Evolutionary and environmental drivers of dry-season deciduousness in a legume genus.

Leaf deciduousness is a key drought-avoidance strategy in tropical flora, reducing water loss during seasonal dry periods. While winter-deciduousness in temperate regions is well-understood, the evolutionary and environmental drivers of dry-season deciduousness remain poorly explored. Using the genus Mimosa, a species-rich and morphologically diverse lineage, we applied an eco-evolutionary framework to investigate the role of dry-season deciduousness across time and space. We combined a time-sliced trait-dependent diversification model, analyses of joint evolution of environmental niches in relation to leaf habit (deciduous vs evergreen), and phylogenetic multilevel models to test whether deciduousness influenced diversification and to identify its environmental drivers. Lineages switch from evergreen to deciduous habit more frequently before c. 7 Ma, whereas deciduous to evergreen transitions and increased speciation rates of evergreen lineages became dominant after this time. Deciduous lineages exhibited faster evolutionary rates along gradients of vapor pressure deficit, which also emerged as the strongest environmental predictor of deciduousness. However, most variation in leaf habit was explained by species-level (nonphylogenetic) variation and phylogeny. Although deciduous lineages respond rapidly to atmospheric dryness, dry-season deciduousness appears to be primarily shaped by the interplay between species identity and shared ancestry rather than by environmental conditions alone.