Above ground biomass changes in the mountain birch forests and mountain heaths of Finnmarksvidda, northern Norway, in the period 1957–2006

Birch forests cover large areas of the northern Fennoscandia and the mountain birch (Betula pubescens ssp. czerepanovii) often forming the altitudinal and Arctic forest and tree lines. Interpreting the factors leading to landscape changes in mountain birch forest involves disturbance from anthropogenic use and climate as important factors. Using vegetation maps based on aerial photographs and satellite images from 7 years in combination with statistical data and ground estimation data of biomass in the period 1957–2006, we were able to assess the transitions among mountain heaths and different types of forest, the displacement of the altitudinal forest line and hence the change in biomass. The tree biomass as well as the growing stock for birch in Finnmarksvidda doubled in the period 1957–2006. Only lichen biomass has been significantly reduced in the period 1957 to 2000, with a subsequent slight increase of lichen biomass in the period 2000 to 2006. The results presented in this paper show significant and positive relationships between measured tree biomass and the increase in reindeer population in the period 1957–2006 in the study area. The hypothesis concerning the removal of the “lichen barrier effect” by heavy reindeer grazing which leads to increased success for birch seeds to germinate and sprout is therefore considered to be valid. However, also, climate change effects like increased precipitation, moth attacks, freezing and thawing events during winter and long-transported air pollution (e.g. nitrogen) may also have reinforced the changes in biomass. Climatic variables from regional weather stations for the period 1955–2006 did not, however, reveal any consistent change except for that the increased snow depth had a negative impact on the reindeer population while increase in snow depth hindering the reindeer in grazing seemed to have a positive impact on the lichen biomass. The ongoing development of increased forest cover and hence elevated forest line will lead to reduction of open habitats, and hence decreased grazing accessibility for the reindeer. The effects on forestry and carbon sequestration are also discussed, and here one of the suggestions is to let the northern birch forests act as carbon sinks contributing to the reduction of total net emission of CO2 in the Nordic countries. Land use like reindeer husbandry has shown to strongly affect relationships between ecological processes like tree-growth and climate. Moreover, reindeer husbandry is such a widespread human activity in the arctic and boreal region that it might affect the global carbon budget.