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Changes in composition of annual plant communities in semi-arid shrubland due to long-term sheep grazing.
ADONIS, CARLY*,1, BOEKEN, BERTRAND1, 1 Jacob Blaustein Institute for Desert Research, Sede Boqer, Israel
ABSTRACT- Semi-arid shrublands consist of shrub patches amid a matrix of crusted soil. The crust generates runoff, while shrub patches absorb it and accumulate organic matter, soil, litter and nutrients. Due to high resource and microsite availability and seed capture, density, productivity and diversity of annual plants are higher in shrub patches than on the crust. Sheep grazing can affect the annual plant communities in both patch types directly by herbivory, and indirectly by trampling. I examined the effects of long-term sheep grazing on the diversity and composition of annual plant communities in semi-arid shrubland of the northern Negev desert of Israel. Data were collected from shrub and crust patches in grazed and ungrazed plots, on a north- and a south-facing slope. DCA ordination showed that patch type and location caused greater differences in species composition than grazing, and that grazing affected the slopes and the patch types differently. The first axis reflected increasing environmental severity from the north-facing shrubs to crust to the south-facing slope. The second axis represented differences between the shrub and crust patches. Grazing increased environmental severity for communities of north-facing shrub patches, but decreased it for north-facing crust and south-facing shrub patches. Grazing did not affect south-facing crust communities. Under north-facing shrubs, grazing reduced the abundance of the dominant grass Avena barbata, while several subdominant species increased. In north-facing crust and south-facing shrub patches, grazing reduced several locally dominant species, and increased abundance of a number of subdominant species. Thus, grazing caused compositional changes in the annual plant communities due to 1) suppression of dominant species directly by herbivory, 2) enhanced density and growth of subdominant species, by relaxed competition, ameliorated microclimate or increased site availability due to trampling.
KEY WORDS: grazing, dominance, species composition