Background/Question/Methods Several environmental factors considerably influence the success of an annual plant community in a desert ecosystem. Composition and structure of such a community can be influenced by abiotic factors and strongly influenced by plant interactions. Due to the important contribution of perennial plants in desert ecosystems, e.g., they act as physical barriers and as organic matter donors and soil moisture moderators, we attempted to distinguish between these two functions of perennial plants. The study site was located in the northern Negev Desert, Israel, where 50 Hammada scoparia shrubs and 50 artificial plants were randomly marked. During the study period, soil samples were collected monthly in the vicinity of the canopies of both shrubs and from the open areas between them. Individuals of all annual plant species were identified and counted during three growing seasons (2006, 2007, and 2008) at the peak of annual biomass, and annual plant biomass was determined.

Results/Conclusions Throughout the three years of study, we observed that some annual species were more associated with perennial shrubs while others were more associated with open areas. This 'species-specific' phenomenon elucidates the role of perennial plants in the regulation of annual communities in arid ecosystems. In addition, a positive effect of H. scoparia shrubs on the biomass of individual annual plants during the drier growing seasons (2006, 2008) was found, while a similar effect was not observed in the control and artificial plant canopies. In contrast, during the wet growing season of 2007, no differences in biomass of individual annual plants between the three locations (canopies of H. scoparia, artificial plant, and control), were found.
These findings suggest that annual plants 'prefer' to grow in open spaces, despite the fact that perennial shrubs create a fertile microniche with a large nutrient supply. We assume this might be due to the abstention of annuals from competition with perennial plants for soil resources. Annual species that manage to cope with the competitive conditions, 'enjoy' the state of having less competition, and utilize the nutrient pools, as per the results showing larger biomass for each individual plant observed under the H. scoparia shrubs. This long-term study will allow us to elucidate the importance of perennial shrubs as moderators of annual communities in a desert ecosystem.