Trees withstand a surprising amount of abuse in the name of pruning. Properly pruned, fertilized and watered, a tree may outlast a human lifetime, sometimes many lifetimes. Pruning involves the removal of branches detrimental to the tree or to its appearance in a way that permits rapid healing of wounds.

Some of the pruning can be done by the homeowner. But to select many branches for removal and retain a healthy, esthetic tree requires knowledge of the tree's growth habits.

For example, when removing a branch from the tree, the practice has been to make the cut flush with the trunk and then paint the wound. Research by Dr. Alex L. Shigo, chief scientist, USDA Forestry Sciences Laboratory, Durham, N.H., has shown that it may cause serious injury to the tree.

"Underneath the branch where it enters the trunk is a collar which contains a chemical zone that inhibits the spread of decay into the trunk," he says. "When decay develops in a branch, it moves downward until it reaches the protective zone. It stops there, and after the branch falls off, new wood forms a callus.

"If the branch is pruned so that the collar is cut off or injured, the chances of infection by decay and canker-causing microorganisms are greatly increased. In addition, it sometimes causes internal cracks that weaken the tree.

"When making the cut, place the saw or shears just outside the bark ridge in the branch crotch. Cut from the outer portion of the bark ridge down and slightly out so as not to injure or remove the collar.

"Do not paint except for cosmetic reasons and then use a very thin coat.

"If too many branches are removed, then the tree will not have enough leaves to produce the energy it requires to maintain itself, and enough energy to use for protection after injury and infection.

"Energy or food for the tree comes from the sun. The tree traps the sun's energy in a molecule of sugar. The trapping process brings together carbon dioxide and water. The food of the tree is sugar, and the food is required to run all of the tree's processes.

"It always disturbs me when we call fertilizer tree food. Fertilizer does supply needed minerals. These minerals are used as building blocks for the plant. But the fertilizer is not food. The plant cannot burn fertilizer as it does sugar.

"Trees are generating systems that are bound by their genetic program to continue to produce new cells in new locations or positions every growing season.

"The tree survives as long as it has enough energy to recognize and wall off the injured and infected tissues, and to still have enough energy to generate new tissues that will continue to maintain the tree.

"I believe the tree dies because it runs out of energy. Our recent studies show that trees that are dying have no reserved energy in their wood. We can test this easily by making a simple test for starch in the wood. As starch depletes, the tree begins to wane and die.

"Take the Dutch elm disease, for example. The elm trees can wall of the infecting fungus very effectively when they have a full amount of energy. As the tree walls off the infected wood, it also walls off wood that would normally be used for storage of energy reserves -- starch. There comes a time when the tree requires a great amount of energy to continue to wall off the fungus. When that energy is not there, the fungus then moves to the cambium. Trees die when the cambium is killed. The cambium is the generator."