Imperial College London

DrAlienorLavergne

Faculty of Natural SciencesDepartment of Physics

Research Associate (Marie Skłodowska-Curie Fellow)
 
 
 
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719Sir Alexander Fleming BuildingSouth Kensington Campus

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Publications

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16 results found

Belmecheri S, Lavergne A, 2020, Compiled records of atmospheric CO<inf>2</inf> concentrations and stable carbon isotopes to reconstruct climate and derive plant ecophysiological indices from tree rings, Dendrochronologia, Vol: 63, ISSN: 1125-7865

© 2020 The stable carbon isotopic composition (δ13C) measured in tree rings is a standard proxy for paleoclimate reconstructions and is increasingly being used as a paleophysiological proxy. To fully exploit the potential of tree ring δ13C proxy, atmospheric CO2 concentration and δ13C (δ13CO2) data are required to correct tree ring δ13C from the declining trend of δ13CO2 due to fossil fuel burning since 1850 CE, and to derive physiological parameters using biochemical models that link photosynthesis to δ13C. These atmospheric data are available from direct measurements or can be inferred from indirect proxies such as ice cores covering the Common Era (CE) at variable temporal resolutions. For almost two decades, tree-ring researchers have relied on a dataset derived from fitted linear regressions of ice core measurements available through the seminal McCarroll and Loader (2004) article for the 1850−2003 CE period. However, new calibrations and compilations of ice core data and direct measurements are now available as part of Earth System Modelling efforts which remain overlooked by the tree ring research community.Here, we present an overview of the new and freely available datasets and provide recommendations for their use in ecophysiology and paleoclimate research, that we expect will stimulate cross-disciplinary collaborations.

Journal article

Lavergne A, Voelker S, Csank A, Graven H, de Boer HJ, Daux V, Robertson I, Dorado-Linan I, Martínez-Sancho E, Battipaglia G, Bloomfield KJ, Still C, Meinzer FC, Dawson TE, Camarero JJ, Clisby R, Fang Y, Menzel A, Keen RM, Roden JS, Prentice Iet al., 2020, Historical changes in the stomatal limitation of photosynthesis: empirical support for an optimality principle, New Phytologist, Vol: 225, Pages: 2484-2497, ISSN: 0028-646X

The ratio of leaf‐internal (ci) to ambient (ca) partial pressure of CO2, defined here as χ, is an index of adjustments in both leaf stomatal conductance and photosynthetic rate to environmental conditions. Measurements and proxies of this ratio can be used to constrain vegetation models uncertainties for predicting terrestrial carbon uptake and water use.We test a theory based on the least‐cost optimality hypothesis for modelling historical changes in χ over the 1951‐2014 period, across different tree species and environmental conditions, as reconstructed from stable carbon isotopic measurements across a global network of 103 absolutely‐dated tree‐ring chronologies. The theory predicts optimal χ as a function of air temperature, vapour pressure deficit, ca and atmospheric pressure.The theoretical model predicts 39% of the variance in χ values across sites and years, but underestimates the inter‐site variability in the reconstructed χ trends, resulting in only 8% of the variance in χ trends across years explained by the model.Overall, our results support theoretical predictions that variations in χ are tightly regulated by the four environmental drivers. They also suggest that explicitly accounting for the effects of plant‐available soil water and other site‐specific characteristics might improve the predictions.

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Lavergne A, Graven H, De Kauwe MG, Keenan FT, Medlyn BE, Prentice Iet al., 2019, Observed and modelled historical trends in the water use efficiency of plants and ecosystems, Global Change Biology, Vol: 25, Pages: 2242-2257, ISSN: 1354-1013

Plant water‐use efficiency (WUE, the carbon gained through photosynthesis per unit of water lost through transpiration) is a tracer of the plant physiological controls on the exchange of water and carbon dioxide between terrestrial ecosystems and the atmosphere. At the leaf level, rising CO2 concentrations tend to increase carbon uptake (in the absence of other limitations) and to reduce stomatal conductance, both effects leading to an increase in leaf WUE. At the ecosystem level, indirect effects (e.g. increased leaf area index, soil water savings) may amplify or dampen the direct effect of CO2. Thus, the extent to which changes in leaf WUE translate to changes at the ecosystem scale remains unclear. The differences in the magnitude of increase in leaf versus ecosystem WUE as reported by several studies are much larger than would be expected with current understanding of tree physiology and scaling, indicating unresolved issues. Moreover, current vegetation models produce inconsistent and often unrealistic magnitudes and patterns of variability in leaf and ecosystem WUE, calling for a better assessment of the underlying approaches. Here, we review the causes of variations in observed and modelled historical trends in WUE over the continuum of scales from leaf to ecosystem, including methodological issues, with the aim of elucidating the reasons for discrepancies observed within and across spatial scales. We emphasize that even though physiological responses to changing environmental drivers should be interpreted differently depending on the observational scale, there are large uncertainties in each data set which are often underestimated. Assumptions made by the vegetation models about the main processes influencing WUE strongly impact the modelled historical trends. We provide recommendations for improving long‐term observation‐based estimates of WUE that will better inform the representation of WUE in vegetation models.

Journal article

Allen KJ, Villalba R, Lavergne A, Palmer JG, Cook EC, Fenwick P, Drew DM, Turney CSM, Baker PJet al., 2018, A comparison of some simple methods used to detect unstable temperature responses in tree-ring chronologies, DENDROCHRONOLOGIA, Vol: 48, Pages: 52-73, ISSN: 1125-7865

Journal article

Daux V, Michelot-Antalik A, Lavergne A, Pierre M, Stievenard M, Breda N, Damesin Cet al., 2018, Comparisons of the Performance of delta C-13 and delta O-18 of Fagus sylvatica, Pinus sylvestris, and Quercus petraea in the Record of Past Climate Variations, JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES, Vol: 123, Pages: 1145-1160, ISSN: 2169-8953

Journal article

Lavergne A, Daux V, Pierre M, Stievenard M, Marina Srur A, Villalba Ret al., 2017, Past summer temperatures inferred from dendrochronological records of Fitzroya cupressoides on the eastern slope of the northern Patagonian Andes, Journal of Geophysical Research: Biogeosciences, Vol: 123, Pages: 32-45, ISSN: 2169-8961

Estimating summer temperature fluctuations over long timescales in southern South America is essential for better understanding the past climate variations in the Southern Hemisphere. Here we developed robust 212 year long basal area increment (BAI) and δ13C chronologies from living temperature‐sensitive Fitzroya cupressoides on the eastern slope of the northern Patagonian Andes (41°S). After removing the increasing trend from the growth records likely due to the CO2 fertilization effect, we tested the potential to reconstruct past summer temperature variations using BAI and δ13C as predictors. The reconstruction based on δ13C records has the strongest predictive skills and explains as much as 62% of the total variance in instrumental summer temperature (n = 81, p < 0.001). The temperature signal recorded in tree‐ring growth is not substantially different to that present in δ13C and consequently does not provide additional information to improve the regression models. Our δ13C‐based reconstruction shows cold summer temperatures in the second part of the 19th century and in the mid‐20th century followed by a warmer period. Notably, the 20th and the early 21st centuries were warmer (+0.6°C) than the 19th century. Reconstructed summer temperature variations are modulated by low‐latitude (El Niño–Southern Oscillation) and high‐latitude (Southern Annular Mode) climate forcings. Our reconstruction based on δ13C agrees well with previous ring width based temperature reconstructions in the region and comparatively enhances the low‐frequency variations in the records. The present study provides the first reconstruction of summer temperature in South America south of 40°S for the period 1800–2011 entirely based on isotopic records.

Journal article

Lavergne A, Gennaretti F, Risi C, Daux V, Boucher E, Savard MM, Naulier M, Villalba R, Begin C, Guiot Jet al., 2017, Modelling tree ring cellulose delta δ¹⁸O variations in two temperature-sensitive tree species from North and South America, Climate of the Past, Vol: 13, Pages: 1515-1526, ISSN: 1814-9324

Oxygen isotopes in tree rings (δ18OTR) are widely used to reconstruct past climates. However, the complexity of climatic and biological processes controlling isotopic fractionation is not yet fully understood. Here, we use the MAIDENiso model to decipher the variability in δ18OTR of two temperature-sensitive species of relevant palaeoclimatological interest (Picea mariana and Nothofagus pumilio) and growing at cold high latitudes in North and South America. In this first modelling study on δ18OTR values in both northeastern Canada (53.86° N) and western Argentina (41.10° S), we specifically aim at (1) evaluating the predictive skill of MAIDENiso to simulate δ18OTR values, (2) identifying the physical processes controlling δ18OTR by mechanistic modelling and (3) defining the origin of the temperature signal recorded in the two species. Although the linear regression models used here to predict daily δ18O of precipitation (δ18OP) may need to be improved in the future, the resulting daily δ18OP values adequately reproduce observed (from weather stations) and simulated (by global circulation model) δ18OP series. The δ18OTR values of the two species are correctly simulated using the δ18OP estimation as MAIDENiso input, although some offset in mean δ18OTR levels is observed for the South American site. For both species, the variability in δ18OTR series is primarily linked to the effect of temperature on isotopic enrichment of the leaf water. We show that MAIDENiso is a powerful tool for investigating isotopic fractionation processes but that the lack of a denser isotope-enabled monitoring network recording oxygen fractionation in the soil–vegetation–atmosphere compartments limits our capacity to decipher the processes at play. This study proves that the eco-physiological modelling of δ18OTR values is necessary to interpret the recorded climate signal more reliably.

Journal article

Lavergne A, Daux V, Villalba R, Pierre M, Stievenard M, Srur AMet al., 2017, Improvement of isotope-based climate reconstructions in Patagonia through a better understanding of climate influences on isotopic fractionation in tree rings, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 459, Pages: 372-380, ISSN: 0012-821X

Journal article

Lavergne A, Daux V, Villalba R, Pierre M, Stievenard M, Vimeux F, Srur AMet al., 2016, Are the oxygen isotopic compositions of Fitzroya cupressoides and Nothofagus pumilio cellulose promising proxies for climate reconstructions in northern Patagonia?, JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES, Vol: 121, Pages: 767-776, ISSN: 2169-8953

Journal article

Lavergne A, Daux V, Villalba R, Barichivich Jet al., 2015, Temporal changes in climatic limitation of tree-growth at upper treeline forests: Contrasted responses along the west-to-east humidity gradient in Northern Patagonia, DENDROCHRONOLOGIA, Vol: 36, Pages: 49-59, ISSN: 1125-7865

Journal article

Lavergne A, Gennaretti F, Risi C, Daux V, Boucher E, Savard MM, Naulier M, Villalba R, Bégin C, Guiot Jet al., Modelling tree-ring cellulose δ&lt;sup&gt;18&lt;/sup&gt;O variations of two temperature-sensitive tree species from North and South America

<jats:p>Abstract. Oxygen isotopes in tree-rings (δ18OTR) are widely used to reconstruct past climates. However, the complexity of climatic and biological processes controlling isotopic fractionation is not yet fully understood. Here, we use the MAIDENiso model to decipher the variability of δ18OTR of two temperature-sensitive species of relevant paleoclimatological interest (Picea mariana and Nothofagus pumilio) and growing at cold high-latitudes in North and South America. In this first modelling study on δ18OTR values in both northeastern Canada (53.86° N) and western Argentina (41.10° S), we specifically aim at: (1) evaluating the predictive skill of MAIDENiso to simulate δ18OTR values, (2) identifying the physical processes controlling δ18OTR by mechanistic modelling and, (3) defining the origin of the temperature signal recorded in the two species. Although the linear regression models used here to predict daily δ18O of precipitation (δ18OP) may need to be improved in the future, the resulting daily δ18OP values adequately reproduce observed (from weather stations) and simulated (by global circulation model) δ18OP series. The δ18OTR values of the two species are correctly simulated using the δ18OP estimation as MAIDENiso input, although some offset in mean δ18OTR levels is observed for the South American site. For both species, the variability of δ18OTR series is more likely linked to the effect of temperature on isotopic enrichment of the leaf water rather than on the isotopic composition of the source water. We show that MAIDENiso is a powerful tool for investigating isotopic fractionation processes but that the lack of a denser isotope-enabled monitoring network recording oxygen fractionation in the soil-vegetation-atmosphere compartments limits our capacity to decipher the processes at play. This study proves that the eco-physiological modelling of &del

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Lavergne A, Response to Anonymous Referee #2

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Lavergne A, Response to Anonymous Referee #1

Journal article

Belmecheri S, szejner P, Frank D, Voelker S, Lavergne Aet al., Global trends of tree-ring carbon isotope discrimination under rising atmospheric CO2 and changing climate

<jats:p> &amp;lt;p&amp;gt;Rising atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; concentrations are expected to stimulate plant carbon uptake (A) while also reducing transpiration via a decrease in stomatal conductance (g&amp;lt;sub&amp;gt;s&amp;lt;/sub&amp;gt;), resulting in an increase in the intrinsic water use efficiency (iWUE, i.e. the ratio of A to g&amp;lt;sub&amp;gt;s&amp;lt;/sub&amp;gt;). While there is overwhelming evidence of a secular iWUE increase in response to rising CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; over the 20th-21st century, the magnitude of changes in iWUE reported so far in the literature strongly varies across climatic regions and biomes. Moreover, increasing iWUE has not systematically been translated into tree growth increment at many forested ecosystems, challenging the CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fertilization theory. There is thus a need to track down the key physiological and environmental mechanisms driving changes in iWUE.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Here we estimate the carbon isotopic discrimination (&amp;amp;#916;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C) - defined as the difference between the stable carbon isotopic compositions (&amp;amp;#948;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C) measured in atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and in tree rings &amp;amp;#8211; from 147 tree-ring &amp;amp;#948;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C chronologies to: 1) investigate the physiological responses of woody C3 plants to increasing atmospheric CO&amp;lt;sub&amp;gt;2 &amp;lt;/sub&amp;gt;and, 2) disentangle climate vs CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; effects on A and g&amp;lt;sub&amp;gt;s&amp;lt;/sub&amp;gt;. We specifically stu

Journal article

Lavergne A, Graven H, Prentice IC, Disentangling the relative contributions of atmospheric demand for water and soil water availability on the stomatal limitation of photosynthesis

<jats:p> &amp;lt;p&amp;gt;Plants open and close their stomata in response to changes in the environment, so they can absorb the CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; they need to grow, while also avoid drying out. Since the activities of leaf stomata determine the exchanges of carbon and water between the vegetation and the atmosphere, it is crucial to incorporate their responses to environmental pressure into the vegetation models predicting carbon and water fluxes on broad spatial and temporal scales. The least-cost optimality theory proposes a simple way to predict leaf behaviour, in particular changes in the ratio of leaf internal (&amp;lt;em&amp;gt;c&amp;lt;/em&amp;gt;&amp;lt;sub&amp;gt;i&amp;lt;/sub&amp;gt;) to ambient (&amp;lt;em&amp;gt;c&amp;lt;/em&amp;gt;&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt;) partial pressure of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, from four environmental variables, i.e. &amp;lt;em&amp;gt;c&amp;lt;/em&amp;gt;&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt;, growing-season temperature (&amp;lt;em&amp;gt;T&amp;lt;/em&amp;gt;&amp;lt;sub&amp;gt;g&amp;lt;/sub&amp;gt;), atmospheric vapour pressure deficit (&amp;lt;em&amp;gt;D&amp;lt;/em&amp;gt;&amp;lt;sub&amp;gt;g&amp;lt;/sub&amp;gt;), and atmospheric pressure (as indexed by elevation, &amp;lt;em&amp;gt;z&amp;lt;/em&amp;gt;). However, even though the theory considers the effect of atmospheric demand for water on &amp;lt;em&amp;gt;c&amp;lt;/em&amp;gt;&amp;lt;sub&amp;gt;i&amp;lt;/sub&amp;gt;/&amp;lt;em&amp;gt;c&amp;lt;/em&amp;gt;&amp;lt;sub&amp;gt;a&amp;lt;/sub&amp;gt;, it does not predict how dry soils with reduced soil water availability further influence &amp;lt;em&amp;gt;c&amp;lt;/em&

Journal article

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