Facilitation and succession under the canopy of a leguminous shrub, Retama sphaerocarpa, in a semi-arid environment in south-east Spain

Retama sphaerocarpa is a leafless leguminous shrub found in most regions of Spain, frequently with a growth of herbs in its understorey which contrasts with the surrounding areas by its higher biomass and diversity. We examined the relationships between the shrub and the herbs underneath along a gradient of shrub age. A total of fifty individuals were selected to fit five age classes and shrub characteristics, soil properties and flora under the canopy were examined along the estimated chronosequence. All shrub size variables increased with time, as did the amount of nutrients stored by the shrub, but differences were often significant only between the three oldest classes. Concentration of nitrogen and phosphorus in photosynthetic stems remained constant. but nutrient pools in stem biomass increased with time. The shrubs changed the soil environment under their canopies with age by ameliorating soil texture, nutrient content and capacitance of water. The improvement was most pronounced between the two oldest classes (IV and V), and was probably due to the high biomass of perennial species in the understorey which increased the production of litter, and the interception of wind-blown dust. Plant diversity in the understorey increased with shrub age, likely due to a greater heterogeneity under larger canopies. Drought-resistant species, typical of the open areas between shrubs, were displaced from the centre of the understorey by taller, more mesic species over the age gradient. Retama sphaerocarpa shrubs benefited from the increase of resources in the understorey and showed a higher reproductive output with shrub age, but decreased at the beginning of senescence. The indirect interactions between R. sphaerocarpa and its understorey herbs could be considered as a two-way facilitation in which both partners benefit from their association.