| Summary: | Both the northern (latitudinal) and the upper (altitudinal) timberlines are phytogeographic transitions between the forested boreal vegetation zone and treeless areas. These two timberlines, which intermingle in northern Fennoscandia, are mainly controlled by the cold climate, although other natural and anthropogenic factors are of noticeable importance at least locally. The roles of the various controlling factors are discussed in this article. The timberline and tree line in northern Finland are usually formed by the mountain birch (Betula pubescens ssp. tortuosa), which extends further than the Scots pine (Pinus sylvestris) or the Norway spruce (Picea abies). The northern coniferous timberline in Finland follows approximately an isoline that represents an effective temperature sum of 600 degree days. Hazardous events also regulate the growth and occurrence of trees in addition to average conditions. Trees growing at the timberline have adapted themselves in many ways to the prevailing harsh circumstances. Pollen research and megafossil analyses (such as tree ring studies) show that the timberline has moved and its tree species composition has changed, primarily due to climatic fluctuations. This is one clear indication that the timberline is a dynamic ‘combat zone’.
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