Effects of Simulated Winter Browsing by Moose on Morphology and Biomass of Two Birch Species

Abstract

(1) Morphological responses and biomass changes of birches (Betula pendula and B. pubescens) following simulated moose browsing were studied on trees which were 8 and 5 years old, respectively, at the beginning of the study. (2) The current annual long-shoots were removed at different clipping intensities in March-April 1981. The treatments were repeated on the same trees for another three years and at the same time control (earlier unclipped) trees were clipped. Plant responses were studied in all three years. (3) An increased clipping intensity induced the following morphological changes: the number of long-shoots decreased; the frequency of branched shoots increased; the base diameter, length and dry weight of individual shoots increased; the number of buds per long-shoot increased, but the total number of buds on all current annual long-shoots decreased. Rate of increase in tree size, measured as height and main stem diameter, decreased with clipping. (4) Simulated browsing during four years caused the long-shoot biomass to increase at a lower rate on treated trees than on control trees. The long-shoot biomass decreased during the last two years only for the highest clipping intensity on B. pubescens. The values never fell below those at the beginning of the trial. (5) The number of female catkins decreased with higher clipping intensities, but individual viable seeds increased in weight.