An exclosure experiment to assess the impact of ungulates on plant diversity in Belgium

The erosion of biodiversity is caused by many factors. Large ungulates, which occupy a large part of the Walloon forests, can also influence the distribution and dynamics of plant species. Therefore, they impact the biodiversity of our temperate forests. The aim of the present study is to identify changes in forest understory vegetation due to the pressure exerted by wild ungulates thanks to an exclosure-enclosure experiment. This study was conducted on a high plateau of the Ardenne, in the forest of St Michel Freyr (South Belgium). The forested area is dominated by beech and spruce where red deer, wild boar and roe deer are living in sympatry. 150 sampling plots were established in 2000. Each plot consisted of two closed enclosures of 4 m² each and one exclosure of 4 m² as control under pressure of ungulates. Sampling was carried out systematically by a 500m-square mesh. The cover rate and height of some easily identifiable species were measured regularly since 2000 (2001, 2002, 2003, 2004, 2007, 2011-2012). In addition, we identified each herbaceous species in 2011-2012 to analyze the species richness after more than 10 years exclosure. Ligneous species showed an increase of developpement in the fenced area. These species are more impacted when they are not much represented in the study site.Species richness and diversity are higher in areas impacted by herbivory. Monocotyledons are mainly favorised while tree seedlings are unfavorised. Browsing on the trees keeps the area free from high vegetation and allows light to reach the ground. In the same time, herbaceous species have a lower cover rate in the enclosure because of the competitive exclusion induced by shrub and tree seedlings layer. If the apparition of most species is favorised by ungulates, they induce a lag of gowth on dicotyledons which have a higher palatability than monocotyledons. Ungulates have the ability to impact the distribution of vegetal species by the modification of the competitive interaction. By their pressure on tree and shrub species, they induce a higher diversity of the ground flora. However, this change in vegetation composition is slow and requires a long-term studie, specially in areas of low biomass productivity like the St-Hubert forest