In article , Archimedes Plutonium
writes

Anyway getting back to the issue of test of strength of woods, the white
ash I have is heavier and denser than the hickory and both are reported to
be stronger than oak. So England had no hickories for trebuchets but they
surely had ash trees. Whether they had both White and Green Ash, I do not
know. But if Ash is nearly equivalent to hickory in terms of strength, then
why did they build the trebuchets out of oak when ash was available?

England has neither white nor green ash; both are American species.
Rather than speculating you could find a web site which lists the trees
native to Britain (there's appreciably less than 100). You might also
note that there's very few (perhaps ever zero) species of tree native to
both Britain and North America.
--
Stewart Robert Hinsley