Lost the second tree last night. This morning I took a stroll around the property (gun in hand) and heard the teeth clacking (Black Bear) again. Talked to a couple of neighbors on the hill behind me and they've had bear sightings in the last couple of days.
I'm not sure why the trees have been attacked, but I've put a fence around the last one and will fence the new ones (those replacing the ones trashed) when I get them.
Steve, our local UPS driver had the answer. Since he had the same problem and actually saw it happen, I'm taking his answer as gospel. Apparently it's rutting season and the bucks are using my wife'a Aspen as a rubbing post. Being such a little tree, all they are doing is trashing the it. I've put a cage around the other tree in hopes it will survive.