Trait–abundance relationships in tree communities along temperature and successional gradients

Questions: Functional traits of species, which are responsible for demographic characteristics, environmental preferences, and competitive abilities, may have essential linkages with species abundance in a community, but we lack a general understanding of trait–abundance relationships in tree communities. This study investigated three questions: (a) is the relative abundance of tree species monotonically correlated with their functional traits; (b) are abundant species functionally similar or dissimilar to each other; and (c) do trait–abundance relationships change along temperature or successional gradients?. Location: A total of 3,799 lowland natural forest sites along a 1,500-km latitudinal gradient in Japan. Methods: We analyzed trait–abundance relationships (correlations and functional similarity/dissimilarity of abundant species) and their responses to temperature and successional gradients in 3,799 tree communities across multiple biomes in Japan. We focused on five functional traits whose relationships with plant performance are well known: adult height, leaf mass per area (LMA), seed mass, wood density, and leaf size. Results: Across the temperature and successional gradients, the abundance of a tree species in a local community was often positively correlated with adult height, LMA, seed mass, and wood density and negatively correlated with leaf size. In contrast, whether abundant species are functionally similar or dissimilar to each other was clearly context-dependent for all five traits. Changes in the similarity/dissimilarity patterns across the temperature and successional gradients were not consistent among the five traits. Conclusions: The trait–abundance relationships suggest that a long life, a high survival rate, stress tolerance, and/or competitive ability are key properties for higher abundance in a tree community across temperature and successional gradients. However, the fact that dominant species are not always functionally similar to each other suggests that mechanisms that prevent these species from competitively excluding other species, such as gap dynamics, are also important.