Tree Maintenance.

Pruning is the most common tree maintenance procedure. Although forest trees grow well with only nature’s pruning, landscape trees require a higher level of care to maintain their structural integrity and aesthetics. Pruning must be done with an understanding of tree biology because improper pruning can create lasting damage or shorten the tree’s life.
Reasons for Pruning;
 Because each cut has the potential to change the growth of the tree, no branch should be removed without a reason. Common reasons include: • Removing dead branches, improving form, and to reduce risk.
• To increase light and air penetration to the inside of the tree’s crown or to the landscape below.
• Generally, mature trees are pruned as corrective or preventive measures. Routine thinning does not necessarily improve the health of a tree. Trees produce a dense crown of leaves to manufacture the sugar used as energy for growth and development. Removal of foliage through pruning can reduce growth and stored energy reserves. Heavy pruning can be a significant health stress for the tree. When to Prune Most light, routine pruning to remove weak, dead, or diseased limbs can be accomplished at any time during the year with little effect on the tree.
 As a rule, growth and wound closure are maximized if pruning takes place before the spring growth flush. Some trees, such as maples and birches, tend to “bleed” if pruned early in the spring. It may be unsightly, but it is of little consequence to the tree.
Heavy pruning of live tissue just after the spring growth flush should be avoided, especially on weak trees. At that time, trees have just expended a great deal of energy to produce foliage and early shoot growth. Removal of a large percentage of foliage at that time can stress the tree. A few tree diseases, such as oak wilt, can be spread through pruning wounds and provide access to pathogens (diseasecausing agents). Susceptible trees should not be pruned during active transmission periods.
How Much Should Be Pruned? The amount of live tissue that should be removed depends on the tree’s size, species, age, and pruning objectives. Younger trees tolerate the removal of a higher percentage of living tissue better than mature trees. Generally, no more than 25% of the crown should be removed at once, and less for mature trees. Removal of a single, large-diameter limb can create a wound that may not be able to close. Care should be taken to meet pruning objectives.
Making Proper Pruning Cuts
A correct pruning cut removes the branch just outside of the collar. Do not make cuts flush to the trunk. Trunk tissues above and below a flush cut branch often die, creating dead spots. If a collar has grown out on a dead limb, make the cut just beyond the collar. Do not cut the collar. There are many outside considerations that make it necessary to prune trees such as: safety, clearance, and compatibility with other components of a landscape. Proper pruning, with an understanding of tree biology, can maintain good tree health and structure while enhancing the aesthetic and economic values of our landscapes.  

(treesaregood.com)