The term "northern hardwoods" was used in the early 1900's to separate the hardwoods of the northern region from those growing in the South. With continued usage in the North the term now represents all dense hardwood species both in the Lake States and Northeast. Unfortunately, this has complicated describing and applying silvicultural practices for good northern hardwood management.
Godman, Richard M. 1992. What are northern hardwoods?. In: Hutchinson, Jay G., ed. Northern hardwood notes. St. Paul, MN.: U.S. Dept. of Agriculture, Forest Service, North Central Forest Experiment Station. 1.01