Traits uncover quasi-neutral community assembly in a coastal heath vegetation

Trait-based approaches are increasingly being used in ecology due to their potential to explain and predict community structure and dynamic regardless of species identities. Here, we asked whether coastal heath vegetation established in stressful habitats could be assembled by quasi-neutral mechanisms based on functional trait distribution. Communities are organized through quasi-neutral dynamics if abiotic gradients and/or species interactions impose sorting on functional traits and species performances, but not on species phylogenetic relatedness and occurrence. The study was conducted in a coastal area containing tall dunes near the sea, and relatively flat areas punctuated by short palaeodunes further inland, Northeastern South America. We evaluated 10 traits of the 21 most abundant species, as well as eight abiotic variables in 70 plots (25 m²) established in dune tops, valleys, flat and steep microenvironments. The effect of environmental filter was tested by linear mixed models (LMM) between each community-weighted mean (CWM) trait and the abiotic variables, and by two indices of environmental filtering based on intra- and interspecific-trait variability relative to null models. To evaluate the effect of species interactions, we used an index of niche overlap relative to null models, and LMMs between this index and the species richness of plots. The environmental filtering index did not deviate from null expectations for any trait. Models that included individual variability performed similarly to models that used species mean traits. However, the explicit consideration of environmental gradients by CWM analyses revealed functional patterns that would remain undetected if trait distribution was analyzed only. In addition, intraspecific variation in the stem length was an important driver of species diversity as suggested by the relationship between species richness and the niche overlap index. Based on the occurrence and phylogenetic relatedness of species, previous work in the same system found neutral community assembly. We concluded that to ignore functional traits may lead to underestimations of niche-based processes, and that analyses of species occurrence and phylogenetic relatedness should be complemented with functional traits to reinforce our interpretation of processes underpinning community assembly. In this regard, the study heath vegetation is influenced by quasi-neutral effects on functional trait distribution.