Since purchasing our Littleton, Colorado, farm in 1990, we have observed significant damage to our Siberian elms by resident fox squirrels. Since these "weed trees" pop up frequently and grow rapidly, the squirrel activity has generally been tolerated. However, upon returning to the farm this month, I found that the squirrels had inflicted the same bark damage on our lone apricot tree.
While theories abound regarding this destructive behavior (from the agitation of late pregnancy to teeth maintenance activity), most experts note that bark stripping is especially common in late winter or early spring when other food sources are scarce; once the outer bark is removed, the squirrel has access to the nutritious cambium. In addition, bark strips may be used to build or repair nests before the newborns arrive.
It may seem counterintuitive that these mammals, so dependent on trees, would inflict damage on plants that provide food, safety and nesting sites. Of course, this behavior developed long before humans established parks, suburbs and orchards and healthy forests rely on natural enemies (fire, insects, cavity makers and other arboreal residents) to remain healthy. Unfortunately, what is surely a natural process in forests is an unsightly and destructive behavior in the eyes of homeowners and human arborists.