ell, you're partly right... the prairies are full of wide open spaces. However there are many kinds of trees that can be found in and around a prairie field. These leaves are from a box elder tree, which is a member of the maple family.

Pictured to the left are leaves of a silver maple tree. Its seeds fall in late spring and are fun to watch as they float in the air in spiraling motion. 

Shown to the right are flowers from a red bud tree. This small tree is actually in the pea family! The fruit is shaped like a pea pod, but is flat.