Direct and indirect effects of browsing on forest biodiversity: evaluating the intermediate disturbance hypothesis in boreal forest ecosystems

Large mammalian herbivores (ungulates) are important drivers in forest ecosystems, but their effects on biodiversity and regeneration remain contested. This thesis examined the direct and indirect effects of ungulates on regeneration, vegetation structure, and biodiversity in Swedish boreal and boreonemoral production forests, with particular emphasis on the intermediate disturbance hypothesis (IDH). Using observational studies and ungulate-exclosure experiments, the thesis tested whether ungulate disturbance promoted biodiversity by reducing competition and increasing structural heterogeneity. Ungulate effects could not be explained by density alone. Moose density influenced browsing damage on Scots pine, but forest composition, competition, forage availability, and site productivity were also important. Browsing also constrained the development of palatable broadleaves. The biodiversity results provided partial support for the IDH. Vascular plant richness was highest at intermediate ungulate pressure, consistent with the idea that moderate disturbance can promote coexistence. This pattern was mediated largely through changes in vegetation structure, light availability, and habitat heterogeneity. Beetles also benefited from ungulate disturbance, but mainly through structurally open conditions and microclimatic change rather than a clear unimodal pattern. Overall, ungulates generated both ecological costs and benefits. The IDH helped explain some biodiversity responses, but not all, suggesting that co-management should move beyond density-based approaches and better account for vegetation structure, site conditions, and trade-offs between production and biodiversity.