In South-Moravian floodplain forests (altitude of 150-170 m; 48degree 43 northern latitude and 16degree 57 eastern longitude), regular spring floods occurred till 1976 when an extensive hydroengineering structure was put into service. After the gradual drying and degradation of floodplain ecosystems, artificial flooding is practised since the 90s. In spite of this, natural regeneration of pedunculate oak stagnates for several decades and, if some seedlings occur, they die within two or three years. In a selected stand aged 130-150 years, two plots were established after the seed year 1999 for study causes of the problem. A control plot with natural seeding left under crown canopy and a trial plot where full-grown trees were felled and completely removed. Forest weed control and spraying of seedlings against oak mildew are carried out at the trial plot continuously. Through the analysis of environmental conditions (groundwater table dynamics, soil water content, mean air temperatures, precipitation totals) and comparing the seedlings of both variants on the basis of differences in morphological/physiological characteristics of leaves (particularly in pigment pool and transpiration area reduction) it has been demonstrated that the main cause of the young progeny mortality is neither soil drought caused by competition for water with full-grown trees nor other climatic conditions but the critical deficiency of solar radiation. In case of rich acorn crop, it is necessary to carry out either intensive opening up of the stand (it means that the rest of a parent stand is left as the protection of young seedlings against late frosts) or it is possible to risk and to fell all already in the course of the following winter. It is necessary to take care of the natural seeding by systematic reduction of forest weed (both herbs and woody species), to protect it against game and oak mildew attack.