BROWSING BY MOOSE ON SCOTS PINE IN RELATION TO PLANT RESOURCE AVAILABILITY

The availability of resources for growth is believed to have a significant impact on the chemical defense of plants against herbivores. However, the means by which resource availability affects different plant traits, and the way in which these factors in turn affect diet selection, are not well understood. I addressed the relation between plant biomass, morphology, and tissue nutritional quality and browsing by moose (Alces alces (L.)) on Scots pine (Pinus sylvestris L.). In two experiments nutrient and light availability were manipulated and trees were subsequently exposed to free-ranging moose at feeding stations. Treatments included fertilization, shading, clipping, or root-cutting for 1-2 growing seasons. Treatments had a significant effect on tree height, twig biomass, shoot morphology, carbon/nitrogen ratio, and nitrogen concentration in current-year shoots. Twig biomass and shoot morphology explained most of the variation in per-tree twig biomass consumption by moose, whereas the carbon/nitrogen ratio had a significant impact only when between-treatment differences in biomass were removed. These findings show that variation in resource availability induces changes in a variety of plant traits other than tissue nutritional quality, to which moose respond by adjusting foraging behavior. This may have important implications for plant-animal interactions: the plants' chemical defense could be less effective against herbivores like moose that adjust foraging behavior at a variety of spatial and temporal scales.