The site comprises a temperate woodland in central Belgium, at the crossroad of the three Regions (Brussels, Flanders, Wallonia). Its unique history has led to a dominance of Fagus sylvatica and mixed Fagus-Quercus forest, whereby the majority of Fagus are exceptionally old for this region (>150 yrs). The wide range of other habitats (valley forests, grasslands, open waters) in combination with the old forest stands makes the Sonian Forest a biodiversity hotspot for a wide range of fauna and flora. There is a long history of research by different institutions and universities in Belgium, and the long-term management is well documented. The site comprises an ICP Forests Level II plot, a strict forest reserve of >200 ha (core area >25 years of vegetation monitoring), several Marteloscopes, scientific arboretum, tree-species experiments, large historical archive, up-to-date detailed remote sensing (aircraft-borne) etc...