Allocation of resources within mountain birch canopy after simulated winter browsing

As a response to browsing, birches are known to produce fewer but larger, m ore nutritious leaves, with enhanced palatability for herbivores. We simula ted winter browsing in ramets of mountain birch (Betula puhescens ssp. czer epanovii) to find out whether it decreases subsequent foliage biomass and a lters the number and type of shoots. After removal of a considerable propor tion of buds (up to 35%) in late winter, the birches were able to compensat e for the lost leaf biomass in the following summer; there were no differen ces in total leaf biomass between winter-clipped and control ramets. This i ndicates that foliage growth was limited by the total amount of stored reso urces, not by the number of buds. Depending on the position of the buds rem oved, different mechanisms were responsible for the compensation. After rem oval of apical buds, the number of leaves decreased significantly but leave s were larger than in control ramets. Removal of the same mass of basal bud s - containing similar amount of carbohydrates and proteins as in the treat ment removing apical buds activated dormant buds, especially in apical loca tions, so that leaf number was similar as in the controls; consequently, si ze of individual leaves increased only slightly. Thus, while the total leaf biomass in a tree seems to be limited by resources from source organs, the distribution of resources among different canopy sections is controlled by their relative sink strengths. In terms of leaf biomass, epical Darts are able to compensate for bud loss by increasing shoot number, basal parts onl y by increasing leaf size.