Long-term effects of ungulate browsing on forest composition and structure

The impact of ungulate herbivores on tree regeneration and its possible consequences for long-term forest dynamics has raised concerns worldwide. In many countries, ungulate management aims at constant animal densities, whereas unmanaged ungulate populations tend to fluctuate over time. The ecosystem consequences of constant vs. varying ungulate densities are largely unknown, and the exact density that is acceptable from a forestry point of view is highly uncertain as well. We used the gap model FORCLIM v2.9.5 to examine the effects of three browsing-related phenomena: (a) temporal changes in animal densities and thus oscillations in browsing intensity; (b) changes in the importance of browsing as a limiting factor relative to other limitations for ingrowth; and (c) growth suppression by browsing and hence different ingrowth rates for slow- vs. fast-growing trees. Results showed that ungulate herbivory can induce profound compositional and structural changes in forest stands: (a) oscillations in the browsing intensity led to compositional shifts that were less severe than under the corresponding constant browsing intensity; (b) an increase in the importance of browsing relative to other environmental factors caused a decrease in the incidence of palatable species; and (c) growth suppression strongly affected the numbers and composition of small trees of all species. We conclude that browsing can cause a shift not only in the structure and composition of tree regeneration, but also of the upper canopy in the long term. Management can manipulate forest ecosystems through the control of animal densities, and our results suggest that alternative management strategies for ungulate populations may be worth considering so as to provide "windows of opportunity" for forest regeneration in time and/or space. (C) 2009 Elsevier B.V. All rights reserved.