Interactive effects of grazing and shrubs on the annual plant community in semi-arid Mediterranean shrublands

Question: We studied the interactive effects of grazing and dwarf shrub cover on the structure of a highly diverse annual plant community. Location: Mediterranean, semi-arid shrubland in the Northern Negev desert, Israel. Methods: Variation in the biomass and plant density of annual species in the shrub and open patches was monitored during four years, inside and outside exclosures protected from sheep grazing, in two contrasting topographic sites: north and south-facing slopes that differed in their dominant dwarf shrubs species: Sarcopoterium spinosus and Corydothymus capitatus, respectively. Results: Above-ground biomass, density and richness of annual species were lower under the canopy of both shrub species compared to the adjacent open patches in the absence of grazing. Grazing reduced the biomass of annuals in open patches of both topographic sites, but not in the shrub patches. On the north-facing slope, grazing also reduced plant density and richness in the open patches, but increased plant density in the shrub patches. At the species level, various response patterns to the combined effects of grazing and patch type were exhibited by different annuals. Protection against the direct impacts of grazing by shrub cover as well as species-specific interactions between shrubs and annuals were observed. A conceptual mechanistic model explaining these interactions is proposed. Conclusion: In semi-arid Mediterranean shrublands grazing and dwarf shrub cover interact in shaping the structure of the annual plant community through (1) direct impacts of grazing restricted to the open patches, (2) species-specific facilitation/interference occurring in the shrub patches and (3) subsequent further processes occurring among the interconnected shrub and open patches mediated through variation in seed flows between patches.