Of course I didn't mean to suggest that Oklahoma has the tree coverage that Alabama does on the whole, but in the case of the 1999 Moore/OKC tornado, I remember plenty of footage that showed a considerable number of trees. Any city is going to have a substantial number of trees which may not be captured well on a national map. Trees definitely factor in, but I think OKC having longer time to realize that a monster was bearing down (among other factors) surely helped.