These tiny galls covered all the hackberry leaves one year (and probably in other years, though I forgot to check). They may look unsightly, but in a place like the FWG, that doesn't really matter, as the tree itself is not harmed. Furthermore, the larvae inside the galls are eaten by chickadees, and no doubt, other birds too. The galls are likely made by a small bug (Hemiptera)in the Psyllidae family (jumping plant lice), possibly by Pachypsylla celtidisvesicula.

This image shows the topside of the leaf (L), and the underside (R).