Modeling the reduction in sapling growth, density and diversity along a gradient of wild ungulate density

Wild ungulates have determinant impacts on temperate forest regeneration. They causes shifts in species composition, alter forest structure and functioning. Their abundance has strongly increased for the last decades in the forests of the northern hemisphere. Although these effects have already been documented for some particular study cases, a deeper and more general understanding of these effects is needed across varying environmental conditions, and particularly across varying densities of wild ungulates.
In 2016, a vast network of 970 enclosure-exclosure experiments was set up in southern Belgium. Each of the 970 plots was established in forest areas where natural generation was present or expected. The plots were composed of one fenced subplot of 6 m2 (control) and two unfenced square subplots of 6 m2 at a 4 meter between centers distance. The heights of the highest seedlings, the number of seedlings of each species, the herbaceous and shrub covers were measured at the installation, and in 2017 and 2018. That network covers a region of 3000 km2 and a wide gradient of abundance of red deer (Cervus elaphus)and wild boar (Sus scrofa).
To model the effects of red deer and wild boar on forest regeneration, we fitted non-linear models to predict regeneration height growth, density and diversity in the unfenced plots in response to observations, carried out in the adjacent fenced plot: red deer abundance, wild boar abundance and different environmental conditions as altitude, soil and basal area. A relationship was then established between the severity of ungulates impacts on forest ecosystem and the density of ungulates in different environmental conditions. Such understanding will be particularly helpful to design adequate forest and wild game management guidelines.