Twentieth-century Scots pine growth variations in the central Scandinavian Mountains related to climate change

Climate-sensitive trees are valuable for reconstructing past climates, but they also can be used to assess the impacts of environmental change, such as global warming, on forest ecosystems. Growth variability and growth responses to climate of a Scots pine (Pinus sylvestris L.) tree-ring width chronology, from the treeline in the central Scandinavian Mountains, were studied throughout the 20th century. Summer temperatures, especially in July, were the most influential climate factor for tree growth. Correlation analyses in three 30-yr periods showed that growth responses to climate varied through time, being particularly low in 1931 to 1960. Nevertheless, tree growth around 1950 was the highest for 320 yr, implying optimal growth conditions. This growth increase appears to be a response to increased summer temperatures, a lengthening of the growing season, and a temporal change in the atmospheric circulation pattern. Despite a continuation of seemingly favorable growth conditions in the latter half of the 20th century, pine growth decreased after the 1950s. It appears that high-altitude pine experienced stress that surpassed the positive effect of improved growth season climate. Since pine growth decline coincided with an unprecedented strong and positive period of wintertime North Atlantic Oscillation (NAO), a measure of strength of westerly winds bringing mild and moist air masses over Scandinavia, it is proposed that milder and wetter winters caused growth stress, and hence reduced growth, in high-altitude central Sweden.