After several hard frost in a row with Temps in the 20s these leaves on the trees (especially the Maple trees)  just aren't dropping. They seem to be turning a shade of brown instead of yellow. I hope they are not freeze drying on the trees. That happened several  Years ago. Some remained on the trees till spring and some fell during  the winter, creating a mess that had to be raked up in winter. There's nothing like having shade from a tree in the middle of January with Temps in the teens.