Conifer dispersion on subalpine pastures in northeastern Spain: Characteristics and implications for rangeland management

Pinus sylvestris L. and Pinus mugo Turra originating from two plantations established during the 1960s are invading subalpine heathlands higher than 1 500 m above sea level in Montseny Natural Park (northeast Spain). Both species are native at a regional scale but may not have been present in the park's subalpine vegetation previously. In addition, Abies alba P. Mill., which is in regression in many areas in Europe, is also colonizing the area from a neighboring natural forest. This invasion appears to be stimulated by a combination of natural and human factors, including differences between vegetation components, climate (i.e., drought periods), changes in land use due to conservation policies (i.e., suppressing fire or grazing practices), the creation of the plantations, and probably the nurse role played by accompanying dwarf shrubs (Calluna vulgaris [L.] Hull and Juniperus communis subsp. nana [Willd.] Syme). We examined the effects of this process in terms of the spatial dispersion and characteristics of the established conifers and deduce implications for the conservation of isolated subalpine pastures in Mediterranean Basin mountains. P. sylvestris was the most successful invading species in this area. The P. mugo invasion is distributed mainly near the plantation. The only native conifer species, A. alba, appears to be colonizing only the eastern slope. The invasion process is related to the diversity and species richness recorded on each slope. Conserving valuable subalpine heathlands at the latitude of the Montseny mountain range implies suppressing propagule pressures from the plantations. The option of removing conifers that are nonnative, at a local scale, must be considered. However, in the case of the native A. alba this option leads to a management conflict between conserving former pastureland and the dispersion of A. alba.