Hybridisation and colonisation dynamics in European oaks

Summary Extensive hybridisation between the two sympatric species Quercus petraea and Q.robur is suggested by the near lack of genetic differentiation between the two species and supported by controlled crosses and mating system analysis in mixed stands. Further ecological and genetic evidence suggest that hybridisation does not impede the ecological specialisation of the two species, raising the issue of its evolutionary significance in oaks. Preferential unidirectional hybridization (pollen Q. petraea to ovule Q. robur) has been shown in various mixed stands and facilitates the introduction of sessile oak in existing pedunculate stands. If this unidirectional trend is reinforced in later backcrosses, then hybridisation leads to the dispersal of Q. petraea in existing stands of Q. robur. Hybridisation can therefore be seen as a ‘pollen-mediated’ dispersal mechanism, and has most likely contributed to the rapid migration of Q. petraea in Europe. Given the extant distribution of the species in Europe, migration through pollen swamping should be seen at the edges of the natural distribution of Q. petraea where the demographic imbalance of the two species will reinforce backcrosses.