Functional traits and phenotypic plasticity modulate species coexistence across contrasting environments

Functional traits are expected to modulate plant competitive dynamics. However, how traits and their plasticity in response to contrasting environments connect with the mechanisms determining species coexistence remains poorly understood. Here, we coupled field experiments under two contrasted climatic conditions to a plant population model describing competitive dynamics between 10 annual plant species in order to evaluate the associations of niche and fitness differences with 19 functional traits covering physiological, morphological and reproductive characteristics. We found a rich diversity of univariate and multidimensional relationships, in which outstand the primary role of traits related to water- and light-use-efficiency in modulating the determinants of competitive outcomes. Importantly, such traits and their plasticity promoted species coexistence across climatic conditions by enhancing stabilizing niche differences and by generating competitive trade-offs between species. Our study represents a significant advance showing how leading dimensions of plant form and function connect to the mechanisms determining the maintenance of biodiversity.