To the south of the Old Wood Yard, not far from the Austford coach house is this fine example of a twin-trunked birch covered in burrs (or burls as they call them in North America).
The causes are explained (sic) in this abstract from CABI:
"The numerous buds that cover the surface of the burr develop from meristematic foci formed in the phloem. The burr phloem is also characterized by the irregularity (waviness) of the elements of its conducting zone and the irregular thickness and distribution of the mechanical tissue. The wood of burrs is distinguished by its curly grain and numerous included bud traces. The rapid growth of burrs and their numerous surface buds are probably caused by local disturbance in the balance of growth substances."
So now you know!