Statistical analysis of ground water table measurements showed that significant differences could be assessed in ground water regimes of the following plant communities: Caricetum gracilis typicum, Caricetum curto-echinatae typicum. Cirsio-Molinietum peucedanetosum and Senecioni-Brometum racemosi caricetosum nigrae. Drainage induced changes in a Cirsio-Molinietum vegetation, were studied during 12 years. The results were compared with changes in the ground water regime. Measurements of the ground water table during a five year period, preceding an interference in hydrology, were compared with measurements of a succeeding seven year period. Significant differences were obtained in median and mean ground water levels, not in extreme values. However, the vegetation responded to extreme ground water levels quite clearly. Therefore it is concluded that semi-natural vegetation can be sensitive to changes in ground water levels that can hardly be proved. It is suggested not to concentrate on mere conditional relationships in field studies, but to consider positional relationships as well.