Integrating Historical Imagery and Sediment Radioisotopes to Shed Light on Long-Term Rangeland Dynamics and Ecosystem Services at the Watershed Scale
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Rangelands are widespread and include grasslands, shrublands, and woodlands, covering more than two thirds of the earth’s land surface. This cover type represents half of the land area in the United States. These systems are extremely dynamic and respond to natural processes as well as varying degrees of human impact. In the southern Great Plains, land cover and land use have changed dramatically over the last century. However, there is tremendous uncertainty as to the timing and magnitude of such transitions and much more so regarding the effects on hydrology and sediment dynamics in these areas. Using a watershed approach in the Lampasas Cut Plain of Texas, we applied object-oriented classification methods and hand-digitizing of historical aerial photos to track the extent of woody plant cover, cropland area, and small ponds through time. We compared these results with population trends to determine the relationship between social and environmental variables. Finally, we conducted sediment analyses of cores from constructed reservoirs in each watershed to establish a chronological sequence of rangeland processes. Woody plant cover displayed very complex responses between areas yet was very similar among watersheds in the same setting. Prolonged decreases occurred over the first half of the study period before rebounding in Lampasas County. By contrast, shrub cover decreased and remained low and stable in Mills County over the last several decades. Woody cover in an urbanized watershed consistently increased. Cropland area showed marked decreases over all areas, declining by 77% between the 1930s and 2012. The number of small ponds increased by over 250% over the same period. Trends in woody plant cover were closely related to population in each context. The opposing trends of cropland and pond density were strongly correlated. Precipitation, streamflow, and baseflow were largely unchanged over the last 90 years, suggesting a minimal impact of land use and land cover on local hydrology. Sediment delivery did increase immediately after drought periods, when intense rainfall caused soil loss as a result of drought-induced vegetation loss. These findings are critical to understanding the implications for future sustainability of rangeland landscapes and the ecosystem services they provide.
Berg, Matthew (2014). Integrating Historical Imagery and Sediment Radioisotopes to Shed Light on Long-Term Rangeland Dynamics and Ecosystem Services at the Watershed Scale. Doctoral dissertation, Texas A & M University. Available electronically from