The effect of shrubs on the seed, rain of annuals in a semiarid landscape

Sarcopoterium spinosum (prickly burnet) is a dominant shrub species. in many semiarid shrublands of the eastern Mediterranean region. The annual plant community under the shrub canopy differs significantly from that in open (intershrub) patches in terms of abundance, biomass, and species diversity. This shrub/open contrast in vegetation. closely matches the contrast in abiotic conditions, which is mainly attributed to the engineering effects of the shrub. However, S. spinosum may also act as a seed barrier between patch types, thus controlling which seeds are exposed to the contrasting conditions. As the canopy of S. spinosum is shaped like a tightly packed cushion, we hypothesized that it prevents or slows the movement of seeds across patch boundaries. We tested this hypothesis by comparing the seed rains under intact shrubs, under shrubs whose canopies were removed, and in open patches. We compared the distribution among the. patch types of species at the seed stage with that of the same species at the adult stage. We found a sharp contrast in seed population between seed traps under intact shrubs and those in open patches, and a much weaker contrast between seed traps under removed shrub canopies and those in open patches. However, between-patch differences at the seed stage tightly paralleled those seen at the adult stage, strongly suggesting that within-patch dispersal is much more probable than between-patch dispersal. The implications of these dispersal patterns for population and community dynamics are discussed. In addition, we discuss the consequences of our findings for shrub-annuals interaction.