After 6 years some of the trees are outgrowing the spiral tree protectors. I took some of the spiral protectors off last fall from the largest trees and most of the rest from the 2009 trees this spring. Unfortunately, as a result, one of the last remaining sugar maple saplings was completely girdled by the field mice over winter. But if I don't take them off I am worried that the trees will be choked by the tree protectors themselves. On some of the trees, as in the picture above, the plastic spirals were starting to constrict the growth of the trunk.
Sometimes the tree protectors fail to fully protect the trunk yet the tree is lucky because of some small factor like the stick in the picture below protecting a portion of the trunk, other times they are lucky one year but not the next. Sometimes they will come back and sprout up from below where they were girdled. In that situation the new bark of the sprouts is very attractive to the field mice the next winter. Occasionally the tree is super lucky and the field mice spare one or two just because they can't reach all sides of the sprout.
In the end I'm not sure there is much that can be done for some species of trees except not plant them in a location that is attractive to meadow voles (field mice). It occurred to me today that one might have more success planting sugar maples in a raspberry bush than in an old field.