How does climate change affect Sweden’s mountain vegetation?

In 1997, SLU made an inventory of the vegetation in a number of sample plots located in two areas north and south of Lake Torneträsk near Abisko in Norrbotten County (northern Sweden). In the summer of 2010, some of the sample plots were revisited.

‘The results show that mainly the mountain birch (Betula pubescens ssp. czerepanovii) is responsible for the increase in biomass*. Trees from the first inventory have grown and become larger. Thus, the increase in biomass isn’t a result of more trees being established,’ says Henrik Hedenås, environmental analyst at SLU and one of the scientists behind the study.

Denser vegetation

Apart from an increase in biomass, the tree coverage has increased. Again, it is the mountain birch canopy which now covers more of the surface. Shrub coverage by willow (Salix ssp.), dwarf birch (Betula nana) and common juniper (Juniperus communis) has also increased. In 1997, these bushes covered 25 per cent of the surface. In 2010 the figure was 32 per cent. In addition, meadows with low herbs have become more common while snowbed vegetation has decreased by almost 50 per cent during the same time period.

‘Densification of tree and shrub vegetation has probably been ongoing for a long time. Already in the 1960s, densification of shrub vegetation was discussed,’ says Henrik Hedenås.

Many possible causes

Henrik Hedenås points out that the study does not answer the question of what is causing the changes. Around the year 1900 a period of colder climate ended known as the “Little Ice Age”. Since then, the temperature has gradually increased and accelerated in the last 15 years. The annual mean temperature in the region has increased by 2.5° C since 1913, from 1.8° C to 0.7° C. The rise in temperature stimulates forest growth, partly by extending the growing season.

‘Small-scale animal husbandry and a need of fuel wood when building the railway line around the turn of 1800/1900 have held back the forest. Grazing reindeer and outbursts of geometrid moths (mainly Epirrita autumnata) continue to this day to hold it back. There are many factors determining the forest’s growth,’ says Henrik Hedenås.

Where are we heading?

Henrik Hedenås gives two examples of what a warmer climate and more vegetation may generate. The carbon balance of the forest may change with consequences for the “ecosystem service” carbon storage. A higher temperature increases forest growth which means more carbon is stored. However, at the same time the degradation in the soil increases, which releases carbon dioxide, one of the greenhouse gases. The net result is hard to tell.

Reindeers' grazing may be affected, with consequences for reindeer husbandry.

‘A dense mountain forest may make it difficult for the reindeers to graze when leaving the mountain top, above the tree line, for reindeer pasturages on the hillsides. Also, less snowbed vegetation means less nutrient-rich plants for the reindeers to graze,’ says Henrik Hedenås.

The researchers will now continue the study by going through mountain data from the National Inventory of Landscapes in Sweden (NILS). The project started in September 2011 and is expected to be completed in 2012.

*The study was conducted in collaboration with researchers at Abisko Scientific Research Station.