Interactions between Browsing Moose and Two Species of Birch in Sweden

In Sweden, moose (Alces alces) feed to a large extent on birches (Betula pendula and B. pubescens). This study examined responses of juvenile birches to moose browsing and the responses of moose to browsed trees. Following browsing, the birches produced larger shoots with larger and more chlorophyll—rich leaves than did unbrowsed birches. Shoots on browsed trees had a longer growing season and contained less Ca, Mg, and crude fat than did shoots on unbrowsed trees. After browsing, some resting buds and buds on some of the short shoots developed into long shoots, a response that was more frequent in an open habitat than in a shaded one. The birches thus showed substantial compensatory growth following browsing by moose. Most of the potential buds were not transformed into long shoots, even though more growth was allocated to each individual shoot. For moose this mean that at moderate browsing pressures (1) the production of shoot biomass was maintained, (2) the available forge increased because the shoot growth was kept at lower positions, and (3) browsing was facilitated by the larger shoot size, but (4) the contents of Ca, Mg, and crude fat in the shoots decreased. Of the two similar birch species, moose preferred B. pendula to B. pubescens. This supports the hypothesis that plants with slow growth rates (B. pubescens) are less preferred by browsers than plants with high growth rates (B. pendula). Further, we found no support for the occurrence of induced defense in juvenile trees; those that had been browsed during the previous winter were more palatable than previously unbrowsed trees.