Shrubs as a source of spatial heterogeneity-the case of Retama sphaerocarpa in Mediterranean pastures of central Spain

The processes underlying the positive effect exerted by woody species on other plant species can be grouped into a physical effect due to the canopy itself (mainly amelioration of temperature and light extremes), and an edaphic effect (nutrient enrichment). Both groups of effects are present in the savanna-like systems originated by Retama sphaerocarpa in the Iberian Peninsula. The consequences of these shrubs for the spatial heterogeneity induced in the herbaceous community are not well known. The herbaceous community was sampled by means of radial transects around 20 adult shrubs, from the canopy centre to the open areas among the shrubs, both northwards and southwards. Floristic composition, diversity and pasture mean height change both along the inside-outside gradient, and from north to south. As a nutrient enriched environment, the centre of the understorey showed the lowest species richness and evenness, and the highest standing crop. In contrast, the environmentally stressful open areas showed the highest number of species and evenness, and the lowest standing crop. This general effect is only partially due to nutrient availability, and is not homogeneous around the shrub, as open area conditions penetrate into the understorey driving the herbaceous community towards that of open areas. An individual shrub effect is also suggested by our results. Different conditions are thus provided by R. sphaerocarpa understoreys, favouring different subsets of species from the general pool, and therefore defining shrub canopies as sources of spatial heterogeneity in the whole savanna-like system.