When tree limbs are cut at the proper spot on the tree limb then the tree will respond by compartmentilizing behind the wound and will help the tree heal and will stop the spread of decay at the site of the wound . The tree then builds walls behind the wounded area and stops the spread of decay. Sort of like when a dying branch falls off and you see the little round swelling behind the branch in what is called the branch bark ridge. This area closes off the wound to stop the spread of decay throughout the the tree.
This is why you do not want to ever top trees. Not only is this bad for the tree but it cuts off that part of the tree that helps the tree heal. Even in tree's like Crape Myrtles This type of pruning practice is discouraged. The best possible way to cut or prune the tree properly is to hire a tree service like Beechnut tree service to properly prune your tree, so it will heal over like it should and stop the spread of decay.
If you can't make a proper cut on a tree limb or the limb has a cut from a long time ago then i would suggest calling Beechnut tree service out to your home to evaluate the limb or limbs that have been cut improperly to see if they can be repaired or simply need to be removed. Sometimes the wound was made through nature , possibly lightning or animals . In that case there is not much you can do. Or when the tree has bad structure, then a more drastic approach must be taken , but these are rare cases and should be taken as a last resort. In Closing i must tell you to please don't let anyone work on your tree's that is not a Certified Arborist . Once you start trimming or having a tree trimmed it must be done properly or the results can end up in disaster. Call us Beechnut tree Service for your free Estimate and make your trees the talk of the neighborhood.