Elm, Ulmus spp.
A very early pollen source, perhaps in March.
I have seen American Elm and Slippery Elm in Iowa, and I only know them from the seeds when they fall in April, which means that most of us need to identify them after their flowers have opened (March) and the leaves are out, then remember their location for the next year. Slippery Elm seeds are rounder without a slit; American Elm seeds are elongated with a slit.
The leaves are very textured and the two trees are different from each other, but generally the trees are so tall that I do not see the leaves well enough to know the difference. Overall, the leaves are oval and come to a point, with serrated edges.