Even if your horse has never been inclined to chew on fences and trees, keep an eye on him this winter. Studies have shown that horses are more likely to gnaw on wood during wet, cold weather, perhaps because of an instinctive urge for more roughage as temperatures fall.
If you discover that your horse has begun this destructive habit, ask your veterinarian to perform a complete workup to rule out physical causes such as a nutritional deficit. If she finds nothing amiss, you can treat wood chewing as a behavioral issue and take some steps to discourage it:
Provide more long-stem forage. This is the easiest and most effective method of stopping wood chewing. In addition, consider using a slow feeder, which will help reduce the potential for boredom by making hay meals last longer.
Eliminate access to the wood source. Of course you can't replace your fences or cut down your trees, but you might be able to cover them with PVC. Stringing an electric "hot" wire just to the inside of the fence line will keep your horse away as well.
Make the wood distasteful. Treat the surface of the wood with an unappetizing substance. Many commercial formulas are available; just be sure to follow instructions.
Step up your horse's exercise program. Regular activity, whether part of a training program or casual trail rides, provides an outlet for excess energy that might otherwise go to chewing.