Thiérache is a small natural region with a bocage and forest landscape which takes up the north-eastern corner of department Aisne. Forest vegetation has been studied according to the synusial phytosociological methodology. In this last part the author presents the phytosociological data for hygrophilous forests. Two arborescent associations, three schrubby associations and eight herbaceous associations with their synfloristic and synecological characteristics are described. The main phytocœnosis are then reconstructed.