Water on the move: Hydraulic redistribution of underground stream water and rainfall by deep-rooted woody plants in central Texas

We investigated the occurrence of hydraulic redistribution (HR) in a live oak woodland in central Texas (630 mm MAP) to improve our understanding of the ecohydrological consequences of woody plant encroachment in water-limited biomes. We measured sap flow in the stems, lateral roots and deep roots (at 20 m depth) of Quercus, Bumelia and Prosopis spp., and characterized HR patterns in relation to environmental conditions and the size and location of roots. HR occurred continuously throughout the year in one form or another, even in dormant deciduous trees, indicating that patchy dry soil is a constant feature of this system. We observed distinct switches in the type of HR, from steady hydraulic lift of deep water during drought to rapid redistribution of surface soil water following rain. During peak HR activity, the amount of water redistributed at night was a large proportion of daytime uptake, up to 70 % in some roots. Deep roots supplied large amounts of underground stream water during drought, but dramatically reduced their contribution following rain. Rainfall-triggered HR was complex, and the response of shallow roots to wetting and drying of topsoil was highly variable, but smaller roots were generally more dynamic than larger roots. Our results suggest that deep roots are highly responsive to surface soil moisture conditions, and they coordinate tightly with lateral roots to supply water for transpiration and HR. Overall, this study reveals strong evidence that deep-rooted woody plants are important drivers of water cycling in dry ecosystems and they can significantly influence landscape hydrology.