Surface Hydrological Dynamics in Inselbergs Favour the Occurrence of Microrefugia in the Caatinga
Inocencio de Oliveira Borges Neto, Leonardo José Cordeiro Santos, José João Lelis Leal de Souza, Rafael Albuquerque Xavier, Bartolomeu Israel de Souza, Aureliana Santos Gomes, Maria do Socorro Silva Salvador, Phillipe Valente Cardoso, Vinícius da Silva SeabraABSTRACT
Water is the main factor limiting vegetation persistence in drylands. Despite the dry landscape, inselbergs are generally surrounded by dense vegetation cover. The hydrological mechanisms by which inselbergs mitigate the water scarcity are still poorly understood. This study assessed how surface runoff dynamics on granite inselbergs sustain the formation of microrefugia in the semiarid Caatinga, integrating geomorphological, pedological, hydrological and floristic analyses. In three inselbergs, soil and colluvial sediment profiles were opened, described and sampled along a fragmented catena. Mean annual precipitation (2017–2023) was 270 mm, concentrated between February and July (82.8% of events), peaking at 460 mm in 2020. Observed textures ranged from sandy loam to clay, and surface horizons reached 18.96% organic carbon. It was observed that convex surfaces (summit and shoulder), due to diverging surface runoff, have shallower soils with predominantly sandy sediments. In contrast, concave areas (backslope and footslope) function as deposition areas due to the convergence of surface runoff, hosting more clayey soils with higher organic carbon content, greater water retention capacity and greater depth. In these soil pockets, the network of fractures in the granitic bedrock favours localized infiltration and groundwater recharge pulses, even under torrential rains. These areas are home to denser, more diverse vegetation composed of species typical of humid phytogeographic domains, functioning as ecological refuges amid the xeric matrix of the Caatinga. In general, the volumes of water captured by the rocky slopes of inselbergs during precipitation events, followed by downstream runoff that allows for accumulation, are fundamental to the formation of microrefugia. This demonstrates that inselbergs act as “islands of moisture” essential to the conservation and resilience of the ecosystem in the face of seasonal rainfall and climate change projections.