Convergent Hydraulic Redistribution in a Semi-Arid Ecosystem with Groundwater Access

Abstract:

In water-limited environments, plants develop adaptive strategies to maximize the use of water and nutrient resources in the soil. One of the strategies is the transport of water from wet to dry soil layers through plant roots as preferential pathway, known as hydraulic redistribution In our studies, we evaluate how hydraulic redistribution impacts water use dynamics in two semiarid ecosystems in Arizona: upland savanna (without groundwater access) and riparian ecosystem (with groundwater access). An interesting bi-directional convergent hydraulic redistribution was detected in the riparian site. The convergent flow occurred after a concentrated precipitation event following an extended dry period. When the topsoil layers are wet and taproots have access to groundwater at deeper depths, trees absorb water from both near surface and groundwater, releasing water to intermediate soil layers through their roots. Convergent hydraulic redistribution and a presence of groundwater in the riparian site promote more facilitative water use between co-existing grasses and trees. In the upland site, hydraulic redistribution supports 47% of tree transpiration and 9% of grass transpiration. In the riparian site, hydraulic redistribution supports 53% and 14% of tree and grass transpiration, respectively. The percent of hydraulic redistribution supporting grasses in the riparian site increased by ~50% compared to that in the riparian site. We further perform comparative assessment of the two sites to compare the role of hydraulic redistribution on long-term water storage and carry-over by running multi-year (2006-2016) simulation. Examining inter-annual water budget dynamics and water storage mechanism through vegetation roots provides insight to how ecosystems will respond to periods of prolonged stress or altered precipitation pattern in water-limited ecosystems.