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 foo d plant quality and quantity. Although experiments have examined the respon ses of herbivores at lower levels, few have examined how herbivores respond to such variation at the stand level. We assessed the response of a herbiv ore 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 s ubsequent year. 2. We used stands at 25 sites in an area 35 x 45 km. At each site we had on e 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-amm onium-nitrate at 600 kg ha(-1) (200 kg N ha(-1)). Fertilization significant ly improved browse quality (nitrogen concentration) in both downy birch Bet ula pubescens and Scots pine Pinus sylvestris. Furthermore, the amount of b rowse significantly increased for birch, and Scots pine showed a similar tr end. 4. Considering the animal community as a whole, we found that 10 of 13 anim al species/groups made more tracks during winter in fertilized than in cont rol plots. The total number of tracks was greater in fertilized plots, foll owed by close and then distant controls. 5. In the summer following fertilization, moose Alces alces strongly select ed fertilized plots over controls. Furthermore, during the following winter moose again selected fertilized plots over controls. Hares Lepus timidus s imilarly left more pellets in the fertilized plots. Other mammals used the fertilized and the close controls similarly, and both were used more than d istant controls. The number of grouse pellets did not differ among the trea tments, although they followed a similar trend. 6. Several lines of evidence suggest that moose browsed fertilized plots mo re. Although not statistically significant, both heavy and light browsing w ere more common in fertilized than in control plots. Trees with the top sho ot removed, or with bark stripped from the stem, did not differ significant ly, 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 fertiliz ation may be a useful tool in modern forestry to manage herbivores. 8. Fertilization might be useful as a tool to alter the location of herbivo re feeding, but buffer strips around any fertilized areas appear necessary, and other potential environmental effects should be evaluated.