An analysis of variation of leaf characters in Quercus robur L. and Quercus petraea (Matt.) Liebl. population samples from Northern Ireland.

Abstract

The levels of hybridity in 35 population samples of oak (Quercus spp.) from Northern Ireland were assessed using leaf characters analysed by Principal Components Analysis and Cluster Analysis. Eight population types were recognised : 1. Populations of pure Q. robur (two populations); 2. Mixed populations with no hybrids and a predominance of Q. robur trees (two populations) ; 3. Predominantly pure Q. robur with a small number of hybrids (four populations); 4. Predominantly pure Q. robur with a small number of Q. petraea and hybrid trees (five populations); 5. Approximately equal proportions of Q. robur and Q. petraea with a small number of hybrids (nine populations); 6. Mixed populations with a high proportion of hybrids (three populations); 7. Predominantly Q. petraea with a small number of hybrid and Q. robur trees (four populations); 8. Predominantly Q. petraea with a small number of hybrid trees (six populations). It is argued that the level of hybridity (13.3%) observed is not substantially different from earlier surveys of the two species in England and Wales. The pattern of hybridisation suggests relatively ancient hybridisation although some populations showed evidence of more recent crossing. There was also evidence that some hybrids had been planted along with the parental species. Mixed populations predominated in the survey possibly owing to the inclusion of obviously planted stands. In some mixed populations, there was evidence of a mosaic pattern with Q. petraea on the drier areas.