Holocene pine tree-line evolution in the Swedish Scandes: Recent tree-line rise and climate change in a long-term perspective

Abstract

BOREAS Kullman, L. & Kjällgren, L. 2006 (February): Holocene pine tree-line evolution in the Swedish Scandes: Recent tree-line rise and climate change in a long-term perspective. Boreas, Vol. 35, 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 millennialcentennial 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 2.58C warmer than by the late-19th century. Projection of the 20th-century maximum tree-line rise by 150 Á/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.

Recent forest limit changes in south-east Norway: Effects of climate change or regrowth after abandoned utilisation?

“…The three findings of the present study point to increased future forest growth and higher actual and UPCEFL. Such findings are also supported in other parts of the Nordic region, but many of these studies have not clearly separated the effect of regrowth after human disturbance from that of recent climate change (Kullmann 2000; Kaplan et al 2003, Dalen & Hofgaard 2005Wielgolaski 2005;Kullman & Kjällgren 2006). This separation is crucial in order to understand and single out the effects of present and future climate change on the uppermost forest limits.…”

“…During warmer periods of the Holocene, the forest limit stretched to considerably higher elevations (Kullman & Kjällgren 2006). This variation has been linked to natural climatic changes, mainly variations in temperature (Miller et al 2008).…”

A Richer, Greener and Smaller Alpine World: Review and Projection of Warming-Induced Plant Cover Change in the Swedish Scandes

“…In response to multi-millennial climate cooling, the extension of alpine tundra gradually expanded to a maximum about a century ago (Kullman and Kjällgren 2006). Over several prior millennia, alpine species spread downhill and centrifugally from the highest mountain massifs and behind the retreating treeline (Smith 1920;Danielsson 1984;Kullman 2006).…”

“…Associated with secular temperature rise by 1.4°C, the balance between alpine and subalpine habitats has shifted in favour of the latter (Kullman and Kjällgren 2006;Kullman and Ö berg 2009).…”

“…Based on integrated actual observations and paleoecolocical data (Kullman and Kjällgren 2006;Kullman 2006), some tentative projections of future vegetation change in Scandinavian high-mountain regions are presented. Of course, this endeavour is hypothetical, since a deterministic mechanism for centennial climate evolution at the spatial scale here concerned is still elusive.…”