The role of geologic grazing refuges in structuring Mediterranean grassland plant communities

摘要

We tested the hypothesis that small rock-enclosed geologic refuges have an important role in maintaining grazing-sensitive species in grassland with a long history of intense grazing. The study was carried out in Mediterranean grassland in a basalt landscape in northern Israel. Community composition was compared in 63 sites between samples of 1 m2 quadrats in two microhabitats: (1) rock-enclosed, presumed “refuges” and (2) rock-adjacent, “near refuges” but just outside them, accessible to cattle grazing. Median refuge area was 2.75 m2, median rock height and diameter around refuges were 1.10 m and 1.50 m. Median height of residual dry herbage was 1.41 m in refuges, compared to 0.38 m outside, indicating the difference in grazing intensity. Species richness at three scales (quadrat, site, all sites) was significantly greater in the near-refuge than in the refuge habitat. In the latter, many annuals were excluded by dominance of tall perennials. Twelve species (of 103) had significantly higher cover in refuges, including tall perennial grasses, tall annuals, climbers, and a shrub. A total of 53 species with a strong significant negative response to refuges were mostly small and medium height annuals. The intermediate group of 38 species with weak or non-significant responses to refuges included, among others, dominant tall grasses that were abundant both in refuges and just outside them. The latter, as well as most refuge-positive species had shown a positive response to protection in exclosures. The results support the hypothesis that small rock-enclosed habitats—more so than artificial exclosures—are effective grazing refuges for rare, grazing-susceptible species. The contribution of refuges to species richness at the landscape scale is much greater than their proportion of the area. Dispersion from refuges maintains small populations of rare species near refuges and can initiate expansion into the landscape when grazing pressure is lowered.