Rainwater runoff, soil, and vegetation interactions in inselbergs provide microrefugia in Brazil's Caatinga drylands

The Caatinga is one of the most diverse dry forests in the world. The knowledge of the influence of environmental drivers on the floristic patterns in this dry forest is limited, particularly in areas surrounding inselbergs. This study investigates the interactions between rainwater runoff, soil properties, and vegetation surrounding inselbergs in the Caatinga, a semi-arid region of Brazil. We tested the hypothesis that water availability, associated with deeper soils surrounding granite outcrops, facilitates the formation of mesic habitats, potentially serving as ecological microrefugia for biota associated to resource-rich environments. We analyzed hydrological contribution areas patterns, soil profiles, and floristic composition in areas influenced by inselberg (surrounding inselbergs) and areas not influenced by inselberg (located more than 200 m away from inselbergs). Floristic diversity was evaluated through species richness and compositional similarity, while soil properties were examined in terms of depth, chemical composition, and physical characteristics. Runoff analyses was conducted across four hydrological contribution areas (HCAs) encompassing both toe slopes and rocky slopes of the inselbergs. Our findings indicate that runoff is largely directed down the slopes of inselbergs, accumulating at their toe slope and contributing to the development of soils with elevated carbon and nutrient levels. This increased water and nutrient availability supports a more diverse vegetation composition, including species typical of wetter environments. These results suggest that areas surrounding inselbergs in the Caatinga serve as ecological microrefugia and create exceptional wetter areas within these dry forests.