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 (π 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 (π 0 ) is tightly correlated with π tlp . Estimating π tlp from osmometer measurements of π 0 is much faster than the standard pressure-volume curve approach of π tlp determination. We used this technique to estimate π tlp for 165 trees of 71 species, at three sites within forests in French Guiana. Our dataset 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, π 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 π tlp (i.e., with greater leaf-level drought tolerance) occurred less frequently across the region than drought-sensitive species. Accepted ArticleThis article is protected by copyright. All rights reserved. 5.Across individuals, π 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 π tlp among species indicates a potential for highly diverse species responses to drought within given forest communities. Given the weak correlations between π 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.

Forest dynamic models predict the current and future states of ecosystems and are a nexus between physiological processes and empirical data, forest plot inventories and remote‐sensing information. The problem of biodiversity representation in these models has long been an impediment to a detailed understanding of ecosystem processes. This challenge is amplified in species‐rich and high‐carbon tropical forests. Here we describe an individual‐based and spatially explicit forest growth simulator, TROLL, that integrates recent advances in plant physiology. Processes (carbon assimilation, allocation, reproduction, and mortality) are linked to species‐specific functional traits, and the model was parameterized for an Amazonian tropical rainforest. We simulated a forest regeneration experiment from bare soil, and we validated it against observations at our sites. Simulated forest regeneration compared well with observations for stem densities, gross primary productivity, aboveground biomass, and floristic composition. After 500 years of regrowth, the simulated forest displayed structural characteristics similar to observations (e.g., leaf area index and trunk diameter distribution). We then assessed the model's sensitivity to a number of key model parameters: light extinction coefficient and carbon quantum yield, and to a lesser extent mortality rate, and carbon allocation, all influenced ecosystem features. To illustrate the potential of the approach, we tested whether variation in species richness and composition influenced ecosystem properties. Overall, species richness had a positive effect on ecosystem processes, but this effect was controlled by the identity of species rather by richness per se. Also, functional trait community means had a stronger effect than functional diversity on ecosystem processes. TROLL should be applicable to many tropical forests sites, and data requirement is tailored to ongoing trait collection efforts. Such a model should foster the dialogue between ecology and the vegetation modeling community, help improve the predictive power of models, and eventually better inform policy choices.