Response of a herbivore community to increased food quality and quantity: an experiment with nitrogen fertilizer in a boreal forest

1. Herbivores make decisions at different spatial levels in response to food plant quality and quantity. Although experiments have examined the responses of herbivores at lower levels, few have examined how herbivores respond to such variation at the stand level. We assessed the response of a herbivore community in a boreal forest to manipulations of food resources at the stand level by performing a large replicated experiment where we fertilized young forest stands and then followed their use by herbivores during the subsequent year. 2. We used stands at 25 sites in an area 35 x 45 km. At each site we had one fertilized plot, a close control (100 m away) and a distant control (300 m away), all 50 x 50 m. 3. Before the growing season we fertilized treatment plots with calcium-ammonium-nitrate at 600 kg ha(-1) (200 kg N ha(-1)). Fertilization significantly improved browse quality (nitrogen concentration) in both downy birch Betula pubescens and Scots pine Pinus sylvestris. Furthermore, the amount of browse significantly increased for birch, and Scots pine showed a similar trend. 4. Considering the animal community as a whole, we found that 10 of 13 animal species/groups made more tracks during winter in fertilized than in control plots. The total number of tracks was greater in fertilized plots, followed by close and then distant controls. 5. In the summer following fertilization, moose Alces alces strongly selected fertilized plots over controls. Furthermore, during the following winter moose again selected fertilized plots over controls. Hares Lepus timidus similarly left more pellets in the fertilized plots. Other mammals used the fertilized and the close controls similarly, and both were used more than distant controls. The number of grouse pellets did not differ among the treatments, although they followed a similar trend. 6. Several lines of evidence suggest that moose browsed fertilized plots more. Although not statistically significant, both heavy and light browsing were more common in fertilized than in control plots. Trees with the top shoot removed, or with bark stripped from the stem, did not differ significantly, but once again fertilized plots were browsed more than controls. 7. Our results are discussed in light of understanding both how herbivores in general respond to changes in food quality and quantity, and if fertilization may be a useful tool in modern forestry to manage herbivores. 8. Fertilization might be useful as a tool to alter the location of herbivore feeding, but buffer strips around any fertilized areas appear necessary, and other potential environmental effects should be evaluated.