That! Is the question! I would hazard a guess that the percentage of the world’s trees that have been pruned is probably less than one tenth of one percent. Is it really necessary? When is it needed? How much should be removed? What about timing?
I have long since lost count of the number of trees that I have pruned, but every tree has taught me something! Initial pruning when the tree is just starting out in life may be the single most important influence in the life of your tree. Developing a strong, well formed structure certainly needs to be done sooner rather than later. This is more properly described as “training” rather than “pruning.” As the tree matures, the ability to be able to correct defects the tree may have starts to diminish. Once a tree reaches maturity, it really is more of a mitigation game rather than eliminating or correcting a problem your tree may have.
“Tricking” the tree into “believing” that it has never been pruned is an art form!
That is the true test of skilled arboriculture. With the exception of fruit production, the last thing you want your patient to do is respond to the pruning by a flush of new growth. Excessive sprouting is hard on the tree as it eats up a tremendous amount of the stored starch reserves the tree relies on for its startup every spring.
Rather than rehash the correct “branch bark ridge” or “collar cut,” let it suffice to say that trees do not heal! When you make a pruning wound, that wound will be with the tree for the rest of its life. After a tree sustains an injury, it will attempt to chemically wall off, or isolate the injury, and throw a new layer of material over itself.
Removing dead branches is probably all your mature tree needs. A dead branch is an injury of sorts, and correct removal of dead branches (not “dead wooding”) will facilitate the isolation of decay, your tree’s worst enemy! All trees respond to correct pruning in pretty much the same way, just at different rates, and degrees of ability to compartmentalize decay.
In a nutshell: Deciduous hardwoods may be pruned any time of the year; conifers never during the hot summer months. Fruit trees just before bud break in spring, and mid to late summer. Try to keep your cuts small and off the main stem, when possible.
Good luck, and have fun!