CLIMATE CHANGE AND THE NORWAY SPRUCE TREELINE IN THE CENTRAL SCANDINAVIAN MOUNTAINS Treeline evolution, tree growth and a Blue Intensity summer temperature reconstruction

Sammanfattning: In the context of global change, mountain regions have received increased interest in recent times with researchers trying to answer how such regions may be affected by a changing climate. Treeline environments are sensitive to the impacts of climate change which may significantly alter the characteristics of the regions. But there are discrepancies regarding the effects in different settings and no generalizations can be made. Therefore, there is a need for more research to answer questions of the impacts in different regions. The aim of this study was to investigate the impacts of climate change on the Norway spruce (Picea Abies (L.) Karst) treeline in the central Scandinavian mountains regarding treeline evolution and tree growth. Moreover, a goal was to present a Blue Intensity summer temperature reconstruction using Norway spruce and a subgoal was to highlight variations in the impacts between slopes that face different cardinal directions. The study area is located in central Sweden where three sites were selected, one facing northwest, one southeast and one north. Trees were sampled at different elevations, following a standard dendroecological approach to reconstruct treeline evolution by examining differences in tree age at different elevations. The samples were later used for measurement of tree ring width (TRW) and Blue Intensity (BI) to provide results regarding variations of tree growth and to reconstruct summer temperatures. The full TRW- and BI-chronologies extend from 1650-2021 and May-August average temperature were reconstructed in the full period. The results show that the treeline have advanced at all studied sites, though the magnitude of the advance differs depending on the cardinal direction of the slope. Tree growth have increased at all sites, and wider ring widths have also become more common at all studied elevations. The summer temperature reconstruction revealed an increasing temperature trend comparable to the observed trend of global warming and the warming in the last 20 years have been the highest in the whole period. The conclusions are that climate change as a main driver of temperature rise seems to positively affect tree growth and recruitment of Norway spruce at the treeline, allowing Norway spruce to grow further up the slopes. Moreover, it is possible to claim that the rate of recruitment is increasing. Further, the results suggest that cardinal direction is important for the microclimatic conditions that allow recruitment and establishment of the species at higher altitude. The treeline advanced more to the south and less to the north. The trends of ring widths also increased more towards the south compared to the north.