Greetings Terry,
Interesting question! Well, rabbit and vole damage during periods of high snow pack may explain the damage up to 3-5 feet, but certainly not to a height of 20 feet. Damage from beavers could also be a factor, but again, not to a height of 20 feet. Woodpeckers could also be a culprit.
Trail maintenance (park staff) by selectively girdling the generally considered undesirable honeylocust could also explain some "intentional" damage, but again, not to a height of 20 feet.