Annual plant–shrub interactions along an aridity gradient

Annual plants in semi-arid and arid areas are often closely associated with shrubs. The degree of association largely depends on the balance of negative and positive effects between these contrasting plant life-forms, ranging from interference to facilitation. Since positive interactions are predicted to become less important with increasing rainfall, the interaction balance is expected to shift along aridity gradients. However, this prediction has not been tested on a community level and for different life-history stages across large geographical gradients. Here, we show such changes for annual plant populations and communities in four contrasting sites along a steep climatic gradient, ranging from the arid desert to mesic Mediterranean regions in Israel. Above-ground productivity, richness, seedling density, and seed bank density of the annual plant community, as well as fecundity of annual plant populations, were generally higher under shrubs than in areas between shrubs at the arid end of the gradient, but significantly lower at the humid end. Net effects of shrubs on annuals expressed as relative interaction intensity indicated a steady and consistent shift from net positive or neutral effects in the desert to net negative effects in the mesic part of the gradient. These findings emphasize the usefulness of studies along large-scale gradients encompassing a wide range of environmental conditions for understanding community level interactions among coexisting species.