Factors influencing the distribution and structure of tropical vascular epiphyte communities at multiple scales

Understanding the factors that determine species distributions is a central question in ecology. Niche-based theories stress the importance of environmental heterogeneity in influencing species distributions while neutral-based theories emphasize the effects of dispersal limitation. The relative importance of these factors in influencing species distributions may depend on spatial scale -- deterministic factors may be more important at small spatial scales where fine-scale habitat factors become more relevant and stochastic factors may be more important at larger spatial scales where dispersal limitation becomes more relevant. I examined the influence of deterministic and stochastic factors on the distribution and structure of vascular epiphyte communities in lowland tropical forests at multiple scales. Vascular epiphytes, non-parasitic plants that often inhabit tropical tree canopies, contribute up to 35% of the local floral diversity and up to 25% of the floral biomass in tropical forests. Yet our understanding of how habitat selection or random colonization events related to dispersal influence the distribution and floristic composition of epiphytes lags far behind that of terrestrial-based plant communities. I surveyed epiphytes among different-aged forests, different-sized trees, and within emergent tree crowns and examined whether habitat characteristics influenced epiphyte community structure. Among different-aged forests, forest structure and age influenced epiphyte species composition as density and species richness increased with forest age, and many epiphyte species were confined to microhabitats unique to old-growth forests. Among different-sized trees, epiphyte species exhibited significant associations to particular tree sizes and microhabitats. Emergent canopy trees had steep environmental and resource gradients that created a high diversity of microhabitats to which many epiphyte species were specialized. Environmental filtering played a role in epiphyte species distributions as species found in the same microhabitat showed convergence in ecological strategy. Among closely related species within a functional group, there was evidence of trait divergence, supporting the hypothesis of niche differentiation. At large spatial scales, habitat structure and dispersal influenced epiphyte community structure among forest stands. At smaller spatial scales, habitat specialization and differences in plant ecological strategies along environmental gradients suggest niche-based processes in driving local patterns of epiphyte diversity.