TREE-LIMITS AND MONTANE FORESTS IN THE SWEDISH SCANDES - SENSITIVE BIOMONITORS OF CLIMATE-CHANGE AND VARIABILITY

The elevational tree-limit constitutes an ideal and sensitive proxy in dicator of climate change and variability, i.e. an essential part of m onitoring systems focusing on global climate change. That contention i s purported by multi-scale records and reconstructions of changes in a ltitudinal tree-limits and northern boreal forests. Climatically force d trends in their position, structure and composition have occurred at all temporal scales throughout the Holocene. A progressive elevationa l descent of Pinus sylvestris tree-limit since the earliest Holocene, concurs with the deterministic theory of millennial climate forcing by changes in the Earth's orbital parameters. The successively less seas onal climate with cooler, more humid summers and winters with increasi ng snow cover has preconditioned the emergence of a subalpine birch fo rest belt during the past ca. 7000 yrs BP as well as the growing gee-e cological prominence of Picea abies. Superimposed on this longterm tre nd, climatic anomalies of shorter duration have been inferred from the tree-limit chronology. Some exceptionally warm and stable centuries, with high tree-limits and dense montane forests occurred during the Me dieval period. Thereafter, the Little Ice Age prevailed until the late 19th century. Northern and high-elevation ecosystems were profoundly stressed, disturbed and destabilized by cold, windy and highly variabl e climate conditions. An episode of warmer climate during the first ha lf of the present century imposed some recovery of structures decayed by the Little Ice Age. However, tree-limits and high-elevation forests were far from restored to their medieval levels. During the past 4-5 decades, a more martime and slightly cooler climate has been instrumen tally recorded. High-elevation arboreal vegetation has responded retro gressively by defoliation, retarded growth, ceasing regeneration and l ocally some tree-limit retraction. Neoglacial processes have been resu med, e.g. dieback of subalpine/alpine dwarf shrub heaths, followed by deflation of humus and surface mineral soils. These processes are read ily monitored in a unique regional network, with baseline data since t he early 20th century.