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

Authors: BARRON-GAFFORD, GREG - University Of Arizona; Knowles, John; PEREZ SANCHEZ-CANETE, ENRIQUE - Universidad De Granada; MINOR, REBECCA - University Of Arizona; LEE, ESTHER - University Of Illinois; SUTTER, LELAND - University Of Arizona; TRAN, NEWTON - Morton Arboretum; MURPHY, PATRICK - University Of Arizona; Hamerlynck, Erik; KUMAR, PRAVEEN - University Of Illinois; Scott, Russell - Russ

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2020
Publication Date: 12/13/2020
Citation: Barron-Gafford, G.A., Knowles, J.F., Perez Sanchez-Canete, E., Minor, R.L., Lee, E., Sutter, L., Tran, N., Murphy, P., Hamerlynck, E.P., Kumar, P., Scott, R.L. 2020. Hydraulic redistribution buffers climate variability and regulates grass-tree interactions in a semiarid riparian savanna. Ecohydrology. 14(3). Article e2271. https://doi.org/10.1002/eco.2271.
DOI: https://doi.org/10.1002/eco.2271

Interpretive Summary: Many woody plant species move water from soils with high soil water content through their roots into soils with low soil water content, a process called hydraulic redistribution. Hydraulic redistribution can move soil water both upwards when shallower soils are dry, or downwards, when these soils are moist. To date, no one has quantified how these different kinds of hydraulic redistribution vary between years, and how this affects the performance of plants growing under or near redistributing plants. We found that over a wetter year, mesquite trees mainly moved water down into deeper soil layers to use later, and that this had a negative impact on understory grasses. In contrast, in a drier year, mesquite tended to move water upwards, and understory grasses were able to use this water to sustain biological activity. Over both years, mesquite growth and photosynthesis were nearly identical, showing that while hydraulic redistribution had varying effects on understory plants, it provided effective buffering against annual rainfall variation for the dominant woody species in Southwestern US semi-arid riparian ecosystems.

Technical Abstract: 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.