This tree is native in Southern England but elsewhere has been planted or escaped from parsk and gardens. The tree on which these catkins were photographed on the banks of the River Ouse might have been planted hundreds of years ago to stabilise the banks of the river but there are also trees nearby in Bishopthorpe Palace grounds from which it could have seeded itself - impossible to tell really. It has a silvery bark not unlike Beech and a similar habit too but the flowers are quite different.