Never.
Sorry, not what you wanted to hear. Many of us have seen some pretty old trees chewed horribly. Exploring how technical we can get with the "girdled" term, it may be unlikely the older trees will be killed outright by a complete removal of a longitudinal length of cambium. A trunk with a 2 foot diameter is a lot to eat for a one ounce critter. Any damage is unacceptable, especially down low where it is likely to become infected with a pathogen. Guards are so easy to install, there should be no reason to take the chance.
On a related note, we are beginning to wrap our guards much higher due to rodent attack. A snowshoe hare completely debarked a few rows of 2" caliper trees in a few days, above the wire guards. Live and learn.
Walden Heights Nursery & OrchardZone 3 in Vermont
Edited 1 time(s). Last edit at 11/09/2013 03:27AM by Todd Parlo.