The habitat of fell biotopes and species shrinks as climate change progresses


The anthropogenic climate change caused by man is one of the most significant threats to the survival of fell ecosystems. The survival of biotopes is affected, among other things, by the shortening period of snow cover, the advance of the tree line, and the fact that not all biotopes are able to move further up the slopes of fells as the climate becomes warmer. The shrinking of habitats leads to the endangerment of many fell species.

Fells are sensitive biotopes

The fell ecosystems typical of Fennoscandia are one of Finland's national character areas as well as an important tourist attraction for Lapland. They include a range of biotopes from treeless fell tops to the mineral soils found in the mountain birch zone. Threatened fell biotopes account for approximately one tenth and near threatened for approximately 77% of the total area. Examples of threatened biotopes include perennial snow patches, xeric lichen-rich mountain birch forests, and one type of seasonal snow patches. Reindeer grazing is currently the biggest threat to fell ecosystems, as it causes a loss of reindeer lichen and damages the ability of mountain birch forests to regenerate [1].

Climate change scenarios indicate that the effects of global warming will be the greatest in northern regions such as the Fennoscandian fells [2]. Species and biotopes will be faced with major changes, and adaptation to the changed conditions will be complicated by the Arctic Ocean, which prevents organisms from moving northwards with the climate [1]. Most species of plants found in fell ecosystems appear to tolerate a relatively wide temperature range [3], and the forecasted changes in the prevalence of different species are likely to relate to changes in the structure of vegetation and competition between species. The numbers of a few species with limited temperature tolerance and a narrow geographic range may nevertheless dwindle [1].

The advance of the tree line shrinks the area of treeless fell tops

The advance of the tree line and the resulting loss of treeless fell tops is one of the most significant projected effects of climate change on fell ecosystems. Even an optimistic scenario (B1) predicts an increase of between two and three degrees in the average summer temperatures of Finnish Lapland. The tree line advances by 100 metres every time the temperature increases by 0.6 °C, which means that the tree line will advance by at least 350 metres by the end of the current century. At that point, the kind of treeless fell tops that we have now would only remain in Enontekiö, the outermost northwest tip of Finland [1].

Figure 1. Fells in mist.

© Seppo Tuominen

Natural regeneration is a particular threat to the biotopes found along the lower parts of treeless fell tops as well as to mesic and hydric habitats near the tree line, where conditions for the spread of trees are favourable. Heather moorland, for example, is susceptible to both afforestation and changes in the prevalence of different species as a result of changes in rainfall. Rocky and bouldery areas, such as the tops and craggy slopes of fells, are likely to remain treeless despite changes in temperature and may prevent or slow down the spread of biotopes. The rate at which the tree line advances also depends on other factors, such as rainfall and winter temperatures, reindeer grazing, pests, soil conditions, and the distribution of snow cover. The advance of the tree line is likely to be delayed due to these factors [1].

There is some uncertainty about the effect of climate change on mountain birch forests. As the tree line advances, the mountain birch zone moves further and further up towards the fell tops. The advance of some biotopes is nevertheless restricted by the stress caused by reindeer grazing. If warm winters become more common, cold winter temperatures will no longer help to prevent the sometimes extremely extensive damage caused by autumnal moths and winter moths to mountain birch forests. This would slow down the natural regeneration of fell tops [1]. On the other hand, the ability of birch trees to recover during warm summers will improve, and the parasites and other predators that regulate moth populations may reduce the damage caused by moths [4].

Figure 2. Mountain birch trees during the autumn colours season.

© Aarno Torvinen

Biotopes that depend on snow will decrease

Global warming shortens the period when the ground is covered by snow, which affects the structure and vegetation of biotopes found in snowy depressions on treeless fell tops, known as seasonal and perennial snow patches [1]. Many species of plants, such as the near threatened glacier buttercup (Ranunculus glacialis), are dependent on these cool and hydric habitats where snow dominates [5]. The prevalence of perennial snow patches is believed to have decreased considerably over the last 50 years, and both biotopes are expected to dwindle with climate change. As snow patches melt, unvegetated perennial snow patches may turn into mossy seasonal snow patches [1].

Global warming reduces ground frost, which is a threat to biotopes that are affected by frost phenomena. Frozen moorland, for example, may become increasingly boggy and turn into peatland as the prevalence of ground frost decreases [1].

No escape for the species of fell ecosystems

Climate change is estimated to be the second biggest threat to the future survival of threatened fell species and the number one threat to the survival of near threatened fell species. Arctoa moss (Arctoa hyperborea) is believed to have already disappeared as a result of climate change [5]. Southern species compete with fell species; for example, the red fox (Vulpes vulpes) has already practically replaced the arctic fox (Alopex lagopus) in Finnish fells.

The climatic ranges of fell birds studied by means of modelling are expected to shrink by 77% on average by 2080, if climate change progresses according to the optimistic B1 scenario. In reality, this is likely to be an underestimation, because the absence and fragmentation of suitable habitats reduces the availability of areas suitable for fell species even further. The ranges of the Rock Ptarmigan (Lagopus muta), the Long-tailed Skua (Stercorarius longicaudus), and the Eurasian Dotterel (Charadrius morinellus), among others, are likely to shrink considerably [6].

As southern species move northwards, fell species will ultimately not be able to move further north or higher up on fells. Many fell species are expected to become extinct in Finland in the near future due to their shrinking habitats [5].