Functional Trait Variation Promotes Tree Species Coexistence and Ecosystem Functioning: Insights from A Vegetation Demographic Model

In the era of global environmental and biodiversity changes, understanding the ecological consequences of biodiversity loss has become a central task in ecology. Forest ecosystems harbor over half of terrestrial plant diversity and play a pivotal role in global carbon cycle. Accumulating evidences from observational and experimental data have demonstrated generally positive effects of tree diversity on key forest functions (e.g., productivity). These positive diversity-function relationships were often explained by niche complementarity among tree species, namely that coexisting species with distinct functional traits can exploit resources (e.g., light, nutrient) in complementary ways, enhancing the overall efficiency of forest biomass production. That said, mechanistic understanding on how trait variation mediates tree species coexistence and complementary resource exploitation is still lacking. Vegetation demographic models explicitly simulate the growth, mortality, and reproduction of trees by taking into account realistic plant traits and physiological parameters, which provide a promising platform to understand the mechanisms underlying tree diversity-function relationships. Using a stand-alone BiomeE model, we simulate tree species with different functional traits (involving empirically established trade-offs) and determine the conditions under which tree species can coexist in long terms. Then we explore whether tree diversity contributes to higher ecosystem functions and how trait variations modulate the biodiversity-ecosystem functioning (BEF) relationship. Our current results reveals that leaf trait covariations can promote species existence under light and nitrogen competition, by (i) introducing trade-offs in species’ minimum light and nitrogen requirements; (ii) modulating monoculture resource environment through litter-mediated plant–soil feedback. Notably, despite the complexity of this model, the competition outcomes can be accurately predicted by R* theory. Meanwhile, we found that species coexistence can lead to higher ecosystem productivity, due to both complementary resource use among species and competition advantage of more productive species. Overall, our work demonstrates that plant functional traits can offer key insights into the ecological mechanisms underlying the maintenance of biodiversity and their consequences for ecosystem functioning. We also show how vegetation demographic models can serve as a new powerful tool to link empirical trait observations to classic community theory, and to help us better understand observed biodiversity-ecosystem functioning relationships.