COS 44-2 - Hydraulic redistribution as a driver of tree and grass competition versus facilitation in a semiarid savanna

Tuesday, August 8, 2017: 8:20 AM
D138, Oregon Convention Center
Greg A. Barron-Gafford1, Enrique Sanchez-Cañete2, Rebecca L. Minor3, Sean Hendryx4, Esther Lee5, Leland F. Sutter3, Elizabeth Parra6, Anthony Colella3, Patrick Murphy3, Erik P. Hamerlynck7, Praveen Kumar5 and Russell L. Scott8, (1)School of Geography & Development; B2 Earthscience / Biosphere 2, University of Arizona, Tucson, AZ, (2)(IISTA-CEAMA), Centro Andaluz de Medio Ambiente, Granada, Spain, (3)School of Geography & Development, University of Arizona, Tucson, AZ, (4)School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, (5)Department of Civil and Environmental Engineering, University of Illinois at Urbana−Champaign, Urbana, IL, (6)College of Science, Biosphere 2, University of Arizona, Tucson, AZ, (7)Eastern Oregon Agricultural Research Center, USDA-ARS, Burns, OR, (8)Southwest Watershed Research Center, United States Department of Agriculture, Agricultural Research Service, Tucson, AZ

Hydraulic redistribution (HR) is an important ecohydrological process in dryland environments by which plants preferentially move water from wet to dry soil layers. How does this water movement by the overstory influence physiological activity in the understory? Are there periods of facilitation when the tree is lifting water and periods of competition when the water is being moved deeper in the profile? We combined trunk, lateral root, and taproot sap flow data, and linked these measures with shallow and deep soil moisture data to show that soil moisture gradients control hydraulic redistribution in overstory mesquite trees. We created plots under mesquite that experienced HR and plots where HR was physically prohibited to quantify the impacts of HR on understory performance. We measured carbon and water exchange at the leaf-level on mesquite and understory grass and for entire understory ecosystem using a large, portable chamber.


We found that HR provided a drought-buffering capacity for the overstory mesquite and a significant decrease in mesquite photosynthesis in trees where the capacity for HR was reduced. While we had hypothesized that water lifted by the mesquite in periods of drought would facilitate understory grass function, we found no evidence for this. In fact, we found that grasses actually conducted higher rates of photosynthesis in plots where HR was eliminated - and this was true at the annual growing season scale and around a single precipitation event. Ultimately, we found that HR in upland savannas, where there is little to no access to deep water, yields a competitive interaction between overstory mesquites and understory grasses at the scale of individual precipitation pulse events and across entire growing seasons.