Impacts of hydraulic redistribution on grass-tree competition versus facilitation in a semiarid savanna

Authors: BARRON-GAFFORD, G.A. - University Of Arizona; SANCHEZ-COHEN, E.P. - University De Granada; MINOR, R.L. - University Of Arizona; HYENDRYZ, S. - University Of Arizona; LEE, E. - University Of Arizona; SUTTER, L.F. - University Of Arizona; TRAN, N. - University Of Guelph; PARRA, E. - University Of Arizona; COLELLA, T. - University Of Arizona; MURPHY, P. - University Of Arizona; Hamerlynck, Erik; KUMAR, P. - University Of Illinois; Scott, Russell - Russ

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/28/2017
Publication Date: 8/4/2017
Citation: Barron-Gafford, G., Sanchez-Cohen, E., Minor, R., Hyendryz, S., Lee, E., Sutter, L., Tran, N., Parra, E., Colella, T., Murphy, P., Hamerlynck, E.P., Kumar, P., Scott, R.L. 2017. Impacts of hydraulic redistribution on grass-tree competition versus facilitation in a semiarid savanna. New Phytologist. 215(4):1451-1461. https://doi.org/10.1111/nph.14693.
DOI: https://doi.org/10.1111/nph.14693

Interpretive Summary: A key challenge for hydrology is how plants affect the hydrological cycle and how the hydrological cycle drives key plant functions like growth. We investigated how the movement of water in tree roots affect tree and understory grass photosynthesis across individual precipitation events to an entire growing season. We measured water flow within roots, tree and grass photosynthesis, ecosystem-level net carbon exchange, and soil carbon dioxide efflux with local meteorology data. The movement of water in tree roots was predominantly from shallow soil layers to deeper ones. We found that this resulted in a competitive interaction between the grass and trees. Trees used nearly all water that was moved in their roots to meet their own water needs to support photosynthesis. Our work suggests that a common semiarid hydrological paradigm, which suggests that grasses supplement some of their water needs by locating under trees, is incorrect and that other stresses besides water, like excessive light and temperature, are being ameliorated.

Technical Abstract: -A long-standing ambition in ecosystem science has been to understand the relationship between ecosystem community composition, structure and function. Differential water use and hydraulic redistribution have been proposed as one mechanism that might allow for the coexistence of overstory woody plants and understory grasses. -Here, we investigated how patterns of hydraulic redistribution influence overstory and understory ecophysiological function and how patterns vary across timescales of an individual precipitation event to an entire growing season. To this end, we linked measures of sap flux within lateral and tap roots, leaf-level photosynthesis, ecosystem-level carbon exchange and soil carbon dioxide efflux with local meteorology data. - The hydraulic redistribution regime was characterized predominantly by hydraulic descent relative to hydraulic lift. We found only a competitive interaction between the overstory and understory, regardless of temporal time scale. Overstory trees used nearly all water lifted by the taproot to meet their own transpirational needs. -Our work suggests that alleviating water stress is not the reason we find grasses growing in the understory of woody plants; rather, other stresses, such as excessive light and temperature, are being ameliorated. As such, both the two-layer model and stress gradient hypothesis need to be refined to account for this coexistence in drylands.