|Title||Productivity and Desertification|
|Publication Type||Book Chapter|
|Year of Publication||2002|
|Book Title||Encyclopedia of Soil Science|
|Publisher||Marcel Dekker, Inc.|
Rangeland degradation frequently occurs in desert environments throughout the world. Although productivity in desert rangelands is 1 to 3 orders of magnitude lower than in forest ecosystems, desert rangelands cannot be viewed as either simple or unproductive systems. Productivity is strongly coupled to precipitation, but nutrients and their availability strongly regulate both primary and secondary production. Degradation of desert grasslands is often characterized by replacement of perennial herbaceous species by long-lived woody shrubs and by an associated reduction in the capacity of the ecosystem to perform some functions. These reductions may be irreversible on human time scales. Numerous causes have been proposed for this grassland-to-shrubland transition including overgrazing, exclusion of fire, dispersal of seeds of woody species by herbivores and a combination of natural and anthropogenic stressors. It is not likely that the historical effects of any one factor can be clearly and singularly associated with desert grassland degradation. We can not necessarily differentiate between cases in which certain stressor(s) are directly responsible for changes and cases in which they simply facilitate the transition from a community that evolved under a previous climatic regime to one that is better adapted to current conditions. It is now understood that these changes can be long-lived, non-linear and resistant to remediation. The dynamics of desert rangeland degradation have been conceptualized for the Chihuahuan Desert of North America. Allogenic forces contributed to reductions in herbaceous cover and to increased dispersal of seeds of competitive woody species. Autogenic forces then reinforced spatial redistribution patches.