Topographic gradient as a determinant of phylogenetic diversity patterns in the understory of a neotropical forest

Topography is a well-known driver of changes in plant community organization in the tropics. However, we have a limited understanding of how the evolutionary history of component species shapes the organization of these plant communities along topographic gradients. We investigated how the phylogenetic structure of an understory plant community changes along a topographic gradient. We expect closely related species to occupy higher elevations along the gradient, leading to clustering, and to observe overdispersion at lower elevations. The study was conducted in a Neotropical Forest fragment in southeastern Brazil. Twenty plots (10 m x 10 m) were surveyed along a topographic gradient, distributed across four topographic habitats: valley, southwest slope, northwest slope, and ridge. We applied ordination methods to species abundance and soil data. We calculated phylogenetic diversity (PD), the mean pairwise distance (MPD), the mean nearest taxon distance (MNTD), and their SES (standardized effect sizes). Generalized linear models were fitted with phylogenetic metrics as outcomes and soil nutrients as predictors. We found 1,695 individuals belonging to 70 species, 53 genera, and 24 families. The valley was characterized by Rubiaceae, Piperaceae, Myrtaceae, and Lauraceae, while representatives of Lecythidaceae, Bignoniaceae, and Salicaceae were more abundant at the ridge. Notably, the valley habitat exhibited phylogenetic clustering, while the ridge habitat demonstrated phylogenetic overdispersion. Soil moisture was negatively associated with phylogenetic metrics, indicating clustering in the wetter valley and a tendency toward overdispersion on the ridge. Our findings suggest that environmental filtering plays a crucial role in structuring the understory of this tropical community.