Without question Dr. Alex Shigo was the best thing that ever happened to the tree service industry. He started as a plant pathologist. Later, with the US Forest Service, he headed up a long term project to study decay in trees.
Until the availability of modern chain saws, most work questioning what happened inside trees used pieces of the trunk cut crosswise. While this was helpful to a certain degree, what Dr Shigo did was to cut tree trunks longitudinally, fully revealing the workings of the interior of the tree. He then began a decades-long work, wounding test trees in many ways and then later dissecting the tree and learning the full extent of the tree’s reaction. Dr Shigo was the first to do forensic examination and the depth of his finding is plain to read in any of his published works.
Many of the experiments he performed were standard tree care practices at the time; most notable was the flush cut. A flush cut removes a powerful natural defense mechanism called the branch collar. He found that when a flush cut is made, decay was given the best opportunity to invade the trunk. Perhaps the most important idea Dr. Shigo produced was what he called CODIT: Compartmentalization Of Decay In Trees.
After retirement, Dr. Shigo published many books on his insights and discoveries, the most important being his New Tree Biology. He then traveled around the world, meeting curious students. He conducted 24 of his New Tree Biology Workshops. These were four-day classes. The mornings were in the lab and the afternoon was spent walking through trees and discussing with his students anything that caught their eyes.
Dr. Shigo was a very generous and good man. His vast intelligence and knowledge poured out of him like a bright light. No one meeting Dr Shigo could help being impressed; indeed, many were inspired.
See Shigo and Trees for more.