A network of strict forest reserves is currently being installed in Flanders (Belgium). Their main objectives are nature conservation and scientific research. Belgium is densely populated and has a long history of intensive forest management, so no natural forests are left. Even the forest reserves are composed of man-made, semi-natural forest stands. In the strict reserves, an intensive monitoring programme is performed in order to study the spontaneous development of the,woody and herbal layer when managed forests are left for free development. The methodology involves measurements in a grid of nested circular sample plots, combined with an intensive full inventory in a core zone. The results for typical dendrometric and structural variables (total woody biomass, necromass, gap size and distribution) from the forest reserve of Kersselaerspleyn (230 year old, man-made beech forest, 20 years of non-intervention) are compared to other beech forest reserves in Europe, both natural and man-made. It is remarkable to see how quickly the stands from Kersselaerspleyn have developed towards a dynamic equilibrium in total living and dead biomass, comparable to that in natural beech stands. The analysis shows that the spontaneous development of a man-made forest to a more natural state can be relatively fast, provided that the original stand is old enough to reach an age where natural decay normally starts to occur. For beech stands, this can be the case for stands of 200 years and older. The consequences of this development on species richness and the composition of mosses, vascular plants, saproxylic invertebrates and fungi were also studied: 48 plant species, 55 mosses, 244 species of fungi and over 100 species of saproxylic invertebrates have up to now been registered on the dead wood, including many rare indicator species of valuable old-growth sites.