Exotics are more complementary over time in tree biodiversity–ecosystem functioning experiments

The Biodiversity–Ecosystem Functioning (BEF) literature proposes that ecosystem functioning increases with biodiversity because of complementarity in resource-use among species, associated with functional diversity. In this study, we challenge the trait-based ecology framework by comparing congeneric exotic (European) and native (North American) tree species showing similar resource-use functional trait values. The trait-based framework suggests that two functionally equivalent species should play similar roles in a community, resulting in similar interactions and performances. However, several studies showed that when growing in mixtures, exotic species that are functionally equivalent to native species benefitted from enemy release, resulting in a reduced apparent competition. We hypothesize that exotic species should be more productive than native species because the exotic species benefit from reduced apparent competition due to enemy release rather than from possessing more competitive resource-use functional traits. We study a diversity experiments, part of the International Diversity Experiment Network with Trees (IDENT), composed of two identical sites, each with two orthogonal diversity gradients: species richness and functional diversity. The functional gradient consists of species combinations of equal richness but increasing functional diversity, using different combinations of species provenance to assess the relationship between productivity, functional diversity and species provenance, independently of species richness. We grew a total of 12 species (six native and six exotic) in different combinations of one-, two- and six-species mixtures. The exotic species were selected based on their functional equivalence to their native congeneric species. Eight years after planting, we found that exotic species were more productive than native species, but only at high functional diversity. Results indicate that exotic species overall benefit from a reduced apparent competition, and that exotic-increased productivity at high functional diversity is consistent with the enemy release hypothesis. After 8 years, exotic species were more productive overall than their native counterparts, but only in the most functionally diverse communities. This study represents a first step in understanding the relative importance of complementarity in resource-use and apparent competition in a context of an exotic tree species invasion