Effect of the canopy of <i>Retama sphaerocarpa</i> on its understorey in a semiarid environment

1. A dense understorey of annual and perennial herbs grow under the canopy of Retama sphaerocarpa shrubs in semiarid environments of south‐east Spain, influencing plant productivity and diversity at a regional scale. We investigated the facilitation by the shrub on its understorey 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 in soil chemical fertility under the shrub canopy, with organic matter and soil nitrogen contents higher at the centre than at the edge of the canopy. Dry mass production of Barley was also higher in soils from intermediate positions, and lower in soils from both the centre and edge of the canopy. 3. In the field, pots sown with Barley placed near the centre, at an intermediate position and at the edge of the canopy of Retama shrubs showed significant differences in productivity, suggesting a mulching effect of the canopy that also affects seedling establishment. 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. These results and field observations suggest that the optimal association of climatic factors under the canopy would combine with a high soil fertility mediated by litter decomposition to increase biomass production mid‐way between the centre and the edge of the canopy. Overstorey and understorey thus interact to increase nutrient retention locally, which benefits both the shrub and the herb layer.