Feeding behaviour of red deer (Cervus elaphus) offered Sitka Spruce saplings (Picea sitchensis) grown under different light and nutrient regimes

1. In addition to exhibiting preferences for particular plant species, vertebrate herbivores select particular individuals of these species whilst leaving others undamaged. This pattern of diet selection may reflect differences in the chemical composition (and hence nutritional quality) between individual plants, and/or variability in the physical constraints on intake rate, such as plant structure. 2. An experiment was conducted to test the effects of environmental manipulations on the morphology and chemical composition of Sitka Spruce saplings, and to evaluate the consequences for herbivory by Red Deer. Fertilizing the trees increased tree height, branch span and leader length, twig width and needle width, and decreased the concentrations of total phenolics, condensed tannins, fibre and lignin but monoterpene content was not altered. Shading also reduced phenolic and tannin concentrations. 3. When the fertilized and shaded trees were offered to deer in feeding trials, the probability of a tree being visited by a deer and the biomass removed were influenced by tree morphological variables, as were the bite rate and intake rate of the deer. More biomass was removed from larger trees. 4. Once the effects of tree morphology had been taken into account, there was no effect of the fertilizer and shade treatments on deer browsing behaviour that could be attributed to changes in chemical composition of the trees. The relationship between intake rate and bite size at each tree varied between individual deer, but the functional response relationship between intake rate and bite size was not influenced by the treatments applied to the trees. 5. Tree morphology may have a larger influence on deer feeding behaviour than tree chemical composition.