Herbivory on young tree seedlings in old‐growth and managed mountain forests

Abstract

AbstractHerbivory of invertebrates and rodents on tree seedlings in Northern temperate forests has been largely underestimated in the past and rarely addressed in studies. There is evidence that masting‐cycles of forest trees and subsequent rodents outbreaks become more frequent, leading to enhanced browsing pressure on forest regeneration. Consequently, studies exploring tree seedling mortality caused by different herbivores are required. We conducted a study on tree seedling losses in the Wilderness Area Dürrenstein, Austria on two old‐growth and one managed forest site. We conducted an exclosure experiment in two subsequent study years, using transplanted two‐month‐old tree seedlings of Picea abies, Abies alba, and Fagus sylvatica. Exclosures allowed access for invertebrates, invertebrates/rodents, and all potential herbivores. We also installed total exclosures, recording seedling losses due to climatic factors or pathogens. We calculated ZINB regressions assuming that seedling losses and seedling survival are distinct processes. Our results did not reveal fully consistent trends for both study years. The factor “treatment”, however, was significant in the ZINBs in both study years. Herbivory was a driving factor for tree regeneration in the studied forests depending on specific conditions in different years or tree species. In 2005, losses in the total exclosures and herbivory of rodents and invertebrates in the other treatments reached comparable extent. Loss rates differed not only in terms of forest management, but also between the two old‐growth forests. This, in connection with different herbivory of different tree species (higher losses of A. alba), strongly suggests the necessity of considering different herbivore guilds as drivers of tree species composition in late successional mountain forests.