Runoff and erosion after cutting western juniper

Authors: Pierson Jr, Frederick; Bates, Jonathan - Jon; Svejcar, Anthony; Hardegree, Stuart

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2006
Publication Date: 5/1/2007
Citation: Pierson Jr, F.B., Bates, J.D., Svejcar, T.J.,and Hardegree, S.P. 2007. Runoff and erosion after cutting western juniper. Rangeland Ecology and Management 60:285-292.

Interpretive Summary: Western juniper has become the dominant tree within many sagebrush/bunchgrass communities of the Great Basin and interior Pacific Northwest. Western juniper can increase the amount of bare ground by reducing the cover and density of understory plants. We used artificial rainfall to compare how hillslopes dominated by western juniper and hillslopes where the juniper had been removed ten years earlier compared in runoff and erosion amounts. The hillslopes dominated by juniper rapidly produced runoff from even small thunderstorms that commonly occur on the site. Hillslopes without juniper only produced runoff from large thunderstorms that may occur one out of every fifty years. Juniper covered hillslopes had far greater exposure of bare ground to the impact of raindrops resulting in much higher levels of sheet erosion. During simulated thunderstorms juniper hillslopes produced significant amounts of runoff resulting in rill erosion rates over fifteen times higher than hillslopes without juniper. Large interconnected patches of bare ground provided the opportunity for runoff to concentrate into rills with much higher flow velocities and consequently more detachment and down slope transport of soil particles. Cutting the juniper trees stimulated herbaceous plant recovery and improved the hydrologic stability of the site. Surface soil cover was improved and infiltration capacity was sufficiently increased to protect the site from even large thunderstorms. When runoff was generated the improved surface cover conditions significantly reduced rill flow velocity resulting in very low rill erosion rates.

Technical Abstract: Western juniper (Juniperous occidentalis spp. occidentalis Hook.) has expanded its dominance in sagebrush/bunchgrass communities of the Great Basin and interior Pacific Northwest. On many sites western juniper dominance increases the amount of bare ground by reducing the cover and density of understory plant species. We used rainfall and rill simulation techniques to compare differences in hydrologic responses between hillslopes dominated by western juniper and hillslopes where the juniper had been removed ten years earlier. Juniper covered hillslopes had significantly lower surface soil cover of understory plants and litter resulting in higher amounts of bare ground. Hillslopes dominated by juniper rapidly produced runoff from even small thunderstorms that commonly occur on the site. Hillslopes without juniper only produced runoff from large thunderstorms that may occur one out of every fifty years. Juniper hillslopes had far greater exposure of bare ground to the impact of raindrops resulting in much higher levels of sheet erosion (250 kg/ha compared to zero kg/ha on the hillslopes without juniper). During simulated thunderstorms juniper hillslopes produced significant amounts of runoff resulting in rill erosion rates over fifteen times higher than hillslopes without juniper. Large interconnected patches of bare ground provided the opportunity for runoff to concentrate into rills with much higher flow velocity and consequently higher erosive force. Cutting juniper stimulated herbaceous plant recovery and restored the site to a hydrologically stable condition. Surface soil cover was improved and infiltration capacity was sufficiently increased to protect the site from even large thunderstorms. When runoff was generated the improved surface cover conditions significantly reduced rill flow velocity resulting in very low rill erosion rates.