Hydraulic redistribution buffers climate variability and regulates grass-tree interactions in a semiarid riparian savanna

Anticipating the ability of ecosystems to maintain functional integrity across predicted altered precipitation regimes remains a grand ecohydrological challenge. Overstory trees and understory grasses within semiarid savannas vary in their structure and sensitivity to environmental pressures, underscoring the need to examine the ecohydrological implications of this climatic variability. Whereas precipitation has long been recognized as a key driver of landscape ecohydrology, understanding a site's hydraulic redistribution regime (the balance in downward and upward movement of water and the seasonality of these bidirectional flows) may be equally important to understanding moisture availability to vegetation in these dryland ecosystems. As a result, we linked measures of ecosystem-scale carbon exchange, overstory tree sap flux and leaf-level gas exchange to understory whole-plot and leaf-level carbon and water exchange within intact and trenched plots (isolating trees from grasses) in a riparian savanna ecosystem. We maintained measurements across 2 years with distinct precipitation regimes. We found that interannual precipitation variability yielded a categorical shift in the directionality and magnitude of the hydraulic redistribution regime—even within this single site. Additionally, we found that connectivity between overstory trees and understory grasses through hydraulic redistribution created a short period of competition within an average rain year but that facilitation of understory function by overstory trees was much greater and lasted longer during drier years. Together, these findings suggest that hydraulic redistribution can serve as a hydrologic buffer against interannual precipitation variability. Given current climate projections of more variable precipitation within and across years, understanding how hydraulic redistribution regimes vary through time will greatly enhance our capacity to anticipate future ecohydrological function.