Hybridisation and introgression are expected to be common in plant populations composed of various species of the same genera. These processes, however, frequently have one direction or are asymmetric, when one of hybridising species is a donor and other recipient of pollen. The asymmetric hybridisation may be also an important manner of migration by pollen. According to the theory of asymmetric hybridisation in mixed oak populations of European white oaks (Quercus robur and Q. petraea) the number of hybrid saplings should be higher under a canopy of Q. robur than under Q. petraea trees. The aim of this study was to determine, which of the two species in the mixed populations has a greater degree of success in regeneration and colonization, measured by the proportion and density of saplings and young individuals higher than 0.3 m. The taxonomic composition of saplings under the crown ranges of trees was analyzed on the basis of morphological characters of leaves in two distantly located mixed oak stands in Poland, in Jamy and Legnica Forests, where young generation to about 17–18 years old was observed under canopy of 125–140 years old mother trees. The first population is located about 100 km of Q. petraea geographic range and covers area of 6 km, the second about 500 km of Q. petraea range and covers more than 10 hectares. The density of 60 and 134 adult trees per hectare, were found in analysed populations, respectively. A significantly higher proportions of Q. petraea than Q. robur saplings were found in both compared stands. A hybrid saplings proportion were higher than hybrid adult trees. The hybrid saplings were observed more frequently under canopy of Q. robur only in the stand closer to the range of Q. petrea. Gene flow from Q. petraea to Q. robur by cross-pollination was found to be likely. The number of hybrid saplings found under crown projections of Q. petraea trees in both populations suggested also reverse gene flow, at least in the stand located close to the north-eastern limit of the Q. petraea range. A higher regeneration success of Q. petraea was observed in both stands suggesting expansion of this species.
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