Convergent hydraulic redistribution and groundwater access supported facilitative dependency between trees and grasses in a semi-arid environment

Authors: LEE, E. - University Of Illinois; KUMAR, P. - University Of Illinois; Knowles, John; MINOR, R.L. - University Of Arizona; TRAN, N. - University Of Guelph; BARRON-GAFFORD, G.A. - University Of Arizona; Scott, Russell - Russ

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/26/2021
Publication Date: 5/27/2021
Citation: Lee, E., Kumar, P., Knowles, J.F., Minor, R., Tran, N., Barron-Gafford, G., Scott, R.L. 2021. Convergent hydraulic redistribution and groundwater access supported facilitative dependency between trees and grasses in a semi-arid environment. Water Resources Research. 57(6), Article 028103. https://doi.org/10.1029/2020WR028103.
DOI: https://doi.org/10.1029/2020WR028103

Interpretive Summary: Often in savanna and woodland ecosystems, deep rooted trees and shallow-rooted grasses coexist. The degree to which these different species compete for or share soil-water derived from precipitation or groundwater, as well as how these interactions are altered by hydraulic redistribution, the movement of soil water by plants, is unknown. We use a multilayer canopy model coupled with field observations to examine how the presence of deep, but tree-root accessible, groundwater impacts seasonal patterns of hydraulic redistribution and interaction between coexisting vegetation species in a semiarid riparian woodland. Hydraulic redistribution supports more than half of the tree water use and about 15% of the grass water use, demonstrating a growth facilitation effect. Compared to a similarly structured upland savanna without accessible groundwater, the riparian site shows increased amount of hydraulically redistributed water and more facilitative water use between coexisting grasses and trees. These results shed light on the linkage between accessible groundwater and the role of hydraulic redistribution on the interaction between deep-rooted and shallow-rooted vegetation.

Technical Abstract: Hydraulic redistribution is the transport of water from wet to dry soil layers, upward or downward, through plant roots. Often in savanna and woodland ecosystems, deep-rooted trees, and shallow-rooted grasses coexist. The degree to which these different species compete for or share soil-water derived from precipitation or groundwater, as well as how these interactions are altered by hydraulic redistribution, is unknown. We use a multilayer canopy model and field observations to examine how the presence of deep, but tree-root accessible, groundwater impacts seasonal patterns of hydraulic redistribution, and interaction between coexisting vegetation species in a semiarid riparian woodland (US-CMW). Based on the simulation, trees absorb moisture at the water table (~10 m depth) and release it in the shallow soil depth (0–3 m) during the dry pre-monsoon season. We observed the occurrence of a new convergent hydraulic redistribution pattern during the monsoon season, where moisture is transported from both the near-surface (0–0.5 m) and the water table to intermediate soil layers (1–5 m) through tree roots. We found that hydraulic redistribution demonstrates a growth facilitation effect at this site, supporting 49% of growing season tree transpiration and 14% of the grass transpiration. Compared to a similarly structured upland savanna without accessible groundwater, the riparian site shows an increased amount of hydraulically redistributed water and more facilitative water use between coexisting grasses and trees. These results shed light on the linkage between accessible groundwater and the role of hydraulic redistribution on the interaction between deep-rooted and shallow-rooted vegetation.