Résumé
Climate anomalies, such as extreme drought events, have been shown to affect tropical rainforest dynamics. However, tree growth response to drought may depend on differences in species water-use strategies at the neighbourhood scale. The capacity of local neighbourhoods to mitigate or exacerbate negative drought effects on individual tree performance likely depends on the mechanisms driving local species competition for limiting water resources. Leveraging species traits that are good predictors of water use has the potential to disentangle the relative role of niche differentiation and hierarchical competition in shaping local neighbourhoods. A better understanding of the separate and joint effects of climate, species traits and neighbourhood interactions on individual tree growth may be crucial to improve our predictions of tropical forest dynamics under climate change. In this study, we test how individual tree growth responds to (1) the separate and joint effects of drought and neighbourhood interactions in a tropical rainforest. We further investigate (2) whether neighbourhood interactions are driven by species trait dissimilarities or hierarchies in water use. We use an exceptional dataset covering 30 years of biennial, spatially-explicit inventories of individual tree growth from permanent forest plots at the Cirad experimental site of Paracou, French Guiana, combined with species water use traits, including water potential at turgor loss point and minimal conductance. We test the effects of atmospheric and soil drought via mean anomalies in atmospheric evaporative demands and soil water availability over the census intervals. We use different neighbourhood crowding indices, capturing trait similarities and hierarchies, to evaluate the role of species differences in water use. We use Bayesian hierarchical models for the most abundant 100 species to explore tree growth response to climate anomalies and neighbourhood crowding. Preliminary results demonstrate that both drought and neighbourhood crowding decline tree growth at Paracou. We further show that these two drivers can interact to shape individual tree growth. In particular, we find that high neighbour densities exacerbate negative effects of soil drought, suggesting that competitive interactions play an important role in the individual drought response of trees. Species responses to climate, neighbourhood crowding and their interaction vary greatly at Paracou. Here, we will detail the role of dissimilarities and hierarchies in leaf water use traits, in shaping these neighbourhood effects on individual tree response to climate. Our study highlights the urgent need to integrate biotic interactions at the neighbourhood scale to improve our understanding on how tropical forests will respond to ongoing climate change.