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ABSTRACT. Drylands cover 41% of the terrestrial surface and support > 36% of the world's population. However, the magnitude of dryland degradation is unknown at regional and global spatial scales and at 15-30-yr temporal scales. Historical archives of > 30 yr of Landsat satellite imagery exist and allow local to global monitoring and assessment of a landscape's natural resources in response to climatic events and human activities. Vegetation indices (VIs), i.e., proxies of vegetation characteristics such as phytomass, can be derived from the spectral properties of Landsat imagery. A dynamical systems analysis method called mean-variance analysis can be used to describe and quantify dynamic regimes of VI response to disturbance using characteristics of ecological resilience, particularly amplitude and malleability, from a change detection perspective. Amplitude is the magnitude of response of a VI to a disturbance; malleability is the degree of recovery of a resource after a disturbance. Spatially aggregate and spatially explicit (image) differencing are methods whereby a VI image or statistic from one time period is subtracted from a VI image or statistic from another time period. To illustrate this method, we used a time series of Landsat imagery from 1972 to 1987 to measure the response of vegetation communities that are managed by subsistence agropastoral communities to the severe 1982-1984 El Niño-induced drought on the Bolivian Altiplano. We found that the entire landscape had decreased vegetation cover, increased variance (diagnostic of a regime shift), and thus, increased susceptibility to soil erosion during the drought. The wet meadow vegetation cover class had the lowest amplitude and thus the most resilience relative to other vegetation cover classes. This response identified the wet meadow as a key resource, as well as a harbinger of climate change for agropastoral communities in areas where drought is an endemic stressor.

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The impact of grazing anhnals and plant protection on shrub seedling establiihment was studied ln 2 separate experiments. A total of 3,665 seedlings were monitored for survival during a sheep grazing trial in 1984, and 5,755 seedlings were monitored durhrg a cattle grazing trial in 1986. Approximately l/2 of the seedlings were located under the canopy of mature plants and l/2 were located in the interspaces between plants. The presence of domestic livestock and the seedling location affected both the overall survival at the end of the growing season and the pattern of survival during the growing season. The interaction between these independent variables resulted In the highest survival (0.11) for sheltered seedlings in the grazed pastures and the lowest survival (0.009) for unprotected seedlings in the grazed pastures. Seedlings in the ungrazed pastures had survival rates intermediate between these 2 rates. The pattern of seedling survival was similar in both experiments. Seedlings in the grazed pastures experienced high mortality during the actual grazing event and immediately after grazing. Seedlings which were unsheltered experienced the lowest survival due to trampling. Survival rates late in the summer were not affected by grazing but were dependent on receiving precipitation during this normally dry period of the year. The interaction between grazing and seedling location may partially explain the aggregated distribution of Artemisia found in many communities. This aggregation should affect interspecific competition and may play a role in later stages of plant succession within these shrub-dominated communities.

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