Indirect effects in a desert plant community: is competition among annualsmore intense under shrub canopies?

An unresolved discussion in contemporary ecology deals with the relative im portance of competition along environmental gradients. In deserts, local-sc ale differences in environmental productivity may be caused by the presence of shrubs, which represent a favorable habitat for annual populations with in a nutrient-poor matrix. In this study, we attempted to test the hypothes is that facilitation of desert annuals by shrubs increase the intensity of competition among the annual plants. Such negative indirect effects have so far been ignored in studies about plant-plant interactions. We tested our hypothesis by measuring seedling survival and fecundity of four abundant an nual plant species with and without neighbors in open areas and under shrub canopies in a sandy desert area. Our findings did not indicate indirect ne gative effects of shrubs on their understory annuals. Sensitivity to the pr esence of neighbors varied between species and surprisingly, the species wi th the smallest seeds was the only one which was not negatively affected by the presence of neighbors. In contrast to our hypothesis, there was no dif ference between the habitat types shrubs and openings in absolute and relat ive competition intensity. Our overall results suggest that negative indire ct effects of shrubs are unimportant in determining demographic response of understory annual plants.