Copini, P.; Buiteveld, J.; Ouden, J. den ; Sass-Klaassen, U (2005).
CGN Report 1, Centre for Genetic Resources(CGN), Wageningen, the Netherlands 45 pp.
This report is about oak clusters, which are found at many places in the Netherlands. It was thought that these oak clusters (up to 12m in diameter) are genetically identical and originated by continuous coppicing over many ages. In this report three main questions were studied: (1) are oak clusters genetically identical, (2) can leaf morphology be used to identify clonal structures of oak and (3) what is the origin of oak clusters.
The study sites were situated in a drift sand area (Maanschoten) as well as in an area with pre-glacial material (Wilde Kamp). In both areas microsatellite analyses were used to describe clones of Quercus petraea and Q. robur with sizes up to 12.4m in diameter. Some of the studied clusters were genetically identical, while others contain a mixture of genotypes. Leaf morphology analysis show that leaves of stems belonging to different genotypes are sometimes comparable in morphology. Thus leaf morphology analysis can not be used to identify clonal structures.
In the drift sand area soil analyses showed that these clusters are overblown. In literature drift sand areas as Maanschoten are described as areas containing oak shrubs. Besides that, it is mentioned these oak shrubs produce new roots when the twigs are overblown (layering). Probably the harsh environment in these drift sand caused the oak grew as shrubs. It is likely that when the drift sand areas were afforestated the oak shrubs became the oak clusters as they can be found nowadays.
In historical documents the Wilde Kamp is described as heath land containing oak shrubs. Also the many horizontally growing stems indicate a shrub-like past and maybe an origin in which layering is an important factor. It is likely that when sheep herding became less interesting the oak shrubs became the oak clusters as they can be found nowadays. Since in both areas no indications are found that clonal growth of oak clusters occurred because of coppicing, an alternative hypothesis is formulated: clonal growth of oak could occur because of the shrub-like growth form with many horizontally growing twigs, in combination with the ability of oak to make natural layerings of horizontally growing or overblown twigs