Final answer:
The likely cause of the observed structures on oak seedlings in northern Turkey is wasp galls, possibly Andricus genus.
Step-by-step explanation:
The likely cause of the observed structures on oak seedlings in northern Turkey is wasp galls, possibly Andricus genus. Wasp galls are abnormal growths on plants that are caused by wasps laying eggs inside the plant tissue. As the eggs develop, they stimulate the plant to form these structures to protect and provide nutrients to the developing wasp larvae. Wasp galls can vary in appearance depending on the species of wasp and the plant they are infesting.