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Process-based management of downy brome in salt desert shrublands: assessing pre- and post- rehabilitation soil and vegetation attributes

Hirsch, M. C. 2011. M.S. thesis. Utah State University


As an ecosystem driver, downy brome (Bromus tectorumL.) presents obstacles to land rehabilitation efforts, including restoring desirable species cover. Because damaged ecosystems may have crossed both abiotic and biotic thresholds, ecologically-based control strategies may assist with altering successional trajectories and restoring desirable plant species. My thesis research had three objectives: 1) assess soil and vegetation relationships in degraded salt desert ecosystems prior to implementing downy brome control treatments, 2) determine the effects of control treatments on soil properties and resident plant species, and 3) evaluate the relative importance of shrubland soil type, herbicide type, and herbicide rate on seedling germination, growth, and establishment of perennial grasses and downy brome in a greenhouse experiment. Two salt desert shrubland sites in Box Elder County, Utah were chosen for objectives 1 and 2. Objective 1 evaluated the relationships between downy brome, soil water, water infiltration, pH, electrical conductivity, sand, nitrate, phosphate, potassium, and magnesium using principal components analysis (PCA), factor analysis, and Pearson product-moment correlation coefficients. Objective 2 determined the effectsof burning, herbicide, and mowing on downy brome, litter, bare ground, soil water, nitrate, carbon, phosphate, and magnesium using analysis of variance (ANOVA) and Wilcoxon sign rank tests. Objective 3 evaluated the effects of rimsulfuron and imazapic on emergence and growth of downy brome and two revegetation grass species using ANOVA. For objective 1, I observed negative correlations between downy brome abundance and soil nitrate and water content which suggests that downy brome abundance is significantly impacting the content of these two soil properties. Objective 2 data indicate that control treatments significantly impacted downy brome abundance, soil resource availability, and resource fluctuation, which implies that downy brome can be reduced by control treatments, that its control can increase the availability of soil resources like nitrate and water, and can stabilize soil resource fluctuations. The results for objective 3 illustrate the importance of shrubland soil properties when using herbicides to reduce annual grasses and show that organic matter content and pH can significantly impact the relative performance of herbicides. Collectively this research helps identify the impacts on soil properties when using process-based management practices that reduce downy brome abundance.