An explanation for the isotopic offset between soil and stem water in a temperate tree species

A growing number of field studies report isotopic offsets between stem water and its potential sources that prevent the unambiguous identification of plant water origin using water isotopes. We explored the causes of this isotopic offset by conducting a controlled experiment on the temperate tree species Fagus sylvatica. We measured d2H and d18O of soil and stem water from potted saplings growing on three soil substrates and subjected to two watering regimes. Regardless of substrate, soil and stem water d2H were similar only near permanent wilting point. Under moister conditions, stem water d2H was 11 ± 3 more negative than soil water d2H, coherent with field studies. Under drier conditions, stem water d2H became progressively more enriched than soil water d2H. Although stem water d18O broadly reflected that of soil water, soil stem d2H and d18O differences were correlated (r = 0.76) and increased with transpiration rates indicated by proxies. Soil stem isotopic offsets are more likely to be caused by water isotope heterogeneities within the soil pore and stem tissues, which would be masked under drier conditions as a result of evaporative enrichment, than by fractionation under root water uptake. Our results challenge our current understanding of isotopic signals in the soil plant continuum. © 2020 The Authors. New Phytologist © 2020 New Phytologist TrustThis work was supported by the French national programme EC2CO-Biohefect (RootWater), the French national research agency (projects Hydrobeech, Climbeech and Micromic within the Cluster of Excellence COTE with grant agreement ANR-10-LABX-45; project ORCA with grant agreement ANR-13-BS06-0005-01), the European Research Council (ERC) under the EU Seventh Framework Program (FP7/2007-2013, with grant agreement no. 338264, awarded to LW) and the Aquitaine Region (project Athene with grant agreement 2016-1R20301-00007218). AB also acknowledges an IdEx Bordeaux postdoctoral fellowship from the Universite de Bordeaux (contract no. 22001162)

Predicting resilience through the lens of competing adjustments to vegetation function

There is a pressing need to better understand ecosystem resilience to droughts and heatwaves. Eco-evolutionary optimization approaches have been proposed as means to build this understanding in land surface models and improve their predictive capability, but competing approaches are yet to be tested together. Here, we coupled approaches that optimize canopy gas exchange and leaf nitrogen investment, respectively, extending both approaches to account for hydraulic impairment. We assessed model predictions using observations from a native Eucalyptus woodland that experienced repeated droughts and heatwaves between 2013 and 2020, whilst exposed to an elevated [CO2] treatment. Our combined approaches improved predictions of transpiration and enhanced the simulated magnitude of the CO2 fertilization effect on gross primary productivity. The competing approaches also worked consistently along axes of change in soil moisture, leaf area, and [CO2]. Despite predictions of a significant percentage loss of hydraulic conductivity due to embolism (PLC) in 2013, 2014, 2016, and 2017 (99th percentile PLC > 45%), simulated hydraulic legacy effects were small and short-lived (2 months). Our analysis suggests that leaf shedding and/or suppressed foliage growth formed a strategy to mitigate drought risk. Accounting for foliage responses to water availability has the potential to improve model predictions of ecosystem resilience. © 2022 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.MEBS, MDK, and AJP acknowledge support from the Australian Research Council (ARC) Centre of Excellence for Climate Extremes (CE170100023). MEBS was also supported by the UNSW Scientia PhD Scholarship Scheme. MDK and AJP acknowledge support from the ARC Discovery Grant (DP190101823) and MDK also acknowledges Eucalypt Australia and the NSW Research Attraction and Acceleration Program, which separately supported the EucFACE infrastructure. EucFACE was built as an initiative of the Australian Government, as part of the Nation-building Economic Stimulus Package, and is supported by the Australian Commonwealth in collaboration with Western Sydney University. BEM acknowledges support from the ARC Laureate Fellowship FL190100003. Finally, we thank the Editor, Dr Danielle Way, and two anonymous reviewers for their constructive comments. Open access publishing facilitated by University of New South Wales, as part of the Wiley - University of New South Wales agreement via the Council of Australian University Librarians. All model, analysis code, and data files are freely available from https://doi.org/10.5281/zenodo.6717290 (Sabot, 2022) and the code is also available from https://github.com/ManonSabot/Competing_Optimal_Adjustments. Previously published data sets used in this study can be accessed at: http://doi.org/10.4225/35/563159f223739 (Duursma et al., 2016). http://doi.org/10.4225/35/57ec5d4a2b78e (Ellsworth et al., 2017). http://doi.org/10.4225/35/55b6e313444ff (Gimeno et al., 2016). http://doi.org/10.4225/35/5ab9bd1e2f4fb (Gimeno et al., 2018). MEBS, MDK, and AJP acknowledge support from the Australian Research Council (ARC) Centre of Excellence for Climate Extremes (CE170100023). MEBS was also supported by the UNSW Scientia PhD Scholarship Scheme. MDK and AJP acknowledge support from the ARC Discovery Grant (DP190101823) and MDK also acknowledges Eucalypt Australia and the NSW Research Attraction and Acceleration Program, which separately supported the EucFACE infrastructure. EucFACE was built as an initiative of the Australian Government, as part of the Nation‐building Economic Stimulus Package, and is supported by the Australian Commonwealth in collaboration with Western Sydney University. BEM acknowledges support from the ARC Laureate Fellowship FL190100003. Finally, we thank the Editor, Dr Danielle Way, and two anonymous reviewers for their constructive comments. Open access publishing facilitated by University of New South Wales, as part of the Wiley ‐ University of New South Wales agreement via the Council of Australian University Librarians

Incorporating non-stomatal limitation improves the performance of leaf and canopy models at high vapour pressure deficit

Vapour pressure deficit (D) is projected to increase in the future as temperature rises. In response to increased D, stomatal conductance (gs) and photosynthesis (A) are reduced, which may result in significant reductions in terrestrial carbon, water and energy fluxes. It is thus important for gas exchange models to capture the observed responses of gs and A with increasing D. We tested a series of coupled A-gs models against leaf gas exchange measurements from the Cumberland Plain Woodland (Australia), where D regularly exceeds 2 kPa and can reach 8 kPa in summer. Two commonly used A-gs models were not able to capture the observed decrease in A and gs with increasing D at the leaf scale. To explain this decrease in A and gs, two alternative hypotheses were tested: hydraulic limitation (i.e., plants reduce gs and/or A due to insufficient water supply) and non-stomatal limitation (i.e., downregulation of photosynthetic capacity). We found that the model that incorporated a non-stomatal limitation captured the observations with high fidelity and required the fewest number of parameters. Whilst the model incorporating hydraulic limitation captured the observed A and gs, it did so via a physical mechanism that is incorrect. We then incorporated a non-stomatal limitation into the stand model, MAESPA, to examine its impact on canopy transpiration and gross primary production. Accounting for a non-stomatal limitation reduced the predicted transpiration by ~19%, improving the correspondence with sap flow measurements, and gross primary production by ~14%. Given the projected global increases in D associated with future warming, these findings suggest that models may need to incorporate non-stomatal limitation to accurately simulate A and gs in the future with high D. Further data on non-stomatal limitation at high D should be a priority, in order to determine the generality of our results and develop a widely applicable model. © The Author(s) 2019. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected]. was supported by a PhD scholarship from Hawkesbury Institute for the Environment, Western Sydney University. M.G.D.K. acknowledges funding from the Australian Research Council (ARC) Centre of Excellence for Climate Extremes (CE170100023), the ARC Discovery Grant (DP190101823) and support from the NSW Research Attraction and Acceleration Program. EucFACE was built as an initiative of the Australian Government as part of the Nation-building Economic Stimulus Package and is supported by the Australian Commonwealth in collaboration with Western Sydney University. It is also part of a Terrestrial Ecosystem Research Network Super-site facility

Species richness influences the spatial distribution of trees in European forests

The functioning of plant communities is strongly influenced by the number of species in the community and their spatial arrangement. This is because plants interact with their nearest neighbors and this interaction is expected to be stronger when the interacting individuals are ecologically similar in terms of resource use. Recent evidence shows that species richness alters the balance of intra- versus interspecific competition, but the effect of species richness, and phylogenetic and functional diversity on the spatial pattern of the plant communities remain less studied. Even far, how forest stand structure derived from past management practices can influence the relationship between species richness and spatial pattern is still unknown. Here, we evaluate the spatial distribution of woody individuals (DBH >7.5 cm) in 209 forest stands (i.e. plots) with an increasing level of species richness (from 1 up to 10 species) in six forest types along a latitudinal gradient in Europe. We used completely mapped plots to investigate the spatial pattern in each forest stand with point pattern techniques. We fitted linear models to analyze the effect of species richness (positively correlated with phylogenetic diversity) and functional diversity on tree spatial arrangements. We also controled this relationship by forest type and stand structure as a proxy of the management legacy. Our results showed a generalized positive effect of species richness and functional diversity on the degree of spatial clustering of trees, and on the spatial independence of tree sizes regardless of the forest type. Moreover, current tree spatial arrangements were still conditioned by its history of management; however its effect was independent of the number of species in the community. Our study showed that species richness and functional diversity are relevant attributes of forests influencing the spatial pattern of plant communities, and consequently forest functioning. © 2019 Nordic Society Oikos. Published by John Wiley & Sons LtdThis research was supported by the FunDivEUROPE project, receiving funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement no.265171, the Spanish‐funded project REMEDINAL TE‐CM S2018/EMT‐4338 and COMEDIAS FEDER/Ministerio de Ciencia, Innovación y Universidades – Agencia Estatal de Investigación/_Proyecto CGL2017‐83170‐R. RB was funded by a Marie Skłodowska‐Curie Intra‐European fellowship (grant agreement no. 302445)

Ecosystem transpiration and evaporation: Insights from three water flux partitioning methods across FLUXNET sites

We apply and compare three widely applicable methods for estimating ecosystem transpiration (T) from eddy covariance (EC) data across 251 FLUXNET sites globally. All three methods are based on the coupled water and carbon relationship, but they differ in assumptions and parameterizations. Intercomparison of the three daily T estimates shows high correlation among methods (R between .89 and .94), but a spread in magnitudes of T/ET (evapotranspiration) from 45% to 77%. When compared at six sites with concurrent EC and sap flow measurements, all three EC‐based T estimates show higher correlation to sap flow‐based T than EC‐based ET. The partitioning methods show expected tendencies of T/ET increasing with dryness (vapor pressure deficit and days since rain) and with leaf area index (LAI). Analysis of 140 sites with high‐quality estimates for at least two continuous years shows that T/ET variability was 1.6 times higher across sites than across years. Spatial variability of T/ET was primarily driven by vegetation and soil characteristics (e.g., crop or grass designation, minimum annual LAI, soil coarse fragment volume) rather than climatic variables such as mean/standard deviation of temperature or precipitation. Overall, T and T/ET patterns are plausible and qualitatively consistent among the different water flux partitioning methods implying a significant advance made for estimating and understanding T globally, while the magnitudes remain uncertain. Our results represent the first extensive EC data‐based estimates of ecosystem T permitting a data‐driven perspective on the role of plants’ water use for global water and carbon cycling in a changing climate.We acknowledge insightful discussions with Dario Papale and apologize for having a cappuccino after lunch. We further acknowledge Ulrich Weber for preparing the cappuccino. M.G. acknowledges funding by Swiss National Science Foundation project ICOS‐CH Phase 2 20FI20_173691. L.Š. was supported by the Ministry of Education, Youth and Sports of the Czech Republic within the CzeCOS program, grant number LM2015061, and by SustES‐Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions (CZ.02.1.01/0.0/0.0/16_019/0000797). G.W. acknowledges support by the Austrian National Science Fund (FWF, project I03859) and the Province of South Tyrol (“Cycling of carbon and water in mountain ecosystems under changing climate and land use”). R.P. was supported by grants CGL2014‐55883‐JIN, RTI2018‐095297‐J‐I00 (Spain), and by a Humboldt Research Fellowship for Experienced Researchers (Germany). This work used eddy covariance data acquired and shared by the FLUXNET community, including these networks: Ameri‐Flux, AfriFlux, AsiaFlux, CarboAfrica, CarboEuropeIP, CarboItaly, CarboMont, ChinaFlux, Fluxnet‐Canada, GreenGrass, ICOS, KoFlux, LBA, NECC, OzFlux‐TERN, TCOS‐Siberia, and USCCC. The ERA‐Interim reanalysis data are provided by ECMWF and processed by LSCE. The FLUXNET eddy covariance data processing and harmonization was carried out by the European Fluxes Database Cluster, AmeriFlux Management Project, and Fluxdata project of FLUXNET, with the support of CDIAC and ICOS Ecosystem Thematic Center, and the OzFlux, ChinaFlux, and AsiaFlux offices. Open access funding enabled and organized by Projekt DEAL

The fate of carbon in a mature forest under carbon dioxide enrichment

Atmospheric carbon dioxide enrichment (eCO2) can enhance plant carbon uptake and growth1 5, thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO2 concentration6. Although evidence gathered from young aggrading forests has generally indicated a strong CO2 fertilization effect on biomass growth3 5, it is unclear whether mature forests respond to eCO2 in a similar way. In mature trees and forest stands7 10, photosynthetic uptake has been found to increase under eCO2 without any apparent accompanying growth response, leaving the fate of additional carbon fixed under eCO2 unclear4,5,7 11. Here using data from the first ecosystem-scale Free-Air CO2 Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responded to four years of eCO2 exposure. We show that, although the eCO2 treatment of +150 parts per million (+38 per cent) above ambient levels induced a 12 per cent (+247 grams of carbon per square metre per year) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone accounting for half of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO2, and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO2 fertilization as a driver of increased carbon sinks in global forests. © 2020, The Author(s), under exclusive licence to Springer Nature Limited

A novel optimization approach incorporating non-stomatal limitations predicts stomatal behaviour in species from six plant functional types

The primary function of stomata is to minimize plant water loss while maintaining CO 2 assimilation. Stomatal water loss incurs an indirect cost to photosynthesis in the form of non-stomatal limitations (NSL) via reduced carboxylation capacity (CAP) and/or mesophyll conductance (MES). Two optimal formulations for stomatal conductance (g s) arise from the assumption of each type of NSL. In reality, both NSL could coexist, but one may prevail for a given leaf ontogenetic stage or plant functional type, depending on leaf morphology. We tested the suitability of two g s formulations (CAP versus MES) on species from six plant functional types (C 4 crop, C 3 grass, fern, conifer, evergreen, and deciduous angiosperm trees). MES and CAP parameters (the latter proportional to the marginal water cost to carbon gain) decreased with water availability only in deciduous angiosperm trees, while there were no clear differences between leaf ontogenetic stages. Both CAP and MES formulations fit our data in most cases, particularly under low water availability. For ferns, stomata appeared to operate optimally only when subjected to water stress. Overall, the CAP formulation provided a better fit across all species, suggesting that sub-daily stomatal responses minimize NSL by reducing carboxylation capacity predominantly, regardless of leaf morphology and ontogenetic stage. © 2019 The Author(s).This work was funded by the IdEx programme of the Université de Bordeaux (project USIFlux) and a Marie Skłodowska-Curie individual fellowship (grant no. 653223). Additional support was provided by the French Agence National de la Recherche (grant agreements ANR-13-BS06-0005-01 and ANR-13-BS06-0005-01) and the European Research Council, under the EU FP7 framework programme (grant agreement 338264)

Disentangling above and belowground drivers of stomatal response to drought stress on a temperate deciduous tree

Ever since the formulation of the stomatal optimization theory four decades ago, plant physiologists have been searching for a unified formulation to model the response of plant stomata to environmental and endogenous drivers. In the meantime, most Dynamic Global Vegetation Models (DGVM) incorporate some form of Leuning’s empirical formulation (Leuning 1995 Plant Cell and Environment 18:339) that enables to predict the coupling of carbon uptake and water use under a gradient of temperature, air moisture and atmospheric CO2 concentration. However, there is still great diversity in the way DGVM represent carbon and water trade-offs in response to soil water availability. Theoretical and semi-empirical formulations arising from optimization theory in combination with hydraulic modelling provide a novel framework for predicting stomatal response in the face of climate and global change. Here, we tested for the ability to predict stomatal behavior of two recently proposed formulations where stomata are hypothesized to maximize carbon gain instantaneously, irrespective of water availability. We measured leaf water potential, photosynthesis and stomatal conductance under a vapor pressure deficit gradient (VPD) in European beech (Fagus sylvatica) saplings subjected to three levels of water availability. The conductivity of the soil-plant-atmosphere continuum was manipulated by potting plant in three different types of soil with contrasting texture. Our results allow to disentangle the effect of atmospheric (VPD) and soil drought on stomatal behavior. Our findings support that a unified theoretical approach combing optimization and hydraulic modelling allows to predict stomatal behavior irrespective of the temporal scale considered

Disentangling the Role of Forest Structure and Functional Traits in the Thermal Balance of the Mediterranean–Temperate Ecotone

The thermal balance of forests is the result of complex land–atmosphere interactions. Different climate regimes and plant functional types can have contrasting energy budgets, but little is known about the influence of forest structure and functional traits. Here, we combined spaceborne measurements of surface temperature from ECOSTRESS with ground-based meteorological data to estimate the thermal balance at the surface (∆Tcan−air) during four summers (2018–2021), at the Mediterranean–temperate ecotone in the NE Iberian Peninsula. We analyzed the spatiotemporal drivers of ∆Tcan−air by quantifying the effects of meteorology, forest structure (stand density, tree height) and ecophysiology (hydraulic traits), during normal days and hot spells. Canopy temperatures (Tcan) fluctuated according to changes in air temperature (Tair) but were on average 4.2 K warmer. During hot spells, ∆Tcan−air was smaller than during normal periods. We attribute this decrease to the advection of hot and dry air masses from the Saharan region resulting in a sudden increase in Tair relative to Tcan. Vapor pressure deficit (VPD) was negatively correlated with ∆Tcan−air, since the highest VPD values coincided with peaks in heat advection. Nonetheless, Tcan increased with VPD due to decreased transpiration (following stomatal closure), even though sufficient soil water availability enabled some degree of evaporative cooling. Our findings demonstrate that plot-scale forest structural and hydraulic traits are key determinants for the forest thermal balance. The integration of functional traits and forest structure over relevant spatial scales would improve our ability to understand and model land–atmosphere feedbacks in forested regions. © 2023. The Authors.AB acknowledges a Beatriu de Pinós MSCA-COFUND postdoctoral grant from the Government of Catalonia (2019BP00193). AB and JC received funding from the Spanish Ministry of Science (Grant MICROCLIM, PID2020-117636GB-C21). DGM is supported by the European Research Council (ERC), grant agreement 101088405 (HEAT). TEG received funding from the Spanish Ministry of Science (Grant PHLISCO, PID2019-107817RB-I00). We would like to thank Teresa Rosas, Jordi Martínez-Vilalta and Maurizio Mencuccini by providing the data on plant hydraulic traits and Víctor Granda for his help with the data from the Catalan Forest Laboratory