Vegetation, hydrologic, and erosion responses of sagebrush steppe 9 yr following mechanical tree removal

Authors: Williams, Christopher - Jason; PIERSON JR, FREDERICK; KORMOS, PATRICK; AL-HAMDAN, O.Z. - TEXAS A&M UNIVERSITY; NOUWAKPO, S.K. - UNIVERSITY OF NEVADA; Weltz, Mark

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/3/2018
Publication Date: 1/9/2019
Citation: Williams, C.J., Pierson, F.B., Kormos, P.R., Al-Hamdan, O., Nouwakpo, S., Weltz, M.A. 2019. Vegetation, hydrologic, and erosion responses of sagebrush steppe 9 yr following mechanical tree removal. Rangeland Ecology and Management. 72(1):47-68. https://doi.org/10.1016/j.rama.2018.07.004.
DOI: https://doi.org/10.1016/j.rama.2018.07.004

Interpretive Summary: Pinyon and juniper tree encroachment into sagebrush vegetation communities have altered the ecological conditions and delivery of ecosystem goods and services on millions of hectares of rangelands in the western US. Land managers across the western US are faced with selecting and applying effective tree-removal treatments on woodland-encroached sites, but current understanding regarding long-term ecological responses of sagebrush rangelands to the various tree removal methods remains inadequate for guiding management. This study used vegetation measures, rainfall simulations, and overland flow experiments to evaluate the impact of tree cutting and shredding treatments on vegetation and hydrology and erosion processes at two sites 9 yr after tree removal. The tree removal treatments effectively removed trees and increased sagebrush vegetation at both sites, but high runoff and erosion rates for the degraded sites were not substantially reduced after tree removal. However, increases in vegetation at the sites clearly indicate the sites are slowly recovering sagebrush and native grasses and that improved hydrologic function is forthcoming as vegetation continues to increase. The study advances understanding regarding the impacts of tree removal on sagebrush sites with dense pinyon and juniper cover and provides insight into additional strategies to increase the rate of vegetation and hydrologic recovery on these sites in the years after tree removal.

Technical Abstract: Land managers across the western US are faced with selecting and applying effective tree-removal treatments on woodland-encroached sites, but current understanding regarding long-term ecohydrologic responses of sagebrush rangelands to the various tree removal options remains inadequate for guiding management across the vast eco-physiological domain in which woodland-encroachment occurs. This study applied a suite of vegetation and soil measures (0.5 m2 to 990 m2), small-plot rainfall simulations (0.5 m2), and overland flow experiments to quantify the effects of mechanical tree removal (tree cutting and mastication) on vegetation and runoff and erosion processes at two mid- to late-succession woodland-encroached sagebrush sites in the Great Basin, USA, 9 yr after treatment. Low amounts of hillslope shrub (3-15%) and grass (7-12%) canopy cover and extensive intercanopy bare ground (69-88% bare, 75% of area) in untreated areas at both sites facilitated high levels of runoff and sediment from high-intensity (102 mm h-1, 45 min) rainfall simulations in interspaces (~45 mm runoff, 60-381 g m-2 sediment) between trees and shrubs and from concentrated overland flow experiments (15, 30, and 45 L min-1, 8 min each) in the intercanopy (371-501 L runoff, 2343-3015 g sediment). Tree cutting increased hillslope-scale density of sagebrush by 5% and perennial grass cover by 2-fold at one site while tree cutting and mastication increased hillslope-scale sagebrush density by 36% and 16% and perennial grass cover by 3-fold at a second more-degraded site over nine growing seasons. Cover of cheatgrass was <1% at the sites pre-treatment and 1-7% 9 yr after treatment. Bare ground remained high across both sites 9 yr after tree removal and was reduced by treatments solely at the more degraded site. Increases in hillslope-scale vegetation following tree removal had limited impact on runoff and erosion for rainfall simulations and concentrated flow experiments at both sites due to persistent high bare ground. The one exception is reduced runoff and erosion rates for intercanopy plots with downed trees. Downed trees provided ample litter cover and tree debris to reduce the amount and erosive energy of concentrated overland flow. Trends in hillslope-scale vegetation responses to tree removal in this study demonstrate the effectiveness of mechanical treatments to recruit sagebrush steppe vegetation without increasing cheatgrass for mid- to late-succession woodland-encroached sites along the warm-dry to cool-moist soil temperature-moisture threshold in the Great Basin. Our results indicate improved hydrologic function through sagebrush steppe vegetation recruitment after mechanical tree removal on mid- to late-succession woodlands can require more than 9 yr. We anticipate intercanopy runoff and erosion rates will decrease over time at both sites as shrub and grass cover continue to increase, but follow up tree removal will be needed to prevent pinyon and juniper re-colonization. The low intercanopy runoff and erosion measured underneath isolated downed trees in this study clearly demonstrate that tree debris following mechanical treatments can effectively limit microsite-scale runoff and erosion over time where tree debris settles in good contact with the soil surface.