Landscape approach for detecting and evaluating change in a semiarid environment

Vegetation change in the American West has been a subject of concern throug hout the twentieth century. Although many of the changes have been recorded qualitatively through the use of comparative photography and historical re ports, little quantitative information has been available on the regional o r watershed scale. It is currently possible to measure change over large ar eas and determine trends in ecological and hydrological condition using adv anced space-based technologies. Specifically, this process is being tested in a community-based watershed in southeast Arizona and northeast Sonora, M exico using a system of landscape pattern measurements derived from satelli te remote sensing, spatial statistics, process modeling, and geographic inf ormation systems technology. These technologies provide the basis for devel oping landscape composition and pattern indicators as sensitive measures of large-scale environmental change and thus may provide an effective and eco nomical method for evaluating watershed condition related to disturbance fr om human and natural stresses. The project utilizes the database from the N orth American Landscape Characterization (NALC) project which incorporates triplicate Landsat Multi-Spectral Scanner (MSS) imagery from the early 1970 s, mid 1980s, and the 1990s. Landscape composition and pattern metrics have been generated from digital land cover maps derived from the NALC images a nd compared across a nearly 20-year period. Results about changes in land c over for the study period indicate that extensive, highly connected grassla nd and desertscrub areas are the most vulnerable ecosystems to fragmentatio n and actual loss due to encroachment of xerophytic mesquite woodland. In t he study period, grasslands and desertscrub not only decreased in extent bu t also became more fragmented. That is, the number of grassland and deserts crub patches increased and their average patch sizes decreased. In stark co ntrast, the mesquite woodland patches increased in size, number, and connec tivity. These changes have important impact for the hydrology of the region , since the energy and water balance characteristics for these cover types are significantly different. The process demonstrates a simple procedure to document changes and determine ecosystem vulnerabilities through the use o f change detection and indicator development, especially in regard to tradi tional degradation processes that have occurred throughout the western rang elands involving changes of vegetative cover and acceleration of water and wind erosion.