FORBIO is an innovative, large-scale forest biodiversity experiment established in Belgium between 2009 and 2012. The acronym stands for assessment of the effects of tree species diversity on FORest BIOdiversity and ecosystem functioning. At three sites with contrasting site conditions, plots were planted with one up to four tree species. Various aspects of ecosystem functioning will be compared between plots that differ in tree species richness but have developed under the same abiotic conditions. The sites are located in Gedinne (Gribelle, Gouverneurs), Hechtel-Eksel and Zedelgem.