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To assess the spatial patterns of forest expansion (encroachment and densification) for mountain pine (Pinus uncinata Ram.) during the last 50 years at a whole mountain range scale by the study of different topographic and socio-economic potential drivers in the current context of global change. The study area includes the whole distributional area of mountain pine in the Catalan Pyrenees (north-east Spain). This represents more than 80 municipalities, covering a total area of 6018 km². Forest cover was obtained by image reclassification of more than 200 pairs of aerial photographs taken in 1956 and 2006. Encroachment and densification were determined according to changes in forest cover, and were expressed as binary variables on a 150 x 150 m cell-size grid. We then used logistic regression to analyse the effects of several topographic and socio-economic variables on forest expansion. In the period analysed, mountain pine increased its surface coverage by 8898 ha (an increase of more than 16%). Mean canopy cover rose from 31.0% in 1956 to 55.6% in 2006. Most of the expansion was found on north-facing slopes and at low altitudes. Socio-economic factors arose as major factors in mountain pine expansion, as encroachment rates were higher in municipalities with greater population losses or weaker primary sector development. The spatial patterns of mountain pine expansion showed a good match with the main patterns of land-use change in the Pyrenees, suggesting that land-use changes have played a more important role than climate in driving forest dynamics at a landscape scale over the period studied. Further studies on forest expansion at a regional scale should incorporate patterns of land-use changes to correctly interpret drivers of forest encroachment and densification.
