Phylogenetic and functional community structure of trees in a seasonal tropical forest

Abstract

Habitat partitioning is important for understanding the drivers that influence the assembly of tropical forest communities. Understanding the local-scale effects of environmental gradients on diversity (taxonomic, functional, and phylogenetic) involves understanding the role of each species within the plant communities. In this context, our objectives were as follows: (1) to understand how topography can influence taxonomic and phylogenetic diversity and which variables most influence community assembly on a local scale; (2) to understand how the topographic gradient shapes the functional identity of species, and how they contribute to understanding the mechanisms of community assembly on a local scale; (3) and to verify the functional trait composition of vertical strata and whether the strategies adopted by trees along vertical stratification are related to functional groups, leaf phenology and dispersal syndrome. The study was conducted in three permanent plots of 1 ha within a fragment of the Atlantic Forest. From the floristic list, a phylogenetic tree was created based on 211 species, and phylogenetic diversity was calculated: the average paired distance between all the taxa in the community, the average distance of the closest taxon in the community, and the standardized effect size for these metrics. We calculated the metrics of community weighted average, functional richness, functional uniformity, functional divergence, and metrics of phylogenetic diversity for each plot. Four leaf traits and two wood traits were selected: leaf area (cm²), specific leaf area (cm²/g), dry mass content (g/g), wood density (mg/m³), maximum height (m), and leaf thickness (cm). Multiple linear and generalized linear models were used to evaluate the effects of environmental variables on taxonomic, functional, and phylogenetic diversity. We analyzed the functional identity and diversity across topographic habitats. To analyze the relationship between leaf phenology and dispersal syndrome with the vertical stratification we performed the Qui-square test. We compared the mean values of the traits to understand the functional patterns within the functional groups of leaf phenology, dispersal syndrome, and vertical strata. The MRT grouped the subplots of the whole community into three habitats: lower, intermediate, and high topographic habitats. The diversity of the elevation habitats was significantly different; the intermediate habitats showed higher diversity than the other habitats, and the high habitats had the lowest diversity values. The Low habitats were always different from the other habitats, indicating that PD and MPD decreased with increasing altitude. The regression results showed that richness was negatively influenced by elevation and SB and positively influenced by soil moisture, coarse sand, and organic matter. The topographic gradients showed different functional compositions along the habitats; a high habitat was characterized by species with conservative leaf traits, higher functional divergence, and lower functional richness. The low habitat was characterized for acquisitive leaf traits, less functional divergence, and higher functional richness. Functional richness and functional divergence were the categorical traits that were best explained; at the local scale, edaphic variables appeared to be more important for the variation of the traits and functional diversity, and the texture of the soil was the main driver. Leaf phenology showed a functional pattern of trait mean values of acquisitive-conservative traits between deciduous and evergreen species. The traits mean values of dispersal syndrome shows that zoochory species are correlated with conservative traits and non-zoochory species are correlated with acquisitive traits. Our study demonstrates the importance of local-scale studies, showing that habitats found on local gradients can affect plant diversity at the taxonomic, phylogenetic, and functional levels. This result is very important for understanding how Seasonal Tropical Forests are structured horizontally and vertically and the patterns of functional composition. Keywords: Topography, community assembly, habitats, environmental variables, vertical stratification