Question
I have 3 Sweet Gum trees that live along a chain link fence in North Georgia.  I really enjoy the shade & privacy they provide in the summer months. The problem I see is the tree trunks are going to start hitting the fence which will eventually cause damage.  Right now the trees are about 25-30Ft high and are about the ideal size.

Is there a way to prune these trees so that the diameter of the trunk does get any larger?  

Answer
I have not seen any method to "prune" the base or side trunk off of a mature tree.  (By the way, your 30-footer is just a baby; it will eventually get to be 80-100 feet tall, and the trunk will get another 2-1/2 times thicker.)

This situation happens frequently in commercial plantings when the tree is not planted in the center of a planter hole and covered with a tree-grate, with an offset trunk - or the tree hole is too small.  It starts to rub, and eventually grow around/over the rim of the tree grate.  Also happens when the tree stakes or guy wires are left on and they are too close to the tree trunk.

Since the tree's "living" part of the trunk is just under the bark (the cambium layer), any damage to the trunk will result in the poor performance of the tree above.  If you completely remove the bark in a circle around the base of the tree, you effectively kill the tree. Removing the bark on one side is like giving the tree an amputation. That side will die back or worse...allow infections to start attacking the woody center of the trunk. Trying to "squeeze" the trunk with wraps or something won't work either.

But how to prevent this from happening with your fence?  Boy, I don't know.  Remove one or the other.  Maybe its time to "re do" the fence, and this time set it in back or in front of the tree's trunk by about a foot.  Moving a 30-foot tree is risky.  It can be done, but it抯 a lot more expensive than a chain link fence.  I've seen some very attractive fences with "jogs" in them to accommodate obstacles like trees or big boulders.  My guess is, however, that they were planted right on the property line, and that isn't an option for you.  

If containing kids or small pets is not needed, then maybe consider a split-rail fence replacement.  As the tree grows to meet the fence, you remove the rails in the future to accommodate the trunk's lateral increase.  Just set the posts closer together.  Posts are usually 8 feet apart, so skooch then in to about 5 feet, with the trunks centered between these posts.  Then, removing them later won't look quite so awkward.  Also, planting climbing roses or large shrubs along this future split-rail fence (or no fence!) can afford you a similar sense of "barrier" that the chain link might be giving you, from the neighbors.

You can also just leave it. The tree will eventually grow "through" the chain links, but when it comes time to remove the fence of the tree in, say, 50-100 years, I don't want to be the guy on the chainsaw!  Plus, it will weaken the tree.  When I rode out my last hurricane (2004), I remember seeing a tree that had fallen over, and inside the break was an old, rusted, metal pipe, used as a tree stake right at the break point.  It was left on, and the tree grew around it, dooming the tree to break right there.

Sorry.  I wish I had a magic tree-moving wand! -Marc