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. / Graduation date: 1997

Since European American settlement of the Intermountain Region, dramatic changes in vegetation composition and structure have occurred in the sagebrush steppe ecosystem. Western juniper (Juniperus occidentalis spp. occidentalis Vasek), although indigenous to the Intermountain Region, has increased since the late 1800s. Considerable work has been done documenting juniper woodland expansion in the Intermountain West, however, little is known about the environmental variables that influence rates of tree establishment and structural attributes of woodlands across landscapes. Most studies of western juniper have addressed site-specific questions at limited spatial scales. Consequently, there is a lack of research on broader scale patterns of woodland development occurring across heterogeneous landscapes. In addition, changes in the amount, composition, and structure of fuels during the transition from open sagebrush steppe communities to closed juniper woodlands have profound influences on the size, intensity, frequency, and behavior of fire. However, limited data exist quantifying changes in fuels during this transition, thus, consequences to fire behavior have been difficult to predict. The major impetus for the study was to determine the influence of environmental variables on rates and structural attributes of woodland development and associated changes in fuel loading characteristics during the transition from sagebrush steppe communities to closed juniper woodlands in the High Desert and Humboldt ecological provinces. The proportion of trees greater than 150 years old relative to trees less than 150 years suggest western juniper has greatly expanded in the Owyhee Mountains and on Steens Mountain since settlement of the areas. Ninety-five percent of the trees established after the 1850s. As evidenced by the presence of western juniper in 96% of plots sampled in this study, juniper is able to encroach upon a variety of plant alliances and under a broad range of environmental conditions over diverse landscapes. Although it appears the occurrence of western juniper within the woodland belt is not spatially limited by environmental or vegetative conditions, stand structural and fuel loading characteristics do vary considerably across heterogeneous landscapes. Total juniper density, density of dominant trees comprising the primary canopy, and certain live and dead fuels biomass very substantially with site potential. Spatial variation in stand structure and fuels may have significant implications to management of juniper at the landscape scale. / Graduation date: 2005