Biomass and Production on a Landscape Level in the Mountain Birch Forests

Abstract

Low summer temperatures limit the distribution of mountain birch in latitude and altitude, but it was found early (Helland 1912) that the Nordic mountain birch normally grows at lower summer temperatures than spruce and pine in Fennoscandia (see Chap. 1). Odland (1996) stated that the average maximum temperature isotherm 13.2 °C of the four warmest months correlates better with the mountain birch forest line than any other temperature. There is often a decrease in the final height of mountain birch trees with increasing elevation which is caused by several environmental factors often even more important than temperature, e.g. strong winds, reduced length of the growing season etc. The nutrient and water conditions are also very important for growth. Generally, there is an increasingly limited nitrogen supply with elevation, often also a low humus content in the soil (in other cases, an extremely high content), causing a low water holding capacity. Most of the Fennoscandian birch forests are relatively oligotrophic and dry with a ground vegetation often closely related to the grass–Vaccinium heaths under more extreme climatic conditions with no trees. In northern Fennoscandia, reindeer lichens and Empetrum hermaphroditum often dominate the understorey of birch forests. Vare (2001) gives a classification of mountain birch woodlands, while Wehberg et al. (see Chap. 3) have presented a new classification of the Nordic mountain birch communities at Finnmarksvidda in northernmost Norway according to plant sociological methods. The lowest production is found on a lichen-rich type common in the north-eastern inland districts of Fennoscandia and in the inner parts of the mountain region. The mountain birch on this type is on average only 2–3 m tall. This is a type extensively grazed by reindeer. Somewhat higher productivity is found in more grass(Deschampsia flexuosa) and moss-rich woodlands, which are common on relatively oligotrophic ground in suboceanic districts