Vegetation in deserts: II. Environmental influences on regional abundance

A remote-sensing approach was used in conjunction with field measurements to examine local and regional-scale environmental processes that covary with the abundance and distribution of vegetation in a semiarid ecosystem. Images of the fractional abundances of vegetation and soils were constructed by spectral mixture analysis of Landsat Thematic Mapper (TM) satellite images, covering a 150-km segment of Owens Valley, California. These images, along with a TM image of the radiant temperature, a digital elevation image and ground-based measurements of precipitation and evapotranspiration, were examined to isolate the effects on vegetation of the covarying factors, net radiation, temperature, elevation, soil type, and precipitation. On a regional scale the abundance of desert scrub on the bajadas of Owens Valley appears to be influenced most by the mean annual precipitation. Also regionally, vegetation cover is sensitive to the differences between the gravelly fanglomerates of the bajadas and the alluvium of the valley floor. Other edaphic and ground-water effects are important but localized, and are most pronounced on the valley floor. They produce patterns in vegetation abundance that are independent of and superposed on the regional precipitation-controlled pattern. Temperature covaries with vegetation less well than precipitation, and appears not to be the major influence on either the amount of vegetation or the boundaries between major vegetation communities. The image-derived measure of vegetation cover correlates closely with ground-based measurements of evapotranspiration. The study demonstrates that local observations cannot be extrapolated safely to the regional scale, and that a combination of local field measurements and the regional measurements provided by remote sensing is required to determine the environmental factors that control vegetation distribution.