The contribution is dealing with comparison of the herb layer biomass and floristic changes of an original willow-poplar forest and secondary poplar monoculture on the low stretch of the Morava river. An original willow-poplar forest (Salici-Populetum typicum) and a secondary poplar monoculture with Populus canadensis, arisen on the original habitat after an artificial reforestation are analysed in detail. Changes in cover and biomass of the herb layer of above-mentioned communities are discussed as well as production and floristic changes caused by a high flood in 1991.