Trees need their leaves in order to produce energy through photosynthesis. In order to produce energy most efficiently, they make intelligent decisions about where to place their leaves.
Sometimes, a person believes that a tree’s branch is in an unintelligent place, and responds by cutting the branch off, a practice generally known as pruning. This leads to three negative effects.
First, the tree loses some of its energy-producing capacity, which negatively affects its ability to support itself and remain healthy.
Second, a tree normally responds to the sudden loss of a branch that was productively photosynthesizing by growing a new branch in the same place. This allows the tree to continue taking advantage of the patch of sunlight it had found. However, the regrowth typically leads to a recurrence of whatever perceived problem led the person to cut off the branch in the first place. In addition, these secondary branches are always weaker than the original branch they are replacing. Thus, pruning tends to lead to a reappearance of the original problem, plus additional problems.
Finally, pruning – especially of large branches – leaves trees with open wounds that take a long time to heal. This makes the tree vulnerable to infection, creating even more opportunity for new problems that harm and weaken the tree.
Pruning rarely solves any problems with trees, and often causes additional problems. Training is a more effective way of dealing with branches in undesirable places – as is understanding that trees have spent millions of years learning how to grow, and our perception of what is an undesirable location for a branch may simply be misguided.