Understory vegetation response and nitrogen cycling following cutting of western juniper Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/w0892g41f

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  • Since the late 1880's western juniper has expanded in range and increased in density in sagebrush-bunchgrass, riparian, and forested plant communities of the Pacific Northwest. Succession to western juniper woodland has been shown to reduce the productivity and diversity of the understory component, result in concentration of soil nutrients beneath juniper canopies, and reduces soil moisture storage. This study assessed understory plant succession, soil nitrogen (N) cycling, litter decomposition, and soil moisture availability following cutting of a western juniper woodland on Steens Mountain, Oregon. Cutting of western juniper reduced below-ground competition for water and N, increasing soil moisture storage and N availability for understory species. Leaf water potentials were less negative, and N concentration and biomass in understory plants were greater in the cut treatment. Understory species responded to improved growth conditions with increased cover, biomass, density, and diversity. In 1993, total understory biomass and canopy cover were 870% and 300% greater, respectively, in the cut treatment than the uncut woodland. Understory succession was dominated by plants present on the site prior to cutting. Results indicated initial that bunchgrass densities of 2 plants/m2 were sufficient for perennial grasses to dominate following juniper cutting. Juniper cutting is an effective method for restoring the understory component in sagebrush rangelands that are currently dominated by western juniper woodland. Nitrogen availability was greatest in cut-interspace locations the first year following treatment and in cut-duff locations in the second year. Nitrification was lowest in cut-slash and woodland-duff locations, areas receiving fresh litter inputs and experiencing lower temperatures than interspace (cut and woodland treatments) and cut-duff locations. Decomposition of juniper litter was two times faster in the cut treatment, however the release of litter N occurred earlier in the woodland. Large inputs of N poor litter from cut juniper slash were hypothesized to have increased microbial demand for N, resulting in immobilization of litter N. Immobilization of litter N may be important in conserving N on sites following cutting.
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