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A century of tree line changes in sub-Arctic Sweden shows local and regional variability and only a minor influence of 20th century climate warming

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Abstract Aim 

Models project that climate warming will cause the tree line to move to higher elevations in alpine areas and more northerly latitudes in Arctic environments. We aimed to document changes or stability of the tree line in a sub-Arctic model area at different temporal and spatial scales, and particularly to clarify the ambiguity that currently exists about tree line dynamics and their causes. Location 

The study was conducted in the Torneträsk area in northern Sweden where climate warmed by 2.5 °C between 1913 and 2006. Mountain birch (Betula pubescens ssp. czerepanovii) sets the alpine tree line. Methods 

We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics. Results 

Since 1912, only four out of eight tree line sites had advanced: on average the tree line had shifted 24 m upslope (+0.2 m year−1 assuming linear shifts). Maximum tree line advance was +145 m (+1.5 m year−1 in elevation and +2.7 m year−1 in actual distance), whereas maximum retreat was 120 m downslope. Counter-intuitively, tree line advance was most pronounced during the cooler late 1960s and 1970s. Tree establishment and tree line advance were significantly correlated with periods of low reindeer (Rangifer tarandus) population numbers. A decreased anthropozoogenic impact since the early 20th century was found to be the main factor shaping the current tree line ecotone and its dynamics. In addition, episodic disturbances by moth outbreaks and geomorphological processes resulted in descent and long-term stability of the tree line position, respectively. Main conclusions 

In contrast to what is generally stated in the literature, this study shows that in a period of climate warming, disturbance may not only determine when tree line advance will occur but if tree line advance will occur at all. In the case of non-climatic climax tree lines, such as those in our study area, both climate-driven model projections of future tree line positions and the use of the tree line position for bioclimatic monitoring should be used with caution.
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Keywords: Climate warming; Sweden; dendrochronology; herbivory; human impact; mountain birch; reindeer; sub-Arctic; tree line dynamics

Document Type: Research Article

Affiliations: 1: Flemish Heritage Institute, Koning Albert II-laan 19 bus 5, B-1210 Brussels, Belgium, formerly Laboratory of Wood Technology, Ghent University Coupure Links 653, 9000 Ghent, Belgium 2: Department of Forest Sciences, University of Helsinki, PO Box 27, FI-00014 Helsinki, Finland 3: Department of Geography, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium

Publication date: 01 May 2011

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