Turkey oak (Quercus cerris)
Buddleia

Turkey oak comes from the Balkans and Western Asia. It was introduced in 1735 for timber, but this was not a success because the wood warps and splits badly. However this is a handsome ornamental tree, often planted in parks and gardens. It is naturalized in southern Britain and continuing to spread.

The two natives, pedunculate (Quercus robur) and sessile (Q petraea) oak are the two most valuable British trees in terms of insect species supported. Turkey oak grows faster, more regularly produces large crops of acorns, and also hybridizes with them. It is a particular threat to the pedunculate oak because it is a host of the knopper gall wasp Andricus quercuscalicis, another alien species first recorded in Britain in 1956 and also spreading widely.

The wasp has a complicated life-cycle with two generations per year. The first eggs are laid on Turkey oak roots, and develop into small wasps. In June, these lay their own eggs in the flowers of the pedunculate oak. A gall grows around the developing acorn to provide a home for the next generation of wasp pupae, but in the process the acorn is destroyed, or damaged badly enough to prevent it germinating. Turkey oaks growing near pedunculate oak therefore reduce the regeneration potential of native oak woodland.

On the commons, some Turkey oaks have been felled, and the trunks used to mark out bays in the car parks and to prevent unauthorised vehicle access at some points (incidentally providing useful dead-wood habitat).