Ground vegetation as an indicator of site quality: effect of non-site factors on the productivity of newly established chestnut plantations in northwestern Spain

The growth of 67 young hybrid (Castanea × coudercii A. Camus) and pure sweet chestnut (Castanea sativa Mill.) plantations was studied in relation to ground vegetation (GV) and non-site factors such as type of site preparation, weed control (WC), disease-related damage and prior land use. The classification and regression trees procedure was used to analyse the data. Ground vegetation communities accounted for a large amount of variability in the site index (SI) and were found to be useful as predictors of chestnut site quality. The presence of broom communities indicated the best site for establishing chestnut plantations, whereas bracken and bramble communities indicated poorer conditions, although those sites can still be considered suitable for growing chestnut. Terrain covered with herbaceous communities also indicated intermediate or even good site conditions. The poorest sites were the heathlands. As regards non-site factors, only weed control was found to be a significant variable. Ground vegetation caused a significant reduction in tree growth, indicating the importance of carrying out regular weeding to improve productivity. The most positive effect of weeding on height growth was detected in terrain covered by gorse and broom, and the least positive effect was observed in heathland. The findings are important for forest management purposes, as evaluation of ground vegetation will enable rapid prediction of site productivity classes in afforested areas and help in selecting the best land for planting this species in reforestation programmes.