Role of spatial and environmental factors in structuring vascular epiphyte communities in two neotropical ecosystems

Abstract The great heterogeneity of tropical systems challenges our understanding of how niche and neutral processes structure epiphyte communities. Dispersal, phorophyte identity and the environmental conditions created by phorophytes are the main determinants of vascular epiphyte structure and diversity. However, these forces comprise multiple causal factors that need to be decomposed to address the complex patterns of independent and confounded effects acting upon epiphyte communities. We tested the hypothesis that the environment created by phorophytes (size, bark characteristics, microenvironmental conditions, and substrate type) is more important than phorophyte species identity and space in determining the structure of epiphyte communities, with phorophyte size being a major determinant within the environmental component. We also expected larger phorophytes to exhibit greater epiphyte richness and diversity. The study was carried out on an inselberg within a semi-deciduous montane forest and a cloud forest in southeastern Brazil. Partial redundancy analysis was used to hierarchically decompose the relative abundances of vascular epiphytes into three components: association with particular phorophyte species, the environment created by phorophytes, and space (Moran’s eigenvector maps). Linear models were used to analyze the relationship between epiphytic diversity and richness with size descriptors. We show that neutral and niche-based processes act simultaneously on the dynamics of vascular epiphyte species, but distinctly on the inselberg and cloud forest. The potential for an epiphyte to occupy all favorable phorophytes was larger in the cloud forest due to the higher connectivity offered by trees. Abundance, richness and diversity of epiphytes increased with phorophyte size, probably due to an increase in the area, time of colonization or increase in microhabitat heterogeneity within phorophytes. Larger phorophytes are a major determinant of local environmental conditions and the spatial organization of epiphytes, as they promote their abundance and maintain their diversity in tropical systems.