Abstract
Millions of hectares of rangeland in the western United States (US) are undergoing vegetation transitions with important hydrologic ramifications. At low elevations, annual grass invasions have increased wildfire frequency and size. Infilling of pinyon (Pinus spp.) and juniper (Juniperus spp.) woodlands and their encroachment of shrub steppe at mid-elevations have increased the modern occurrence of high-severity fires. Conversion of shrubland communities to woodlands throughout much of the western US has altered the ecological structure and function of these ecosystems. These disturbances elicit hydrologic and erosion responses that pose hazards to ecological resources, property, and life. This dissertation addresses these impacts in a series of papers focused on: 1) current knowledge regarding wildfire effects on hydrology and erosion, 2) fire impacts on infiltration, runoff, and erosion processes across point to hillslope scales, 3) hydrologic and erosion process connectivity as a driver of post-disturbance erosion, and 4) tool development for evaluating ecohydrologic impacts of vegetation transitions, management practices, and wildfire. Results demonstrate that knowledge has advanced regarding disturbance effects on runoff and erosion, but the ability to forecast hydrologic responses in the wake of ongoing transitions on western rangelands remains limited. This study presents a conceptual model for evaluating hydrologic vulnerability. A review of literature indicates quantitative population of the model requires improved understanding in several key areas: 1) spatial scaling of post-fire hydrologic process responses across diverse landscapes, 2) quantification of interactions between varying storm intensities and measures of site susceptibility, and 3) quantification and prediction of soil water repellency effects. Runoff and erosion experiments in this study demonstrate that hillslope hydrologic vulnerability and recovery following disturbance is strongly governed by runoff and erosion process connectivity, and that connectivity of processes is dictated by the magnitude of water input and the spatial connectivity of ground-surface susceptibility to runoff generation and sediment detachment. This study concludes with a framework for integrating these key ecohydrologic relationships into a commonly applied rangeland management tool, Ecological Site Descriptions. The proposed framework increases the utility of Ecological Site Descriptions to assess rangelands, target management practices, and predict hydrologic responses to disturbances such as fire and plant community transitions.