Evidence of hydraulic redistribution and its water sharing mechanism in a karst field of Southwest China

Hydraulic redistribution (HR) might serve as a crucial drought-tolerance mechanism to address dry and harsh environments in mixed-species communities. However, it remains unclear just how the unique water supply capacity of karst ecosystems (higher potential water content in karst water bearing media and lower effective water content in rock stratum) impact HR patterns. Here, dominant evergreen trees and their neighboring species were selected to determine the occurrence of HR and its water sharing mechanism by employing three methods (soil water content (SWC)/soil water potential (psi s), deuterium labeling, and thermal technique methods) in a subtropical karst ecosystem with shallow, patchy, and skeletal soils. The results revealed that the water potential gradient (>-1.28 Mpa) between 0 and 20 cm and 20-40 cm soil depths provide favorable conditions for the occurrence of HR. A marked increase in the isotopic composition of soil water following a deuterium labeling experiment showed the distinct occurrence of HR between sampled species. In contrast, the stable isotopic composition of xylem water in sampled species implied that there was no uptake of water released from HR. The downward flow rate of sap largely explained the quantity and maintenance of HR. A weak increase in the isotopic composition of the xylem water in neighboring species following a deuterium labeling experiment indicated that water sharing between species was contingent on the distance between the sampled and neighboring species. The water sharing kinetics of HR between species was observed to depend on the distance between the trees, as well as the HR volume and sustainability. This HR was initially confirmed in a karst field of Southwest China. The water released through the sampled plant roots was not for their own use, but for that of neighboring species. Our results provide multiple research strategies and indicated that HR is a significant mechanism in facilitating species coexistence, which sheds light on the restoration of vegetation in karst areas.