Hi, It's just a guess but it looks like a siberian elm (Ulmus siberiensis) to me. They are often called 'chinese elm' and are naturalized where I grew up in western Nebraska. I've seen them advertised as the "wall of china elm" for their supposed ability to form a quick hedge. Did you ever notice it producing seeds while it was still growing? If so what time of year and what did they look like? Siberian elms produce a winged seed, that is slightly oval over all, and produces them early in the spring before leafing out.
Ron