Effects of afforestation with pines on woody plant diversity of Mediterranean heathlands in southern Spain

Open heathlands of the Strait of Gibraltar region constitute a singular vegetation type within the Mediterranean Basin owing to their high biodiversity levels and distinct features in the biological aspects of endemism. However, they have been traditionally depicted as low profitable, tree-less, barren lands and have been either overlooked in conservation policies or included in extensive pine-tree afforestation programmes. Nowadays, most of the existing pine plantations are no longer exploited and have become dense pine woodlands. Here we present the results of a comparative analysis aimed to ascertain the effects of pine stands on the structure and diversity of the woody component of these mediterranean heathlands. Ten sampling sites were located within the study area, each consisting of two adjacent subsamples: an open heathland and a neighbouring heathland under pine-tree cover. Woody species richness and overall shrub cover was much lower in pine-tree heath understoreys. This drop in cover was more marked for short-lived species (seeders and withering resprouters), implying a decrease of the life-history diversity and also having noticeable consequences in homogenizing the floristic composition of heath patches under pine trees. Heathland narrow endemics dropped significantly in pine stands, showing greater sensitivity to afforestation than nonendemics. We suggest that the open heathlands of the Strait of Gibraltar should be given further attention in EU and IUCN conservation policies. Programmes should be implemented to progressively eliminate abandoned pine plantations so as to restore and preserve these highly diverse mediterranean heathlands.