Ungulates and succession dynamics reduce tree species richness in temperate unevenaged forests

Wild ungulate populations have increased throughout the northern hemisphere exacerbating the impact of browsing on understory vegetation. Ungulates could thus become an important driver of future forest diversity with cascading effects on forest resistance to future climatic conditions, forest resilience and provision of ecosystem services. Such an observation would be of particular concern in forests that are managed under continuous cover silviculture relying on natural regeneration that can hardly be protected against browsing. To better understand the impact of ungulate on future forest diversity, we conducted a large experiment with 734 pairs of fenced and unfenced 6-m² plots set across a broad gradient of red deer abundance in oak-beech forests managed though continuous cover forestry, in Belgium. Height of the dominant seedlings, seedling density, and vegetation cover were monitored annually from 2016 to 2021. Species diversity and ecological affinity for light, temperature, and atmospheric humidity conditions were inferred from these measures and the literature. Ungulates were found to strongly reduce seedling growth, density and cover of understory vegetation. Among the species studied, the early successional species (Betula pendula and Sorbus aucuparia) were the most affected. These species failed to grow in height when unprotected from ungulates but grew faster than the other species in fenced plots. In contrast, without protection against browsing, the late successional species (Fagus sylvatica and Picea abies) rapidly dominated the other admixed species. In contrast to the two previous species groups, oak (Quercus petreae and Quercus robur) was rapidly dominated in both treatments (fenced and unfenced). Ungulates can thus be a key factor in oak-beech forest succession, seriously impeding the regeneration of several species, including those most adapted to the warmer and drier conditions to come.