Comparative investigations within the two different plant communities of Croatia (Fagetum silvaticae croaticum abietetosum Ht and Querco-Carpinetum croaticum erythronietosum Ht) indicate that (1) the two communities have their own range of water deficit values, (2) the Dw values are dependent on the capability of the species to regulate their water regime, (3) the same species behave differently within the two communities, (4) water deficit of leaves can be used as an indication of the water status of the site and plants, and consequently may have a significant place in the synecology and synchorology of plant communities.