Bottom-up versus top-down control of tree regeneration in the Bialowieza Primeval Forest, Poland

We tested the interactions between biotic and abiotic factors in structuring temperate forest communities by comparing tree recruitment after 7 years inside 30 pairs of exclosure (excluding ungulates: red deer, roe deer, bison, moose, wild boar) and control plots (7 × 7 m each) in one of the most natural forest systems in Europe, the Bialowieza Primeval Forest (eastern Poland). The strictly protected part of that forest hosts the complete native variety in trees, ungulates and their carnivores and excludes human intervention.2. We analysed whether the exclosure effect interacted with abiotic factors, relevant for tree recruitment (canopy cover, ground vegetation cover, soil fertility and soil wetness) at different stages of tree regeneration (seedlings, saplings = 50 and > 50 cm).3. Contrary to our expectations, a single factor dominated at each stage of tree regeneration. Herbaceous vegetation cover was the main factor determining the number of seedlings with an optimum at 38% of cover. Soil fertility determined the density of saplings = 50 cm, with on average three times higher density on eutrophic than on oligotrophic soils. Herbivory was the main factor determining recruitment rate of trees into > 50 cm size classes only.4. The density of saplings that grew into the > 50 cm size class was more than three times higher in the exclosures than in the control plots during 7 years. In the absence of ungulates, on average 3.1 species recruited into the > 50 cm size class compared to 1.7 in control plots. Tree species occurred in more equal proportions inside exclosures, whereas species composition was pushed towards strong dominance of a preferred forage species, Carpinus betulus, in the presence of ungulates. This suggests that preference of species by ungulates can coincide with tolerance to browsing.5. Synthesis. The study showed that abiotic conditions dominated the early stages and ungulate impact the later stages of tree regeneration, indicating the context-dependence of herbivore top-down effects. Heterogeneity in abiotic and biotic conditions may, therefore, have an important influence on the strength of top-down effects and the role that herbivores play in natural ecosystems