|Miller, Richard - OREGON STATE UNIVERSITY|
Submitted to: Western North American Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 30, 2007
Publication Date: December 1, 2007
Citation: Bates, J.D., Miller, R.F., Svejcar, A.J. 2007. Long-term zonal vegetation dynamics in a cut western juniper woodland. Western North American Naturalist. 67:(4)549-561 Interpretive Summary: Western juniper removal by mechanical treatments has increased the past decade in Eastern Oregon and Northern California, with a major goal of restoring native shrub-grassland communities. Cutting treatments produce large amounts of debris which occupies a significant portion of treated areas and can impact community recovery. In this study we evaluated the effects of juniper debris to plant succession spanning thirteen years following juniper cutting. The results showed that; 1) juniper debris did not increase establishment and growth of native plants compared to areas without debris, and 2) non-native weeds had increased under juniper debris compared to areas without debris. This evaluation demonstrated management practices can alter traditional successional pathways and that long-term research is necessary to properly assess management activities and detect significant ecological changes.
Technical Abstract: Western juniper (Juniperus occidentalis spp. occidentalis Hook.) expansion into northern Great Basin plant communities has reduced shrub-steppe productivity and diversity. Chainsaw cutting of western juniper woodlands is a commonly applied practice to remove tree interference and restore shrub steppe plant communities. Studies assessing understory response following cutting have been limited to early successional stages and have not evaluated zone dynamics. Cutting treatments produce large amounts of debris, which is commonly left on site and occupies a significant portion of treated areas. This study evaluated spatial successional dynamics spanning thirteen years following juniper cutting. Prior to cutting, juniper cover averaged 26% and mature juniper density averaged 250 trees ha-1. Four 0.45 ha blocks were selected on Steens Mountain in southeastern Oregon. Blocks were cut in late summer 1991. Understory standing crop, cover, and density were compared among three zones in cut woodlands: old canopy litter mats (canopy), interspace, and under downed juniper (debris). In the interspace zone, perennial grasses increased in cover and standing crop relative to other functional groups. Species composition shifts in canopy and debris zones developed in the sixth year after cutting as annual grass cover, density, and standing crop increased. This result contrasts with studies which document large increases in annual grass in the first two years after mechanical treatments and fire. However, by 2003 perennial grass biomass was two times greater than annual grass in these zones. Retaining juniper debris on this site did not increase establishment and growth of perennial grasses compared to interspace zones. Because annual grasses increased in areas of debris accumulation, managers need to be cognizant of juniper treatments that create safe sites that are favorable to the establishment of weedy species. Our results demonstrate that long-term vegetation evaluations are necessary to properly assess management activities and disturbance.