WINTER BROWSING ON PINE AND BIRCH IN RELATION TO MOOSE POPULATION DENSITY

During a 10-year period we have studied the relationships between an enclosed moose (Alces alces) population of a known density (range 1.3-5.7 moose per km2) and browsing on Scots pine (Pinus sylvestris) and birch (Betula spp.). In this paper we focus on three aspects: 1) the percentage of pine stems browsed, 2) browsing induced mortality of pine and 3) stem-breakage of pine and birch (Betula pendula) by moose. these aspects were studied on permanently marked plots and stems in two different stand types (8-15 years and 16-40 years old respectively). There was a significant correlation between moose density and the number of browsed Pines in the youngest stand type but not in the older stand type. Stem-breakage of the two tree species showed similar patterns over the years, although the frequency of breakage was considerably higher on birch than on pine. A sharp decline in stem-breakage of birch appeared some years before the moose density was decreased. Mortality of pine in the youngest stand type reached about two percent of the total number of stems. The highest accumulated mortality (≈8 %) over the ten year period was recorded on subdominant pines in the oldest stand type