EFFECTS OF SIMULATED SUMMER BROWSING BY MOOSE ON LEAF AND SHOOT BIOMASS OF BIRCH, BETULA-PENDULA

Woody plants respond in many ways to browsing by large herbivores. Changes in morphology and/or chemistry may appear during and beyond the season of impact. The plant responses may, in turn, affect the probability and magnitude of future herbivory. Responses following defoliation were studied on Betula pendula. This species is a deciduous tree, the leaves of which are utilized as summer food by moose (Alces alces). We used two intensities (50 and 100%) of simulated defoliation of long-shoots, in mid-June and mid-July, respectively, to study responses of juvenile trees. On defoliated and undefoliated (control) trees, we recorded leaf and shoot biomass, number of branched and damaged long-shoots and tree height and diameter growth. In autumn, 3-4 months after defoliation, the leaf biomass was lower on defoliated trees than on undefoliated trees. However, the total leaf biomass produced during the season was about the same on treated and untreated trees, indicating a refoliation, at least on trees defoliated in mid-June. Defoliation did not affect the total number of long-shoots produced during the treatment season. The treatment caused considerable damage (tip drying) to long-shoots, especially on trees defoliated in July. The percentage of branched long-shoots was also reduced by defoliation. There was a trend towards a lower total leaf biomass per tree one year after defoliation. The main reason for this trend was a significantly lower leaf biomass on short-shoots. The increase in tree height was significantly reduced by the 100% defoliation in June and July, respectively. By affecting leaf and shoot biomass, as well as tree height, leaf-stripping of birch may have significance for the feeding pattern of a large herbivore, both during the year of impact and the following year.