Different moose densities and accompanied changes in tree morphology and browse production

Large herbivores affect the morphology and productivity of their food plants, which in turn affects food availability and foraging efficiency. These effects can occur directly, through tissue removal, or indirectly through additions of nutrients in dung and urine. Although influences of herbivores on plant communities are well established for grazers, less is known about the effects of browsers. We studied how tree morphology (number of shoots, shoot mass, and mean yearly height increase) and production of winter browse of Scots pine (Pinus sylvestris) and birch (Betula pubescens and B. pendula) responded to simulated moose herbivory in Swedish boreal forests. We simulated browsing, defecation, and urination to represent effects of four levels of moose density in each of eight exclosures. To be able to study the whole range of moose impact and obtain better mean estimates from the ecosystem perspective, the eight exclosures were placed along a forest productivity gradient. Treatments were applied for three consecutive years. For both birch and pine, the mean average number of shoots per tree and the mean yearly height increase decreased with increasing biomass removal. Mass of individual shoots increased for birch, but not for pine. The mean yearly production of winter browse (per m(2)) was significantly reduced by biomass removal. The responses of birch followed a threshold model predicting no effect up to a low to moderate level of biomass removal, and thereafter a linear decrease, although the model describing constant-linear decrease not could be ruled out (model selection based on Akaike Information Criterion). The response of pine followed a threshold model. We concluded that winter browse might be in shortage at high browsing pressure over extended time, but most woody plants eaten by moose should sustain browsing pressure at moose densities common in many areas in Sweden.