Multifaceted Assessment of Amazonian Tree Diversity Reveals Pervasive Impacts of Human Modification.

Tropical forests harbour the majority of tree species on the planet but are increasingly subjected to deforestation and human-driven disturbances. Understanding how human modifications impact various facets of diversity-i.e., taxonomic, functional, and phylogenetic-is crucial, as their responses can differ significantly. Additionally, the influence of species dominance and individual size class on the recovery trajectories of future forests is often overlooked. Here, we address these knowledge gaps by comparing the taxonomic, functional, and phylogenetic diversities of large (≥ 10 cm DBH) and small (≤ 2 cm DBH < 10 cm DBH) trees in undisturbed and human-modified Amazonian forests, considering different weights of species dominance using Hill Numbers. We sampled 25,313 large and 30,070 small trees across 215 forest plots distributed in two different regions of Eastern Amazonia and representing a range of human modification (i.e., undisturbed, logged, logged-and-burned, and secondary forests). Our findings indicate that human modifications significantly reduce the taxonomic, functional, and phylogenetic diversities of both large and small trees, regardless of dominance weightings. Secondary forests exhibited the lowest alpha-diversity and were the most dissimilar to undisturbed forests, while logged-and-burned forests were as distinct from undisturbed forests as they were from secondary forests across all diversity facets. Taxonomic and functional diversity showed similar sensitivity to human modification, while phylogenetic diversity was the least sensitive in alpha-diversity but equally sensitive in community composition analyses. Overall, we showed that human modification explained 55% of the effect size variation found in alpha-diversity and 42% of that found in community composition, with diversity facet, tree size and dominance weighting explaining either ≤ 5%. Given the deleterious impacts of human modification on the diversity of tropical forests, it is imperative to protect remaining undisturbed areas from selective logging and wildfires. Nevertheless, even disturbed primary forests still harbour more taxonomic, functional and phylogenetic diversity than secondary forests.