Abstract
Vegetation and land-cover changes are not always directional but follow complex trajectories over space and time, driven by changing anthropogenic and abiotic conditions. We present a multi-observational approach to land-change analysis that addresses the complex geographic and temporal variability of vegetation changes related to climate and land use. Using land-ownership data as a proxy for land-use practices, multitemporal land-cover maps, and repeat photography dating to the late 19th century, we examine changing spatial and temporal distributions of two vegetation types with high conservation value in the southwestern United States: grasslands and riparian vegetation. In contrast to many reported vegetation changes, notably shrub encroachment in desert grasslands, we found an overall increase in grassland area and decline of xeroriparian and riparian vegetation. These observed change patterns were neither temporally directional nor spatially uniform over the landscape. Historical data suggest that long-term vegetation changes coincide with broad climate fluctuations while fine-scale patterns are determined by land-management practices. In some cases, restoration and active management appear to weaken the effects of climate on vegetation; therefore, if land managers in this region act in accord with on-going directional changes, the current drought and associated ecological reorganization may provide an opportunity to achieve desired restoration endpoints.
Highlights
Monitoring and characterizing the interacting effects of land use and climate on land surface processes is a primary focus of land change science [1], and of particular concern in arid environments where both landscapes and livelihoods can be impacted by short-term climate variability [2,3]
NLCD land cover was mapped for the conterminous United States only, whereas the LULC maps we developed of Santa Cruz Watershed included portions of Mexico
Sixteen camera stations depict grasslands and, of these, nine images recorded either decline of grass cover or conversion of grassland to savanna through woody plant encroachment
Summary
Monitoring and characterizing the interacting effects of land use and climate on land surface processes is a primary focus of land change science [1], and of particular concern in arid environments where both landscapes and livelihoods can be impacted by short-term climate variability [2,3]. A considerable amount of research conducted in the western United States documents and describes a wide range of anthropogenic and climate-driven vegetation changes over the past several centuries [4,5,6,7]. Much of the focus in the southwest has been on: (1) the conversion of native grassland to shrubland and savanna through the process of woody plant encroachment [8,9], and (2) changes in the distribution and cover of species-rich riparian vegetation related to ground and surface water use [10,11]. Spatial models describing past landscape changes to upland and riverine systems can provide important baseline information for restoration of arid watersheds. In western North America, several key drivers have been identified including overgrazing, wildfire suppression, climate change, and recent
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