The role of intraspecific variability (IV) in shaping community dynamics and species coexistence has been intensively discussed over the past decade and modelling studies have played an important ...role in that respect. However, these studies often implicitly assume that IV can be represented by independent random draws around species-specific mean parameters. This major assumption has largely remained undiscussed, although a great part of observed IV is structured in space or time, in particular when environmental dimensions that influence individual performance are imperfectly characterised or unobserved in the field. To test the impact of this strong assumption on the outcome of community dynamics models, we designed a simulation experiment where we varied the level of knowledge of the environment in virtual communities, resulting in different relative importance of explained vs unexplained spatial individual variation in performance. We used a community dynamics simulator to generate communities where the unexplained individual variation is, or is not, added as an unstructured random noise. Communities simulated with unstructured IV never reached the community diversity and composition of those where all the variation was explained and structured (perfect knowledge model). This highlights that incorporating unstructured IV (i.e. a random noise) to account for unexplained (but structured) variation can lead to incorrect simulations of community dynamics. In addition, the effects of unstructured IV on community diversity and composition depended on the relative importance of structured vs unstructured IV, i.e. on the level of knowledge of the environment, which may partly explain the contrasting results of previous studies on the effect of IV on species coexistence. In particular, the effect of unstructured IV on community diversity was positive when the proportion of structured IV vs unstructured IV in the model was low, but negative when this proportion was high. This is because unstructured random noise can either limit the competitive exclusion of inferior competitors in low dimensions or destabilise tight niche partitioning in high dimension. Our study suggests that it is crucial to account for the sources and structure of observed IV in real communities to better understand its effect on community assembly and properly include it in community dynamics models.
La faible représentation de la biodiversité dans les modèles de végétation a longtemps été un obstacle à la compréhension et à la projection des processus écosystémiques. La forte biodiversité des ...forêts tropicales, leur rôle clé dans les cycles biogéochimiques globaux, ainsi que leur vulnérabilité aux perturbations anthropiques directes et indirectes, amplifient les difficultés et enjeux de ces questions de recherche. En particulier, l'augmentation prédite de la fréquence et de l'intensité des sécheresses pourrait impacter la structure et composition floristique de ces forêts, comme dors et déjà observé au cours d'expériences naturelles et artificielles. Cette thèse explore ces questions de recherche à travers deux approches complémentaires, de modélisation et de mesures écophysiologiques. Dans le premier chapitre, je décris un simulateur de croissance forestière individu-centré et spatialement-explicite, TROLL, qui intègre les progrès récents en physiologie des plantes. Les processus sont paramétrés à l'aide de traits fonctionnels espèce-spécifiques, pour une forêt tropicale amazonienne. Une régénération forestière est simulée, et validée par des observations faites en Guyane française. La sensibilité du modèle à plusieurs paramètres globaux clés est évaluée. Enfin, l'influence de la variation de la richesse et composition spécifiques sur les propriétés écosystémiques est explorée. La réponse des forêts tropicales à la sécheresse est mal connue, empêchant la représentation pertinente des processus en jeu dans les modèles de végétation. Les chapitres 2 à 5 de cette thèse ont ainsi pour but de documenter la tolérance à la sécheresse et sa diversité dans une forêt amazonienne. Une méthode récente et rapide de détermination d'un trait de tolérance des feuilles à la sécheresse, le potentiel hydrique des feuilles au point de perte de turgescence (ptlp), est validée et utilisée, permettant de quantifier pour la première fois un tel trait de tolérance à la sécheresse dans une forêt amazonienne à l'échelle de la communauté. Ce jeu de données permet l'exploration des déterminants de la tolérance à la sécheresse des feuilles, à travers les espèces d'arbres, les tailles des individus, les stades de succession, les expositions à la lumière, ainsi que les lianes. La variabilité de ptlp observée suggère une large diversité de réponses à la sécheresse au sein des communautés de plantes amazoniennes. Ceci est confirmé par le suivi direct du flux de sève au cours d'une saison sèche sur divers arbres de canopée. Enfin, je discute les implications de ces résultats pour le développement des futurs modèles de végétation.
A great part of uncertainties in our current understanding and projections of the carbon cycle lies in the vegetation compartment. The problem of biodiversity representation in vegetation models has long been an impediment to a detailed understanding of ecosystem processes. The high biodiversity of tropical forests, their disproportionate role in global biogeochemical cycles, together with their vulnerability to direct and indirect anthropogenic perturbations, amplify the relevance of this research challenge. In particular, the predicted increase in drought intensity and frequency in the tropics may impact forest structure and composition, as already observed in natural and artificial experiments. This thesis explores how new advances in modelling and ecophysiology should help improve our understanding of these processes in the future. In the first chapter, I describe an individual-based and spatially-explicit forest growth simulator, TROLL, that integrates recent advances in plant physiology. Processes are linked to species-specific functional traits parameterized for an Amazonian tropical rainforest. This model is used to simulate a forest regeneration, which is validated against observations in French Guiana. Model sensitivity is assessed for a number of key global parameters. Finally, we test the influence of varying the species richness and composition on ecosystem properties. Tropical forest response to drought is not well understood, and this hampers attempts to model these processes. In chapters 2 to 5 I aimed at documenting drought-tolerance and its diversity in an Amazonian forest. A rapid method of determination of a leaf drought tolerance trait, the leaf water potential at turgor loss point (ptlp), was validated and applied to a range of plant species. We established the first community-wide assessment of drought tolerance in an Amazonian forest. These results inform on the drivers and determinants of leaf drought tolerance, across tree species and lianas, tree size, successional stages, light exposition, and seasons. Variability in ptlp among species indicates the potential for a range of species responses to drought within Amazonian forest communities. This is further confirmed by direct monitoring of whole-plant water use on diverse canopy trees during a marked dry season. Finally, I discuss the implications of these results to increase the dialogue between the vegetation modeling community and ecology, to enhance model's predictive ability, and to inform policy choices.
Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological, energy, and carbon budgets at the land–atmosphere interface. However, despite being the ...main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observations. Here we introduce the first global compilation of whole-plant transpiration data from sap flow measurements (SAPFLUXNET, https://sapfluxnet.creaf.cat/, last access: 8 June 2021). We harmonized and quality-controlled individual datasets supplied by contributors worldwide in a semi-automatic data workflow implemented in the R programming language. Datasets include sub-daily time series of sap flow and hydrometeorological drivers for one or more growing seasons, as well as metadata on the stand characteristics, plant attributes, and technical details of the measurements. SAPFLUXNET contains 202 globally distributed datasets with sap flow time series for 2714 plants, mostly trees, of 174 species. SAPFLUXNET has a broad bioclimatic coverage, with woodland/shrubland and temperate forest biomes especially well represented (80 % of the datasets). The measurements cover a wide variety of stand structural characteristics and plant sizes. The datasets encompass the period between 1995 and 2018, with 50 % of the datasets being at least 3 years long. Accompanying radiation and vapour pressure deficit data are available for most of the datasets, while on-site soil water content is available for 56 % of the datasets. Many datasets contain data for species that make up 90 % or more of the total stand basal area, allowing the estimation of stand transpiration in diverse ecological settings. SAPFLUXNET adds to existing plant trait datasets, ecosystem flux networks, and remote sensing products to help increase our understanding of plant water use, plant responses to drought, and ecohydrological processes. SAPFLUXNET version 0.1.5 is freely available from the Zenodo repository (https://doi.org/10.5281/zenodo.3971689; Poyatos et al., 2020a). The “sapfluxnetr” R package – designed to access, visualize, and process SAPFLUXNET data – is available from CRAN.
Amazonian tree communities have already been seriously impacted by extreme natural droughts, and intense droughts are predicted to increase in frequency. However, our current knowledge of Amazonian ...tree species’ responses to water stress remains limited, as plant trait databases include few drought tolerance traits, impeding the application and predictive power of models. Here we explored how leaf water potential at turgor loss point (πtlp), a determinant of leaf drought tolerance, varies with species life history, season, tree size and irradiance within a forest in French Guiana. First, we provided a further direct validation of a rapid method of πtlp determination based on osmometer measurements of leaf osmotic potential at full hydration for five Amazonian tree species. Next, we analysed a dataset of 131 πtlp values for a range of species, seasons, size (including saplings), and leaf exposure. We found that early-successional species had less drought-tolerant leaves than late-successional species. Species identity was the major driver of πtlp variation, whereas season, canopy tree size and leaf exposure explained little variation. Shifts in πtlp from saplings to canopy trees varied across species, and sapling leaf drought tolerance was a moderate predictor of canopy tree leaf drought tolerance. Given its low within-species variability, we propose that πtlp is a robust trait, and is useful as one index of species’ drought tolerance. We also suggest that measuring this trait would considerably advance our knowledge on leaf drought tolerance in hyperdiverse communities and would thus likely shed light on the resilience of such vulnerable species-rich ecosystem.
1. Amazonian droughts are predicted to become increasingly frequent and intense, and the vulnerability of Amazonian trees has become increasingly documented. However, little is known about the ...physiological mechanisms and the diversity of drought tolerance of tropical trees due to the lack of quantitative measurements.2. Leaf water potential at wilting or turgor loss point (pi(tlp)) is a determinant of the tolerance of leaves to drought stress and contributes to plant-level physiological drought tolerance. Recently, it has been demonstrated that leaf osmotic water potential at full hydration (pi(o)) is tightly correlated with pi(tlp). Estimating pi(tlp) from osmometer measurements of pi(o) is much faster than the standard pressure-volume curve approach of pi(tlp) determination. We used this technique to estimate pi(tlp) for 165 trees of 71 species, at three sites within forests in French Guiana. Our data set represents a significant increase in available data for this trait for tropical tree species.3. Tropical trees showed a wider range of drought tolerance than previously found in the literature, pi(tlp) ranging from -1.4 to -3.2 MPa. This range likely corresponds in part to adaptation and acclimation to occasionally extreme droughts during the dry season.4. Leaf-level drought tolerance varied across species, in agreement with the available published observations of species variation in drought-induced mortality. On average, species with a more negative pi(tlp) (i.e. with greater leaf-level drought tolerance) occurred less frequently across the region than drought-sensitive species.5. Across individuals, pi(tlp) correlated positively but weakly with leaf toughness (R-2 = 0.22, P = 0.04) and leaf thickness (R-2 = 0.03, P = 0.03). No correlation was detected with other functional traits (leaf mass per area, leaf area, nitrogen or carbon concentrations, carbon isotope ratio, sapwood density or bark thickness).6. The variability in pi(tlp) among species indicates a potential for highly diverse species responses to drought within given forest communities. Given the weak correlations between pi(tlp) and traditionally measured plant functional traits, vegetation models seeking to predict forest response to drought should integrate improved quantification of comparative drought tolerance among tree species.
1. Amazonian droughts are predicted to become increasingly frequent and intense, and the vulnerability of Amazonian trees has become increasingly documented. However, little is known about the ...physiological mechanisms and the diversity of drought tolerance of tropical trees due to the lack of quantitative measurements.2. Leaf water potential at wilting or turgor loss point (pi(tlp)) is a determinant of the tolerance of leaves to drought stress and contributes to plant-level physiological drought tolerance. Recently, it has been demonstrated that leaf osmotic water potential at full hydration (pi(o)) is tightly correlated with pi(tlp). Estimating pi(tlp) from osmometer measurements of pi(o) is much faster than the standard pressure-volume curve approach of pi(tlp) determination. We used this technique to estimate pi(tlp) for 165 trees of 71 species, at three sites within forests in French Guiana. Our data set represents a significant increase in available data for this trait for tropical tree species.3. Tropical trees showed a wider range of drought tolerance than previously found in the literature, pi(tlp) ranging from -1.4 to -3.2 MPa. This range likely corresponds in part to adaptation and acclimation to occasionally extreme droughts during the dry season.4. Leaf-level drought tolerance varied across species, in agreement with the available published observations of species variation in drought-induced mortality. On average, species with a more negative pi(tlp) (i.e. with greater leaf-level drought tolerance) occurred less frequently across the region than drought-sensitive species.5. Across individuals, pi(tlp) correlated positively but weakly with leaf toughness (R-2 = 0.22, P = 0.04) and leaf thickness (R-2 = 0.03, P = 0.03). No correlation was detected with other functional traits (leaf mass per area, leaf area, nitrogen or carbon concentrations, carbon isotope ratio, sapwood density or bark thickness).6. The variability in pi(tlp) among species indicates a potential for highly diverse species responses to drought within given forest communities. Given the weak correlations between pi(tlp) and traditionally measured plant functional traits, vegetation models seeking to predict forest response to drought should integrate improved quantification of comparative drought tolerance among tree species.
Musiciens, musicologues et spécialistes de littérature médiévale proposent, dans ce volume, une mise en synergie de leurs compétences pour réfléchir, à partir de quelques exemples, sur la manière ...dont la musique et le spectacle donnent à voir et à entendre un Moyen Âge légendaire, archétypal mais revisité au fil des époques, adapté à de nouveaux publics et à de nouvelles techniques. La démarche relève de l’histoire des représentations : l’enquête, destinée à éclairer les principes, esthétiques ou idéologiques, à l’œuvre dans les essais d’interprétations, de transpositions et d’inventions, s’effectue en deux étapes. La première partie interroge les modes de transmission de la musique médiévale, qu’il s’agisse du répertoire choral ou filmique. Dans la seconde partie, les analyses portent sur la mise en musique de textes ou thèmes médiévaux. Manuscrits, livrets d’opéras, partitions, adaptations audio-visuelles : tous les supports d’expression sont convoqués, permettant de faire résonner le potentiel artistique d’une époque qui, pour être lointaine, reste en accord avec les sensibilités les plus diverses.
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