Grazing history affects the tree-line ecotone: a case study from Hardanger, Western Norway

Expansion of subalpine forests and upslope shift of tree-lines are expected in temperate and boreal mountain ecosystems due to global climate change. On a local scale, climatic forces are modified by a number of controlling factors. Site history is of central interest because it impacts on the importance of air and soil temperature vs. other factors. In many mountain areas, the grazing record is an important part of the site history. This study compares the tree and ground coverof two localities having different grazing histories in the tree-line ecotone of Western Norway. Data from the sampling of trees, saplings, seedlings and ground cover, tree age determinations, aerial photographs, interviews and literature research are used. The results show differences in the structure of the tree-line ecotone with regard to tree age, height, circumference and the structure of the understory. Ground cover differs in species composition, mainly with regard to light preferences. Together, the differences stress the importance of site history consideration in order to understand changes in the tree-line ecotone, because dissimilar site histories may create differing local responses to future global and regional climatic changes