While splitting some logs that we cut from dead trees, we found one with an inner portion darker than its outer portion, as if the tree had been burned. What has happened?—Marj Barnden, Go Home Lake, Ont.
The tree wasn’t burned. The darkened inner section of wood was most likely caused by a pathogen that got into the tree through an injury—probably a broken or badly pruned branch.
When a tree is wounded, decay organisms can get in through the opening and attack, explains John McLaughlin of the MNR’s Ontario Forest Research Institute. The tree responds by producing barriers around the wounded area in an effort to “wall off” the tissue associated with the injury—in this case, the layer of wood connected to the branch that was lost—so that future layers aren’t affected. “It’s sacrificing its old wood to keep its new wood protected,” says McLaughlin.
This scientific theory of how a tree responds to injury and keeps decay from spreading is called “compartmentalization of decay in trees,” or codit, says Ken Farr, a dendrologist (tree specialist) with Natural Resources Canada’s Canadian Forest Service. It’s a normal, natural defence mechanism that the tree has evolved, because it has no way of repairing or regenerating cells to heal an injury. “A tree can’t grow new tissue in the same way that animals can,” says Farr, “so it seals that part off.”