Young sapling of elm (Ulmus glabra) stablished close to the local birch treeline. (Credit: Leif Kullman, Umeå University)

Sweden's mountains are growing greener. At the border between woods and bare mountain, trees that require warm temperatures, such as oak, elm, maple, and black alder, have become established for the first time in 8,000 years. This is shown in current studies led by Leif Kullman, professor of physical geography at Umeå University in Sweden.

Over the last century, the temperature has risen by more than one degree. The cooling trend over several thousand years is broken, and this has triggered changes in flora, fauna, and landscapes. In important respects, the present state is similar to what occurred directly after the latest ice age.

“Most noticeable, alongside the melting of glaciers, is an elevating of the timberline by 200 meters. Bare alpine areas are shrinking, and typical Nordic mountain birch forests are losing ground to spruce and pine, which are more competitive in a warmer and drier climate,” says Leif Kullman.

The alpine landscape is becoming generally greener and more inviting. Many mountain plants have produced profuse blossoms as well as prodigious amounts of seeds and fruits in the last few years.

Plants that were previously limited to the borderline between woods and bare mountain are now rapidly climbing alpine slopes.

“The changes are so rapid that plants like fireweed (rose bay) and rowan have even taken root in the gravel up on melting glaciers. Even wood anemones are appearing higher up the mountain,” says Leif Kullman.

The alpine flora and biodiversity are thus burgeoning dramatically. More and more plants are migrating to the high mountains since the warmer climate is conducive to them, including contorta pine and cembra pine, which are not native to Scandinavia.

The distribution of the mountain landscape’s various plant communities is in flux. Certain plants, such as mosses and low-growing herbs, are adapted to a short growing period after the snow melts. As the snow thaws earlier and earlier, these plants have been replaced by brush and grass heaths, which has lent the mountain slopes a steppe-like appearance. Mountain fens are drying up, which means that sedge and grass vegetation is growing denser, new species are migrating in, and in some places glorious alpine meadows are appearing. At the highest elevations, formerly the domain of sterile gravel and boulders, fens are occurring.

Changes in flora impact the conditions for the mountain fauna. Leif Kullman has observed new bird and butterfly species, such as wrens and admirals, at ever higher elevations.

The knowledge generated by the current monitoring system is a precondition for models that describe the development of a possibly warmer future.

“The alpine world is evincing truly major changes despite the modest increase in temperature. Present prognoses of a temperature increase of three degrees by 2100 will entail considerably more sweeping changes. We can expect fewer bare mountain areas, even more lush vegetation, and a richer flora,” says Leif Kullman.

The studies were carried out primarily in Sweden’s southern mountain regions in the provinces of Jämtland, Härjedalen, and Dalarna. Data from more than 200 sites have been recorded at various times since 1915. There is no other series of this scope in the world.