Leaf area index (LAI) of both tree and shrub layers in an adult Robusta poplar forest has been studied during one vegetation period. The research area is situated in the neighbourhood and within the reach of the Gabcikovo power-station on the Danube. After an intensive increase of LAI values at the end of April, a moderate and permanent increase was observed in May and in two first decades of June up to the second decade of August. A decrease of LAI values started in the last decade of August and got intensive in the middle of September and in October.There were some differences in the dynamics of LAI values of tree and shrub layers.The percentage of LAI values at various dates of a vegetation period is applicable to other ecosystems, when LAI is assessed at one time-point of the vegetation period and information on LAI values at a different date is necessary.