Abstract
Climate warming during the past century has imposed recession of glaciers and perennial snow/ice patches along the entire Swedish Scandes. On the newly exposed forefields, subfossil wood remnants are being outwashed from beneath ice and snow bodies. In Scandinavia, this kind of detrital wood is a previously unused source of postglacial vegetation and climate history. The present study reports radiocarbon dates of a set of 78 wood samples, retrieved from three main sites, high above modern treelines and stretching along the Swedish Scandes. In accord with previous studies, pine (Pinus sylvestris) colonized early emerging nunataks already during the Late Glacial. Around 9600-9500 cal. yr BP a first massive wave of tree establishment, birch and pine, took place in "empty" glacier cirques. Both species grew 400-600 m above their present-ay treeline position and the summer temperatures may have been 3.5 oC warmer than present. In respons to Neoglacial cooling, treelines of both birch and pine descended until their final disappearance from the record 4400 and 5900 cal. yr BP, respectively. During the entire interval 9600 to 4400 cal. yr BP, birch prospered in a 100-150 broad belt above the uppermost pines. The recent emergence of tree remnants in the current habitats relates to the contemporary episode of climate warming, possibly unprecedented for several past millennia. It is inferred, by an anology with the past, that in a future scenario with summers 3.5 °warmer than present, the birch treeline may rise by 600 m or so.
Highlights
A data set of 78 radiocarbon-dated subfossil tree remants from all three main study areas constitutes the core of this study (Table 1)
It is startling to witness that high-alpine sites in the Scandes, 500-600 m above the contemporary treeline and currently occupied by glacier ice and perennial snow, have for long periods of the early- to mid- Holocene harboured stands of trees
The present study suggests that pine was a more frequent constituent of the early Holocene tree flora at high elevations than previously assumed (e.g. Barnekow 1999; Bigler et al 2002; Seppä et al 2004b)
Summary
World-wide, post-Little Ice Age climate warming has fundamentally altered the preconditions for physical and biological systems in alpine and subalpine/subarctic regions (Kullman 2002a, 2010a,b; Fagre et al 2003; Moore 2003; Smol et al 2005; Oldfield 2005; DArrigo et al 2006; Barry 2006; Shiyatov et al 2007; Kaufmann et al 2009; Nesje 2009; Thompson et al 2009; Nagy & Grabherr 2009; Akasofu 2010; Callaghan et al 2010; Hallinger et al 2010). Provided that currently observed climatic and biotic trends continue or accelerate, there is an urgent need to project the consequences for future landscape evolution. In this context, it is our conviction that such an endeavour has to draw essentially on experiences of past ecological performances (cf Davis 1989; Petit et al 2008). Responses of the alpine treeline and the forest-alpine tundra ecotone are crucial in this respect, since presence/absence of a tree cover has a steering effect on the entire plant cover structure and biodiversity patterns. Understanding of the dynamics of the forest-tundra interface is crucial for realistic regional and global climate modelling, since the location, extent and structure of this boundary may feed back on the entire climate system (Betts et al 2000; Grace et al 2002; Harding et al 2002; Salonen et al 2011)
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