Here is a picture of Box Elder leaves. The bark is the dead giveaway even before you see the leaves.

http://www.cnr.vt.edu/DENDRO/DENDROL...sheet.cfm?ID=3

I have all the trees here Reds.Black and Sugars plus the Silvers and Box Elder's. The Blacks hold their buds longer then the Reds do and since they are all mixed toghether I end up haveing trees beside each other that are tapped and not tapped.