- Similarity between seed bank and vegetation in a semi-arid annual plant community - 29 Journal of Vegetation Science 17: 29-36, 2006 © IAVS; Opulus Press Uppsala. Abstract 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 similar- ity in Mediterranean semi-arid annual plant communities re- lated to variation in primary productivity; 2. How is this productivity-similarity relationship affected by exclusion from grazing. Location: Mediterranean, semi-arid rangeland in the North- ern Negev desert, Israel. Methods: Density of seeds and plants, and species composi- tion 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 be- tween 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 produc- tivity 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 with- out grazing, is positively related to productivity up to a thresh- old 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. Keywords: Grassland; Mediterranean; Rangeland. Nomenclature: Feinbrun-Dothan & Danin 1998. Similarity between seed bank and vegetation in a semi-arid annual plant community: The role of productivity and grazing Osem, Yagil 1* ; Perevolotsky, Avi 2 & Kigel, Jaime 1 1 Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot Campus, PO Box 12, Rehovot, Israel; 2 Department of Natural Resources, ARO-The Volcani Center, Bet Dagan, Israel; * Corresponding author; syosem@zahav.net.il Introduction Similarity between the seed bank and standing vege- tation has been studied frequently in different plant communities, aiming to improve understanding of the role of seed banks in successional and regeneration processes. In grasslands, the degree of similarity differs across habitats and environmental conditions (Marañon 1998; Peco et al. 1998; Egan & Unger 2000; Grandin 2001; Funes et al. 2001), successional stages (Milberg 1995; Bekker et al. 2000; Grandin 2001), and type of disturbance (Levassor et al. 1990; Bakker & de Vries 1992; Jutila 1998; Chang et al. 2001). However, the processes by which abiotic stresses and resource avail- ability interact with disturbances (e.g. grazing) in deter- mining plant community dynamics and seed bank - vegetation similarity are not fully understood (Kinloch & Friedel 2005). Grazing, for instance, was found to either increase (Bakker & de Vries 1992; Unger & Woodell 1996), decrease (Jutila 1998) or have no effect (Peco et al. 1998) on seed bank - vegetation similarity. These contrasting patterns are probably due to differ- ences in grazing regimes, environmental conditions and vegetation characteristics. Usually, low similarity occurs in grasslands domi- nated by perennial species (Thompson & Grime 1979; Rabinowitz 1981; Schenkeveld & Verkaar 1984; Milberg 1995; Bakker et al. 1996), while higher similarity has been found in communities dominated by annuals, as in early successional stages (Moore 1980; Ungar & Woodell 1993, 1996; Chang et al. 2001) and in Medi- terranean grasslands (Levassor et al. 1990; Lavorel & Lebreton 1992; Peco et al. 1998). In these communi- ties most of the vegetation develops each year from the available seed bank and, therefore, the seed bank at the onset of the growth season should correspond to species composition and plant density in the vegetation. Yet, large variation in seed bank - vegetation similarity occurs within, as well as among annual plant commu- nities (Thompson 2000) hampering the prediction of