Small Mammal Communities of the North Scandinavian Birch Forest

Abstract

This paper presents an analysis of a collection of small mammals from a number of habitats in Troms County of north Norway. The primary purpose of the work was to obtain live specimens of Clethrionomys for a genetical study of species relationships in this genus, but since the animals were collected by a standardized live-trapping technique the records yield some new information on the composition of the small mammal communities of this relatively little studied region, in a form which permits comparison with collections by similar methods from other districts. The reproductive activity of the animals is the subject of a separate study by Hoyte (1955). The collection was made in the neighbourhood of Rosta at latitude 68? 58' N., longitude 19? 42' E. This is a district of high relief on the western side of the main watershed of northern Scandinavia, where deeply cut U-shaped river valleys penetrate far inland and are separated by mountains rising to summits of about 1700 m. The floor of the valley at Rosta is only about 100 m above sea-level, although it lies some 80 km inland from the head of its fjord. To the east and south the general level of the ground rises progressively to the plateau of Swedish Lapland (Lappmark). There is in consequence a considerable variety of types of vegetation, arranged for the most part in sharply defined horizontal bands across the mountain sides, and providing major differences of habitat for the animal communities. Forests of birch, and in some places conifer, cover the valley floor and lower slopes of the mountains, except where areas have been cleared for farming. The cleared areas are discontinuous and rarely more than a few acres in extent. Above the upper limit of trees are various sub-alpine associations, which give way at still higher altitudes to a more sparse arctic-alpine type of vegetation. The area studied included a part of the valley floor and the northern slope and summit plateau of Kamnasfjell, a low mountain mass to the south of Rosta. The estimated vertical range of this area was about 500 m, and its topography is represented in the profile diagram, Fig. 1, which shows the relations between the principal zones of vegetation in which trapping was carried out. These included birch forest, meadowland and several types of lower mountain associations above the tree line, but not coniferous forest or high mountain associations. The forest covered approximately the lower third of the mountain slope and was succeeded by a well-defined belt dominated by heath plants, termed here the Empetrum nigrum zone. The latter extended a further 100-150 m vertically up the slope before giving way to a fjaeldmark type of vegetation on the summit plateau. The following descriptions of the vegetation of the areas trapped have been com-