Spatial turnover of taxonomic, functional, and phylogenetic tree diversity across broad environmental gradients in Puerto Rico

Background/Question/Methods

Enhancing plant community surveys with functional and phylogenetic data can help resolve the influence of the various processes that govern species distributions and community assembly.  Applying this approach to broad environmental gradients may reveal how communities respond to environmental change. We examined variation of Puerto Rican tree diversity across precipitation and soil gradients to understand how the relative influence of processes governing these patterns changes with respect to environmental conditions.  Specifically, we evaluated the evidence that environmental filtering and biotic interactions govern these patterns at a regional scale.

We censused all freestanding, woody stems ≥1 cm diameter in 24, 0.25-ha plots located along Puerto Rico’s broad precipitation gradient.  For the species included in these plots (N=221), we measured 6 functional traits based on a priorihypotheses about their ecological relevance and response to environmental conditions (specific leaf area, leaf area, leaf thickness, the ratio of leaf lamina to petiole mass, maximum height, and wood density).  In addition, we quantified the evolutionary relationships among all native and naturalized trees in Puerto Rico (N=564) using a highly resolved molecular phylogeny.  Using null models, we evaluated how taxonomic, functional, and phylogenetic dimensions of tree community composition varied with respect to environmental gradients.

Results/Conclusions

Pairwise taxonomic dissimilarity of tree assemblages increased significantly with respect to dissimilarity of mean annual precipitation among sites.  Phylogenetic and functional dissimilarity metrics were also positively associated with environmental dissimilarity between sites but only some of these comparisons differed significantly from a random expectations based on taxonomic dissimilarity.  In general, community mean functional trait values varied in ways consistent with our hypothesized ecological significance of traits.  For example, community mean wood density and leaf thickness were negatively associated with mean annual precipitation while leaf area, the ratio of leaf lamina to petiole mass, and maximum height increased with mean annual precipitation.  Soil type had a strong influence on taxonomic, functional, and phylogenetic components of community composition but its effects were somewhat confounded with precipitation.

Our study extends recent developments in community phylogenetics and functional ecology to address community variation at an intermediate spatial scale and along broad environmental gradients.  Integrating multiple dimensions of biodiversity helps illuminate the processes underlying community variation along environmental gradients.  Broadening the spatial and taxonomic scale of analysis to address these issues at a regional scale integrates ecological and evolutionary processes structuring community diversity.