What was done
Working on an east-facing slope of the Slattatjakka/Njulla mountains (68°21'N, 18°49'W) in the Abisko Valley about 200 km north of the Arctic Circle, the authors documented - in September of 2009 and August of 2010 - the presence of several shrub and tree species in three 50 x 50-m plots that had been similarly documented in 1976 and 1977, two of which were located at the tree-line (characterized by sparse occurrences of solitary trees or small groups of trees), while the other one was located just below the forest-line, parts of which consisted of a denser mountain birch forest.

What was learned
Rundqvist et al. write that they documented "rapid and substantial increases in the abundance of prominent tree and shrub species near tree-line and forest-line in sub-Arctic Sweden," and that they "recorded an invasion by a thermophilic tree species not present in the plots 34 years ago."

What it means
In discussing their findings, the seven Swedish scientists describe how they mesh well with those of many other researchers, noting that "there is an indication that the shrub layer near the tree-line has expanded, since the 1930s, in the Abisko area (Enquist et al., 1933; Sandberg, 1963)," while "data from Canada, Fennoscandia, Alaska and Russia reveal that there is a Pan-Arctic expansion of shrubs and trees in progress (e.g. Kullman, 2002; Tommervik et al., 2004; ACIA, 2005; Tape et al., 2006; Karlsson et al., 2007; Olofsson et al., 2009; Hallinger et al., 2010; Hendenas et al., 2011)." And they state that the change in shrubs and small trees they observed is "consistent with anticipated changes due to climate change and reduced herbivory," which change in climate, in their words, "could be interpreted as an ongoing natural re-establishment of plants at higher altitudes due to a natural increase in the temperature since the 'Little Ice Age' (Kammer et al., 2007)."

References
ACIA. 2005. Arctic Climate Impact Assessment. Cambridge University Press, Cambridge, United Kingdom.

Hallinger, M., Manthey, M. and Wilmking, M. 2010. Establishing a missing link: warm summers and winter snow cover promote shrub expansion into alpine tundra in Scandinavia. New Phytologist 186: 890-899.

Hedenas, H., Olsson, H., Jonasson, C., Bergstedt, J., Dahlberg, U. and Callaghan, T.V. 2011. Changes in tree growth, biomass and vegetation over a 13-year period in the Swedish Sub-Arctic. Ambio 40: 672-682.

Kammer, P.M., Schob, C. and Choler, P. 2007. Increasing species richness on mountain summits: Upward migration due to anthropogenic climate change or re-colonization? Journal of Vegetation Science 18: 301-306.

Karlsson, H., Hornberg, G., Hannon, G. and Nordstrom, E.-M. 2007. Long-term vegetation changes in the northern Scandinavian forest limit: A human impact-climate synergy? The Holocene 17: 37-49.

Kullman, L. 2002. Rapid recent range-margin rise of tree and shrub species in the Swedish Scandes. Journal of Ecology 90: 68-77.

Olofsson, J., Oksanen, L., Callaghan, T., Hulme, E.P., Oksanen, T. and Suominen, O. 2009. Herbivores inhibit climate-driven shrub expansion on the tundra. Global Change Biology 15: 2681-2693.

Sandberg, G. 1963. Vaxtvarlden I Abisko nationalpark. In: Curry-Lindahl, K. (Ed.). Natur i Lappland, II. Bokforlaget Svensk Natur, Uppsala, Sweden.

Tape, K., Sturm, M. and Racine, C. 2006. The evidence for shrub expansion in Northern Alaska and the Pan-Arctic. Global Change Biology 12: 686-702.

Tommervik, H., Johansen, B., Tombre, I., Thannheiser, D., Hogda, K. and Gaare, E. 2004. Vegetation changes in the Nordic mountain birch forest: The influence of grazing and climate change. Arctic, Antarctic, and Alpine Research 36: 323-332.