Investigations on genetic variation, geographic and hierarchic distribution of Quercus robur L. and Q. petraea (Matt.) Liebl. are indispensable conditions for the rearrangement of Lithuanian genetic resources conservation system. The occurrence area, spreading nature, and introgression zone of Q. petraae are still not known. Under the processes of global climate warming the conditions for sessile oak, as for the species of warmer climate comparing to Q. robur, would become more favorable in Lithuania. Trakas forest has the status of the Strict Nature Reserve in Meteliai Regional park (southern Lithuania) and is famous because of abundant presence of sessile oak, which has natural limit stretching farther south. The forest was investigated for the composition of oak species and possible interspecific hybrids by the aid of 11 morphological traits of fallen leaves. Trees possessing features of hybrids comprised up to 38%, while Q. petraea only 18 % of the total number of trees. The relief and soil types in the area are very altering and could be considered incentive for the processes of hybridisation. The diversity of the classified tree forms indicates that these hybridisation processes have a long history in Trakas forest.
Keywords: Quercus robur, Quercus petraea, discrimination of species, infraspecific hybrids, morphological traits of leaves.