Density-related effect of red deer browsing on palatable and unpalatable tree species and forest regeneration dynamics

Large herbivores play a key role in terrestrial ecosystems, and their populations alter the plant composition in many ecosystems. One example is deer, whose populations in the Northern Hemisphere have increased over the last decades. This has resulted in increased browsing pressure on saplings and, as a consequence, has hampered forest regeneration in some areas. Such herbivore-induced changes in the regeneration of forest stands have changed their composition and biodiversity. Although there is an established link between the deer population density and the level of damage caused by browsing, the question remains open: What is the effect of increasing deer density over years on the regeneration of palatable and less browsing-resistant tree species? In this study, we investigated the relationships between the regeneration dynamics of five tree species (European beech Fagus sylvatica, silver fir Abies alba, sycamore Acer pseudoplatanus, hornbeam Carpinus betulus, and oak Quercus spp.) and browsing pressure in two years with low and high red deer (Cervus elaphus) density and at similar roe deer (Capreolus capreolus) density. We set up experimental plots in Roztocze National Park (central-eastern Poland), a protected area where natural predators of deer are constantly present. In addition to deer density, we analysed parameters that may affect browsing intensity: height of saplings, forage availability (sapling density), sapling diversity and light intensity (canopy openness). We found that increasing red deer density significantly increased browsing on all tree species, with saplings taller than 50 cm being under the strongest pressure. Moreover, higher deer density altered deer forage selection – it increased their selection for unpalatable European beech and decreased for highly preferred sycamore. Additionally, more species-diverse patches and closed canopy attracted deer and increased the probability of browsing on saplings. Contrary to our predictions, sapling density decreased browsing intensity. The study confirms that high ungulate density has significant effects on tree recruitment and may alter tree species composition of the forest stand. We provide clear evidence that deer herbivory plays a crucial role in tree regeneration in top-down processes in natural ecosystems.