Different degrees of water-related stress affect evolutionary diversity in a seasonally dry biome.

Environmental gradients play a key role in shaping diversity in tropical forests. However, we have a little understanding of how evolutionary diversity is affected by gradients and the role of niche persistence in flooded forests in dry biomes. Here, we assessed the evolutionary diversity across a flooding gradient in the Caatinga Domain of South America. We established 120 plots across four tributaries of the São Francisco River, eastern Brazil, consisting of 72 plots in flooded, 24 in occasionally flooded, and 24 in unflooded forests. We computed richness, phylogenetic diversity (PD), mean nearest taxon distance (MNTD), and mean pairwise phylogenetic distance (MPD) and their standardized effect sizes (ses.PD, ses.MNTD, and ses.MPD). We found low richness, low PD, and high MNTD values in flooded forests relative to unflooded and occasionally flooded forests. MPD did not differ across the flooding gradient. The standardized effect size metrics were higher in flooded forests. Despite the unflooded and occasionally flooded forests being rich in terms of species and correlated phylogenetic structure, flooded forests showed more lineage diversity than expected by chance. We assessed whether this pattern could be driven by resprouting ability testing its phylogenetic signal. Resprouting is randomly distributed across phylogeny, but plant communities are likely assembled from random draws of the resprouters' lineage pool. Quantifying evolutionary diversity across flooding gradients in dry environments brought new insights to how the same environmental filters may lead to disparate patterns of evolutionary diversity and the role of response traits in allowing certain clades to persist in flooded habitats.