I suspect a white ash tree. Due to mild winters and overplanting of male ash trees there is an eriophyid mite causing an ash staminate flower fall. They are abnormal growths of the ash flower plant tissue. The mites are feeding on tiny flowers. The mites spend the winter under the flower buds and begin feeding and laying eggs in the developing flowers in spring. The ash tree, in response to the mites feeding, grows new malformed plant tissue (gall) around the mite. The galls in turn provide some protection for the mite against weather, predators and parasites. Research has shown that this particular gall does not harm the tree’s health or growth rate. The galls are usually an aesthetic problem and chemical control is not necessary and rarely works. Perhaps a cooler winter will help limit the future damage, but the best answer is to diversify the species of trees planted. Our ash trees are under attack by many problems. The gall problem is not particularly harmful to the tree.