Site classification in a mixed hardwood forest (Hallerbos, Belgium) with a homogeneous ground vegetation dominated by Hyacinthoides non-scripta (L.) Chouard ex. Rothm.

Abstract

The forest of Halle (560 ha), situated 20 km south of Brussels is covered by a beech (Fagus sylvatica) forest, locally mixed with secundary species (Tilia, Fraxinus, Acer, Quercus,... ). In almost all stands, herbal vegetation is dominated by bluebell (Hyacinthoides non-scripta). The research intended to classify 36 plots of different tree species composition according to their site quality. Three classification methods were compared: the first one based on the indicator value of the understorey vegetation, a second one on the humus morphology and a last one on some quantitative soil characteristics. According to the plant sociological site classification, the plots have the same site quality. However, humus forms differ apparently and significant differences were found in pH value and base cation saturation of the soil, abundance and biomass of earthworms and biomass of the ectorganic horizon. Tree species proved to be the main cause of these differences. The results illustrate that the herbal vegetation is not always a reliable indicator of site quality. In the case of a homogeneous vegetation dominated by one or more indifferent species, classification on humus morphology or soil analysis are more appropriate. In the forest of Halle, the tree species is probably the main cause of the observed differences in site quality.