Since the introduction of Dutch elm disease to North America in the 1930s, American elms (Ulmus americana, Ulmaceae) have seen their populations decimated. Luckily some have escaped this threat, especially those like the one shown here that grow in floodplain forests (Kershner, Mathews, Nelson, and Spellenberg 2008).
There are a number of trees that resemble the American elm, but a combination of characteristics provide a positive identification. The leaves are simple, alternate, doubly-serrate, and conspicuously asymmetric at the base. The leaves have side veins that are forked less than in slippery elm (U. rubra) and they are spaced farther apart than in rock elm (U. thomasii). Also unlike these other trees, the samaras (fruit) are deeply notched, fringed with hairs, and hang in clusters on long stalks (Kershner et al. 2008):
I always assumed these trees were the more Dutch elm-resistant slippery elm or rock elm, so it’s encouraging to see American elms surviving in the wild.
Kershner, B., D. Mathews, G. Nelson, and R. Spellenberg. 2008. National Wildlife Federation Field Guide to Trees of North America. Sterling Publishing Company, Inc., New York, NY.