The relative importance of phylogeny and environment for sapling community assembly varies from individual to species scale

Background/Question/Methods

Recent studies have shown that accounting for intraspecific trait variability (ITV) is essential to understand community assembly. Even clearer is the importance of phylogeny for community assembly, given that organism relatedness may affect assembly mechanisms (e.g., limiting similarity and environmental filtering). Despite these advances in the understanding of ITV and phylogeny role in community assembly, to our knowledge, no study has addressed the influence of phylogeny on community assembly while accounting for ITV. We assessed the relative importance of phylogeny and environment for sapling community assembly from the scale of individuals to species. Data collection was conducted in 40 plots (4.5m²) across a canopy openness gradient in forest patches spread over a 120-ha area dominated by grassland in southern Brazil. We described 1129 tree saplings by specific leaf area (SLA). Trait data were constrained into three scales: individual, population and species. For each scale, we used trait community-weighted means. Phylogenetic distance matrices were calculated based on APGIII, using fixed branch lengths. We used the first axis from a PCoA of a matrix of phylogenetic-weighted community composition (P) as the phylogenetic factor for each scale. We performed path analyses to assess causal relationships between environment (canopy openness), phylogeny and trait (SLA). 

Results/Conclusions

The first axes of PCoA over matrix P explained most of the variation in phylogenetic composition (>59%). Path analyses indicated that canopy openness and phylogeny independently influenced SLA variation at all scales. Phylogeny was more important than canopy openness in explaining SLA variation at the species scale (phylogeny, β = 0.56, P < 0.001; canopy, β = -0.31, P = 0.02). On the other hand, phylogeny was less important than canopy openness at the scales of individuals (phylogeny, β = 0.37, P < 0.01; canopy, β = -0.47, P = 0.001) and populations (phylogeny, β = 0.37, P < 0.01; canopy, β = -0.46, P < 0.01). Independent effects of phylogeny and environment on trait variation suggest that there is no phylogenetic niche conservatism involved in sapling community assembly in our system. Phylogeny’s greater importance than environment at the species scale is likely because major phylogenetic dissimilarities are found between species rather than between organisms of a same population or between conspecific populations. Phenotypic plasticity of SLA may explain the higher effects of environment on trait variation at population and individual scales. The next step is evaluating the role of phenotypic plasticity of species for sapling community assembly.