Overstorey and topographic effects on understories: Evidence for linkage from cork oak (Quercus suber) forests in southern Spain

The understorey is a critical feature of forest ecosystems, affecting nutrient cycling, biodiversity, regeneration capacity and wildfire regimes. Understanding the interaction of environmental and canopy controls on understorey development is therefore important for forest management. We use airborne lidar to elucidate this interaction in a forest of cork oak (Quercus suber) mixed with the deciduous oak (Quercus canariensis) on complex topographic relief in southern Spain. Solar radiation (controlled by aspect) and topographic position were highly influential in the differential distribution of the two dominant canopy trees, and exerted some control on their canopy density and height. There was a detectable effect of both canopy characteristics and solar radiation on the amount of understorey shrub cover, indicating direct and indirect controls of the environment on this stratum. In cork oak forest, understorey shrubbery increased towards valley bottoms, at lower levels of solar radiation and under shorter canopies. Our results are relevant to the management of these forest systems in the face of future change. They suggest that maturation and closure of these even-aged stands, coupled with climate warming, may lead to impoverishment of the shrub layer, reducing the build-up of fuel but also the biodiversity value of these systems.