The search for phenotypic traits that explain species interactions and compositional turnover along environmental gradients is a major research priority because quantifying the adaptive value of traits will improve our understanding of species coexistence and responses to global change. If correlations between average community-level traits and environmental gradients reflect the adaptive value of traits, then fitness must be driven by strong trait-by-environment interactions. To evaluate this important yet untested assumption, we compiled survivorship data for 13,822 individual plants representing 46 herbaceous perennial species using census data from long-term (2003-2012) permanent plots in northern Arizona. We modelled individual survival as a function of trait-by-environment interactions, plant size, inter-annual climatic variation, and local neighborhood competition.
We found that survival in a given environment does depend on the trait value of the species, but trait-environment relationships were unreliable estimates of the adaptive value of traits. For example, community-level leaf and soil properties were uncorrelated, which would imply that leaf traits have no adaptive value along gradients of soil nutrient availability. However, survival was highest for species with high specific leaf area in fertile soil, a result that supports the widely held view of the importance of leaf traits in determining species distributions along soil fertility gradients. Temporal dynamics were driven by both intra- and inter-specific competition and by inter-annual climatic variation. To forge a deeper understanding of the adaptive value of traits in community assembly, we recommend linking demographic rates with functional traits along gradients of environmental conditions and competitive interactions.