Light mediates the relationship between community diversity and trait plasticity in functionally and phylogenetically diverse tree mixtures

Trait values affect how plants function, with consequences that propagate through scales of ecological organization to affect ecosystem function. However, the pathway connecting trait expression to ecosystem function is complicated by feedbacks: trait expression may vary within species in response to community diversity, and trait expression also determines a community's functional diversity. The predictability and consequences of interactions between trait expression and community diversity are poorly understood, particularly in forest ecosystems. Here we quantify the extent to which light access—which past studies suggest affects trait expression and differs as a result of interactions among plants—differs consistently with community diversity and explains intraspecific trait variation in trees. In a common garden, trees of five angiosperm and seven gymnosperm species were planted to form 37 communities ranging widely in species and functional diversity whereby confounding environmental variation was minimized. We sampled leaves of each species to characterize intraspecific variation within and among communities in three traits—leaf size, specific leaf area and nitrogen concentration—and estimated each leaf's access to light. We found that light access explained a substantial portion of trait variation within all species, species tended to receive less light in more diverse communities and effects of community diversity on trait expression tended to be mediated by light. Synthesis. This study shows that community diversity affects leaf trait plasticity in young trees through its effects on light availability. Moreover, trait responses across angiosperms and gymnosperms were remarkably similar, showing that shared biophysical constraints overwhelm divergent evolutionary histories. Together, these results illustrate how leaf trait expression responds to, and may subsequently shape, diversity in young tree communities in largely predictable ways via resource-based interactions, with consequences that may scale from individuals to ecosystems.