Outcomes of interactions among plants shape communities and ecosystem properties. Functional traits capture aspects of species’ niches, in particular resource strategies, such that traits ought to reflect competitive abilities and predict outcomes of interactions. But translating traits into interaction outcomes may be complicated. Modern coexistence theory tells us that outcomes of interactions are a balance between fitness differences – the position of a species in a hierarchical ranking of trait values – and niche differences – absolute differences in trait values. Moreover, traits can vary within species; traits may even respond to neighborhood context itself. Here we examined how trait values are both shaped by and affect outcomes of interactions among trees. We grew seedlings of twelve temperate-boreal tree species for six years in a functionally diverse set of mixtures in a common garden in Cloquet, Minnesota. We took spectral measurements to derive estimates of specific leaf area and leaf nitrogen content – two leaf traits critical to light and carbon acquisition – and measured the growth of trees across these mixtures. To assess neighborhood effects on tree growth, we compared a series of spatially-explicit models of individual tree growth, which differed in how neighbors and their trait values – both inherent and plastic – influenced growth.
Trait values varied significantly among species and, for some species, varied among mixtures; this resulted in trait convergence or divergence in nearly half of the mixtures examined. The trait values of neighbors affected growth in all species. Models incorporating trait hierarchies – assuming more acquisitive trait values conferred stronger competitive effects – best predicted growth in nine species. Models incorporating absolute trait differences – assuming differentiation in trait values offset competitive effects – best predicted growth in the remaining three species. Incorporating plastic variation in trait values improved the fit of models for half of the species. These results highlight the importance of traits and trait differences for tree growth, showing that trait differences – hierarchical and absolute, inherent and plastic – shape how trees grow in different neighborhoods.