Vegetation structure of planted versus natural Aleppo pine stands along a climatic gradient in Spain

Reforestation programmes have been widely used in many forest areas worldwide but scarcely compared to nearby natural woodlands. As an example, we selected a Pinus species devoted to the Mediterranean basin in order to validate the naturalness of the plantations. We found that natural Aleppo pine stands are more structurally complex with more developed and diverse understory compared to planted stands. Maintaining a large biodiversity is a major concern for reforestation programmes in the Mediterranean basin, which displays a large area of planted forests. We compared mature planted forests (over 50 years old) to nearby natural forests in relation to the structure, composition and diversity of the understory vegetation. An efficient comparison method should provide guidelines to improve the effectiveness of reforestation management programmes. We selected three pairs of nearby planted and natural Pinus halepensis Mill. stands in southeastern Spain along a precipitation gradient and compared stand overstory structure (density, basal area and canopy cover), and understory vegetation (plant cover, species richness and evenness index). Pine tree density was lower but basal area was higher in natural stands than in the nearby planted stands. Natural stands displayed also higher understory plant cover and species richness, but no difference was found regarding evenness. These differences were larger in the most mesic sites and were due to the density and arrangement of the pine tree cover. These results provide (i) valuable information for evaluating the success of reforestation programmes to recreate natural forest ecosystems and (ii) guidelines for silviculture and management.