A multivariate analysis of introgression between Quercus robur and Q. petraea in The Netherlands

Hybridization and introgression between Quercus robur and Q. petraea was investigated in 12 Dutch populations, using multivariate analysis. Compared with the reference groups, the differences between the two taxa are slight, but distinct. The Q. petraea types differ more from their reference group than the Q. robur types, possibly due to greater selection pressure. In most populations a range of specimens occurs between pure Q. robur and pure Q. petraea. The latter category is poorly represented, and actual Fr and F2-hybrids are most uncommon. Products of introgression between primary hybrids and Q. robur as well as Q. petraea constitute an important part of the populations. Hybridization and introgression is partly due to planting of Q. robur by man. In the natural population of the Meinweg a gradient is established between Q. robur in the lower, moist levels and Q. petraea in the higher, relatively dry levels.