I know that there is a strong link between oaks and the English God Thunor (Scandianian 'Thor') - apparently scholars think that this connection between the oak and thunder gods may have something to do with the frequency with which lightneing strikes them. On tree-lore in general its worth pointing out that Germanic heathens saw the worlds as being cradled in the branches of Yggdrasil, a vast ash (or possibly yew) tree.