Shrubs and herbaceous seed flow in a semi-arid landscape: Dual functioning of shrubs as trap and barrier

Shrubs in arid and semi-arid ecosystems are often associated with three distinct patch types: the shrub core, the shrub periphery and surrounding open patches. The distribution of herbaceous seeds in such a patchy system exhibits a well-documented spatial heterogeneity. However, the mechanisms that generate this heterogeneity are poorly understood, not least because of the difficulty of separating possible effects of the shrub on seed production (via the shrub's modification of resource distribution), seed dispersal and post-dispersal processes. We used a well-studied system dominated by a common east Mediterranean shrub (Sarcopoterium spinosum) to directly test the effect of shrubs on herbaceous community seed dispersal. We clipped all potential herbaceous seed sources from plots with or without a shrub at their centre, designated 'shrub' or 'open', respectively. Seed rain was then sampled over a 7-month period, along four directions within a fine-scale radial sampling pattern from the shrub core to its periphery and in corresponding positions in the open plots. Seed predation was monitored likewise. The overall abundance and species richness of herbaceous seeds were similar at all distances from the core in the open plots and at the shrub periphery, but lower under the shrub canopy. However, the observed patterns were clearly directional, with the highest seed abundance and species richness found on the upslope periphery of the shrub patch. These patterns were observed after the elimination of all within-patch herbaceous seed sources, which suggests that the movement of seeds was mainly driven by gravity-related mechanisms whose effect was modified by the shrubs. Synthesis. A shrub can function simultaneously as both a seed trap and a barrier to herbaceous seed flow, with the exact balance determined by location within the patch. Furthermore, whether the effect of the shrub on herbaceous seeds is regarded as facilitatory or competitive is scale-dependent. A mechanistic dissociation of seed dispersal from other processes modulated by the shrub in shaping the herbaceous community, as done in this study, is important for understanding the resilience of semi-arid and arid ecosystems to environmental changes, especially to the increasingly observed drought-induced mortality of shrubs associated with climate change.