Large-scale assessment of regeneration and diversity in Mediterranean planted pine forests along ecological gradients

Abstract

Aim: There is increasing concern regarding sustainable management and restoration of planted forests, particularly in the Mediterranean Basin where pine species have been widely used. The aim of this study was to analyse the environmental and structural characteristics of Mediterranean planted pine forests in relation to natural pine forests. Specifically, we assessed recruitment and woody species richness along climatic, structural and perturbation gradients to aid in developing restoration guidelines. Location: Continental Spain. Methods: We conducted a multivariate comparison of ecological characteristics in planted and natural stands of main Iberian native pine species (Pinus halepensis, Pinus pinea, Pinus pinaster, Pinus nigra and Pinus sylvestris). We fitted species-specific statistical models of recruitment and woody species richness and analysed the response of natural and planted stands along ecological gradients. Results: Planted pine forests occurred on average on poorer soils and experienced higher anthropic disturbance rates (fire frequency and anthropic mortality) than natural pine forests. Planted pine forests had lower regeneration and diversity levels than natural pine forests, and these differences were more pronounced in mountain pine stands. The largest differences in recruitment - chiefly oak seedling abundance - and species richness between planted and natural stands occurred at low-medium values of annual precipitation, stand tree density, distance to Quercus forests and fire frequency, whereas differences usually disappeared in the upper part of the gradients. Main conclusions: Structural characteristics and patterns of recruitment and species richness differ in pine planted forests compared to natural pine ecosystems in the Mediterranean, especially for mountain pines. However, management options exist that would reduce differences between these forest types, where restoration towards more natural conditions is feasible. To increase recruitment and diversity, vertical and horizontal heterogeneity could be promoted by thinning in high-density and homogeneous stands, while enrichment planting would be desirable in mesic and medium-density planted forests.