Mountain birch and the nordic treeline
The treeline in the Nordic region is formed by the mountain birch. This species complex, composed of a variety of forms, subspecies, and hybrids, is found in Fennoscandia and on the Kola Peninsula. Mountain birch and its close relatives extend farther east in Russia and westward to Iceland and Greenland.
A number of factors may explain why the Nordic treeline is formed by a broadleaf tree, unlike the coniferous treeline found across much of the rest of the Arctic. The climate of Fennoscandia, in particular the influence of the ocean, is a major contributor. Higher humidity helps accelerate the bud break of birch to a greater degree than for most other deciduous trees. More importantly, the birch grows at lower summer temperatures than spruce and pine in Fennoscandia. The average-daily-maximum isotherm of 13.2°C for the four warmest summer months has been found recently to be the figure of highest significance in association with the mountain birch forest-line. In most areas of Fennoscandia, this places the upper limit of birch some 150-200 meters above the treeline for conifers, and also farther north.
While climate determines the potential treeline, the actual limit of mountain birch in the Nordic countries has often been depressed by grazing. In northern Finland, northernmost Norway and Sweden, and in some subalpine areas of southern Norway, reindeer are the main grazing animal. In other parts of Fennoscandia, such as the subalpine areas of western Norway and particularly in Iceland, grazing by sheep has been very extensive and has strongly influenced the growth of mountain birch. Grazing by invertebrates, particularly the cyclic outbreaks of geometrid moths, is an important cause of mortality for mountain birch.
In the last 50 to 80 years, many of the outfarms established over the course of several centuries for summer grazing in the mountain birch belt were abandoned, and there may now be a succession towards new mountain birch growth at those locations. Because of the harsh climate, however, the succession is slower than might have been expected, indicating that the position of the current tree line reflects more the influence of past climate than that of current conditions. The slow response may explain why the Nordic mountain birch treeline has only weakly advanced in recent years, despite higher average temperatures. Human influences today include increased tourism and the building of cottages and ski lifts. The slow recovery rate in the region causes the impacts of such activities to be visible for several years.
Frans E. Wielgolaski, University of Oslo, Oslo, Norway. From CAFF publcation, Arctic Flora and Fauna, 2001. www.caff.is