| Land degradation, a decline in land quality caused by human activities, is a growing global problem with adverse impacts on the environment, agronomic productivity, food security, and general quality of life. This dissertation examines the case of Pozo canyon on southwestern Santa Cruz Island, California, to obtain a deeper understanding of the underlying processes that drive land degradation and vegetation recovery processes in Mediterranean landscapes. I present a chronicle of environmental change from the mid-Holocene to the present, with particular emphasis on quantifying the impacts of different land management strategies over the past 200 years. Geomorphic evidence, historic documents, and fluvial deposits, including Chumash Indian shell middens and buried ranching artifacts, indicate that prior to the initiation of ranching in the mid-nineteenth century, Pozo canyon was slowly aggrading with an extensive wetland at its lower reaches. With the introduction of grazing animals, and subsequent explosion in the island sheep population, overgrazing-induced hillslope erosion led to a brief period of accelerated fluvial aggradation, followed by stream entrenchment. With the removal of grazing animals, land cover on southwestern Santa Cruz Island has gone from ~55% bare soil in 1929 to less than 10% today, with re-vegetation patterns largely controlled by the degree of land degradation/soil erosion and distance to pre-existing vegetation. Re-vegetation has affected sediment yields through hillslope stabilization, though the watershed is again aggrading at roughly four times the pre-grazing rate. In spite of the impressive recent vegetation recovery, active gullies still exist across much of the island and represent the dominant mechanism of soil erosion. An evaluation of two different LiDAR systems, airborne and terrestrial, for measuring volumetric gully erosion determined that both are superior to more traditional methods, though the steep geometry of large gullies proved particularly problematic for the side-looking terrestrial system. The environmental changes measured on Santa Cruz Island provide a rare and valuable reference dataset for other areas currently experiencing or at-risk for land degradation. |
