Assessment of the Impact of Ungulate Browsing on Tree Regeneration

Abstract

Ungulates browse on plants and may have an influence on tree regeneration. Browsing percentage (the relative number of browsed terminal shoots) captures little of the effective browsing impact on tree regeneration, such as reduction in stem number or loss of tree species in the future stand. The main objective is to present the most important factors that can influence the impact of browsing and how these factors can be measured objectively. We conducted a literature review of influencing factors, tested these in different areas of the Swiss forest and performed studies to objectively survey them in inventories. Apart from browsing percentage, the following factors are important for estimating the long-term browsing impact: (i) the spatial distribution and density of tree saplings; (ii) the within-tree browsing intensity; (iii) the site-specific height growth of the tree saplings, and thus the time needed to grow out of the reach of browsers and the possible changes in growth rate ranking between the different selected species; (iv) the possible delay in the tree response after browsing; and (v) the tree mortality induced by browsing. The first four of these factors can be assessed easily and should thus be included in future inventories of browsing impact.