Backyard Woods - Which trees should I prune?

Prune trees that pose a safety hazard, threaten to damage property, pose a fire hazard, and will have improved health as a result of pruning. Your objectives for your backyard woods will determine whether you also prune trees that will improve the aesthetic appearance of your woods or that will have increased value for wood products as a result of pruning.
For safety, focus on trees that are in your immediate backyard, and trees that are in high-use areas of your backyard woods, such as near a bench, picnic table, fire pit, or trail. To reduce the risk of property damage, focus on trees that could fall on a vehicle, building, or other structure. Examine trees once a year and after severe storms, being sure to check all parts and sides. Remove any broken branches lodged in the tree crown. Look for and prune branches with the following:
• Dead wood
• A crack that extends through the bark and into the wood
• A weak V-shaped union with the stem or another branch
• Decay—wood that is soft, or crumbly, or a cavity where wood is missing
• A canker—a localized area of sunken or missing bark

Trees that pose a fire hazard
Focus on trees in a safety zone around your home. This zone should be at least 30 feet wide on level or gently sloping ground, and 100 feet or more on slope grades 30 percent or greater down hill from all structures. Outside the safety zone, prune branches near power lines and outbuildings, low hanging limbs, and dead branches.
Trees with health problems
Pruning trees with diseased and insect-infested branches may help alleviate the problem. For example, prune infected lower branches of white pines to reduce damage due to white pine blister rust.
Trees with aesthetic value
If having an aesthetically pleasing woods is one of your objectives, evaluate and prune trees that block your view and thin crowns of broadleaf trees to increase flower production.
Trees with wood products value
If your objectives include producing high-value timber or veneer, prune preferred trees that have good local markets for these products. Pruning is a long-term investment. Even under good conditions, it takes 20 years to add a 5-inch layer of clear wood on a tree.
How big should a tree be when I start?
Begin pruning broadleaf trees, such as oak, maple, and hickory for strong structure shortly after they are planted. Retain branches with strong U-shaped attachments.
Remove branches with narrow V-shaped attachments because they are weak and could fail when the tree matures. As two branches with narrow V-shaped angles of attachment grow, they produce a wedge of inward-rolled bark between them. This included bark prevents strong branch attachment, which often causes a crack at the point below where the branches meet. Removal of one branch will prevent a potential failure of the branch attachment when they are much larger.

Needleleaf trees, such as pine, spruce, and fir that have branches in whorls around the trunk, rarely need structural thinning except to restore a dominant leader. This becomes necessary occasionally, when the leader—branch at the top of the tree—is damaged and multiple branches form at the top of the tree. Select the strongest branch from among them and remove the competing branches to prevent the development of a bushy or forked tree.
Producing strong structure or value or both is the emphasis when pruning young trees. Pruning goals shift to maintaining safety and tree health as trees mature.

Begin pruning for fire protection when trees are 3 to 5 inches in diameter at breast height (d.b.h.). (Breast height is about 4½ feet above the ground.) Through periodic pruning remove all branches to a height of 6 to 10 feet.
Begin pruning needleleaf trees for aesthetics when they are 3 to 5 inches d.b.h. Through periodic pruning remove all branches until the desired view is obtained. Flowering broadleaf trees can be pruned at any age. Crowns should be thinned to increase sunlight penetration and air movement throughout the crown. Avoid unnecessary stress to the tree by removing no more than one-quarter of the living crown.

Begin pruning for value in broadleaf trees such as walnut, oaks, and hickory when they reach 2 to 8 d.b.h. Begin pruning needleleaf trees when they reach 3-5 inches in d.b.h. The sooner you start, the greater the amount of high quality knot-free wood is produced. Usually, it is uneconomical to prune trees larger than 10 inches in diameter. Continue pruning all live and dead lower branches until the trunk is free of branches to a height of 17-18.5 feet. Be sure to leave 50 percent of the total tree height in live branches to maintain healthy tree growth. Following these guidelines and reaching the desired branch-free height will require pruning the tree more than once.