Growth response of Scots pines in polar-alpine tree-line to a warming climate

Coniferous trees at the alpine and polar tree lines of the Northern Hemisphere represent the outermost limit of their ecological range. Under such conditions, even small temperature variations may cause growth responses, which therefore can be used as indicators for changing environmental conditions. In this study we analysed the radial growth of Scots pine (Pinus sylvestris L.) along a latitudinal and altitudinal gradient at six locations in the polar and alpine tree-line ecotone in Finnish Lapland. We evaluated the effect of climatic trends on pine growth in relation to tree age and region, specifying a northern and a southern region in the study area. We found a response of Scots pine to climatic variations until the 1980s, but not to the current warming period. Increasing growth trends could be detected since 2000 in the radial growth of southern located trees, predominantly of juvenile ages, while the northern trees did not respond significantly to the current warming. In the north recent warmer and wetter conditions during winter time, inducing snow loads, wind damages, diseases and frost damages possibly masked the benefits of warmer conditions. The missing link between warming and radial growth would affect the use of tree-rings as proxy for past climate and for predictions for forest extension in polar-alpine tree-line sites.