DISTURBANCE ASSESSMENT AND MITIGATION OF GREAT BASIN RANGELAND
Location: Watershed Management Research
Title: Hydrologic and erosion responses of sagebrush steppe following juniper encroachment, wildfire, and tree cutting
Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 27, 2013
Publication Date: May 21, 2013
Citation: Pierson Jr, F.B., Williams, C.J., Hardegree, S.P., Clark, P., Kormos, P.R., Al-Hamdan, O.Z. 2013. Hydrologic and erosion responses of sagebrush steppe following juniper encroachment, wildfire, and tree cutting. Rangeland Ecology and Management. 66:274-289.
Interpretive Summary: Western juniper now occupies approximately 9 million hectares in the Intermountain West and its encroachment into sagebrush steppe has been linked to understory decline and amplified runoff and erosion. Land owners and management agencies across the western US are actively seeking restoration guidelines and information on alternative restoration methods and treatment effects. This study used rainfall simulation and concentrated flow simulations to evaluate the impacts of western juniper encroachment on runoff and erosion and to evaluate short-term effects of wildfire and tree cutting on woodland hydrologic and erosional processes. Runoff and erosion from rainsplash, sheetflow, and concentrated flow processes were low from the unburned tree subcanopy (areas underneath tree canopies) and were significantly greater for the intercanopy (area between tree canopies). The overall effects of tree encroachment were a reduction in understory vegetation and formation of highly-erodible bare intercanopy area. Severe burning increased runoff and erosion from the subcanopy and the effects of burning persisted two years post-fire. Minor improvements in infiltration were observed in the intercanopy post-fire. Placing cut-downed trees into intercanopy areas had minimal immediate impact on infiltration and soil loss. Results indicate restoration treatments that generate short-term erosion pulses followed by declines in soil loss are potentially more favorable than long-term soil loss and desertification of sagebrush steppe vegetation resulting from severe tree encroachment. The results provide land managers insight into potential responses of restoration treatments in woodland encroached sagebrush steppe.
Extensive woodland expansion in the Great Basin has generated concern regarding ecological impacts of tree encroachment on sagebrush rangelands and strategies for restoring sagebrush steppe. This study used rainfall (0.5 m2 and 13 m2 scales) and concentrated flow simulations and measures of vegetation, ground cover, and soils to investigate hydrologic and erosion impacts of western juniper (Juniperus occidentalis Hook.) encroachment into sagebrush steppe and to evaluate short-term effects of burning and tree-cutting on runoff and erosion responses. The overall effects of tree encroachment were a reduction in understory vegetation and formation of highly-erodible, bare intercanopy between trees. Runoff and erosion from high intensity rainfall (102 mm h-1) were generally low from unburned areas underneath tree canopies (13 mm and 48 g m-2) and were higher from the unburned intercanopy (43 mm and 272 g m-2). Intercanopy erosion increased linearly with runoff and exponentially where bare ground exceeded 60%. Erosion from simulated concentrated flow was 15- to 25-fold greater from the unburned intercanopy than unburned tree canopy areas. Severe burning amplified erosion from tree canopy plots by a factor of 20, but had a favorable effect on soil erosion from the intercanopy. Two years post-fire, erosion remained 20-fold greater on burned than unburned tree plots, but concentrated flow erosion from the intercanopy (76% of study area) was reduced by herbaceous recruitment. The results indicate burning may amplify runoff and erosion immediately post-fire. However, we infer burning that sustains residual understory cover and stimulates vegetation productivity may provide long-term reduction of soil loss relative to woodland persistence. Simply placing cut-downed trees into the unburned intercanopy had minimal immediate impact on infiltration and soil loss. Results suggest cut-tree treatments should focus on establishing tree debris contact with the soil surface if treatments are expected to reduce short-term soil loss during the post-cut understory recruitment period.