Spatial segregation of plant species caused by Retama sphaerocarpa influence in a Mediterranean pasture:: a perspective from the soil seed bank

Retama sphaerocarpa is a leguminous shrub whose important role in the semi-arid regions of south-eastern Spain has already been assessed: shrub canopies reduce light intensity, and thus evapotranspiration; also their extensive radical system take water and nutrients from great volumes of soil, concentrating them in the understorey. Consequently, subcanopy vegetation benefits from these facilitation processes, increasing its productivity. However, these shrublands have been rarely studied at a broader scale, i.e. as a savannah-like system composed of a variable number of shrubs scattered in a herbaceous matrix. As the microenvironmental conditions associated to the understorey are rather different from those of the open spaces among shrubs, species composition of the herbaceous matrix is expected to change accordingly. Thus, R. sphaerocarpa would be an important and still unknown source of spatial heterogeneity to the system. Our main purpose was to evaluate, through the soil seed bank, the heterogeneity in the herbaceous community induced by this shrub species. Soil samples were collected around adult shrubs from three positions relative to the canopy: near the centre of the shrubs, at the edge of the understorey, and completely outside the canopy. Floristic composition was evaluated by germination under greenhouse conditions. The results show that each position has a different floristic composition, characterised by a group of different species. The herbaceous species associated with the external position have functional traits which enable them to resist water stress and herbivore pressure, such as hairs, CAM metabolism, early flowering, horizontal growth or tiny stature. The species associated with the central position lack those traits, and are more competitive in more mesic environments, rich in nitrogen. The lowest number of seedlings and species was found in the internal position, suggesting that in our study the facilitation process may have less importance for community dynamics due to less stressful environmental conditions.