Some authors consider Acer nigrum to be a subspecies of Acer saccharum rather than a separate species. The key characters of Acer nigrum used to separate the two species are the prominent stipules at the base of the petiole (these may fall off later in the growing season), pubescent petiole and undersurfaces of the leaves, and a tendency of fresh leaves to droop at the edges. The petiole bases also tend to be larger than in Acer saccharum, the sinus at the base of the leaf tends to be somewhat "closed" (the lobes overlapping), and the bark is reported to be darker, although this character is not obvious in the trees I have seen in Wisconsin. This is often not a clear taxon in Wisconsin, but some local populations matching the above criteria can be found. The photos above are from individuals found in Brown County.

The range of Acer nigrum in North America is similar to that of Acer saccharum, but it extends a little farther west than A. saccharum and not as far east and north. In Wisconsin it is most common in the southwestern counties, but there are reports from throughout the state. Botanists may contribute to our knowledge of this species by taking good vouchers to help clarify the variability and range.