Holocene pine tree-line evolution in the Swedish Scandes: Recent tree-line rise and climate change in a long-term perspective
This article focuses on the Holocene tree line ( Pinus sylvestris ) and climate change in the Swedish Scandes. A composite of three previously independently published data sets of megafossil tree remains (trunks, stumps and roots) from sites above today's tree line is analysed. Calibration of ages, adjustment for glacio-isostatic land uplift and a larger sample provide new and more conclusive insights. The tree-line altitude peaked about 11?200 cal. yr BP, 425 m higher than the position about a century ago, when it was at its Holocene low. At the millennial-centennial scale, tree-line retraction has been a smooth process, following a linear trend, which largely conforms to the Milankovitch model of orbital forcing of summer temperature evolution. Shorter excursions from this trend cannot be ruled out, as these are beyond the resolution of the analytical approach. During the earliest Holocene, summers may have been about 2.5°C warmer than by the late-19th century. Projection of the 20th-century maximum tree-line rise by 150–190 m upon the entire Holocene tree-line chronology indicates that present-day tree lines may be higher than ever during the past 5000 or even 7000 years. Indirectly, this might reflect an anomalous modern climatic regime for which there is supporting evidence from retreating mountain glaciers and recent evolution of the alpine plant cover and its zonation patterns. With this background, the evolution of the tree-line ecotone in the case of continued warming is considered. Pine is likely to regain the dominant position held in the early Holocene. Its tree line may shift at least 400 m above its present position. Small outlier stands of mountain birch will establish in sheltered, moderately snow-rich habitats high above the new pine tree line, but an overall alpine character of the landscape will prevail above the tree line of pine. Exotic tree species, mainly belonging to the genus Pinus may profit from a new climatic regime and establish in the tree-line ecotone.
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