Similarity between seed bank and vegetation in a semi-arid annual plant community: The role of productivity and grazing

Question: Large variation in the similarity between seed bank and standing vegetation generally occurs along environmental gradients. We asked: 1. How is seed bank-vegetation similarity in Mediterranean semi-arid annual plant communities related to variation in primary productivity; 2. How is this productivity-similarity relationship affected by exclusion from grazing. Location: Mediterranean, semi-arid rangeland in the Northern Negev desert, Israel. Methods: Density of seeds and plants, and species composition of the seed bank and vegetation were compared in grazed and ungrazed subplots, in four neighbouring topographic sites differing in productivity. Seed bank samples were collected in autumn, just before the rainy season, and vegetation samples in the following spring, at peak above-ground biomass, when net primary productivity was assessed. Results: Quantitative and qualitative similarities between seed bank and the vegetation (Sørensen index values) varied between 0.14-0.61 and 0.40-0.68. Quantitative similarity and productivity were positively and linearly related within the low productivity range (up to 160 g.m^-2), in both grazed and ungrazed subplots. In contrast, at higher productivity levels (up to 500 g.m^-2), similarity decreased slightly with productivity in the ungrazed subplots, but no trend occurred in the grazed subplots. At low productivity, grazing did not affect similarity, while at higher productivity grazing prevented the reduction in similarity with increasing productivity. Conclusion: seed bank - vegetation similarity, with and without grazing, is positively related to productivity up to a threshold range, above which soil resource availability is no longer the factor limiting plant density. Above this range grazing prevents a reduction in similarity with increasing productivity, by diminishing vegetation cover and litter accumulation that constrain germination, seedling emergence and plant survival.