Interactions between forests and herbivores: the role of controlled grazing experiments

The use of controlled grazing by large herbivores as a research and management tool in forest systems is reviewed, with particular focus on exclosure-based studies of plant-animal interactions within north-west Europe. Controlled grazing studies have revealed that large herbivores (wild and domestic) have a substantial influence on forest composition and dynamics. We review current knowledge and highlight the deficiencies and potentials for new research, in relation to specific areas identified as of key importance in forest dynamics. Few attempts have been made to assess critical thresholds of grazing intensity in relation to factors such as recruitment and maintenance of different tree species. Thus it is not possible to define appropriate grazing management techniques for specific aims, yet there is an urgent need for this type of knowledge, especially in countries with little remaining natural forest. Gradients of primary productivity have been shown to affect grazing choice and species responses to damage in non-forest vegetation, yet few studies have explored these effects within forest systems. More research on plant-herbivore and herbivore-herbivore interactions is required at a range of both plant and animal densities. The increasing awareness of biodiversity issues has also highlighted deficiencies in our knowledge of forest systems. We conclude that controlled grazing experiments have an important role to play in these key aspects of forest research, and that their potential has not yet been fully utilized.