Rock moisture access contributes to hydrologic niche segregation among co-dominant woody plants

Abstract Background and aims Bedrock can store substantial plant-available water, yet its role in hydrologic niche segregation and drought resilience remains unclear. We investigated this question in the semiarid karst Edwards Plateau (Texas), where differences in woody species composition and drought responses have traditionally been attributed to variation in soil depth. Methods During three consecutive years of below-average precipitation, we monitored predawn water potential (Ψ PD ), vadose-zone water storage (soil and bedrock), and xylem water isotopes of three co-dominant woody species ( Quercus fusiformis , Juniperus ashei , and Juniperus pinchotii ) in two contrasting topoedaphic settings: shallow upslope soils over weathered bedrock and deeper downslope soils over consolidated bedrock. Results Across all species, Ψ PD was consistently less negative and less variable in shallow soils over weathered bedrock, likely reflecting access to stable rock moisture. Quercus fusiformis showed the least negative Ψ PD and the lowest δ 2 H and δ 18 O values in both settings, indicating deeper water uptake. Juniperus ashei maintained the most negative Ψ PD and the highest isotope values, suggesting reliance on shallow, evaporatively enriched soil water. Juniperus pinchotii displayed greater flexibility: in deep soils it resembled J. ashei , whereas in shallow soils it maintained higher water status and showed no isotopic shift following rainfall that primarily recharged surface soils, consistent with access to rock moisture. Conclusions Our findings provide evidence that bedrock weathering and access to rock moisture, rather than soil depth alone, strongly shape belowground water partitioning among co-occurring woody species, with important implications for hydrologic niche segregation and drought resilience in shallow-soil ecosystems.