Short-term effects of tree removal on infiltration, runoff, and erosion in woodland-encroached sagebrush steppe

Authors: Pierson Jr, Frederick; Williams, Christopher - Jason; KORMOS, PATRICK - Boise State University; AL-HAMDAN, OSAMA - University Of Idaho

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2014
Publication Date: 9/1/2014
Publication URL: http://www.srmjournals.org/doi/abs/10.2111/REM-D-13-00033.1
Citation: Pierson Jr, F.B., Williams, C.J., Kormos, P.R., Al-Hamdan, O.Z. 2014. Short-term effects of tree removal on infiltration, runoff, and erosion in woodland-encroached sagebrush steppe. Rangeland Ecology and Management. 67:522-538.

Interpretive Summary: Millions of hectares of sagebrush steppe in the Great Basin, USA, have been altered ecologically by encroachment of pinyon and juniper woodlands. Woodland encroachment of sagebrush rangelands has been linked to losses of understory vegetation and ground cover that, in turn, amplify runoff and soil loss. Land owners and management agencies across the Great Basin are actively seeking guidelines to restore woodland-encroached sagebrush rangelands through tree-removal treatments that promote understory vegetation. This study used rainfall simulations and measures of vegetation, ground cover, and soils to investigate short-term (2 yr post-treatment) woodland response to burning at two woodlands and to tree mastication at one of the woodlands. Prescribed burning increased erosion at both study sites, but the amplified post-fire response was substantially greater at one site than the other due to site-specific soil properties. Erosion at the less erodible site is expected to decrease over time as the plant community re-develops. Erosion from unburned, degraded areas at the more erodible site was greatly reduced by application of masticated tree debris or mulch. The results demonstrate similarly degraded conditions at woodland-encroached sites may elicit differing hydrologic and erosion responses to treatment and that treatment decisions should consider inherent site-specific erodibility when evaluating tree-removal alternatives. Further, the study shows that mastication or mulch treatments may be more appropriate than burning on highly erodible sites if more immediate soil loss reductions are desired. The findings provide land managers insight into potential responses of restoration treatments on sagebrush steppe rangelands encroached by pinyon and/or juniper woodlands.

Technical Abstract: Land owners and managers across the western United States are increasingly searching for methods to evaluate and mitigate the effects of woodland encroachment on sagebrush steppe ecosystems. We used small-plot rainfall simulations (0.5 m2) and measures of vegetation, ground cover, and soils to investigate woodland response to tree removal at two late-succession woodlands. We also evaluated the effects of burning on soil water repellency and effectiveness of aggregate stability indices to detect changes in erosion potential. Erosion from untreated interspaces in the two woodlands differed 5-fold, and the site differences yielded different erosion responses to prescribed burning. High-intensity rainfall (102 mm h-1) on the less erodible woodland generated amplified runoff and erosion from tree microsites post-fire, but elevated erosion (50 to 75 g m-2) was minor relative to burned tree microsites at the highly erodible site (240 to 300 g m-2). Burning the highly erodible woodland generated 3-10-fold increases in erosion from tree and shrub microsites and erosion across all burned microsites ranged 220-350 g m-2. High levels of runoff (40-45 mm) and soil erosion (230-270 g m-2) on unburned interspaces at the more erodible site were reduced 4-5-fold (10 mm and 50 g m-2) by masticated tree material. The results demonstrate similarly degraded conditions at woodland-encroached sites may elicit differing hydrologic and erosion responses to treatment and that treatment decisions should consider inherent site-specific erodibility when evaluating tree-removal alternatives. Strong soil water repellency was detected underneath unburned tree canopies at both woodlands and its strength was not altered by burning. However, fire-removal of litter exacerbated repellency effects on infiltration, runoff generation, and erosion. The aggregate stability index method detected differences in relative soil stability between areas underneath trees and in the intercanopy at both sites, but failed to provide any indication of between-site differences in erodibility or the effects of burning on soil erosion potential.