Early warning indicators of desertification: examples of tests in the Chihuahuan Desert

Perennial grasslands that dominated the south-west United States during the past 10,000 years have been desertified to varying extents. Desertification is a temporal phenomenon, defined in this paper as a change in the scale of the spatial distribution of resources. We report here on several indicators of desertification, using bare patch indices as surrogates for direct measures. A bare patch index based on canopy cover, which is relevant for precipitation and wind-driven erosion, is not equivalent to another bare patch index based on soil surface measurements, which is relevant for overland water flow-driven erosion. Per cent grass cover, per cent long-lived grass cover, per cent cover of vegetative reproducers, and a relative preference index all yielded sensitive indicators of desertification. The usefulness of some indicators, such as organic litter, depends upon recent environmental history. Other indicators, such as per cent cover due to grazing-resistant species, appear not to be sensitive to desertification and need further development. Range site type may also be important in determining indicator responses, where some indicators were sensitive to a range site while others were not. Overall, we have identified several sensitive indicators of desertification based on vegetational characteristics in varied range sites in the northern Chihuahuan Desert.