Remnant shrubs in Mediterranean semi-arid steppes: Effects of shrub size, abiotic factors and species identity on understorey richness and occurrence

Isolated shrub patches are a key element for community structure and dynamics in semi-arid ecosystems, and may act as "hot spots" of understorey species diversity. Despite its importance, the relationships between shrub patch characteristics and understorey species richness, and the relative importance of these characteristics against other factors driving understorey species richness, are still poorly understood. We studied perennial species richness under the canopy of late-successional shrubs in semi-arid Stipa tenacissima steppes of SE Spain. The main aim of this study was to evaluate the relationship between shrub size and understorey richness, and to test if this relationship is modified by species identity. We also aimed to know the relative importance of shrub patch characteristics and abiotic factors as predictors of the distribution of understorey species under shrub canopies. Altitude and geographical co-ordinates were able to significantly explain the patterns of shrub occurrence and abundance in the study area. Understorey species richness was significantly related to the size of individual shrubs according to a simple power relationship for all evaluated species. Slope values of linear regressions with log-transformed data, ranging between 0.22 and 0.37, did not differ between species. Site characteristics, patch characteristics and patch species identity significantly explained the occurrence of species on patches. However, the relative importance of site characteristics was higher than that of patch characteristics and species identity in explaining these patterns. Our results agree with the general expectations of the theory of island biogeography and complement previous studies that emphasise the importance of late-successional shrubs in semi-arid Stipa steppes. Environmental management activities within these steppes should promote the conservation of remnant shrubs, as well as its introduction with restoration activities.