EFFECT OF SHRUBS ON EMERGENCE, SURVIVAL AND FECUNDITY OF 4 COEXISTINGANNUAL SPECIES IN A SANDY DESERT ECOSYSTEM

Demographic responses of four coexisting species of annual plants to l ocal-scale patchiness in habitat conditions caused by the presence of perennial shrubs, were investigated in a sandy desert ecosystem. Diffe rences between the shrub habitat and the openings in emergence density , seedling mortality and fecundity of the studied populations were sta tistically significant, and all of these demographic responses were st rongly species-specific, as indicated by statistically significant hab itat x species interactions. Three of the studied species (Senecio gla ucus, Rumex pictus and Erodium laciniatum) showed higher emergence den sities under shrub canopies, and emergence densities of the fourth spe cies, Ifloga spicata, were much higher in the open areas between shrub s. Seedling mortality was higher under shrubs for all of the species, indicating that mortality responses could not account for the observed patterns of variation in the abundance of the populations. Individual s of Senecio glaucus, Rumex pictus and Erodium laciniatum growing unde r shrub canopies produced significantly more seeds than conspecific in dividuals growing in the openings, whereas an opposite pattern, with h igher fecundity in the openings, was detected for Ifloga spicata. Thes e results indicate that fecundity responses may have been important in determining the spatial structure of the studied populations. Emergen ce densities of individual plants germinating below shrub canopies wer e influenced by the identity of the canopy species, indicating that th e composition of the perennial plant community may influence the compo sition and abundance of the associating annual species. The overall re sults demonstrate that demographic responses to biologically induced p atchiness caused by the presence of canopy plants may be important in structuring communities of desert annual plants.