Forest of pedunculate oak in Croatia cover an area of about 200,000 ha, or about 10 % of the forest area. They are mostly distributed over flat floodplain regions along the rivers Sava, Drava and Danube and their tributaries. The most important plant communities are the forest of pedunculate oak and common hornbeam (Carpino betuli-Quercetum roboris Anić 1959/emend. Rauš 1969), and the forest of pedunculate oak and greenweed (Genisto elatae-Quercetum roboris H t. 1938). Research into these ecosystems has shown that the crucial factor of their stability is water, or the water regime of the soil. The regime is disturbed when there is a surplus or an insufficiency of water over a longer period of time. The research of several years into changed ecological relations in pedunculate oak forests of upper Posavina has indicated that the causes are of antropogenous and climatic character. Not only do older trees and entire stands decline, but the physiographic characteristics of the soil and the floral composition of the vegetation also change. The most important changes occur in the forest of pedunculate oak and greenweed, particularly in the association with quaking sedge (Genisto elatae-Quercetum roboris caricetosum brizoides Ht. 1938), which is characteristic of upper Posavina. The changes in the soil caused by excess (of polluted) water insufficient water are almost imperceptible or hardly distinct, and they do not always reflect the real state of acrobiosis or anacrobiosis. They are most clearly manifested in the change of redox potential and in the status of organic matter in the soil top horizon. An altered floral composition is the accompanyng and recognizable result of changed edaphic and climatic site conditions. In the forest of pedunculate oak and greenweed, a pronounced water deficit results in a progression towards the association of pedunculate oak and common hornbeam, while long-lasting manifold excessive wetting leads to a regression and the occurrence of narrow-leaved ash, black alder and the species of swampy sites.