EFFECT OF THE CANOPY OF RETAMA-SPHAEROCARPA ON ITS UNDERSTOREY IN A SEMIARID ENVIRONMENT

1. A dense understorey of annual and perennial herbs grow under the ca nopy of Retama sphaerocarpa shrubs in semiarid environments of south-e ast Spain, influencing plant productivity and diversity at a regional scale. We investigated the facilitation by the shrub on its understore y in field and laboratory experiments with Barley designed to explore the mechanisms of interaction between both vegetation layers and their spatial variation. 2. There was a gradient of spatial heterogeneity i n soil chemical fertility under the shrub canopy, with organic matter and soil nitrogen contents higher at the centre than at the edge of th e canopy. Dry mass production of Barley was also higher in soils from intermediate positions, and lower in soils from both the centre and ed ge of the canopy. 3. In the field, pots sown with Barley placed near t he centre, at an intermediate position and at the edge of the canopy o f Retama shrubs showed significant differences in productivity, sugges ting a mulching effect of the canopy that also affects seedling establ ishment. 4. Micro-climatic measurements showed significant differences in total radiation reaching the soil, mean air and soil temperatures and maximum temperature among different positions in the understorey, increasing radially from the centre to the edge of the canopy. 5. Thes e results and field observations suggest that the optimal association of climatic factors under the canopy would combine with a high soil fe rtility mediated by litter decomposition to increase biomass productio n mid-way between the centre and the edge of the canopy. Overstorey an d understorey thus interact to increase nutrient retention locally, wh ich benefits both the shrub and the herb layer.