Timberline structure and limited tree recruitment in the Catalan Pyrenees

Background: Pyrenean timberlines, lowered to various extents through ancient land use, are expected to rise due to abandonment of land use and climate warming. Aims: To provide a comprehensive survey of the timberline in the Catalan Pyrenees, at the landscape, plant community, and tree population levels. Methods: We assessed the present location of timberlines by means of GIS techniques and characterised the treeline ecotone in 12 representative plots, where we took small phytosociological releves ( surface cover type, vegetation structure and species cover) and studied the regeneration of Pinus uncinata ( seedlings and saplings). Results: The potential timberline was mostly found between 2300 and 2400 m a. s. l., with complex variation in these values due to bioclimatic and topographic diversity. Most of the present timberline was found at far lower altitudes than its potential location. Present forests reached the potential timberline about 25% along its total length. At natural or semi-natural treelines, the transition from subalpine forest to open alpine landscapes was relatively sharp. It was formed by the irregular alternation of two or three contrasting plant community types (forest, dwarf-shrub, grassland). Most Pinus regeneration was recorded where vegetation mosaics occurred on a fine scale. The highest numbers of Pinus seedlings and saplings were found emerging from bare ground or litter in small vegetation gaps. Over recent years Pinus recruitment showed very slow growth and a strong limitation in reaching the pole stage. Conclusions: In Pyrenean treeline ecotones situated near to their potential altitudes, recruitment and growth of existing Pinus uncinata seedlings and saplings is unlikely to produce pronounced infilling and thus perceptible changes in high-altitude forest cover.