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

Abstract

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. In this study, the response of a herbivore community in a boreal forest in coastal northern Sweden to manipulations of food resources at the stand level was assessed by performing a large replicated experiment in which young forest stands at 25 sites in an area 35×45 km, were fertilized and their use by herbivores monitored during the subsequent year. At each site there was one fertilized plot, a close control (100 m away) and a distant control (300 m away), all 50×50 m. Before the growing season (April 1996) treatment plots were fertilized with calcium-ammonium-nitrate at 600 kg ha^-1 (200 kg N ha^-1). Fertilizing significantly improved browse quality (N concentration) in both downy birch (Betula pubescens) and Scots pine (Pinus sylvestris), and the amount of browse significantly increased for birch, with Scots pine showing a similar trend. Considering the animal community as a whole, 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. In the summer following fertilizing, moose (Alces alces) strongly selected fertilized plots over controls. 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, but both were used more than distant controls. The number of grouse pellets did not differ among the treatments, although they followed a similar trend. Several lines of evidence suggest that moose browsed fertilized plots more. Although not significantly different, 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 between plots, but once again fertilized plots were browsed more than controls. The results are discussed in light of understanding both how herbivores in general respond to changes in food quality and quantity, and if fertilizing may be a useful tool to manage herbivores in modern forestry. Fertilizing 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.

Key words

• animal ecology
• bark
• behaviour
• boreal forests
• browse
• browsing
• browsing damage
• calcium ammonium nitrate
• environmental impact
• feeding behaviour
• food plants
• forest ecology
• forest management
• forests
• herbivores
• multiple use
• nitrogen fertilizers
• pines
• quality
• spatial variation
• trees
• wild animals
• wild birds
• wildlife management
• woody plants