Land-use legacies rather than climate change are driving the recent upward shift of the mountain treeline in the Pyrenees

Abstract

Aim To assess the effects of climate change, past land uses and physiography on the current position of the tree line in the Catalan Pyrenees and its dynamics between 1956 and 2006. Location More than 1000 linear kilometres of sub-alpine tree line in the Catalan Pyrenees (north-east Spain) Methods Using aerial photographs and supervised classification, we reclassified the images into a binary raster with 'tree' and 'non-tree' values, and determined canopy cover in 1956 and 2006. We then determined the change in position of the tree line between 1956 and 2006 based on changes in forest cover. We used the distance from the position of the tree line in 1956 to the theoretical potential tree line - determined from interpretation of aerial photographs, identifying the highest old remnants of forest for homogeneous areas of the landscape in terms of bioclimatic conditions, bedrock, landform and exposure - as a surrogate of intensity of past land uses. Results Our analyses showed that the Pyrenean tree line has moved upwards on average almost 40 m (mean advance ± SE: 35.3 ± 0.5 m, P < 0.001), although in most cases it has remained unchanged (61.8%) or advanced moderately, i.e. between 25 and 100 m (23.7%); only 9.2% of the locations have advanced more than 100 m. Upward shifts of the tree line were significantly larger in locations heavily modified in the past by anthropogenic disturbance (mean advance 50.8 ± 1.1 m) compared with near natural tree line locations (19.7 ± 0.8 m, P < 0.001), where the mean displacement was much lower than expected and was not related to changes in temperature along the study period. Main conclusions Our results stress the impact of the cessation of human activity in driving forest dynamics at the tree line in the Catalan Pyrenees, and reveal a very low or even negligible signal of climate change in the study area.

This study was funded by the Spanish Ministry of Science and Innovation via the project ‘DINAMIX’ (AGL2009‐13270‐C02), and by the Spanish Ministry of Environment via the projects 200/2010 (TREBIO), 1032s (GESCLIMFOR), 69/2005 and 634S/2012 of the Organismo Autónomo Parques Nacionales. Additional funding came from the European Commission through the Marie Curie IRSES project ‘NEWFORESTS’. The authors acknowledge J. Carreras, E. Carrillo and A. Ferré for kindly providing the digitized version of the potential tree line, and the Department of Geography of the Universitat Autònoma de Barcelona for access to aerial photographs. We are also thankful to M. Ninyerola and M. Batalla (Autonomous University of Barcelona) for providing the climatic data in the frame of the MONTES Project (Consolider‐Ingenio Montes CSD2008‐00040).