian Leuzinger

Yao Liu

Natasha Macbean

Yadvinder Malhi

Tim Mcvicar

Josep Penuelas

Julia Pongratz

A Shafer Powell

Terhi Riutta

Manon E B Sabot

Juergen Schleucher

Stephen Sitch

William K Smith

Benjamin Sulman

Benton Taylor

César Terrer

Margaret S Torn

Kathleen Treseder

Anna T Trugman

Susan E Trumbore

Phillip J Van Mantgem

Steven L Voelker

Mary E Whelan

Pieter A Zuidema

S Margaret Torn

Sue E Trumbore

Integrating the evidence for a terrestrial carbon sink caused by increasing atmospheric CO 2

Keywords: global carbon-cycle, land-atmosphere feedback, carbon dioxide, terrestrial ecosystems, free-air CO 2 enrichment (FACE), CO 2 -fertilisation hypothesis, CO 2 -fertilization, beta factor

Atmospheric carbon dioxide concentration ([CO 2 ]) is increasing, which increases leaf-scale photosynthesis and intrinsic water-use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO 2 ] increase and thus climate change. However, ecosystem CO 2 -responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO 2 ]-driven terrestrial carbon sink can appear contradictory. Here we synthesise theory and broad, multi-disciplinary evidence for the effects of increasing [CO 2 ] (iCO2) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre-industry. Established theory, supported by experiments, indicates that iCO2 is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO2-responses are high in comparison with experiments and theory. Plant mortality and soil carbon iCO2-responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO2, albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change.

I. Introduction

Photosynthesis uses the energy in sunlight to bind CO 2 to a five-carbon sugar, transferring CO 2 from the atmosphere to plants [START_REF] Calvin | THE PATH OF CARBON IN PHOTOSYNTHESIS[END_REF][START_REF] Farquhar | A biochemical model of photosynthetic CO 2 assimilation in leaves of C3 species[END_REF]. Sugars produced by photosynthesis provide the building blocks and the primary fuel for much of life on Earth. Plant tissues, many microbes, animals, and dead organic matter are all composed of carbon-rich compounds formed from these photosynthetic sugars. In many environments, an increase in atmospheric CO 2 concentration [CO 2 ] increases photosynthesis. Thus an increase in [CO 2 ] leads to greater plant sugar availability with the potential to increase the total amount of carbon stored in the live and dead organic matter in an ecosystem. These observations have led to the CO 2 -fertilisation hypothesis (Box 1): that plant responses to increasing atmospheric [CO 2 ] drive increases in terrestrial-ecosystem carbon storage creating negative feedback on atmospheric [CO 2 ] growth.

Since the industrial revolution human activities have increased [CO 2 ] by 48 % (1760-2019, 277-411 ppm), an increase in atmospheric CO 2 -carbon of 277 Pg C [START_REF] Friedlingstein | Global Carbon Budget 2019[END_REF]. However, global-scale carbon accounting quantifies anthropogenic emissions to the atmosphere at 645 Pg C and suggests a substantial 'natural' terrestrial carbon sink (a net flux of carbon from the atmosphere to intact terrestrial ecosystems) which currently removes the equivalent of 33±9 % of anthropogenic atmospheric CO 2 [START_REF] Trumbore | Radiocarbon and Soil Carbon Dynamics[END_REF]( -2018( (Friedlingstein et al., 2019)). Along with the ocean carbon sink, this terrestrial carbon sink is mitigating the rate of climate change. Process-based carbon-cycle models attribute increasing [CO 2 ] (iCO2, Table 1) as the primary driver of the terrestrial carbon sink, albeit with substantial uncertainty [START_REF] Huntzinger | Uncertainty in the response of terrestrial carbon sink to environmental drivers undermines carbon-climate feedback predictions[END_REF][START_REF] Arora | Carbon-concentration and carbon-climate feedbacks in CMIP6 models, and their comparison to CMIP5 models[END_REF]. However, iCO2 is not the only global-change factor that can influence terrestrial carbon stocks. Anthropogenic land-use and land-cover change (hereafter land-use change) and recovery (Pugh et al. 2019), nitrogen cycle changes (Fowler et al., 2013), and climate change all affect ecosystem carbon stocks [START_REF] Keenan | The Terrestrial Carbon Sink[END_REF]. A vast and overwhelming literature often disagrees on the size and duration of CO 2driven increases in terrestrial carbon storage and predictive understanding of this process is a longstanding and unresolved scientific goal.

Predictive understanding of how terrestrial ecosystems respond to iCO2 requires knowledge of a range of processes, their interactions, and how these processes scale. For example, terrestrial ecosystem responses begin with photosynthesis inside the leaf, yet scale to have long-term global impacts. All the relevant processes must be understood across scales, and ultimately at the global scale because iCO2 and climate change are global-scale phenomena with decadal to centennial

Accepted Article

This article is protected by copyright. All rights reserved dynamics.

Given that around 50 % of plant biomass is carbon acquired via photosynthesis, it is reasonable to assume that increased photosynthesis increases plant biomass production (BP) and experimentally elevated [CO 2 ] (eCO2) commonly increases BP (e.g. [START_REF] Baig | Does the growth response of woody plants to elevated CO2 increase with temperature? A model-oriented meta-analysis[END_REF]. However, in natural ecosystems iCO2 may not always increase BP, primarily because plant tissues require nutrients, and BP responses to iCO2 will interact with soil nutrient availability and other limiting factors [START_REF] Strain | Terrestrial plant communities[END_REF][START_REF] Rastetter | Responses of N-limited ecosystems to increased CO2: A balanced-nutrition, coupled-element-cycles model[END_REF]. A related argument is that present day [CO 2 ] is likely to supply plants with unprecedented carbon availability that may be surplus to BP requirements (Körner, 2003a). This is because for at least one million years prior to the industrial revolution [CO 2 ] was much lower (170-300 ppm) [START_REF] Bereiter | Revision of the EPICA Dome C CO 2 record from 800 to 600 kyr before present: Analytical bias in the EDC CO2 record[END_REF].

Ecosystem carbon stocks are the result of both inputs (BP for plants or litter production for soils) and outputs. Thus for the CO 2 -fertilisation hypothesis to hold true, the residence time of carbon in an ecosystem must not be reduced by an amount that would negate effects of increased BP on terrestrial carbon pools. However, it has been suggested that both vegetation and soil carbon residence times may be reduced by iCO2 [START_REF] Van Groenigen Kj Van, Qi | Faster Decomposition Under Increased Atmospheric CO2 Limits Soil Carbon Storage[END_REF][START_REF] Körner | A matter of tree longevity[END_REF].

Drawing from multiple disciplines, vast quantities of diverse data have been collected on the [CO 2 ]responses of many processes. Often this evidence can appear conflicting. For example, many Free Air CO 2 Enrichment (FACE) experiments show BP gains [START_REF] Walker | Decadal biomass increment in early secondary succession woody ecosystems is increased by CO 2 enrichment[END_REF], while others show none [START_REF] Bader | Central European hardwood trees in a high-CO2 future: synthesis of an 8-year forest canopy CO2 enrichment project[END_REF][START_REF] Ellsworth | Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil[END_REF]. Many tree-ring studies indicate historical increases in intrinsic water-use efficiency (iWUE) but no detectable change in BP [START_REF] Peñuelas | Increased water-use efficiency during the 20th century did not translate into enhanced tree growth[END_REF][START_REF] Van Der Sleen | No growth stimulation of tropical trees by 150 years of CO2 fertilization but water-use efficiency increased[END_REF], while the majority of forest-inventory analyses suggest biomass gains [START_REF] Brienen | Long-term decline of the Amazon carbon sink[END_REF][START_REF] Hubau | Asynchronous carbon sink saturation in African and Amazonian tropical forests[END_REF]. Flux-tower data, global CO 2 -flask networks, and remotesensing data are now of sufficient timescales (decades) to study CO 2 responses against background variability, but have led to different inferences [START_REF] Smith | Large divergence of satellite and Earth system model estimates of global terrestrial CO2 fertilization[END_REF][START_REF] Fernández-Martínez | Atmospheric deposition, CO 2 , and change in the land carbon sink[END_REF]. This literature represents a wealth of information and inference that can appear fragmented, posing an opportunity for integration. Thus our overall goal is to provide a synthetic review of key lines of evidence related to the CO 2 -fertilisation hypothesis, specifically:

1. overview of theory and potential mechanisms within the CO 2 -fertilisation hypothesis; 2. quantitative evaluation of the evidence, identifying agreement and major conflicts; 3. resolution of apparent conflicts and, where not possible, identification of key knowledge gaps

Accepted Article

This article is protected by copyright. All rights reserved to guide future studies.

We structure this multi-disciplinary review within the mechanistic theory of the five broad processes that are key to the CO 2 -fertilisation hypothesis (Box 1, Fig. 1a): gross primary production (GPP), plant biomass production (BP), vegetation mortality rate (k veg ), soil organic matter decomposition rate (k soil ), and terrestrial carbon storage (C eco ). Within each of these high-level processes, numerous inter-related mechanisms and sub-processes shape terrestrial ecosystem CO 2 responses (Fig. 1b, Section II).

Within these processes we integrate four primary evidence themes (Box 2). eCO2 studies in evidence theme 1 provide the only direct evidence for CO 2 responses but are restricted in space and time. Observation studies (evidence themes 2-4) span a broader range of evidence types covering larger spatial-scales and longer temporal-scales but provide only indirect evidence for the effect of iCO2 on terrestrial ecosystems.

To quantify and standardise CO 2 effects across variables and varying ranges of [CO 2 ] we report data as a relativised β-factor: β = ln(y e /y a ) / ln(CO 2,e /CO 2,a ) (Eq. 1)

where y a and y e are the value of any response variable at lower [CO 2 ] (CO 2,a ) and higher [CO 2 ]

(CO 2,e ). Other methods to calculate the β-factor have been proposed (e.g. [START_REF] Friedlingstein | On the contribution of CO2 fertilization to the missing biospheric sink[END_REF] but we use Eq. 1 for ease of interpretation that results from scale-independence (Supporting Information Notes S1, Fig. S1). A β of 1 represents direct proportionality between a variable's CO 2 response and the change in CO 2 . Where possible (i.e. when reported at source) we report uncertainties as 95 % confidence intervals.

As described above, attributing iCO2 as the cause of trends is confounded by covarying factors which also drive variability in the terrestrial carbon sink. We discuss these other global-change factors in the context of attribution, but do not cover them in depth. The difference between direct evidence from eCO2 experiments and indirect evidence from historical data (concurrent with a suite of global-change factors) motivates our use of two abbreviations: eCO2 and iCO2. As with eCO2 and iCO2, we distinguish direct CO 2 responses (β dir ) from indirect apparent CO 2 'responses' (β app ).

Accepted Article

This article is protected by copyright. All rights reserved Photosynthesis is limited by CO 2 or light [START_REF] Farquhar | A biochemical model of photosynthetic CO 2 assimilation in leaves of C3 species[END_REF]. When CO 2 is limiting, theory predicts that eCO2 increases leaf-scale net carbon assimilation (A net ) (β dir,hist = 0.86, Table S1). The enzyme that fixes CO 2 (RuBisCO) also catalyses an oxygenation reaction, which results in CO 2 loss (photorespiration; [START_REF] Farquhar | A biochemical model of photosynthetic CO 2 assimilation in leaves of C3 species[END_REF]. eCO2 also suppresses photorespiration (Fig. 2a). Given that photorespiration always occurs during C3 photosynthesis, the suppression of photorespiration by eCO2 increases A net also when light is limiting, but with a lower response (β dir,hist = 0.31 [START_REF] Ehleringer | Quantum Yields for CO2 Uptake in C3 and C4 Plants: Dependence on Temperature, CO2, and O2 Concentration[END_REF]. Therefore A net in C4 plants is not directly influenced by [CO 2 ] above c. 200 ppm (Fig. 2a), though water savings from reduced g s may stimulate A net indirectly [START_REF] Leakey | Will photosynthesis of maize ( Zea mays ) in the US Corn Belt increase in future [CO 2 ] rich atmospheres? An analysis of diurnal courses of CO 2 uptake under free-air concentration enrichment (FACE)[END_REF].

Photosynthesis requires the acquisition of other resources and eCO2-stimulation of A net increases A net per unit resource consumption, i.e. increases resource use-efficiencies (UE) of: water (WUE), light (LUE), and leaf nitrogen [START_REF] Cowan | Regulation of Water Use in Relation to Carbon Gain in Higher Plants[END_REF][START_REF] Drake | MORE EFFICIENT PLANTS: A Consequence of Rising Atmospheric CO2?[END_REF]. Increased use-efficiencies imply a shift in a plant's resource-use economy [START_REF] Bloom | Resource Limitation in Plants-An Economic Analogy[END_REF] which is commonly studied using optimisation theory.

Optimisation theory predicts that a change in the ratio of A net to g s (intrinsic WUE, iWUE) in proportion to the change in [CO 2 ] (β dir c. 1; Fig 2d) maximises the benefit of carbon gain while minimising the cost of water lost for C3 [START_REF] Medlyn | Reconciling the optimal and empirical approaches to modelling stomatal conductance[END_REF] and C4 plants [START_REF] Lin | Optimal stomatal behaviour around the world[END_REF].

Canopy-scaling theory predicts that the increase in iWUE is preserved at the canopy scale (Fig 2e).

Where the response of A net to eCO2 is less than proportional (β dir < 1) the increase in iWUE (i.e.

A net /g s ) implies a reduction in g s (canopy-scale iWUE β dir,hist = 1.1±0.1, A net β dir,hist = 0.60±0.3, thus g s β dir,hist = -0.53.±0.2; Fig. 2f,i). Due to the lower predicted A net in the future, the predicted decrease in g s is greater (β dir,fut = -0.62.±0.1).

Accepted Article

This article is protected by copyright. All rights reserved

Optimisation theory also predicts reduction in photosynthetic carboxylation capacity (V cmax ), reducing nitrogen demand [START_REF] Bowes | Growth at elevated CO2: photosynthetic responses mediated through Rubisco[END_REF][START_REF] Drake | MORE EFFICIENT PLANTS: A Consequence of Rising Atmospheric CO2?[END_REF]. A reduction in leaf nitrogen may also occur due to limited plant-available soil nitrogen (Section II.2) or physiological competition for the products of electron transport [START_REF] Bloom | CO2 enrichment inhibits shoot nitrate assimilation in C3 but not C4 plants and slows growth under nitrate in C3 plants[END_REF].

2 Plant biomass production BP of leaf, wood, and root tissues is controlled by the interplay of source (resource acquisition), sink (metabolic tissue production) [START_REF] Muller | Water deficits uncouple growth from photosynthesis, increase C content, and modify the relationships between C and growth in sink organs[END_REF][START_REF] Fatichi | Modelling carbon sources and sinks in terrestrial vegetation[END_REF], and regulatory processes (phenology, hormones) [START_REF] Schwartz | Phenology: an integrative environmental science[END_REF][START_REF] Bahuguna | Temperature regulation of plant phenological development[END_REF]. Within this framework eCO2 can increase BP when BP is either carbon-source limited or when eCO2 can alleviate other limitations. Plant BP is carbon-source limited when in competition with respiration for available carbon and when light limits BP [START_REF] Lloyd | Effects of rising temperatures and [CO 2 ] on the physiology of tropical forest trees[END_REF]. Sustained periods of high growth may also reduce carbon stores [START_REF] Würth | Non-structural carbohydrate pools in a tropical forest[END_REF], potentially leading to carbon-source limitation.

BP is also carbon-sink limited by stoichiometric nutrient requirements [START_REF] Elser | Biological stoichiometry of plant production: metabolism, scaling and ecological response to global change: Tansley review[END_REF]. Thus increased BP requires either increased nutrient acquisition or increased stoichiometric carbon-tonutrient ratios. Increased plant-available carbon may be able to "pay" for increased nutrient acquisition via a number of mechanisms (e.g. increased fine-root BP, mycorrhizal investment, exudation, atmospheric N fixation) [START_REF] Luxmoore | CO 2 and phytomass[END_REF][START_REF] Hungate | Elevated CO2 increases nitrogen fixation and decreases soil nitrogen mineralization in Florida scrub oak[END_REF]Fleischer et al., 2019).

Changing stoichiometry may result in feedbacks that compound nutrient limitations by reducing decomposition rates and nutrient availability [START_REF] Comins | Long-Term Response of Nutrient-Limited Forests to CO2 Enrichment; Equilibrium Behavior of Plant-Soil Models[END_REF], known as progressive nitrogen limitation [START_REF] Luo | Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide[END_REF].

In environments where BP is primarily sink-limited, e.g. tree-lines (temperature-limitation), arid and semi-arid (water-limitation), increased carbon availability may have little effect on BP [START_REF] Kramer | Carbon Dioxide Concentration, Photosynthesis, and Dry Matter Production[END_REF]Körner, 2003b). However, in water-limited environments, increased iWUE could increase BP [START_REF] Mooney | Predicting Ecosystem Responses to Elevated CO2 Concentrations[END_REF][START_REF] Wullschleger | Plant water relations at elevated CO2implications for water-limited environments[END_REF]. Leaf area index (LAI) may also be limited by water availability [START_REF] Woodward | Climate and Plant Distribution[END_REF][START_REF] Yang | Applying the Concept of Ecohydrological Equilibrium to Predict Steady State Leaf Area Index[END_REF] and increased WUE may increase LAI and light absorption leading to indirect positive feedback on GPP and transpiration [START_REF] Fatichi | Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO 2[END_REF][START_REF] Trancoso | CO2-vegetation feedbacks and other climate changes implicated in reducing base flow[END_REF].

If BP is restricted by sink limitation, biomass production efficiency (BP per unit GPP) would decrease and the labile products of photosynthesis would accumulate. If BP is stimulated this may be as shortlived, primary tissues (leaves and fine-roots) or long-lived, secondary tissues (wood) (De Kauwe et

Accepted Article

This article is protected by copyright. All rights reserved al., 2014). Division of carbon among these tissues determines residence time of carbon in plant biomass. Wood has greater residence time and thus greater potential to increase C veg accumulation over multiple years. Greater production of short-lived tissues (i.e. leaves and fine-roots) may increase resource capture and will increase litter carbon inputs to the soil.

Plant mortality

Increases in mortality rates reduce vegetation residence times and have the potential to offset any biomass gains resulting from increased BP (Eq. B3) [START_REF] Bugmann | Will the CO2 fertilization effect in forests be offset by reduced tree longevity?[END_REF][START_REF] Körner | A matter of tree longevity[END_REF].

Hydraulic failure, and less-so, carbon starvation are thought to be interrelated mechanisms of plant death [START_REF] Mcdowell | Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought?[END_REF]. By easing the carbon and hydraulic impacts of abiotic and biotic stressors such as drought, or pest and pathogen attack, eCO2 could potentially decrease mortality.

Greater carbon resources could supply greater maintenance respiration, stored carbon reserves, or synthesis of defense compounds [START_REF] Mcdowell | Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought?[END_REF]. More efficient water use (Section II.1) could delay the onset or the intensity of drought, which could reduce the risk of xylem-conductivity losses.

Indirect influences on mortality may emerge from the acceleration of individual size-growth.

Increased growth could reduce small-size related mortality by speeding individuals out of the hazards of early life (e.g., browsing) and increasing their ability to acquire resources [START_REF] Metcalfe | Herbivory makes major contributions to ecosystem carbon and nutrient cycling in tropical forests[END_REF][START_REF] Hülsmann | How to kill a tree: empirical mortality models for 18 species and their performance in a dynamic forest model[END_REF]. Conversely, increased growth could increase large-size mortality risk, with tall trees being more susceptible to hydraulic stress, windthrow, lightning, and certain pests or pathogens [START_REF] Bugmann | Will the CO2 fertilization effect in forests be offset by reduced tree longevity?[END_REF][START_REF] Bennett | Larger trees suffer most during drought in forests worldwide[END_REF][START_REF] Körner | A matter of tree longevity[END_REF][START_REF] Trugman | Differential declines in Alaskan boreal forest vitality related to climate and competition[END_REF].

At the stand-scale, increased growth may accelerate post-disturbance successional dynamics [START_REF] Mcdowell | Pervasive shifts in forest dynamics in a changing world[END_REF]. Intensified competition for light, water, and nutrients could lead to early reorganisation and transition (self-thinning) phases of development [START_REF] Bormann | Catastrophic disturbance and the steady state in northern hardwood forests: A new look at the role of disturbance in the development of forest ecosystems suggests important implications for land-use policies[END_REF], but also an earlier switch from transition to steady-state phases [START_REF] Miller | Alteration of forest succession and carbon cycling under elevated CO2[END_REF]

Accepted Article

This article is protected by copyright. All rights reserved 4 Organic matter decomposition

Residence times of litter and soil organic matter (SOM) vary from minutes to millenia and can respond rapidly to environmental perturbation [START_REF] Trumbore | Radiocarbon and Soil Carbon Dynamics[END_REF][START_REF] Schmidt | Persistence of soil organic matter as an ecosystem property[END_REF][START_REF] Dwivedi | Abiotic and Biotic Controls on Soil Organo-Mineral Interactions: Developing Model Structures to Analyze Why Soil Organic Matter Persists[END_REF]. Increases in SOM decomposition rates reduces SOM residence times and has the potential to offset any eCO2-related increases in litter inputs. Accelerated decomposition of litter and particulate SOM (i.e. priming) can result from microbial responses to increased labile-carbon availability [START_REF] Kuzyakov | Review of mechanisms and quantification of priming effects[END_REF][START_REF] Blagodatskaya | Microbial Growth and Carbon Use Efficiency in the Rhizosphere and Root-Free Soil[END_REF], including at depth (Fontaine et al., 2007).

Organic acids produced by roots can destabilise mineral-associated SOM [START_REF] Keiluweit | Mineral protection of soil carbon counteracted by root exudates[END_REF].

eCO2 effects on environmental conditions could also affect SOM decomposition. CO 2 -related increases in soil water (Section II.1) would likely stimulate decomposition in water-limited ecosystems [START_REF] Castanha | The effects of heating, rhizosphere, and depth on root litter decomposition are mediated by soil moisture[END_REF], but could reduce oxygen availability (slowing decomposition) in energy-limited ecosystems.

Microbial activity has also been linked to the formation of mineral-associated SOM [START_REF] Cotrufo | The Microbial Efficiency-Matrix Stabilization (MEMS) framework integrates plant litter decomposition with soil organic matter stabilization: do labile plant inputs form stable soil organic matter?[END_REF][START_REF] Liang | The importance of anabolism in microbial control over soil carbon storage[END_REF], and potentially soil aggregates [START_REF] Ge | Soil Aggregation and Organic Carbon Dynamics in Poplar Plantations[END_REF], which might slow decomposition by restricting microbial access to SOM [START_REF] Kögel-Knabner | Organo-mineral associations in temperate soils: Integrating biology, mineralogy, and organic matter chemistry[END_REF]. Changing stoichiometry might slow decomposition (Section II.2). Roots can distribute carbon deeper in the soil where decomposition is slower and capacity for mineral stabilization is higher [START_REF] Jackson | The Ecology of Soil Carbon: Pools, Vulnerabilities, and Biotic and Abiotic Controls[END_REF][START_REF] Hicks Pries | Root litter decomposition slows with soil depth[END_REF].

Greater decomposition rates might also increase soil nutrient availability, potentially reducing plant nutrient limitation [START_REF] Treseder | A meta-analysis of mycorrhizal responses to nitrogen, phosphorus, and Accepted Article This article is protected by copyright. All rights reserved atmospheric CO2 in field studies[END_REF][START_REF] Dijkstra | Long-term enhancement of N availability and plant growth under elevated CO[END_REF] or increasing microbial immobilisation. Over longer timescales, nutrient immobilisation can reduce nutrient losses, leading to accumulation of ecosystem nutrient stocks which may enhance mineralisation and progressively release plants from nutrient limitation [START_REF] Rastetter | Responses of N-limited ecosystems to increased CO2: A balanced-nutrition, coupled-element-cycles model[END_REF][START_REF] Medlyn | Using ecosystem experiments to improve vegetation models[END_REF].

5 Terrestrial ecosystem carbon responses to CO 2

The response of terrestrial carbon storage to eCO2 ( C eco ) is the net result of the above discussed processes. Potential increases in BP and litter production are balanced by potential increases in loss rates (Eqs. B3 and B4). Increased BP of short-lived primary tissues such as leaves and fine-roots could lead to greater biomass of these transient C veg pools and to increased litter inputs to the soil. If wood BP is stimulated by eCO2, over medium timescales (annual to several decades) ecosystem biomass could increase due to the longer residence time of wood. However, wood BP is tied to tree

Accepted Article

This article is protected by copyright. All rights reserved size-growth rates and the effects of tree size on mortality rates may be either positive or negative (Section II.3). Greater wood BP or greater wood mortality rates would result in greater coarse woody debris, which may immobilise nutrients (e.g. [START_REF] Zimmerman | Nitrogen Immobilization by Decomposing Woody Debris and the Recovery of Tropical Wet Forest from Hurricane Damage[END_REF].

Increased plant inputs to litter and soil (e.g., wood, leaf and root litter, root exudates, and mycorrhizal subsidies) could increase C soil . However, the complex processes that drive the formation and decomposition of SOM make the response of C soil to eCO2 difficult to predict [START_REF] Schmidt | Persistence of soil organic matter as an ecosystem property[END_REF][START_REF] Dwivedi | Abiotic and Biotic Controls on Soil Organo-Mineral Interactions: Developing Model Structures to Analyze Why Soil Organic Matter Persists[END_REF]Section II.4). Increased soil mineralisation rates could lead to greater C eco if nutrients are redistributed from soils to plants, which have higher carbon:nutrient ratios and hence can store more carbon per unit nutrient [START_REF] Rastetter | Changes in C-Storage by Terrestrial Ecosystems -How C-N Interactions Restrict Responses to Co2 and Temperature[END_REF][START_REF] Zaehle | Evaluation of 11 terrestrial carbon-nitrogen cycle models against observations from two temperate Free-Air CO 2 Enrichment studies[END_REF].

A one-pool ecosystem carbon model (Box 1) with simplifying assumptions (BP β dir = GPP β dir ;

residence time β dir = 0) provides baseline expected β dir for carbon storage (Fig. 2j-l). The model indicates that when starting carbon storage is non-zero, β dir depends on the time of measurement (Fig. 2j,k et al., 2008;Bader et al., 2010;[START_REF] Ellsworth | Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil[END_REF].

Indirect evidence also suggests increased photosynthesis with iCO2. Deuterium isotopomers of

Accepted Article

This article is protected by copyright. All rights reserved glucose in plant archives indicate that the leaf-level photorespiration:assimilation ratio decreased since pre-industry (β app = -0.99) [START_REF] Ehlers | Detecting longterm metabolic shifts using isotopomers: CO2-driven suppression of photorespiration in C3 plants over the 20th century[END_REF], which translates to an increase in photosynthesis (β app = 1.0) [START_REF] Ehlers | Detecting longterm metabolic shifts using isotopomers: CO2-driven suppression of photorespiration in C3 plants over the 20th century[END_REF]. GPP estimates from eddy-covariance (23 sites, c. 20 years) suggest a recent increase (β app = 1.6±0.9), implicating a substantial iCO2 contribution (β dir,hist = 1.2±0.6) [START_REF] Fernández-Martínez | Atmospheric deposition, CO 2 , and change in the land carbon sink[END_REF]. Eddy-covariance data used to calibrate a model suggests a lower iCO2 response (β dir,hist = 0.5±0.2) [START_REF] Ueyama | Inferring CO2 fertilization effect based on global monitoring land-atmosphere exchange with a theoretical model[END_REF].

Ice-core measurements of atmospheric carbonyl sulfide (OCS) combined with mass-balance analysis suggests that global GPP has increased since pre-industry (β app = 0.95±0.2) [START_REF] Campbell | Large historical growth in global terrestrial gross primary production[END_REF]. As do ice-core measurements of atmospheric O 2 isotopes combined with models (β app = 1.3±2.3) [START_REF] Ciais | Large inert carbon pool in the terrestrial biosphere during the Last Glacial Maximum[END_REF]. Satellite-based evapotranspiration combined with an ecosystem WUE model estimated increased GPP during recent decades (β app = 1.1±0.5) [START_REF] Cheng | Recent increases in terrestrial carbon uptake at little cost to the water cycle[END_REF]. 14 methods to estimate GPP from satellite-based fAPAR resulted in wide-ranging iCO2 sensitivities (β dir range = -0.39±0.34 to 1.6±1, mean = 0.52±0.3; 2000-2014) [START_REF] Sun | Evaluating and comparing remote sensing terrestrial GPP models for their response to climate variability and CO2 trends[END_REF].

Water use efficiency, stomatal conductance, and transpiration

Experimental evidence also supports increased iWUE in response to eCO2 (β dir = 1.2±0.4; 4 sites, 7 species) [START_REF] Ainsworth | What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy[END_REF]. At two FACE experiments (Duke University and Oak Ridge National Laboratory, ORNL), tree-ring δ 13 C implies increased iWUE (β dir = 1.4 and 1.3, respectively) [START_REF] Battipaglia | Elevated CO2 increases tree-level intrinsic water use efficiency: insights from Accepted Article This article is protected by copyright. All rights reserved carbon and oxygen isotope analyses in tree rings across three forest FACE sites[END_REF]. Tree-ring δ 13 C samples from across the globe suggest increased iWUE in many biomes since pre-industry in: northern boreal gymnosperms (β app = 1.2±2 to 1.5±1.6) [START_REF] Saurer | Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years[END_REF], tropical forests (β app = 1.0) (van der [START_REF] Van Der Sleen | No growth stimulation of tropical trees by 150 years of CO2 fertilization but water-use efficiency increased[END_REF], and a wide range of forest biomes (β app = 1.19; [START_REF] Keller | 20th century changes in carbon isotopes and water-use efficiency: tree-ring-based evaluation of the CLM4.5 and LPX-Bern models[END_REF]. Attribution to iCO2 also suggests increases in iWUE in European

Pinus and Quercus (β dir = 1.0±0.6 and 0.67±0.9; 9-14 sites) (Frank et al., 2015). Additional environmental factors have contributed to observed iWUE trends, e.g. drying trends have increased iWUE [START_REF] Saurer | Spatial variability and temporal trends in water-use efficiency of European forests[END_REF].

δ 13 C in atmospheric CO 2 combined with mass-balance modelling suggests a global increase in iWUE since pre-industry (β app = 0.94±0.2) [START_REF] Keeling | Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis[END_REF].

Evidence from Duke and ORNL FACE experiments supports increases in ecosystem-scale plant WUE (annual BP/T; β dir,hist = 0.76 and 1.1, respectively) [START_REF] De Kauwe | Forest water use and water use efficiency at elevated CO 2 : a modeldata intercomparison at two contrasting temperate forest FACE sites[END_REF]. Inferred from eddycovariance, 'inherent' WUE (VPD.GPP/ET) increased in temperate and boreal forests with notably higher magnitude (β app = 4.72; 21 sites) [START_REF] Keenan | Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise[END_REF]. A follow up study reduced this estimate (β app = 2.5) [START_REF] Mastrotheodoros | Linking plant functional trait plasticity and the large increase in forest water use efficiency[END_REF]. An eddy-covariance calibrated, canopy-scale

Accepted Article

This article is protected by copyright. All rights reserved model suggested iCO2 reduced g s (β dir,hist = -0.28±0.09) and increased iWUE (β dir,hist = 0.73±0.2) [START_REF] Ueyama | Inferring CO2 fertilization effect based on global monitoring land-atmosphere exchange with a theoretical model[END_REF]. Satellite-based models (2000Satellite-based models ( -2013) ) of GPP and ET suggest smaller or decreased WUE (GPP/ET) (β app = -0.49 and 0.28) [START_REF] Tang | How is water-use efficiency of terrestrial ecosystems distributed and changing on Earth?[END_REF][START_REF] Xue | Global patterns, trends, and drivers of water use efficiency from 2000 to 2013[END_REF].

Experimental evidence has thoroughly demonstrated reduced leaf-scale g s in response to eCO2

(β dir,fut = -0.22±0.15) [START_REF] Medlyn | Stomatal conductance of forest species after long-term exposure to elevated CO2 concentration: a synthesis[END_REF]. Averaged across FACE experiments (12 sites, 40 species), eCO 2 reduced g s (β dir,fut = -0.60±0.2) but with substantial variability across functional groups [START_REF] Ainsworth | What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy[END_REF] and disturbance history [START_REF] Donohue | A simple hypothesis of how leaf and canopy-level transpiration and assimilation respond to elevated CO2 reveals distinct response patterns between disturbed and undisturbed vegetation[END_REF]. Notably for Eucalyptus saligna in whole-tree chambers, canopy-scale iWUE was very tightly constrained (β dir,fut = 0.98±0.2), and variability in the A net response controlled the g s response [START_REF] Barton | Effects of elevated atmospheric [CO2] on instantaneous transpiration efficiency at leaf and canopy scales in Eucalyptus saligna[END_REF].

Across four FACE experiments (Duke, EucFACE, ORNL, Swiss Canopy Crane), transpiration responses were only reduced by eCO2 at ORNL (β dir,fut = -0.54), an ecosystem that is rarely water limited [START_REF] Leuzinger | Rainfall distribution is the main driver of runoff under future CO2concentration in a temperate deciduous forest[END_REF][START_REF] De Kauwe | Forest water use and water use efficiency at elevated CO 2 : a modeldata intercomparison at two contrasting temperate forest FACE sites[END_REF][START_REF] Gimeno | Elevated CO2 did not affect the hydrological balance of a mature native Eucalyptus woodland[END_REF]. Airborne remote sensing suggested decreased evapo-transpiration with long-term volcanically-derived eCO2 in California [START_REF] Cawse-Nicholson | Ecosystem responses to elevated CO2 using airborne remote sensing at Mammoth Mountain, California[END_REF]. Stream-gauge networks indicate global increases in runoff (Gedney et al. 2006), in agreement with reduced g s over the northern hemisphere extratropics [START_REF] Knauer | The response of ecosystem water-use efficiency to rising atmospheric CO2 concentrations: sensitivity and large-scale biogeochemical implications[END_REF]. However, decreases in runoff have also been observed [START_REF] Ukkola | Reduced streamflow in water-stressed climates consistent with CO 2 effects on vegetation[END_REF][START_REF] Trancoso | CO2-vegetation feedbacks and other climate changes implicated in reducing base flow[END_REF] and modest run-off increases across the tropics have been driven by precipitation increases (Yang et al., 2016).

2 Biomass production eCO2 increased BP in four temperate-forest, stand-scale (25-30 m diameter) FACE experiments in the early years (β dir,fut = 0.56±0.2) (Norby et al., 2005) and over a full decade (β dir,fut = 0.49±0.3) [START_REF] Walker | Decadal biomass increment in early secondary succession woody ecosystems is increased by CO 2 enrichment[END_REF]. These forest ecosystems were in the early phases of secondary succession (initiated 1-13 years following a major disturbance). In three later-succession forests (c. 100 years old), BP did not respond to eCO2 (note fine-root BP was often not measured): deciduous broadleaved trees (β dir,fut = -0.097±1.0 to 0.55±1.7) (8 years; [START_REF] Bader | Central European hardwood trees in a high-CO2 future: synthesis of an 8-year forest canopy CO2 enrichment project[END_REF], Picea abies (β dir,fut = -0.30±0.7, 5 years eCO2; [START_REF] Klein | Accepted Article This article is protected by copyright. All rights reserved Growth and carbon relations of mature Picea abies trees under 5 years of free-air CO2 enrichment[END_REF], and a low productivity Eucalyptus woodland (β dir,fut = -0.26±0.6, 4 years eCO2; [START_REF] Ellsworth | Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil[END_REF][START_REF] Jiang | The fate of carbon in a mature forest under carbon dioxide enrichment[END_REF].

eCO2 consistently decreased specific leaf area (β dir,fut = -0.16±0.07) [START_REF] Ainsworth | What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy[END_REF], which requires increased leaf BP at a given LAI [START_REF] De Kauwe | Where does the carbon go? A model-data intercomparison of vegetation carbon allocation and turnover processes at two temperate forest free-air CO 2 enrichment sites[END_REF]. Synthesis of experiments (19 sites) suggests that eCO2 increased grassland leaf and stem BP (β dir,fut = 0.17±0.07) (Hovenden et

Accepted Article

This article is protected by copyright. All rights reserved al., 2019), related to summer water savings and spring water availability [START_REF] Morgan | Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2[END_REF][START_REF] Hovenden | Globally consistent influences of seasonal precipitation limit grassland biomass response to elevated CO 2[END_REF]. Meta-analysis found eCO2 increased fine-root BP across experiments (β dir,fut = 0.56) (β dir,fut = 0.56), in forests (β dir,fut = 0.92), and to a lesser degree in grasslands (β dir,fut = 0.18) (Nowak et al., 2004).

Tree-ring analysis at CO 2 -springs in Italy (two sites) suggests that eCO2 increased Quercus ilex tree ring-width (a proxy for wood BP) initially (β app = 0.49-0.81), and the increase diminished as trees aged [START_REF] Hättenschwiler | Thirty years of in situ tree growth under elevated CO2: a model for future forest responses?[END_REF]. Basal-area increment (BAI) analysis showed the eCO2 response stabilised at around 10 years (β app = 0.27) (Norby et al., 1999).

A large number of tree-ring studies have found little evidence for increases in wood BP. No detectable trends in BAI were found across tropical forests (3 sites, 12 species) (van der [START_REF] Van Der Sleen | No growth stimulation of tropical trees by 150 years of CO2 fertilization but water-use efficiency increased[END_REF], and both increasing and decreasing trends were found across North American boreal forests (598 sites, 19 species) [START_REF] Girardin | No growth stimulation of Canada's boreal forest under half-century of combined warming and CO2 fertilization[END_REF]. Syntheses across biomes found no significant increase in tree ring-width since 1950 (β app = 0.23±0.8; 40 sites) [START_REF] Peñuelas | Increased water-use efficiency during the 20th century did not translate into enhanced tree growth[END_REF] and variable responses of BAI (β app = 0.45±0.7; 37 sites, 22 species) [START_REF] Silva | Probing for the influence of atmospheric CO2 and climate change on Accepted Article This article is protected by copyright. All rights reserved forest ecosystems across biomes[END_REF]. Conversely, Pinus

and Quercus tree-rings from Missouri showed a positive response to iCO2 that diminished with tree age (β app = 3.3, at age one; β app = 1.1, at age 50) [START_REF] Voelker | Historical Co2 Growth Enhancement Declines with Age in Quercus and Pinus[END_REF].

Evidence from multi-plot inventory data consistently show increasing wood biomass (Section III.5), but few of these studies quantify wood BP. A single census interval of eastern-US Forest Inventory Analysis plots (20,000) suggested very little change in wood BP [START_REF] Caspersen | Contributions of Land-Use History to Carbon Accumulation in U.S. Forests[END_REF], but with high uncertainty [START_REF] Joos | Growth enhancement due to global atmospheric change as predicted by terrestrial ecosystem models: consistent with US forest inventory data[END_REF]. Two large tropical-forest plots showed no change in aboveground wood BP (Clark et al., 2010;Rutishauser et al., 2019). In contrast, tropical forest-plot networks (321 and 244) suggest that aboveground wood BP increased in Amazonia (β app = 1.2±0.6) [START_REF] Brienen | Long-term decline of the Amazon carbon sink[END_REF] and Africa (β app = 0.69±0.63) with a regression-attributed iCO2 response (β app = 0.54±1) [START_REF] Hubau | Asynchronous carbon sink saturation in African and Amazonian tropical forests[END_REF]. Analysis of worldwide forest plots (695) suggested that wood BP increased (β app = 0.94±1.1) in recent decades [START_REF] Yu | Pervasive decreases in living vegetation carbon turnover time across forest climate zones[END_REF].

BP-nutrient interactions and progressive nitrogen limitation

At Duke FACE, nitrogen availability influenced the magnitude of BP responses (McCarthy et al., 2010) and experiments in later-succession systems with no BP response were limited by nitrogen (Flakaliden; [START_REF] Sigurdsson | Growth of mature boreal Norway spruce was not affected by elevated [CO2] and/or air temperature unless nutrient availability was improved[END_REF] and phosphorus (EucFACE; [START_REF] Ellsworth | Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil[END_REF]. Limiting factors were not examined for a number of the other later-succession experiments [START_REF] Bader | Central European hardwood trees in a high-CO2 future: synthesis of an 8-year forest canopy CO2 enrichment project[END_REF][START_REF] Klein | Accepted Article This article is protected by copyright. All rights reserved Growth and carbon relations of mature Picea abies trees under 5 years of free-air CO2 enrichment[END_REF].

Accepted Article

This article is protected by copyright. All rights reserved eCO2 experiments in early-succession ecosystems suggest that BP gains were supported by increased nitrogen acquisition rather than changes in stoichiometry (Finzi et al., 2007;[START_REF] Zaehle | Evaluation of 11 terrestrial carbon-nitrogen cycle models against observations from two temperate Free-Air CO 2 Enrichment studies[END_REF]. Nitrogen acquisition was increased through increased fine-root BP (see above), changing root traits [START_REF] Iversen | Digging deeper: fine-root responses to rising atmospheric CO2 concentration in forested ecosystems[END_REF]Nie et al., 2013;[START_REF] Beidler | Changes in root architecture under elevated concentrations of CO2 and nitrogen reflect alternate soil exploration strategies[END_REF], and below-ground carbon flux to mycorrhizal symbionts and rhizosphere microbial associations (Section III.4; [START_REF] Drake | Increases in the flux of carbon belowground stimulate nitrogen uptake and sustain the long-term enhancement of forest productivity under elevated CO(2)[END_REF][START_REF] Phillips | Enhanced root exudation induces microbial feedbacks Accepted Article This article is protected by copyright. All rights reserved to N cycling in a pine forest under long-term CO2 fumigation[END_REF][START_REF] Terrer | Ecosystem responses to elevated CO2 governed by plant-soil interactions and the cost of nitrogen acquisition[END_REF]. Meta-analysis suggests that eCO2 increased nitrogen fixation in more intensively manipulated experiments but not in more natural settings (total 441 studies, rates were scaled to plant or ground-area units; B.A.

Leaf area, water, and land cover interactions

In some low LAI ecosystems, eCO2 increased LAI, but did not in higher LAI (c. 5) ecosystems (Norby & Zak, 2011;[START_REF] Bader | Central European hardwood trees in a high-CO2 future: synthesis of an 8-year forest canopy CO2 enrichment project[END_REF]. However, low LAI (c. 1) at EucFACE did not respond to eCO2 [START_REF] Duursma | Canopy leaf area of a mature evergreen Eucalyptus woodland does not respond to elevated atmospheric [CO2] but tracks water availability[END_REF]. The LAI response to eCO2 in low LAI systems has been interpreted as

CO 2 accelerating open canopies towards closure [START_REF] Körner | Plant CO2 responses: an issue of definition, time and resource supply[END_REF]. However, evidence from two FACE sites (Duke and Rhinelander) suggests that LAI can be higher at canopy closure [START_REF] Walker | Decadal biomass increment in early secondary succession woody ecosystems is increased by CO 2 enrichment[END_REF]. Higher above-ground biomass in some grasslands [START_REF] Hovenden | Globally consistent influences of seasonal precipitation limit grassland biomass response to elevated CO 2[END_REF] indicates potential LAI increases, though increases in leaf mass per unit area would reduce the LAI response relative to the biomass response. High grassland biomass responses have been linked to low soil matric potential [START_REF] Morgan | Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2[END_REF], though more complex interactions with precipitation seasonality have also been indicated [START_REF] Hovenden | Globally consistent influences of seasonal precipitation limit grassland biomass response to elevated CO 2[END_REF].

Satellite data show "greening" trends over much of the planet, inferred as increasing LAI [START_REF] Zhu | Greening of the Earth and its drivers[END_REF][START_REF] Mao | Human-induced greening of the northern extratropical land surface[END_REF] and with model-based attribution primarily to iCO2 [START_REF] Zhu | Greening of the Earth and its drivers[END_REF].

Consistent with theory, satellite greeness data suggests increased foliage cover in warm and semiarid regions, likely an iCO2 effect via increased WUE [START_REF] Donohue | Impact of CO2 fertilization on Accepted Article This article is protected by copyright. All rights reserved maximum foliage cover across the globe's warm, arid environments[END_REF]. Tree-rings have indicated decreasing sensitivity to rainfall or drought in the Eastern US, possibly indicating WUEmediated iCO2 response [START_REF] Wyckoff | Response of the prairie-forest border to climate change: impacts of increasing drought may be mitigated by increasing CO2[END_REF][START_REF] Helcoski | Growing season moisture drives interannual variation in woody productivity of a temperate deciduous forest[END_REF]. However, less severe droughts, noted in the Eastern US, likely appear as reduced growth sensitivity [START_REF] Maxwell | On the declining relationship between tree growth and climate in the Midwest United States: the fading drought signal[END_REF].

At the Florida scrub oak experiment, eCO2 alleviated drought-related declines in NEP (Li et al.,

Accepted Article

This article is protected by copyright. All rights reserved 2007) but the opposite was observed at the Nevada desert FACE [START_REF] Jasoni | Net ecosystem CO2 exchange in Mojave Desert shrublands during the eighth year of exposure to elevated CO2[END_REF].

Plant mortality

Greenhouse experiments with potted plants have found little benefit of eCO2 on survival during drought or high temperature (e.g. [START_REF] Duan | Elevated [CO2] does not ameliorate the negative effects of elevated temperature on drought-induced mortality in Eucalyptus radiata seedlings[END_REF][START_REF] Bachofen | No carbon "bet hedging" in pine Accepted Article This article is protected by copyright. All rights reserved seedlings under prolonged summer drought and elevated CO2[END_REF]. However, remote sensing evidence shows increased vegetation cover in drylands [START_REF] Donohue | Impact of CO2 fertilization on Accepted Article This article is protected by copyright. All rights reserved maximum foliage cover across the globe's warm, arid environments[END_REF]Section III.2) which possibly suggests a reduction in mortality in those regions.

We are unaware of direct or indirect evidence for CO 2 -related increases in individual-scale mortality, but growth-mortality relationships provide insight. Evidence supports both an inter-specific growthsurvival tradeoff [START_REF] Wright | Functional traits and the growth-mortality trade-off in tropical trees[END_REF][START_REF] Bugmann | Will the CO2 fertilization effect in forests be offset by reduced tree longevity?[END_REF]) and an intra-specific tradeoff [START_REF] Bigler | Increased early growth rates decrease longevities of conifers in subalpine forests[END_REF][START_REF] Filippo | Bioclimate and growth history affect beech lifespan in the Italian Alps and Apennines[END_REF], 2015;[START_REF] Büntgen | Limited capacity of tree growth to mitigate the global greenhouse effect under predicted warming[END_REF]. However, there are common exceptions with some high-growth-rate species with long lifespans (Rüger et al., 2020), and other species that show no, or even negative, growth-mortality relationships [START_REF] Ireland | Slow lifelong growth predisposes Populus tremuloides trees to mortality[END_REF][START_REF] Cailleret | A synthesis of radial growth patterns preceding tree mortality[END_REF].

Experimental evidence for stand-scale mortality responses to eCO2 is rare. In the young, regenerating stand at Rhinelander FACE, over 11 years of eCO2 lowered rates of self-thinning (i.e.

higher stand basal area for any given stem density) [START_REF] Kubiske | Increasing Atmospheric CO2 Concentration Stand Accepted Article This article is protected by copyright. All rights reserved Development in Trembling Aspen Forests: Are Outdated Density Management Guidelines in Need of Revision for All Species?[END_REF].

At broader scales, most inventory networks have shown increases in stand-scale mortality rates.

Increases in biomass mortality have been observed in Amazon forests (β app = 2.4) [START_REF] Brienen | Long-term decline of the Amazon carbon sink[END_REF] and across continents (β app = 1.6-3.9) [START_REF] Yu | Pervasive decreases in living vegetation carbon turnover time across forest climate zones[END_REF]. Tree stem mortality rates have increased, across species, elevation, and tree size, in the western US (β app = 6.2±3; [START_REF] Van Mantgem | Widespread Increase of Tree Mortality Rates in the Western United States[END_REF] and in Canada (β app = 6.1) [START_REF] Peng | A droughtinduced pervasive increase in tree mortality across Canada's boreal forests[END_REF]. However, none of these studies conclusively attribute trends to iCO2 and other global change (e.g. temperature) and biotic (e.g. pest and pathogens) agents have often been attributed drivers of mortality trends [START_REF] Peng | A droughtinduced pervasive increase in tree mortality across Canada's boreal forests[END_REF][START_REF] Luo | Climate change-associated tree mortality increases without decreasing water availability[END_REF]. Finally, several networks observed decreases or non-significant changes, e.g. in stem mortality rates in Germany [START_REF] Pretzsch | Forest stand growth dynamics in Central Europe have accelerated since 1870[END_REF] and biomass mortality in tropical Africa (β app = -0.88±2), although multiple-regression estimated that CO 2 increased mortality (β dir,hist = 1.8±4) [START_REF] Hubau | Asynchronous carbon sink saturation in African and Amazonian tropical forests[END_REF].

Organic matter decomposition

Evidence for changes in SOM-decomposition rates comes primarily from experiments. Many eCO2

Accepted Article

This article is protected by copyright. All rights reserved experiments have demonstrated increased plant litter production and allocation of carbon belowground (e.g. [START_REF] Drake | Increases in the flux of carbon belowground stimulate nitrogen uptake and sustain the long-term enhancement of forest productivity under elevated CO(2)[END_REF][START_REF] Iversen | Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2-enrichment[END_REF]. Meta-analysis (53 experiments, primarily FACE and OTC) showed that eCO2 increased litter production (β dir,fut = 0.4±0.1) and SOMdecomposition rates (β dir,fut = 0.34±0.2) [START_REF] Van Groenigen Kj Van, Qi | Faster Decomposition Under Increased Atmospheric CO2 Limits Soil Carbon Storage[END_REF], yet priming effects are difficult to detect in field studies [START_REF] Van Groenigen Kj Van, Qi | Faster Decomposition Under Increased Atmospheric CO2 Limits Soil Carbon Storage[END_REF][START_REF] Georgiou | Toward improved model structures for analyzing priming: potential pitfalls of using bulk turnover time[END_REF].

Results from ecosystem-scale experiments indicate some heterogeneity and nuance in these responses. For example, in a scrub oak ecosystem, 6 years of eCO2 increased SOM decay despite unchanged microbial biomass [START_REF] Carney | Altered soil microbial community at elevated CO2 leads to loss of soil carbon[END_REF], and at ORNL FACE a decade of eCO2 resulted in a small but non-significant increase in surface-soil SOM decomposition along with a reduction in microbial nitrogen [START_REF] Iversen | Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2-enrichment[END_REF]. In a later-succession forest, eCO2 increased microbial biomass (β dir,fut = 0.40±0.4) but with no change in soil respiration (β dir,fut = -0.18±0.7) (Bader & Körner, 2010). At EucFACE +30 ppm eCO2 increased soil respiration (β dir,fut = 1.3), but a further increase of 120 ppm produced no additional effect after 3 months (β dir,fut = 0.3) or 3 years (β dir,fut = 0.21) [START_REF] Drake | Short-term carbon cycling responses of a mature eucalypt woodland to gradual stepwise enrichment of atmospheric CO2 concentration[END_REF][START_REF] Drake | Three years of soil respiration in a mature eucalypt woodland exposed to atmospheric CO2 enrichment[END_REF]. This 3-year response was non-significant but accounted for about half of the additional carbon acquired under eCO2 [START_REF] Jiang | The fate of carbon in a mature forest under carbon dioxide enrichment[END_REF].

Data on long-term changes in SOM decomposition in response to iCO2 remains limited. Synthesis of 23 flux-towers with increased GPP (Section III.1) suggested a non-significant increase in ecosystem respiration (R e ) (β app =0.58±1) [START_REF] Fernández-Martínez | Atmospheric deposition, CO 2 , and change in the land carbon sink[END_REF]. Synthesis and statistical upscaling of chamber measurements suggested that global soil respiration has increased (β app =0.22) [START_REF] Bond-Lamberty | Temperature-associated increases in the global soil respiration record[END_REF]. Statistical predictors of this trend include temperature anomaly and year (possibly an iCO2 effect). Notably, heterotrophic respiration would be expected to increase if C soil increased, even with no change in decomposition rates.

Accelerated SOM decomposition may release nutrients and feed back onto the activity of plant processes. For example, at Duke FACE increased root exudation (β dir,fut = 1.1±0.6) was coupled with a non-significant but substantial increase in microbial biomass (β dir,fut = 1.1±1.3) and production of nitrogen-acquiring extracellular enzymes [START_REF] Phillips | Enhanced root exudation induces microbial feedbacks Accepted Article This article is protected by copyright. All rights reserved to N cycling in a pine forest under long-term CO2 fumigation[END_REF]. Exoenzyme activity was increased at Duke and Rhinelander FACE [START_REF] Larson | Extracellular Enzyme Activity Beneath Temperate Trees Growing Under Elevated Carbon Dioxide and Ozone[END_REF]Finzi et al., 2006), although no change in nitrogen mineralisation was observed in lab incubations [START_REF] Zak | Soil Nitrogen Cycling Under Elevated Co2: A Synthesis of Forest Face Experiments[END_REF], perhaps suggesting that stimulation of microbial activity required plant inputs. Conversely, leaf δ 15 N suggests that eCO2 may have increased nitrogen mineralisation but not ring-width in mature trees in a European forest [START_REF] Bader | Central European hardwood trees in a high-CO2 future: synthesis of an 8-year forest canopy CO2 enrichment project[END_REF]. eCO2 increased nitrogen and phosphorus mineralisation for a limited period at

Accepted Article

This article is protected by copyright. All rights reserved

EucFACE [START_REF] Hasegawa | Elevated carbon dioxide increases soil nitrogen and phosphorus availability in a phosphorus-limited Eucalyptus woodland[END_REF] and enzyme activity in an alpine forest [START_REF] Souza | Responses of soil extracellular enzyme activities to experimental warming and CO2 enrichment at the alpine treeline[END_REF].

Conversely, meta-analysis suggests eCO2 increased fine root C:N ratios (β dir,fut = 0.13) (Nie et al., 2013), which are associated with lower decomposability.

Contrasting mycorrhizal associations have been linked to biomass responses under low soil nitrogen conditions [START_REF] Phillips | The mycorrhizal-associated nutrient economy: a new framework for predicting carbon-nutrient couplings in temperate forests[END_REF][START_REF] Terrer | Mycorrhizal association as a primary control of the CO2 fertilization effect[END_REF]. Ectomycorrhizal (ECM) fungi are assumed capable of stimulating SOM decomposition, while arbuscular mycorrhizal (AM) fungi are not, resulting in increased nitrogen in aboveground BP in ECM trees but not in AM plants, primarily grasses [START_REF] Terrer | Ecosystem responses to elevated CO2 governed by plant-soil interactions and the cost of nitrogen acquisition[END_REF]. Conversely, AM association with Avena fatua in a lab and field setting increased SOM-decomposition rates under eCO2 (β dir,fut = 1.4) [START_REF] Cheng | Arbuscular Mycorrhizal Fungi Increase Organic Carbon Decomposition Under Elevated CO2[END_REF].

Terrestrial ecosystem carbon

Direct evidence from site-scale studies

In the four longest-running FACE experiments eCO2 over a decade increased C veg increment (β dir,fut = 0.60±0.4) in these early-succession temperate forests [START_REF] Walker | Decadal biomass increment in early secondary succession woody ecosystems is increased by CO 2 enrichment[END_REF]. eCO2 of geological origin increased tree basal area in 30 year old trees (β dir,fut = 0.23-0.39) [START_REF] Hättenschwiler | Thirty years of in situ tree growth under elevated CO2: a model for future forest responses?[END_REF].

Conversely, in the later-succession forest at EucFACE, four years of eCO2 did not increase C veg increment [START_REF] Jiang | The fate of carbon in a mature forest under carbon dioxide enrichment[END_REF], likely because of phosphorus limitation [START_REF] Ellsworth | Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil[END_REF]. Other experiments in later-succession forests did not quantify C veg . Meta-analysis and extrapolation (138 experiments) predicted a global increase in C veg (β dir,fut = 0.22±0.1) related to soil C:N ratio in AMassociated ecosystems and soil phosphorus in ECM-associated ecosystems [START_REF] Terrer | Nitrogen and phosphorus constrain the CO 2 fertilization of global plant biomass[END_REF].

Biomass responses were generally higher in ECM systems than in AM systems [START_REF] Terrer | Mycorrhizal association as a primary control of the CO2 fertilization effect[END_REF], while another meta-analysis showed analogous biomass responses in trees compared with grasses [START_REF] Song | A meta-analysis of 1,119 manipulative experiments on terrestrial carbon-cycling responses to global change[END_REF].

Synthesis of meta-analyses found that eCO2 increased C soil across all (>200) experiments analysed (β dir,fut = 0.039±0.03) but not in field experiments lasting ≥2 years without nitrogen addition (25) (β dir,fut = 0.0054±0.03) [START_REF] Hungate | Assessing the effect of elevated carbon dioxide on soil carbon: a comparison of four meta-analyses[END_REF]. However, C soil responses to eCO2 at individual sites are mixed. For example, a decade of eCO2 increased C soil at ORNL FACE (β dir,fut = 0.51±0.6, 0-90 cm) [START_REF] Iversen | Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2-enrichment[END_REF] and in a desert ecosystem (β dir,fut = 0.59±0.62) [START_REF] Evans | Greater ecosystem carbon in the Mojave Desert after ten years exposure to elevated CO 2[END_REF], but not in a scrub oak ecosystem (β dir,fut = -0.15±0.5) [START_REF] Hungate | Cumulative response of ecosystem carbon and nitrogen stocks to chronic CO2 exposure in a subtropical oak woodland[END_REF]. In the desert ecosystem, inorganic carbonate pools may have contributed to increases in C soil through nocturnal CO 2 uptake (Hamerlynk et al. 2013) though net effects are likely small (Soper et al., 2016).

Accepted Article

This article is protected by copyright. All rights reserved

Given limited data, litter addition experiments can also provide insight. Synthesis of priming responses to litter addition (26 studies) suggested that 32 % of litter inputs accumulate as C soil [START_REF] Liang | More replenishment than priming loss of soil organic carbon with additional carbon input[END_REF]. 10-30 years of doubled aboveground-litter inputs in temperate forests increased C soil at two sites (29±13 % and 33±28 %) but had no effect at three sites [START_REF] Lajtha | The detrital input and removal treatment (DIRT) network: Insights into soil carbon stabilization[END_REF], nor in one tropical forest [START_REF] Sayer | Tropical forest soil carbon stocks do not increase despite 15 years of doubled litter inputs[END_REF]. Based on these responses and assuming doubled CO 2 doubles litter production (which is unlikely), β dir,hist would range from 0 to 0.41±0.3.

Measurement of NEP requires whole-ecosystem enclosure, thus data are few. In a US salt-marsh higher rates of NEP were sustained over 19 years in both C3 and C4 communities (Drake 2014). A data-assimilation approach provided a comprehensive carbon budget at EucFACE showing no change in C eco [START_REF] Jiang | The fate of carbon in a mature forest under carbon dioxide enrichment[END_REF].

Indirect evidence from global and regional studies Spatially-explicit atmospheric [CO 2 ] measurements, fossil-fuel emissions, and other data are integrated using atmospheric transport models to infer terrestrial net biome production (NBP). These "inversions" suggest a global NBP of 2.3±0.9 (MACC-II), 2.3±1.5 (Jena-CarboScope) (1995-2014;[START_REF] Fernández-Martínez | Global trends in carbon sinks and their relationships with CO2 and temperature[END_REF], and 1.9±0.5 PgC y -1 (2010-2014) [START_REF] Li | Recent Changes in Global Photosynthesis and Terrestrial Ecosystem Respiration Constrained From Multiple Observations[END_REF] 1±1.5 (1992-1996) and -0.9±0.9 PgC y -1 (2001-2004) [START_REF] Gurney | Transcom 3 inversion intercomparison: Model mean results for the estimation of seasonal carbon sources and sinks[END_REF][START_REF] Peylin | Global atmospheric carbon budget: results from an ensemble of atmospheric CO 2 inversions[END_REF]. Combined with land-use change-related NBP, these inversion results suggest small 'natural' NBP in the tropics (c. 0.3-0.5).

However, analysis of the vertical atmospheric [CO 2 ] gradient suggested close-to-neutral tropical NBP [START_REF] Stephens | Weak Northern and Strong Tropical Land Carbon Uptake from Vertical Profiles of Atmospheric CO2[END_REF], implying 'natural' NBP of similar magnitude and opposite sign to land-use change-related NBP, attributed primarily to iCO2 [START_REF] Schimel | Effect of increasing CO2 on the terrestrial carbon cycle[END_REF].

Flask, aircraft, and satellite-based measurements show trends in the seasonal-cycle amplitude of [CO 2 ] since c. 1960 [START_REF] Keeling | Increased activity of northern vegetation inferred from atmospheric CO2 measurements[END_REF][START_REF] Graven | Enhanced Seasonal Exchange of CO2 by Northern Ecosystems Since 1960[END_REF][START_REF] Yin | Changes in the Response of the Northern Hemisphere Carbon Uptake to Temperature Over the Last Three Decades[END_REF], implying seasonal intensification of northern NBP (β app = 2.2±0.6) [START_REF] Graven | Enhanced Seasonal Exchange of CO2 by Northern Ecosystems Since 1960[END_REF]. iCO2 has been implicated as a major driver of these trends (Forkel et al., 2016;[START_REF] Bastos | Contrasting effects of CO 2 fertilization, landuse change and warming on seasonal amplitude of Northern Hemisphere CO 2 exchange[END_REF], though increasing crop

Accepted Article

This article is protected by copyright. All rights reserved production [START_REF] Gray | Direct Accepted Article This article is protected by copyright. All rights reserved human influence on atmospheric CO 2 seasonality from increased cropland productivity[END_REF][START_REF] Zeng | Agricultural Green Revolution as a driver of increasing atmospheric CO2 seasonal amplitude[END_REF] and warming-induced increasing vegetation cover [START_REF] Keenan | Greening of the land surface in the world's cold regions consistent with recent warming[END_REF] are also likely candidates.

Carbon budgeting estimated global 'natural' NBP at 3.6±1.0 PgC y -1 (2009-2018) and 141 PgC since 1959 from the budget residual, and 3.2±1.2 PgC y -1 and 130 PgC from process-based models [START_REF] Friedlingstein | Global Carbon Budget 2019[END_REF]. Based on the residual estimate of 'natural' NBP and the lower and upper bounds of either global vegetation or global ecosystem carbon stocks results in β app = 0.93-1.4

(assuming all the sink is in vegetation) or β app = 0.18-0.29 for ecosystem carbon (global vegetation and non-permafrost soils).

Synthesis and extrapolation of global inventory data suggested increased C eco (β app = 1.0±0.6), C veg (β app = 1.9), C soil (β app = 0.31), litter carbon (β app = 0.92), and dead wood carbon (β app = 0.64) (Pan et al., 2011). Little additional data on C soil changes over the historical period are available. Evidence from multi-plot forest-inventory data consistently shows net gains in wood C veg in recent decades in tropical Africa (β app = 0.77) [START_REF] Hubau | Asynchronous carbon sink saturation in African and Amazonian tropical forests[END_REF], the Amazon (β app = 0.69) [START_REF] Brienen | Long-term decline of the Amazon carbon sink[END_REF], Borneo (β app = 0.48±0.3) [START_REF] Qie | Long-term carbon sink in Borneo's forests halted by drought and vulnerable to edge effects[END_REF], and in large 50 ha plots across the tropics (β app = 0.30±0.24) [START_REF] Chave | Assessing Evidence for a Pervasive Alteration in Tropical Tree Communities (GM Mace[END_REF]. Wood C veg also increased in plots across the Eastern US (β app = 2.9±1.5) [START_REF] Mcmahon | Evidence for a recent increase in forest growth[END_REF] and globally (β app = 0.82±0.5) [START_REF] Yu | Pervasive decreases in living vegetation carbon turnover time across forest climate zones[END_REF]. Long-term geological CO 2 release was associated with reduced lidar-estimated aboveground C veg [START_REF] Cawse-Nicholson | Ecosystem responses to elevated CO2 using airborne remote sensing at Mammoth Mountain, California[END_REF].

IV. Synthesis 1 Evidence for the CO 2 -fertilisation hypothesis

In this section we integrate and interpret the evidence for change in the components of the carbon cycle during the historical record concurrent with increasing [CO 2 ] (iCO2; c. 280-400 ppm), in response to elevated [CO 2 ] (eCO2; c. 390-500 ppm), and the probability and magnitude of iCO2 as a driving factor in the historical change. In doing so we acknowledge that we are mixing evidence across scales, measurements, methods of analysis, and in some cases different variables that may not be perfectly comparable. However, this is required for a broad synthesis and a formal meta-

Accepted Article

This article is protected by copyright. All rights reserved analysis is not our intention. We assign confidence as either "high" (all estimates agree), "medium" (estimate means disagree, substantial overlap in confidence intervals), or "low" (estimate means disagree, little overlap in confidence intervals).

Physiology

A number of independent lines of indirect evidence-ice-core OCS [START_REF] Campbell | Large historical growth in global terrestrial gross primary production[END_REF] and O 18 [START_REF] Ciais | Large inert carbon pool in the terrestrial biosphere during the Last Glacial Maximum[END_REF], glucose isotopomers [START_REF] Ehlers | Detecting longterm metabolic shifts using isotopomers: CO2-driven suppression of photorespiration in C3 plants over the 20th century[END_REF], satellite ET [START_REF] Cheng | Recent increases in terrestrial carbon uptake at little cost to the water cycle[END_REF], and flux-partitioned eddy-covariance (Fernández-Martínezet al., 2017)-provide high confidence that terrestrial GPP has increased concurrently with iCO2. Estimates of the GPP increase disagree by a factor of 1.7 (β app = 0.95-1.6, mean = 1.2; Table 2), but overlap in confidence intervals (Fig. 3, Fig. S2) indicates that these estimates are consistent and suggests medium confidence in the magnitude of the increase in GPP concurrent with iCO2. Above the canopy-scale GPP can be measured only indirectly, and most of these estimates are a function of the [CO 2 ] trend (Box 3; isotopomers, satellite, OCS) which introduces a circularity. However, we place less confidence in estimates (usually satellite-based) that omit a CO 2 effect from the theory used in their GPP estimation (Box 3;[START_REF] De Kauwe | Satellite based estimates underestimate the effect of CO2 fertilization on net primary productivity[END_REF]. Flux-partitioned eddy-covariance provides the only estimate of GPP that does not require [CO 2 ] in its calculation and provides the highest β app of 1.6±0.9 [START_REF] Fernández-Martínez | Atmospheric deposition, CO 2 , and change in the land carbon sink[END_REF]. A smaller proportion of this change was attributed to iCO2 (β dir,hist = 1.2±0.6).

Synthesis of direct evidence from experiments provides high confidence that ecosystem-scale eCO2 increases diurnal photosynthesis in leaves (β dir,fut = 0.68±0.2). This increase is very similar to the theoretical value for a light-saturated leaf (β dir,fut = 0.70±0.2, Table S1). The theoretical value for the canopy-scale photosynthesis response to iCO2 (280-410 ppm, β dir,fut = 0.60±0.3, Table S1) is about half the observed mean increase in GPP concurrent with iCO2 (β dir,hist = 1.2). For iCO2 to be the sole driver of the observed responses would require all leaves to be operating at the lightsaturated rate of increase and would require additional positive feedbacks of equivalent magnitude.

The majority of global models tend to follow the theoretical response to iCO2 [START_REF] Keenan | Recent pause in the growth rate of atmospheric CO2 due to enhanced terrestrial carbon uptake[END_REF]. A carbon cycle model was able to replicate the OCS increase in GPP (β app = 0.95±0.2) and change in northern seasonal [CO 2 ] amplitude by hypothesising leaf optimisation and predicting a substantial increase in LAI (note the phosphorus cycle was disabled) [START_REF] Haverd | Higher than expected CO2 fertilization inferred from leaf to global observations[END_REF]. However, it is not clear that leaves optimise as hypothesised (Smith and Keenan, 2020), and models consistently represent allocation and LAI simplistically. For example, LAI trends are inferred in high-LAI tropical

Accepted Article

This article is protected by copyright. All rights reserved rainforests [START_REF] Zhu | Greening of the Earth and its drivers[END_REF]. In these regions models are likely predicting an increase in maximum LAI, which conflicts with experimental evidence and resource investment theory. An alternative hypothesis is that iCO2 accelerates the recovery of forest-gaps such that landscape-scale LAI is greater-a hypothesis not represented by any of the models used for attribution. Outside of tropical forests, changes in LAI are related to both iCO2 [START_REF] Donohue | Impact of CO2 fertilization on Accepted Article This article is protected by copyright. All rights reserved maximum foliage cover across the globe's warm, arid environments[END_REF] and temperature-stimulated increases in growing season length [START_REF] Keenan | Greening of the land surface in the world's cold regions consistent with recent warming[END_REF]). An additional consideration is that models tend to under-estimate GPP relative to solar-induced fluorescence (a GPP proxy) in agricultural regions [START_REF] Zeng | Agricultural Green Revolution as a driver of increasing atmospheric CO2 seasonal amplitude[END_REF]Walker et al., 2017), agriculture being another major factor of global change. Taken together, we place high confidence that the historical GPP increase was primarily driven by iCO2 and also that iCO2 was not the sole driving factor. However, it is unclear which factors might be driving the additional change in GPP.

A number of independent lines of indirect evidence-tree-ring δ 13 C (e.g. [START_REF] Saurer | Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years[END_REF][START_REF] Peñuelas | Increased water-use efficiency during the 20th century did not translate into enhanced tree growth[END_REF]Frank et al., 2015), flux-partitioned eddy-covariance [START_REF] Keenan | Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise[END_REF]Mastrotheodorus et al., 2017), and atmospheric δ 13 C [START_REF] Keeling | Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis[END_REF]-provide high confidence that iWUE (across leaf to global scales) and WUE (across leaf to ecosystem scales) have increased over the historical period (β app = 0.85-3.9, mean = 1.5). There remain large differences (factor of 5) between these estimates of the increase, primarily due to the eddycovariance estimates (β app = 2.4±2.0 and 3.9±2.5). The causes for these differences are not fully understood, though scale (Medlyn et al., 2017), plasticity (Mastrotheodorus et al., 2017), high variability and short time scales (indicated by the high uncertainty), and GPP trends that are higher than expected from iCO2 alone (see above) all play a role. Eddy covariance estimates skew the mean and the modal change is around β app = 1 (Fig. 3), similar to the mean for iCO2-attribution studies (β dir,hist = 0.80) and the theoretical value for iWUE (β dir,hist = 1.1). As with GPP, other than eddy-covariance these indirect methods use [CO 2 ] in their calculation (Box 3). Satellite estimates of WUE suffer from very short time-periods (13 years) with low signal-to-noise ratio, leaving little confidence in these trend estimates. Direct evidence from multiple experiments support iWUE and WUE increases (β dir,fut = 0.65-1.6, mean=1.1) in agreement with predictions from theory (Figure 2).

Taken together this evidence provides high confidence that iCO2 has increased iWUE, medium confidence that the magnitude is in accordance with theory, and low confidence in the magnitude of the historical change in WUE.

How do these changes in iWUE translate to changes in water use? Theory predicts that iWUE (A net /g s ) responses are very tightly constrained (β dir c. 1), so if the change in A net is below 1, g s will

Accepted Article

This article is protected by copyright. All rights reserved decrease [START_REF] Barton | Effects of elevated atmospheric [CO2] on instantaneous transpiration efficiency at leaf and canopy scales in Eucalyptus saligna[END_REF]. The observed changes in GPP (β app c. 1) suggest that widespread and broad-scale reductions in g s may not have occurred. Reductions in stomatal conductance may occur at points in time or space but as spatial and temporal scale increases, iCO2-induced decreases in stomatal conductance likely translate into smaller decreases in transpiration (Field et al. 1995;[START_REF] Körner | CO2 Fertilization: When, Where, How Much?[END_REF].

Increased vegetation cover in semi-arid regions [START_REF] Donohue | Impact of CO2 fertilization on Accepted Article This article is protected by copyright. All rights reserved maximum foliage cover across the globe's warm, arid environments[END_REF][START_REF] Ukkola | Reduced streamflow in water-stressed climates consistent with CO 2 effects on vegetation[END_REF], increased rooting depth (Y. Yang et al., unpublished;[START_REF] Iversen | Digging deeper: fine-root responses to rising atmospheric CO2 concentration in forested ecosystems[END_REF], soil-water feedback on g s , competition, and atmospheric coupling [START_REF] Jarvis | Stomatal Control of Transpiration: Scaling Up from Leaf to Region[END_REF][START_REF] Buckley | Optimal plant water economy[END_REF][START_REF] Sperry | The impact of rising CO2 and acclimation on the response of US forests to global warming[END_REF][START_REF] Sabot | Plant profit maximization improves predictions of European forest responses to drought. New Accepted Article This article is protected by copyright[END_REF]) are all mechanisms that may lead to no change in water use at larger scales. This is especially likely to be the case in water-limited regions where long-term transpiration is primarily precipitation driven [START_REF] Fatichi | Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO 2[END_REF] i.e. plants use the water that is available.

Biomass production

Ecosystem-scale forest-inventory networks suggest increases in wood BP concurrent with iCO2

(mean β app c. 1; [START_REF] Brienen | Long-term decline of the Amazon carbon sink[END_REF][START_REF] Yu | Pervasive decreases in living vegetation carbon turnover time across forest climate zones[END_REF][START_REF] Hubau | Asynchronous carbon sink saturation in African and Amazonian tropical forests[END_REF]. Conversely, evidence from tree-rings is mixed (e.g. [START_REF] Peñuelas | Increased water-use efficiency during the 20th century did not translate into enhanced tree growth[END_REF][START_REF] Silva | Probing for the influence of atmospheric CO2 and climate change on Accepted Article This article is protected by copyright. All rights reserved forest ecosystems across biomes[END_REF]. Both of these methods are subject to potential sampling biases (Box 3). However, the tree-ring biases are potentially larger and can be either positive (Nehbas-Ahles et al., 2014) or negative [START_REF] Brienen | Tree demography dominates long-term growth trends inferred from tree rings[END_REF]. The inventory evidence provides medium confidence in an increase in wood BP over the historical period, with low confidence in the magnitude (β app c. 1). However, this is an area of disagreement among several in our authorship group.

Many studies show increased BP in response to eCO2 (e.g. [START_REF] Baig | Does the growth response of woody plants to elevated CO2 increase with temperature? A model-oriented meta-analysis[END_REF], but these studies are often short-lived and under artificial conditions. Evidence from long-term, large-scale FACE experiments (<10 experiments) is mixed, with both increases (e.g. Norby et al., 2005) and no change in BP observed (e.g. [START_REF] Jiang | The fate of carbon in a mature forest under carbon dioxide enrichment[END_REF][START_REF] Bader | Central European hardwood trees in a high-CO2 future: synthesis of an 8-year forest canopy CO2 enrichment project[END_REF] (β dir,fut = -0.3-0.56, mean = 0.19). Many studies show a BP response to eCO2 that is higher at sites with higher nutrient availability (e.g. [START_REF] Terrer | Ecosystem responses to elevated CO2 governed by plant-soil interactions and the cost of nitrogen acquisition[END_REF], that is greater when nutrients were added (e.g. [START_REF] Sigurdsson | Growth of mature boreal Norway spruce was not affected by elevated [CO2] and/or air temperature unless nutrient availability was improved[END_REF][START_REF] Reich | Nitrogen limitation constrains sustainability of ecosystem response to CO2[END_REF], or no response when nutrients are low (e.g. [START_REF] Sigurdsson | Growth of mature boreal Norway spruce was not affected by elevated [CO2] and/or air temperature unless nutrient availability was improved[END_REF][START_REF] Ellsworth | Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil[END_REF]. However, strong evidence for the widely held progressive nitrogen limitation hypothesis is restricted to two experiments (Biocon, ORNL) [START_REF] Reich | Nitrogen limitation constrains sustainability of ecosystem response to CO2[END_REF]Norby et al., 2010). At both of these experiments nutrient dynamics also caused declining BP in the ambient treatments, indicating that eCO2 responses can be tied, via nutrient availability, to underlying ecosystem dynamics.

Accepted Article

This article is protected by copyright. All rights reserved BP responses were observed in earlier-succession more-disturbed ecosystems, which also tend to have higher nutrient availability [START_REF] Körner | Plant CO2 responses: an issue of definition, time and resource supply[END_REF]. The experiments with no response were often situated in later-succession forests, some of which were also severely limited by nutrients. The forest inventories in which BP increases concurrently with iCO2 were observed in later-succession, primarily tropical, forests that are assumed to be strongly nutrient limited. These inventory responses are high (β app c. 1) compared with the results from experiments even in earlier-succession forests (β dir,fut = 0.49±0.3). However, the evidence is insufficient to robustly evaluate how eCO2 affects latesuccessional and tropical forests. Taken together, this evidence suggests high confidence that eCO2 can stimulate BP (β dir,fut c. 0.5), that the response is diminished by nutrient limitations, and that the observed inventory response is likely due to iCO2 and additional factors.

Vegetation mortality

A number of independent plot networks provide high confidence that tree mortality has increased over the historical period but low confidence in the magnitude (β app = -1.2-7.4, mean = 2.8; Fig.

3, Fig. S3). The greatest changes are primarily attributed to drought. Causes of mortality are often stochastic, multi-factorial, and play out over long time periods, making trend identification and attribution at ecosystem and landscape scales uncertain [START_REF] Mcmahon | The importance and challenges of detecting changes in forest mortality rates[END_REF]. For individual scale mortality, an intra-specific growth-survival tradeoff is apparent for some species (e.g. Di Fillipo et al., 2015), which would reduce lifespans if iCO2 increases wood BP. However an intra-specific growthsurvival tradeoff is not ubiquitous among species (e.g. [START_REF] Cailleret | A synthesis of radial growth patterns preceding tree mortality[END_REF].

Greenhouse eCO2 experiments suggest that eCO2 does not reduce drought-related mortality (e.g. [START_REF] Duan | Elevated [CO2] does not ameliorate the negative effects of elevated temperature on drought-induced mortality in Eucalyptus radiata seedlings[END_REF][START_REF] Bachofen | No carbon "bet hedging" in pine Accepted Article This article is protected by copyright. All rights reserved seedlings under prolonged summer drought and elevated CO2[END_REF]. However, eCO2 commonly increased leaf area in these experiments, increasing transpiration that likely exacerbated mortality risk [START_REF] Duan | CO2 and temperature effects on morphological and physiological traits affecting risk of drought-induced mortality[END_REF]. What does this mean for eCO2 responses in ecosystems? Due to the juvenile growth stage of these plants, leaf area increases were much higher than expected in closed canopy systems (see Box 2), and increased root BP from eCO2 would exacerbate pot-volume constraints on root proliferation.

Inference from these experiments is limited. At the stand scale there is very limited evidence that eCO2 might change self-thinning relationships allowing higher basal area for a given stem density [START_REF] Kubiske | Increasing Atmospheric CO2 Concentration Stand Accepted Article This article is protected by copyright. All rights reserved Development in Trembling Aspen Forests: Are Outdated Density Management Guidelines in Need of Revision for All Species?[END_REF]. Evidence for changes in mortality caused by iCO2 is weak and mostly indirect with limited support for both increases and decreases in individual and stand-scale mortality rates.

Taken together (CS11) the response of mortality to iCO2 and eCO2 is unknown, even the direction of change is unclear.

Accepted Article

This article is protected by copyright. All rights reserved

Organic matter decomposition

The few studies of soil or ecosystem respiration show small (β app = 0.22; Bond-Lamberty & Thompson 2010) or non-significant increases (β app = 0.58±1; [START_REF] Fernández-Martínez | Atmospheric deposition, CO 2 , and change in the land carbon sink[END_REF].

These trends could possibly be related to increasing heterotrophic respiration and decomposition, but increasing temperature is inferred as the cause and not iCO2 (e.g. [START_REF] Bond-Lamberty | Globally rising soil heterotrophic respiration over recent decades[END_REF].

Due to the low number of studies there is low confidence that SOM decomposition has increased over the historical period, but it is unknown whether SOM decomposition rates have increased.

Evidence from eCO2 experiments generally supports the theory that rising [CO 2 ] increases SOMdecomposition rates (e.g. van Greonigen et al., 2015) due to increases in microbial biomass, rhizosphere priming, mycorrhizal association, and increases in soil water content (see refs in Section III.4). Smaller changes in decomposition rates have been associated with lower microbial biomass and higher soil water (Bader & Körner, 2010;[START_REF] Iversen | Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2-enrichment[END_REF]. Taken together, the evidence suggests medium confidence that eCO2 increases rates of SOM decomposition but with low confidence in the magnitude. Increasing SOM decomposition will also release nutrients that may be available for plant growth and BP. Plant nutrient acquisition through mycorrhizal and other rootmicrobe interactions are likely mediators of this process [START_REF] Terrer | Ecosystem responses to elevated CO2 governed by plant-soil interactions and the cost of nitrogen acquisition[END_REF]. Notably, the large stepchange in eCO2 experiments compared with the more gradual iCO2, could lead to a greater imbalance of available resources resulting in a carbon surplus (Box 3) that could fuel greater microbial activity. It is worth noting that increased SOM-decomposition rates do not necessarily imply lower C soil litter inputs are also increasing [START_REF] Liang | More replenishment than priming loss of soil organic carbon with additional carbon input[END_REF].

Terrestrial ecosystem carbon

Multiple independent lines of evidence-global-scale carbon budgeting [START_REF] Friedlingstein | Global Carbon Budget 2019[END_REF], atmospheric inversions (e.g. [START_REF] Peylin | A new stepwise carbon cycle data assimilation system using multiple data streams to constrain the simulated land surface carbon cycle[END_REF][START_REF] Fernández-Martínez | Global trends in carbon sinks and their relationships with CO2 and temperature[END_REF], seasonal [CO 2 ] amplitude trends [START_REF] Graven | Enhanced Seasonal Exchange of CO2 by Northern Ecosystems Since 1960[END_REF], and forest inventories (e.g. Pan et al., 2011;[START_REF] Hubau | Asynchronous carbon sink saturation in African and Amazonian tropical forests[END_REF]-imply a CO 2 sink in terrestrial ecosystems (Fig. 3, Fig. S4). CO2 effects on terrestrial carbon are convolved with the effects of concurrent anthropogenic changes in climate, nitrogen deposition, and land-use change, including agricultural intensification and fire management. Attribution analyses indicate a primary role for iCO2 (e.g., [START_REF] Schimel | Effect of increasing CO2 on the terrestrial carbon cycle[END_REF][START_REF] Keenan | Recent pause in the growth rate of atmospheric CO2 due to enhanced terrestrial carbon uptake[END_REF][START_REF] Bastos | Contrasting effects of CO 2 fertilization, landuse change and warming on seasonal amplitude of Northern Hemisphere CO 2 exchange[END_REF][START_REF] Fernández-Martínez | Global trends in carbon sinks and their relationships with CO2 and temperature[END_REF][START_REF] Haverd | Higher than expected CO2 fertilization inferred from leaf to global observations[END_REF].

These analyses depend on the inclusion of accurate explanatory-variable datasets and accurate process representation in models, which may not be the case. Quantification of the effect of iCO2 on carbon storage in terrestrial ecosystems remains elusive.

As with BP responses, studies of forest inventories show higher C veg responses (β app = 0.3-2, mean = 0.85) than studies of eCO2 experiments (β app = 0.22-0.39) (Fig. 3). However, the highest values come from two analyses: one that includes global forest regrowth (β app = 1.9; Pan et al., 2011) and younger (c. 50-100 years old) temperate forests (β app = 2±1; McMahon et al., 2011). Exclusion of these higher change studies results in a narrower range (β app = 0.3-0.85, mean = 0.57). This exclusion narrows the difference between responses inferred from iCO2 and eCO2 studies, which is consistent with theory as relative stock changes are under-estimated more in short-term experiments than in inventory-type studies (Fig. S2). Responses of vegetation carbon increment may give a more accurate estimate of responses in systems that are far from equilibrium when initially exposed to eCO2 (Fig. S2). Vegetation carbon increment responses estimated from FACE experiments (β app = 0.60±0.4; [START_REF] Walker | Decadal biomass increment in early secondary succession woody ecosystems is increased by CO 2 enrichment[END_REF] are consistent with the reduced range from inventory studies.

However, the theoretical under-estimation of undisturbed forest-inventory responses (Fig. S2) yet similarity of these responses with those from disturbed forests subjected to eCO2 and not the lower values from undisturbed forests (e.g. [START_REF] Jiang | The fate of carbon in a mature forest under carbon dioxide enrichment[END_REF], requires further consideration. Either eCO2 experiments are under-estimating responses or other factors have affected the inventory evidence.

Both of these evidence types are likely missing the full extent of mortality (e.g. [START_REF] Chambers | The steady-state mosaic of disturbance and succession across an old-growth Central Amazon forest landscape[END_REF], and evidence from larger-scale 50 ha plots suggests a lower response for intact tropical forests (β app = 0.30±0.2; [START_REF] Chave | Assessing Evidence for a Pervasive Alteration in Tropical Tree Communities (GM Mace[END_REF].

Evidence for changes in C soil is mixed and context dependent. On average there is no detectable

Accepted Article

This article is protected by copyright. All rights reserved response across experiments [START_REF] Hungate | Assessing the effect of elevated carbon dioxide on soil carbon: a comparison of four meta-analyses[END_REF], though at some individual sites, C soil did accumulate (e.g. [START_REF] Iversen | Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2-enrichment[END_REF][START_REF] Evans | Greater ecosystem carbon in the Mojave Desert after ten years exposure to elevated CO 2[END_REF]. The only study (to our knowledge) of soil carbon changes concurrent with iCO2 suggests a relative response in global forests (β app = 0.31; Pan et al., 2011), that would be substantial if extrapolated to mineral soils globally. As with vegetation carbon stocks, the long-term, relative responses of soil carbon stocks are likely underestimated by short-term eCO2 experiments (Fig. S2). Taken together, evidence suggests medium confidence that eCO2 increases ecosystem carbon stocks over short to medium timescales and iCO2 has contributed to the change over the historical period, but with low confidence in the magnitude.

2 What we need to know

Confidence in the magnitude of CO 2 effects is generally low. In particular, iCO2-attribution is a major challenge in testing the CO 2 -fertilisation hypothesis over the historical period. Attribution often relies on empirical regression which simply indicates correlation, anything with a trend over the historical period will correlate with iCO2. We advocate using log-log β as a stable (Notes S1; Fig. S1), relativised metric for comparison with theoretical expectations and other studies.

Process-based models are also used to deconvolve causation from multiple global-change factors.

Models often represent key mechanisms over-simplistically and yet are also equifinal, while model ensembles represent a non-random sample of non-independent models [START_REF] Beven | A manifesto for the equifinality thesis[END_REF][START_REF] Fatichi | Modelling carbon sources and sinks in terrestrial vegetation[END_REF][START_REF] Sanderson | A fiery wake-up call for climate science[END_REF]. Thus, models need always to be interpreted in the context of the mechanisms they represent, those they do not, how representations might bias results, and how well they reproduce observations (e.g. [START_REF] Medlyn | Using ecosystem experiments to improve vegetation models[END_REF]. Mechanistic models (or modules) of BP, resource acquisition and allocation, how soil and plant water status affect g s , plant-microbe effects on soil decomposition, vegetation structure and demography (e.g. competition, mortality), and land-use need to be applied more extensively to the CO 2 -fertilisation hypothesis. Alternative hypotheses to explain observed phenomena should be evaluated within model ensembles, and calibrated to allow the hypotheses to compete on an equal footing (e.g. Zhang et al., 2015). Agile and extensible models (e.g. [START_REF] Clark | A unified approach for process-based hydrologic modeling: 1. Modeling concept[END_REF][START_REF] Walker | The multiassumption architecture and testbed (MAAT v1.0): R code for generating ensembles with dynamic model structure and analysis of epistemic uncertainty from multiple sources[END_REF] will be needed to rapidly incorporate this understanding, including uncertainty, into the internally-consistent and quantitative systems-level theory that models represent.

It is crucial that future eCO2 experiments are designed and resourced to understand the mechanistic basis for responses (or lack thereof) and do not simply report significance or effect-sizes. Integration

Accepted Article

This article is protected by copyright. All rights reserved with extensible, process-based models will help evaluate and explore the mechanistic basis for observed responses [START_REF] Medlyn | Using ecosystem experiments to improve vegetation models[END_REF]. During the lifetime of long-term experiments, new hypotheses will arise to explain unexpected or key observations that may help to provide context and mechanisms underlying the observed responses. These long-term experiments represent very large investments and for relatively small additional investment, related studies can test mechanistic hypotheses as they arise during an experiment's lifetime.

Suggestions for high-priority future studies:

Understanding the mechanistic basis for GPP increases observed over the historical period and how this relates to water use. GPP, iWUE, and water use are intimately tied. The mechanisms by which plants might adjust to iCO2 (photosynthetic acclimation/optimisation, more and deeper roots, g s responses to water status) are not fully understood and thus not well explored within models. A quantitative synthesis canopy or stand-scale photosynthetic responses in eCO2 experiments would be informative.

Biomass production inferred from tree-rings and forest inventories reach very different conclusions.

Where possible, studies that can integrate these two types of evidence, such as tree-ring sampling at inventory sites (e.g. [START_REF] Dye | Comparing tree-ring and permanent plot estimates of aboveground net primary production in three eastern U.S. forests[END_REF][START_REF] Evans | Fusing tree-ring and forest inventory data to infer influences on tree growth[END_REF], acknowledging respective biases will be fruitful. The mechanisms underlying how increased GPP leads to increased BP and increased nutrient acquisition through plant-microbe associations are key areas for future study, especially over successional gradients. eCO2 studies in mid and late-succession ecosystems, and tropical, boreal, semi-arid, and savannah ecosystems will help to address the young, temperate ecosystem bias in eCO2 studies.

How iCO2 affects mortality is key for understanding C veg and community responses to iCO2. As mortality is a relatively rare event in established vegetation, change detection and attribution of causation require large-scale, long-term monitoring, and ideally, experiments [START_REF] Hartmann | Research frontiers for improving our understanding of droughtinduced tree and forest mortality[END_REF]. Understanding the mechanics of observed growth-mortality tradeoffs and whether iCO2 may be alleviating mortality in semi-arid regions are high-priority.

Studies of the C soil decomposition rate over the historical period are practically non-existent, additional studies are required. As with BP, efforts to fully understand plant-microbe-soil (and likely invertebrate), carbon-nutrient interactions continue to be a high-priority. Furthermore, investigation of responses in deep soil layers are few or non-existent. Understanding how the opposing processes of increased litter production, root-microbe interactions, increased decomposition rates, and rates of

Accepted Article

This article is protected by copyright. All rights reserved [START_REF] Friedlingstein | Global Carbon Budget 2019[END_REF]. Though the boundary between these fluxes is blurred [START_REF] Pongratz | Terminology as a key uncertainty in net land use and land cover change carbon flux estimates[END_REF]. Integrated studies that consider all of these factors, especially land-use change (including iCO2-acceleration of regrowth following disturbance, e.g. et al., 2019), agriculture, and 'natural' fluxes will yield insight.

Pugh

V. Conclusions

To evaluate the CO 2 -fertilisation hypothesis, we synthesised evidence from wide-ranging disciplines within an integrated theoretical framework. We have medium or high confidence that GPP, iWUE, BP, and mortality have all increased over the historical period. However, we often have low or medium confidence in the magnitude, and low confidence in how much of the change is attributable to iCO2.

The complex nature of the problem demands integrated studies, and further integration is required to fully combine the broad evidence in a way that is scale, bias, and uncertainty aware (Box 3).

Inference regarding trends and responses (or lack thereof) should always be grounded in the context-dependence and biases associated with a particular study. Further experiments and observations are needed to help reconcile differences among evidence streams. For example, treering sampling at flux sites or forest-inventory plots, proximal remote sensing at flux and experiment sites, and model-data integration to reconcile diverse data streams would all help to provide an integrated understanding of this complex problem. A holistic, community-based approach will enable the greatest advances and provide the most robust information to decision makers.

The required size of climate-change mitigation efforts depends directly on how future terrestrial

Accepted Article

This article is protected by copyright. All rights reserved carbon storage evolves. Evidence for the CO 2 -fertilisation hypothesis suggests a highly valuable ecosystem service that is buying us time in the fight against climate change, though the size of this subsidy remains unclear. Based on diminishing theoretical GPP responses, likely increasing nutrient limitations, increasing mortality, and other negative temperature-related effects [START_REF] Peñuelas | Shifting from a fertilization-dominated to a warmingdominated period[END_REF] it is highly likely that increases in terrestrial carbon storage due to iCO2 will decline into the future. A decline in this subsidy will result in accelerated climate change on the current trajectory of anthropogenic CO 2 emissions.

Accepted Article

This article is protected by copyright. All rights reserved

Accepted Article

This article is protected by copyright. All rights reserved

Accepted Article

This article is protected by copyright. All rights reserved storage increment (green dashed), and β dir on 30-year change in carbon storage in elevated CO 2 scenario (blue points). Further modelling details in Supporting Information Notes S3. Grey vertical lines (a, b, d, e, g, h) are at pre-industrial, 2010s, and projected end-of-century [CO 2 ] (280, 400, 550 ppm). Grey vertical lines (c, f, i) are at β dir of 0 (solid) and 1 or -1 (dashed). Grey horizontal lines (j-l) are β dir when both ambient and elevated CO 2 carbon pools are at steady state.

Accepted Article

This article is protected by copyright. All rights reserved

Accepted Article

This article is protected by copyright. All rights reserved

Box 1. The CO 2 -fertilisation hypothesis

The stimulation of photosynthesis by CO 2 has been called "CO 2 fertilisation" [START_REF] Ciais | Carbon and other biogeochemical cycles[END_REF], a term that goes back to global carbon cycle modelling in the 1970's [START_REF] Bacastow | Atmospheric carbon dioxide and radiocarbon in the natural carbon cycle: II. Changes from A. D. 1700 to 2070 as deduced from a geochemical model[END_REF].

However, "CO 2 fertilisation" or "CO 2 fertilisation effect" have been used to refer to the [CO 2 ] response of any number of variables across scales. This broad usage has been a source of confusion and more commonly, "fertilisation" is a value-laden, agricultural term that means the addition of nutrients to increase crop yield. Acknowledging the precedence of the term, its multiple uses, and that CO 2 responses of some processes may be neutral or negative, we opt to refer to "CO 2 responses" of explicitly defined variables and scales.

We reserve the term "CO 2 fertilisation" solely to label the hypothesis that: plant responses to For the CO 2 -fertilisation hypothesis to be true, Eq. B1 must be positive at the global scale and over a specified time period:

NEP = C eco = C veg + C soil (Eq. B1)
where NEP is net ecosystem production, C veg and C soil are plant and soil (including litter and coarse woody debris) terrestrial carbon that sum to give total ecosystem carbon (C eco ), and represents change due to increasing [CO 2 ]. A change in carbon storage is the net result of inputs and outputs [START_REF] Olson | Energy Storage and the Balance of Producers and Decomposers in Ecological Systems[END_REF]:

dC / dt = I -kC (Eq. B2)

where C is stored carbon, I is the input, and k is the turnover rate of the pool (the inverse of mean residence time).

Net primary production (NPP) represents the net input of carbon to C veg and is calculated as gross primary production (GPP), which responds directly to iCO2, minus autotrophic respiration (R a ). In practice, NPP is often estimated from total biomass production (BP), the sum of leaf, wood, root, and reproductive tissue production over a given time period [START_REF] Vicca | Fertile forests produce biomass more efficiently[END_REF]. In addition to BP, NPP includes carbon used for the production of volatiles, root exudation, supply to symbionts, and

Accepted Article

This article is protected by copyright. All rights reserved changes in non-structural carbohydrates (NSCs). However, these carbon fluxes are difficult to measure and often have very short residence times, somewhat akin to respiratory carbon. Therefore, to align with measurements and residence time we use BP to decompose changes in C veg :

dC veg / dt = BP -k veg C veg (Eq. B3)
where k veg is the turnover (litterfall and mortality) rate of vegetation biomass. For soils, the inputs to C soil are vegetation litter production and mortality, as well as non-biomass NPP fluxes (S) that include exudation and carbon supply to symbionts:

dC soil / dt = k veg C veg + S -k soil C soil (Eq. B4)
where k soil represents the turnover rate of soil carbon caused by microbial decomposition.

Accepted Article

This article is protected by copyright. All rights reserved Box 2. Evidence themes Theme 2-Tree growth measurements: Tree rings and forest inventories provide long-term estimates of wood BP in forest ecosystems across the globe (e.g. [START_REF] Hember | Tree Ring Reconstructions of Stemwood Biomass Indicate Increases in the Growth Rate of Black Spruce Trees Across Boreal Forests of Canada[END_REF][START_REF] Hubau | Asynchronous carbon sink saturation in African and Amazonian tropical forests[END_REF]. Tree ring data are annually resolved estimates of individual stem growth over the past decades to millennia (e.g. [START_REF] Babst | A tree-ring perspective on the terrestrial carbon cycle[END_REF]. These data provide insight into individual growth variability in relation to environmental changes including soil moisture, temperature and potentially also iCO2. Repeated inventories of forest ecosystems offer an assessment of forest-scale dynamics and the demographic processes of recruitment, growth, and mortality over the past decades and in some cases around century length [START_REF] Pretzsch | Forest stand growth dynamics in Central Europe have accelerated since 1870[END_REF]. Inventories tend to have a coarser temporal resolution (5-10 year resurveys) but represent forest-stand spatial scales, albeit that plot scale varies widely: 0.067 ha forest inventory analysis, c. 1-2 ha (e.g. [START_REF] Brienen | Long-term decline of the Amazon carbon sink[END_REF][START_REF] Hubau | Asynchronous carbon sink saturation in African and Amazonian tropical forests[END_REF], 50 ha ForestGEO network (e.g. [START_REF] Chave | Assessing Evidence for a Pervasive Alteration in Tropical Tree Communities (GM Mace[END_REF].

Theme 3-Ecosystem monitoring: Ecosystem eddy-covariance and global remote-sensing may detect effects of iCO2 on carbon, water, and energy fluxes over the recent decades. Tower-based sensors are used to calculate ecosystem-scale (c. 1 km) carbon, water, and energy fluxes from the covariance of gas concentrations and vertical wind velocity [START_REF] Baldocchi | Assessing the eddy covariance technique for evaluating carbon dioxide exchange rates of ecosystems: past, present and future[END_REF]. A global network of continental networks (http://fluxnet.fluxdata.org), synthesises flux-tower data from 916 sites, some in operation for over two decades, while the majority have run for a decade or less and are located in temperate ecosystems [START_REF] Chu | Fluxes all of the time? A primer on the temporal representativeness of FLUXNET[END_REF]. Satellite and other aircraft-borne Earth observing systems

Accepted Article

This article is protected by copyright. All rights reserved have been measuring the reflectance of electromagnetic radiation from Earth's surface, used to infer changes in vegetation cover, leaf area, and biomass at the global scale [START_REF] Fensholt | Evaluation of MODIS LAI, fAPAR and the relation between fAPAR and NDVI in a semi-arid environment using in situ measurements[END_REF][START_REF] Smith | Constraining estimates of terrestrial carbon uptake: new opportunities using long-term satellite observations and data assimilation[END_REF]. Reflected wavelengths from Landsat (first launched in 1972), MODIS, and other instruments can be used to measure the fraction of absorbed photosynthetically active radiation (fAPAR) and greenness indices, which are further used to infer LAI, GPP, and NPP with the help of simple models [START_REF] Myneni | Increased plant growth in the CO2-do photosynthetic and productivity data from FACE experiments support early predictions?[END_REF]Field et al., 1998). Microwave wavelengths are used to measure vegetation optical depth (VOD, first available in the early 1980s) which can be used to infer vegetation water content and, by extension, vegetation biomass [START_REF] Liu | Recent reversal in loss of global terrestrial biomass[END_REF].

Theme 4-Large-scale constraints: At regional-to-global scales, several long-term data-streams provide constraints on the global carbon budget and its change over time. These data streams include near-surface and vertical profiles of atmospheric CO 2 concentration and δ13C, global watercycle measurements, and atmospheric composition from ice cores. Atmospheric CO 2 measurements can be combined with other data and models to infer the global carbon budget and spatial details of land carbon uptake [START_REF] Friedlingstein | Global Carbon Budget 2019[END_REF][START_REF] Peylin | Global atmospheric carbon budget: results from an ensemble of atmospheric CO 2 inversions[END_REF]. The impact of vegetation responses to iCO2 on the hydrological cycle measured by stream gauges can also act as further indirect evidence [START_REF] Ukkola | Reduced streamflow in water-stressed climates consistent with CO 2 effects on vegetation[END_REF][START_REF] Trancoso | CO2-vegetation feedbacks and other climate changes implicated in reducing base flow[END_REF]. Carbonyl sulphide (OCS) can be used to infer global carbon assimilation because it is taken up by plants through stomata and is transformed by carbonic anhydrase [START_REF] Wohlfahrt | Carbonyl sulfide (COS) as a tracer for canopy photosynthesis, transpiration and stomatal conductance: potential and limitations †[END_REF][START_REF] Whelan | Reviews and syntheses: Carbonyl sulfide as a multi-scale tracer for carbon and water cycles[END_REF].

Accepted Article

This article is protected by copyright. All rights reserved et al., 1999). These experiments represent early post-disturbance "reorganising", and possibly open-canopy, ecosystems but are not representative of closed-canopy ecosystems. Oscillating [CO 2 ] may lessen physiological responses [START_REF] Allen | Fluctuations of CO2 in Free-Air CO2 Enrichment (FACE) depress plant photosynthesis, growth, and yield[END_REF]. The stepchange in [CO 2 ] results in a large shift in the ecosystem resource balance [START_REF] Medlyn | Using ecosystem experiments to improve vegetation models[END_REF], while soil disturbance can increase nutrient availability [START_REF] Körner | Plant CO2 responses: an issue of definition, time and resource supply[END_REF]. Many experiments (and evidence themes more broadly) do not quantify total BP, especially root BP. Even the longest-running experiments are short-lived relative to the lifespan of trees. Landscape-scale atmospheric feedbacks (e.g. increased VPD that could mitigate reductions in transpiration) cannot be accounted for [START_REF] Leuzinger | The 'island effect' in terrestrial global change experiments: a problem with no solution?[END_REF].

Many 'measurements' rely on models in their calculation, thus have the potential to omit or presuppose a CO 2 effect. For example, satellite GPP (e.g. Sun et al., 2018) and NPP (e.g. [START_REF] Smith | Large divergence of satellite and Earth system model estimates of global terrestrial CO2 fertilization[END_REF] are calculated from fAPAR using an LUE model [START_REF] Monteith | Solar Radiation and Productivity in Tropical Ecosystems[END_REF] that often does not include the CO 2 effect on photosynthesis [START_REF] De Kauwe | Satellite based estimates underestimate the effect of CO2 fertilization on net primary productivity[END_REF]. Thus, changes in GPP result only from changes in LAI (fAPAR) or climate. Conversely, measurement-models that include a CO 2 effect are thus not independent of iCO2, e.g. iWUE from δ13C, OCS, or isotopomers, and thus have the potential to pre-suppose a CO 2 -related trend.

Carbon isotope discrimination during photosynthesis reduces the 13 C: 12 C ratio (δ 13 C) in plant material and is used to calculate iWUE from δ 13 C (Farquhar et al., 1982;[START_REF] Farquhar | Ternary effects on the gas exchange of isotopologues of carbon dioxide[END_REF]. The commonly used model neglects mesophyll and photorespiration discrimination (Farquhar et al., 1982;[START_REF] Farquhar | Ternary effects on the gas exchange of isotopologues of carbon dioxide[END_REF], and accounting for these effects can increase iWUE trends by around 50 % [START_REF] Keeling | Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis[END_REF].

Tree-ring trends are subject to sampling and survivorship biases [START_REF] Brienen | Detecting evidence for CO 2 fertilization from tree ring studies: The potential role of sampling biases: CO 2 fertilization from tree rings[END_REF][START_REF] Peters | Detecting long-term growth trends using tree rings: a critical evaluation of methods[END_REF] that can affect growth trends by up to 400 % (Hember et al., 2009;Nehrbass-Ahles et al., 2014), leading some to question whether tree-rings should be used for trend detection at all [START_REF] Brienen | Detecting evidence for CO 2 fertilization from tree ring studies: The potential role of sampling biases: CO 2 fertilization from tree rings[END_REF]. However, tree-rings are the only data that give insight into tree BP since the industrial revolution.

Accepted Article

This article is protected by copyright. All rights reserved

Many studies use tree-ring width as a proxy for wood BP because it is a direct measurement.

However, trees grow in three dimensions and change in the one-dimensional ring-width does not directly scale with wood volume growth and thus BP in different sized trees. Conversion to the twodimensional basal area increment (BAI) helps unify this size mismatch, but again does not account for non-linear change in wood BP with tree size [START_REF] Mcgarvey | Size-related scaling of tree form and function in a mixed-age forest (E Sayer, Ed.)[END_REF]. Allometric scaling should be applied to ring-width and BAI to attempt a best possible estimate of wood BP (e.g. Dye et al., 2014). Static allometric relationships over time can introduce bias where environmental changes have altered resource allocation. For example, shifting allocation from wood to leaves in Russian forests reconciled apparently conflicting inventory data that suggested BP declines while remote sensing suggested increases (Lapenis et al. 2005). Furthermore, wood volume growth does not always scale with BP as wood density can also change [START_REF] Pretzsch | Wood density reduced while wood volume growth accelerated in Central European forests since 1870[END_REF].

Forest inventory plots (c. 1 ha and less) can under-sample mortality, resulting in over-estimates of biomass accumulation [START_REF] Chambers | The steady-state mosaic of disturbance and succession across an old-growth Central Amazon forest landscape[END_REF]. Generally, statistical power for detecting and attributing change in mortality and SOM is often low [START_REF] Hungate | Assessing the effect of elevated carbon dioxide on soil carbon: a comparison of four meta-analyses[END_REF][START_REF] Sulman | Multiple models and experiments underscore large uncertainty in soil carbon dynamics[END_REF][START_REF] Mcmahon | The importance and challenges of detecting changes in forest mortality rates[END_REF]. Statistical power for detection is low due to measurement uncertainty, low signal-to-noise, heterogeneity, and potential pre-treatment differences. Low statistical power presents a real challenge for attribution when using commonly used binary mortality assessments or bulk SOM measurements [START_REF] Sulman | Multiple models and experiments underscore large uncertainty in soil carbon dynamics[END_REF][START_REF] Mcmahon | The importance and challenges of detecting changes in forest mortality rates[END_REF]. Furthermore, satellite data, flux-towers, and experiments all suffer from short time-periods, often with much background variability that can obscure or amplify trends.

Quantification of global 'natural' NBP is confounded with quantification of land-use changerelated NBP which is uncertain (95 % CI is 92 % of the mean flux; [START_REF] Friedlingstein | Global Carbon Budget 2019[END_REF]. Landuse change-related NBP is calculated using bookkeeping models that account for complex legacy effects and many elements of land-use change, which adds to the uncertainty [START_REF] Pongratz | Terminology as a key uncertainty in net land use and land cover change carbon flux estimates[END_REF].

Accepted Article

This article is protected by copyright. All rights reserved multiple years, and concepts of steady-state and non-steady state apply. Ideally we would like to know β from pre-change steady-state to post-change steady-state. However, an ecosystem may not be in steady-state prior to change and post-change ecosystems enter a transient phase and can take a long time to reach steady-state. Calculated during the transient phase, β will be a function of initial stocks and the developmental stage explored (seedling, sapling, mature tree) and signals will accumulate over time. For ecosystems not in steady-state pre-change, β of the changes in the stock increment is not sensitive to initial stocks, but could be large where pre-change increments are small (i.e. when pre-change the system is close to steady-state). For steady-state ecosystems pre-change, acknowledgment that β is non-steady-state is needed and a β that explicitly includes temporal scale would be ideal.

Accepted Article

This article is protected by copyright. All rights reserved

  and all estimated positive trends in global NBP (β app = 19±7, 11±4, 9.8±5). These estimates of NBP include both 'natural' NBP and land-use change-related (instantaneous and legacy) NBP.Global land-use change-related NBP was estimated from bookkeeping models at -1.4±1.4 PgC y -1[START_REF] Caspersen | Contributions of Land-Use History to Carbon Accumulation in U.S. Forests[END_REF][START_REF] Trumbore | Radiocarbon and Soil Carbon Dynamics[END_REF][START_REF] Friedlingstein | Global Carbon Budget 2019[END_REF], and are predominantly in the tropics (-1.4±0.3 PgC y -1 ) with fluxes outside the tropics balancing to a net flux of near zero[START_REF] Houghton | Global and Regional Fluxes of Carbon from Land Use and Land-Cover Change 1850-2015[END_REF]. Regional analysis of NBP show a strong sink in Northern Hemisphere extra-tropics 2.3±0.6 PgC y -1 (1992-1996), 2.2±0.5 PgC y -1 (2001-2004) but a substantial source in the tropics -1.

Flux-towers

  measure NEP directly, yet have been running for a relatively short time. Synthesis of 23 flux-towers indicate increased NEP (β app = 4.3±2), with high CO 2 sensitivity (β dir,hist = 4.6±2) (Fernández-Martínez et al., 2017).

Figure 1 .Figure 2 .Figure 3 .

 123 Figure 1. Conceptual diagrams of the terrestrial carbon cycle and the action of CO 2 Figure 2. Modelled leaf-scale physiological responses to CO 2 Figure 3. Comparison of β values Box 1. The CO 2 -fertilisation hypothesis Box 2. The evidence themes Box 3. Consideration of methods and bias

Figure Captions Figure 1 .

 Captions1 Figure Captions

Figure 2 .

 2 Figure2. Modelled theoretical responses to atmospheric CO 2 concentration ([CO 2 ], ppm) of (a-c) photosynthesis, A net ( mol m -2 s -1 , black) and photorespiration ( mol m -2 s -1 , dark grey, a only); (d-f) iWUE ( mol mol -1 ), (g-i) stomatal conductance (mol m -2 s -1 ), and (j-l) carbon storage (kg C m -2 ) under ambient (blue) and elevated (red)[CO 2 ]. Leaf (a, d, g) and canopy (b, e, h) scale for C3 (solid line) and C4 (dashed, leaf-scale only) plants. Variation in b, e, h from a 1000 member ensemble (mean, sd, and 95 percentile shown)-a factorial combination of 100 top-of-canopy V cmax values (mean = 60, sd = 10) and 10 values of the J max to V cmax slope (mean = 1.63, sd = 0.2), the iWUE response does not vary in this ensemble. Distributions of β dir for historical (purple, 280 to 400 ppm) and future (yellow, 400 to 550 ppm) [CO 2 ] changes (c, f, i) of diurnally-integrated, canopy-scale variables that includes the same leaf physiology variation as in b, e, h, plus three levels of temperature (10, 15, 25 °C) and relative humidity (50, 70, 90 %) combined in factorial. Tri-modality in the GPP β distributions from the temperature variation. β distributions weighted by the variables' absolute response to CO 2 . Carbon storage (j-l) calculated using a simple one-pool model with the mean β dir,fut GPP response applied to BP for [CO 2 ] at 400 and 550 ppm when (j) initial carbon stores are in equilibrium or (k) 10 % of equilibrium. (l) β dir,hist response when initial carbon stores are assumed in equilibrium at 280 ppm and using the observed historical CO 2 record. Instantaneous β dir for absolute carbon storage are shown (j-l, right y-axis, yellow or purple), as well as β dir,hist calculated using carbon

Figure 3 .

 3 Figure 3. β distributions based on data from Table2for GPP, WUE, BP, k veg , k soil , C veg , and C soil .

Furthermore, potentially substantial

  interactions of land-use change-related NBP and iCO2 are not considered by these methods. C veg and C soil changes, loss of storage/sink capacity, and potential CO 2 interactions with secondary succession all convolve land-use change and 'natural' NBP fluxes suggesting a false dichotomy in these flux calculations. Calculating and interpreting β, or any relative response, is challenging for carbon stocks in which pre-change values can be large, change is the product of two opposing fluxes cumulative over

  ). Canopyscale A net results from a mixture of CO 2 and light-limited photosynthesis, thus has an intermediate

eCO2 response that depends on the fraction of light-saturated leaves in the canopy (β dir,hist = 0.60±0.3, c. 280 to 400 ppm; Fig 2c). As [CO 2 ] increases, the fraction of light-saturated leaves in the canopy is expected to decrease and therefore historical eCO2 response of GPP is expected to be higher than the future response (β dir,fut = 0.46±0.2, c. 400 to 550 ppm; Fig 2c). C4 plants have evolved to concentrate carbon, thus saturating photosynthesis and suppressing photorespiration at low [CO 2 ]

  ). Based on the observed [CO 2 ] trend (Le Quéré et al., 2018), the model indicates that β dir

	calculations over a 30 year period (typical of forest-inventory analysis) are generally a little smaller
	(β dir,hist c. 0.5) than steady-state (β dir,hist = 0.6; Fig 2. l). Departures from these expected β's derived
	from GPP responses alone provide a guide to the magnitude of positive and negative feedbacks in
	eCO2 studies and can help to guide iCO2 attribution in historical studies.
	III. The evidence
	1 Physiology
	Carbon assimilation and GPP
	Evidence across FACE experiments (11 sites, 45 species) showed that eCO2 increased leaf-level,
	light-saturated photosynthesis (β dir = 0.73±0.2, see Supporting Information Notes S3 for methods),
	and supports differences between C 3 (β dir = 0.79±0.2) and C 4 species (β dir = 0.27±0.2) (Ainsworth &
	Long, 2005; all reported β's are in Table 2). Evidence suggests that maximum photosynthetic
	capacity acclimated (reduced) to eCO2, primarily maximum carboxylation capacity (β dir = -0.38±0.1)
	(Ainsworth & Long, 2005; Ainsworth & Rogers, 2007). Nevertheless, in many forest eCO2
	experiments photosynthetic stimulation (>5 years) was only minimally affected by acclimation (Crous

  Hungate, unpublished).Experimental evidence for progressive nitrogen limitation is limited to a single forest (ORNL; Norby et al., 2010) and a single grassland (Biocon;[START_REF] Reich | Nitrogen limitation constrains sustainability of ecosystem response to CO2[END_REF]. Paleo-climatic evidence suggests that despite increasing carbon storage the nitrogen cycle became more open between the last glacial

maximum and the industrial revolution

(Fischer et al., 2019;[START_REF] Jeltsch-Thömmes | Low terrestrial carbon storage at the Last Glacial Maximum: constraints from multi-proxy data[END_REF]

.

This evidence provides high confidence that terrestrial ecosystem carbon has increased over the historical period, with substantial changes in the 'natural' carbon sink almost balanced by a net carbon source from land-use change.

  Global carbon budgeting and global forest analysis suggest responses concurrent with iCO2 in the range, β app = 0.18-1.0. The 'natural' carbon store response estimated for global β app =0.18-0.29;[START_REF] Friedlingstein | Global Carbon Budget 2019[END_REF]. Trends observed in eddy-covariance NEP (site-scale 'natural' sink) and inversion NBP (global-scale combined 'natural' and land-use sink) are extremely high (β app = 4.3-19, mean 11). The extremely high β app for global NBP (and to a lesser degree NEP) results from global NBP being near zero as the 'natural' sink is almost balanced by the net source from land-use change, thus small absolute changes can be relatively high (Box 3).

intact forests (β app = 0.66±0.4;

Pan et al., 2011)

is higher than estimated for the 'natural' land surface

Accepted Article

This article is protected by copyright. All rights reserved (

iCO2 affects ecosystem carbon primarily through effects on NEP, thus understanding of C eco

  mineral-associated SOM formation balance to affect C soil throughout the soil profile will be key to predictive understanding. This may be especially relevant in non-forest ecosystems, where the largest potential change in carbon storage is belowground.

	responses to iCO2 will emerge from the above research priorities. Further, NBP is what the
	atmosphere 'sees', which includes additional non-respiratory carbon losses caused by fire
	(anthropogenic and wild), hydrological export, and export of consumer goods. iCO2 may interact with
	some non-NEP fluxes, e.g. greater grassland BP leading to higher fuel loads, greater BP in
	regrowing forests following land-use change. Land-use change NBP is often calculated without
	considering iCO2 and separately from 'natural' NBP caused by iCO2, climate change, nitrogen
	deposition, and other factors (e.g.

Table 1 .

 1 Acronyms and abbreviations Table 2. CO 2 responses across studies.

	Tables, Figures, and Boxes

  Table 2 for GPP, WUE, BP, k veg , k soil , C veg , and C soil .

	Data are organised by CO 2 response category-iCO2 (blue), attribution to iCO2 (green), and eCO2
	(purple). See Supporting Information Figs. S2-S4 for further details.

Table 1 .

 1 Acronyms and abbreviations.

	A net	Net photosynthetic carbon assimilation
	fAPAR	Fraction absorbed photosynthetically active radiation
	BAI	Basal area increment
	BP	Biomass production, the sum of all tissue production over a given time,
		typically a year
	C x	Carbon in pool x (where x is either: veg -vegetation, soil, eco -ecosystem)
	CO 2	Carbon dioxide
	[CO 2 ]	Atmospheric CO 2 concentration
	eCO2	Elevated CO 2 from experiments and CO 2 springs
	FACE	Free air CO 2 enrichment
	GPP	Gross primary production
	g s	Stomatal conductance
	iCO2	Increasing CO 2 from fossil fuel emissions and land-use change
	iWUE	Intrinsic WUE (A net /g s )
	k x	Turnover rate of carbon in pool x (see C x )
	LAI	Leaf area index
	NBP	Net biome production, net land atmosphere exchange
	NEP	Net ecosystem production
	OCS	Carbonyl sulphide
	UE	Use efficiency
	VPD	Vapour pressure deficit
	WUE	Water use efficiency (transpiration/BP)

increasing atmospheric [CO 2 ] lead to increasing terrestrial-ecosystem carbon storage causing negative feedback on atmospheric [CO 2 ] growth.

  This definition of the CO 2 -fertilisation hypothesis is explicit about the feedback on atmospheric [CO 2 ] growth, implying the potential of this process to slow climate change. The hypothesis is therefore defined at climate-change relevant scales, i.e. global in space and decadal to centennial in time.

Theme 1-Direct exposure to elevated CO 2 :

  Experiments in which plants are grown in CO 2enriched air and observations of plants growing close to geological CO 2 sources provide the only direct evidence of plant and soil responses to future [CO 2 ]. The first eCO2 experiments were typically at the scales of hours or days and of leaves or small, individual plants. Ecosystem-scale open-top chambers (OTC) and larger free-air CO 2 enrichment (FACE) experiments have since been implemented over decades in more natural settings. All of these experiments provide evidence for the direct CO 2 -effect on photosynthesis and stomatal conductance. These experiments also provide valuable data on biomass production, allocation to organs, and transpiration. The time scale of most experiments (<10 years), however, is generally much shorter than many ecosystem processes, and evidence for CO 2 -effects on mortality, plant community dynamics, or changes in soil carbon stocks is limited.

  Box 3. Consideration of methods and bias eCO2 experiments. Confinement of roots in pots can limit below-ground resources. While eCO2 can accelerate leaf area gain in open-grown plants leading to compound interest that does not occur with closed canopies (Norby

Acknowledgements

Author contributions

Accepted Article

Acknowledgements

This paper was outlined and informed by the "Integrating CO 2 -fertilisation evidence streams and

public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

theory (ICOFEST)" meeting held at Biosphere II in September 2018. The meeting was supported by the U.S. Department of Energy Office of Science, Biological and Environmental Research through the Free Air CO 2 Enrichment Model Data Synthesis (FACE-MDS) project. ORNL is managed by UT-Battelle, LLC, for the DOE under contract DE-AC05-00OR22725. MGDK acknowledges support from Australian Research Council (ARC) Discovery Grant (DP190101823). DJPM and WKS acknowledge support by NASA Terrestrial Ecosystems Grant 80NSSC19M0103. SZ received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 647204). KJAT acknowledges support from Smithsonian's Forest Global Earth Observatory (ForestGEO). JBF contributed to this research at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration with support from the NASA IDS program. VH acknowledges support from the Earth Systems and Climate Change Hub funded by the Australian. FJ acknowledges support by the Swiss NSF (#200020_172476). TFK was supported by the NASA Terrestrial Ecology Program IDS Award NNH17AE86I. PC and JP were supported by the European Research Council Synergy grant ERC-2013-SyG-2013-610028 IMBALANCE-P. J. Pongratz was supported by the German Research Foundation's Emmy Noether Program. MEBS acknowledges support from the Australian Research Council Centre of Excellence for Climate Extremes (CE170100023). CT was supported by a Lawrence Fellow award through Lawrence Livermore National Laboratory (LLNL) under contract DE-AC52-07NA27344 with the U.S. Department of Energy. MST was supported by the U.S. Department of Energy, Office of Science under contract number DE-AC02-05CH11231. ATT acknowledges funding from the USDA National Institute of Food and Agriculture, Agricultural and Food Research Initiative Competitive Programme Grant No. 2018-67012-31496. SLV was supported by the US National Science Foundation Paleo Perspectives on Climate Change Program. PvM was supported by the U.S. Geological Survey Ecosystems Mission

Accepted Article

This article is protected by copyright. All rights reserved Area. Government sponsorship acknowledged. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Accepted Article

This article is protected by copyright. All rights reserved Field CB, Behrenfeld MJ, Randerson JT, Falkowski P. 1998. Primary production of the biosphere: Integrating terrestrial and oceanic components. Science 281: 237-240. Field CB, Jackson RB, Mooney HA. 1995. Stomatal responses to increased CO2: implications from the plant to the global scale. Plant, Cell & Environment 18: 1214-1225. Finzi AC, Moore DJP, DeLucia EH, Lichter J, Hofmockel KS, Jackson RB, Kim H-S, Matamala R, McCarthy HR, Oren R, et al. 2006. Progressive Nitrogen Limitation of Ecosystem Processes Under Elevated Co2 in a Warm-Temperate Forest. Ecology 87: 15-25. Finzi AC, Norby RJ, Calfapietra C, Gallet-Budynek A, Gielen B, Holmes WE, Hoosbeek MR, Iversen CM, Jackson RB, Kubiske ME, et al. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences 104: 14014-14019. Fischer H, Schmitt J, Bock M, Seth B, Joos F, Spahni R, Lienert S, Battaglia G, Stocker BD, Schilt A, et al. 2019. N2O changes from the Last Glacial Maximum to the preindustrial -Part 1: Quantitative reconstruction of terrestrial and marine emissions using N2O stable isotopes in ice cores. Biogeosciences 16: 3997-4021. Fleischer K, Rammig A, Kauwe MGD, Walker AP, Domingues TF, Fuchslueger L, Garcia S, Goll DS, Grandis A, Jiang M, et al. 2019. Amazon forest response to CO 2 fertilization dependent on plant phosphorus acquisition. Nature Geoscience 12: 736-741. Fontaine S, Barot S, Barré P, Bdioui N, Mary B, Rumpel C. 2007. Stability of organic carbon in deep soil layers controlled by fresh carbon supply. Nature 450: 277-280. Forkel M, Carvalhais N, Rödenbeck C, Keeling R, Heimann M, Thonicke K, Zaehle S, Reichstein M. 2016. Enhanced seasonal CO2 exchange caused by amplified plant productivity in northern ecosystems. Science 351: 696-699. Fowler D, Coyle M, Skiba U, Sutton MA, Cape JN, Reis S, Sheppard LJ, Jenkins A, Grizzetti B, Galloway JN, et al. 2013. The global nitrogen cycle in the twenty-first century. Philosophical Transactions of the Royal Society B: Biological Sciences 368: 20130164.

Accepted Article

This article is protected by copyright. All rights reserved Frank DC, Poulter B, Saurer M, Esper J, Huntingford C, Helle G, Treydte K, Zimmermann NE, Accepted Article This article is protected by copyright. All rights reserved northern high latitudes from 1981 to 1991. Nature 386: 698-702. Nehrbass-Ahles C, Babst F, Klesse S, Nötzli M, Bouriaud O, Neukom R, Dobbertin M, Frank D. 2014. The influence of sampling design on tree-ring-based quantification of forest growth. Global Change Biology 20: 2867-2885. Nie M, Lu M, Bell J, Raut S, Pendall E. 2013. Altered root traits due to elevated CO2: a metaanalysis. Global Ecology and Biogeography 22: 1095-1105. Norby RJ, De Kauwe MG, Domingues TF, Duursma RA, Ellsworth DS, Goll DS, Lapola DM, Luus KA, MacKenzie AR, Medlyn BE, et al. 2016. Model-data synthesis for the next generation of forest free-air CO 2 enrichment (FACE) experiments. New Phytologist 209: 17-28. Norby RJ, DeLucia EH, Gielen B, Calfapietra C, Giardina CP, King JS, Ledford J, McCarthy HR, Moore DJP, Ceulemans R, et al. 2005. Forest response to elevated CO2 is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences of the United States of America 102: 18052-18056. Norby RJ, Kauwe MGD, Walker AP, Werner C, Zaehle S, Zak DR. 2017. Comment on "Mycorrhizal association as a primary control of the CO 2 fertilization effect". Science 355: 358-358. Norby RJ, Warren JM, Iversen CM, Medlyn BE, McMurtrie RE. 2010. CO2 enhancement of forest productivity constrained by limited nitrogen availability. Proceedings of the National Academy of Sciences 107: 19368-19373. Norby RJ, Wullschleger S, Gunderson C. A., Johnson D. W., Ceulemans R. 1999. Tree responses to rising CO2 in field experiments: implications for the future forest. Plant, Cell & Environment 22: 683-714. Norby RJ, Zak DR. 2011. Ecological Lessons from Free-Air CO2 Enrichment (FACE) Experiments. In: Futuyma DJ, Shaffer HB, Simberloff D, eds. Annual Review of Ecology, Evolution, and Systematics, Vol 42. Palo Alto: Annual Reviews, 181-203. Nowak RS, Ellsworth DS, Smith SD. 2004. Functional responses of plants to elevated atmospheric

Accepted Article

This article is protected by copyright. All rights reserved

Data availability statement

Data and analysis scripts used in this publication can be found at ESS-DIVE https://data.essdive.lbl.gov/view/doi:10.15485/1644687

Accepted Article

This article is protected by copyright. All rights reserved

Accepted Article

This article is protected by copyright. All rights reserved

Author contributions

APW conceived and wrote the paper, with major contributions from MGDK, AB, KG, SM. APW, ASP, and BT collated the data. The ICOFEST meeting was organised by APW, SB, KGC, MGDK, RK, BM, DJPM, RJN, and SZ. All authors attended or contributed to the meeting and commented and provided feedback on the manuscript.

Supporting Information

Notes S1. Standardising CO2 responses with a β factor. Notes S2. Calculation of β from different data types.

Notes S3. Modelling leaf and canopy physiology.

Table S1. Theoretical β's for photosynthesis.