PERSISTENCE OF DESERTIFIED ECOSYSTEMS - EXPLANATIONS AND IMPLICATIONS

Studies of rainfall partitioning by shrubs, responses of shrub-dominat ed ecosystems to herbicide treatment, and experiments using drought an d supplemental rainfall were conducted to test the hypothesis that the shrub-dominated ecosystems that have replaced desert grasslands are r esistant and resilient to disturbance. Between 16 and 25% of the inter cepted rainfall is channelized to deep soil storage by stemflow and ro ot channelization. Stemflow water is nutrient enriched and contributes to the ''islands of fertility'' that develop under desert shrubs. Dro ught and rainfall augmentation experiments during the growing season a fter 5 consecutive years of summer drought found that (1) growth of cr eosotebushes, Larrea tridentata, was not significantly affected, (2) p erennial grasses and forbs disappeared on droughted plots, (3) nitroge n mineralization increased in the short term, and (4) densities and bi omass of spring annual plants increased on the droughted plots. Doubli ng summer rainfall for 5 consecutive years had less-significant effect s. Coppice dunes treated with herbicide in 1979 to kill mesquite (Pros opis glandulosa) had the same frequency of occurrence of the shrub as the untreated dunes when remeasured in 1993. These data indicate that the shrub-dominated ecosystems persist because they are resistant and resilient to climatic and anthropogenic stresses.