When humans get a cut, we replace the injured tissues with new cells. But when tree tissue is damaged, the tree can’t just swap them out with new cells. So what does it do?
In the 1970's, a forest pathologist named Alex Shigo helped answer that question. Using the newly invented one-person chainsaw, he cut trees into long vertical sections rather than the traditional horizontal cross cuts. In those long cuts, he could see the dynamics of how decay spreads in trees in new ways.
After dissecting over 10,000 trees, he concluded that tree health lies in “sealing, not healing.” Trees contain damaged and diseased areas by isolating the wound.
They first plug their vascular system above and below the damaged area. Then, they grow a physical barrier zone with “callus wood.” Ever so slowly, that wall grows up and over the wounded area, like a shirt collar, which eventually covers the original injury entirely.
Now, when arborists prune trees, they design their cuts to minimize the risk of decay and reduce the spread of disease.
It’s telling that the word “tree” comes from the Sanskrit word, "deru," which means to endure, and to be firm and strong. Thanks to Shigo’s tree studies — and the invention of the one-person chainsaw — we now know how trees can endure the wounds they sustain over their long, long lives.
