My guess is that Neahawg's theory is correct. I would bet that happened on the southwest side of the tree.................that's where it always happens.
There is a chance it will heal unless the bark separated all the way around the tree. Usually, that doesn't happen, but I have seen it happen. Then you have a serious problem.
Take a sharp knife and trim the damaged bark away around the edge of the split until you just barely expose green bark at a point where the bark is still attached to the trunk. It's important that the bark is still attached at the point you stop trimming. Don't go any further than you need to, but be sure to trim enough to get down to that point.
Unless the problem is a severe one, It should heal over given enough time - but could take a couple of years. While you are waiting for it to heal, stand something up to block the sun during winter on the southwest side. That will keep it from happening again. The more mature the tree becomes the less likely it is that this will happen, but I saw it happen once to a 8 year old semi dwarf apple tree. The paint that Neahawg mentioned (or blocking the sun as I mentioned above) will prevent it from happening in the first place on your other trees out in the open..
