Do Woody Plants Affect Streamflow on Semiarid Karst Rangelands?

Abstract

There is considerable public and political pressure to reduce woody plant cover on rangelands as a means of increasing water yield, despite the lack of studies documenting that such a strategy is effective. In the Texas Hill Country, runoff from the Edwards Plateau recharges the highly productive and regionally vital Edwards Aquifer. The dominant woody plant on the Plateau is Ashe juniper (Juniperus ashei Buchholz). To understand how woody plant cover may affect the amount and timing of runoff in this region, we monitored streamflow from nine small (3‐ to 6‐ha) watersheds over a 13‐year period. After the first two years (initial observations), 100% of the shrub cover was removed from three of the watersheds and ∼70% from another three. Following these treatments we continued to monitor runoff for four years, suspended monitoring for four and a half years, and then resumed monitoring for an additional three years. Runoff from these nine first‐order watersheds generally accounted for <5% of the total precipitation and occurred entirely as stormflow (there was no baseflow before or after treatment). Some runoff was generated as subsurface flow, as indicated by hydrographs showing prolonged runoff (typically lasting hours longer than the rainfall). We evaluated the influence of woody plant cover on streamflow by comparing streamflow during the four‐year treatment period with that during the posttreatment period (when considerable recovery of woody plants had taken place). Our findings indicate that changes in woody plant cover had little influence on the amount, timing, or magnitude of streamflow from these watersheds. On the basis of this work and other observations in the region, we hypothesize that, for small watersheds, changes in shrub cover will have little or no effect on streamflow except where springs are present.

Woody Plant Encroachment: Causes and Consequences

“…Juniper removal had no significant effect on the amount of water captured as cave drip (Bazan et al 2013). Decades of controlled experiments in this region have generally returned the same result, that the effect of removing encroaching woody plants on ET and/or spring flow is small and short-lived (Wilcox et al 2005) (Chapter 3, this volume).…”

Removing Adult Overstory Trees Stimulates Growth and Transpiration of Conspecific Juvenile Trees

“…Concern has been raised that juniper may contribute to aquifer drawdown due to high year-round transpiration rates. Woody plant control may provide potential water savings to the region (Dugas and Mayeux 1991;Wu et al 2001;Wilcox 2002;Wilcox et al 2005). For woody plant management to successfully enhance groundwater recharge, the vegetation remaining after land clearing must use significantly less water than pretreatment vegetation.…”

Biogeography of woody encroachment: why is mesquite excluded from shallow soils?

“…Stromberg et al, 2007;Scott et al, 2008). By contrast, shallowrooted species such as members of the genus Juniperus, by most accounts, have very little if any effect on groundwater recharge or stream-flow (Belsky, 1996;Wilcox et al, 2005). Independent of the species involved, outside of riparian zones or where there is no rapid subsurface flow, impacts of woody plants on groundwater may also be small (Huxman et al, 2005;Wilcox et al, 2006).…”