Elms are the classic yellow morel tree, but there aren't that many of them around anymore. I find plenty on Cottonwood, Soft Maple, Ash, and Sycamore in the bottoms. My early spot is bunch of beat up elms growing next to a large asphalt parking lot. It gets full sun all day, and warms up early. Have picked a bunch from big dead & dying cottonwoods over the years. They will grow in a 30-50' circle around the tree, so keep your eyes open.