Abstract

The generality of the water-limited-nitrogen-regulated plant growth hypothesis for desert ecosystems was tested by applying water and nitrogen to experimental plots set up in a marine terrace at Lagunillas, Coquimbo (30 degrees S). Biomass response of ephemeral plants to increased water and nitrogen fertilization were species and site specific. Depth of a calcium carbonate hardpan present in the terrace modulated plant responses to water. Germination and growth of introduced species was triggered at lower water levels than native plants. This characteristic could be important for the persistance of these weeds in this ecosystem, since low precipitation ir common for this region. This has interesting implications for patterns of community change in areas where accelerated desertification has been accompanied by increasingly erratic and sparser rainfall events. Human impact has differently affected plant communities in Chilean coastal desert and North American deserts. In North American deserts overgrazing seems to be the main factor changing the previously grass-dominated communities to an unpalatable shrub-dominated system. Dry farming (wheat crop) in foothills, overgrazing, and intense use of woody plants as fuel for home and mining activities during the last two centuries have all greatly contributed to the desertification process in north-central Chile. Herbs in chronic disturbed areas do not seem to be more abundant under bushes, forming the ''fertility islands'' which are typical of North American deserts. However, in protected areas some herb species are restricted to areas under bushes, while others are present only in open areas. Chemical analysis of soil samples taken under and between bushes show large differences in nutrients. North of La Serena, where rainfall becomes extremely low, annual herbs appear (in years with rains) only under bushes. I propose that man, by removing trees and shrubs previously present in the Chile coastal desert, has allowed the invasion of introduced annual species and the reduction of species associated with bushes. Additionally, native species inhabiting open areas would have increased their presence in this system.