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

AimTo 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. LocationMore than 1000 linear kilometres of sub-alpine tree line in the Catalan Pyrenees (north-east Spain) MethodsUsing 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. ResultsOur analyses showed that the Pyrenean tree line has moved upwards on average almost 40m (mean advanceSE: 35.3 +/- 0.5m, P<0.001), although in most cases it has remained unchanged (61.8%) or advanced moderately, i.e. between 25 and 100m (23.7%); only 9.2% of the locations have advanced more than 100m. Upward shifts of the tree line were significantly larger in locations heavily modified in the past by anthropogenic disturbance (mean advance 50.8 +/- 1.1m) compared with near natural tree line locations (19.7 +/- 0.8m, 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 conclusionsOur 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.