HOLOCENE TREE-LIMIT AND CLIMATE HISTORY FROM THE SCANDES-MOUNTAINS, SWEDEN

Elevational tree-limit change of Pinus sylvestris, Betula pubescens ssp. tortuosa, and Alnus incana during the Holocene was studied from subfossil wood remains in the Scandes Mountains, Sweden. From 8250 yr BP to the present, the Pinus-limit descended at an average rate of 20 m per millennium (after adjustment for glacioisostatic land upheaval). In contrast, Betula and Alnus ascended until approximate to 3000 yr BP, whereafter an altitudinal decline is apparent. The relative abundance of Pinus in the tree-limit ecotone has decreased, while that of Betula increased throughout much of the Holocene. The present-day subalpine belt of Betula developed after 7000 yr BP in response to long-term cooling interacting with short-term climatic disturbance of pine stands. The inferred tree-limit histories are consistent with climatic model simulations of solar radiation and the atmospheric circulation for the Holocene (the Milankovitch theory). Based on the current climate requirements of the tree-limit species, an early Holocene summer thermal optimum approximate to 1.0 degrees C higher than present is inferred. Subsequently, tree-limit vegetation changed gradually, in dynamic equilibrium with progressive summer cooling and increasing humidity and snow cover, i.e., a decreasingly seasonal climate. Accelerated recession of Finns, indicative of abrupt cooling, is recorded for a short period around 7200 yr BP. A short-term advance of the Pinus tree-limit, presumably a consequence of climatic warming, occurred approximate to 4300 yr BP. Rapid and persistent reductions in the quantitative record of subfossils of all tree species, after 5500-5000 yr BP, may result from fragmentation of the tree-limit ecotone. This response is inferred as the combined effect of long- and short-term cooling passing a threshold, which initiated periglaciation, podzolization, and paludification.