It is a common misconception that if a tree receives a minor wound it will heal itself over time, much like a minor cut or abrasion to an animal’s skin. This is not true. In fact, once a tree is wounded that wound endures for the entire remaining lifespan of the tree. But, you may ask, why are there plenty of wounded trees out there that appear to have healed over a once noticeable injury? This is true, Wounds are eventually covered over by an otherwise healthy tree.
However, the wound is still there and can actually be viewed if you took the time to peel back the layers of new growth.
Dr. George H. Hepting, working as a forest scientist and plant pathologist for the U.S. Forest Service, is credited with making the first recorded observations in 1935 of what is referred to as compartmentalization of decay in trees (CODIT). About 20 years later, Dr. Alex L. Shigo formalized the concept of CODIT in a beautifully illustrated U.S. Forest Service publication: Agriculture Information, Bulletin No. 405, July 1977 (https://www.nrs.fs.usda.gov/pubs/misc/ne_aib405.pdf).
The concept is fairly simple—trees wall off or compartmentalize a wound to protect remaining portions of the tree from further invasion by damaging agents, most notably decay fungi. Tree bark is somewhat analogous to human skin, both protect against harmful microscopic invaders. Wound the skin or wound the bark, however, and the breach becomes an open invitation for infection. Remove a portion of tree bark and the tree immediately mobilizes chemicals to the site in an effort to stem the infectious tide. A layer of cells surrounding the wound actually are filled with gums, resins, and chemicals that stop the spread of decay. The filled cells are no longer functional to the tree but they serve a vital role as a barrier to the invading organism. Once the wound is compartmen- talized, living tissue outside the wound forms a callus layer and given enough time, will cover over and close the wound. But the damage has been done and still remains.
Minor lesions and abrasions occur all the time to trees throughout their lives. Damaging storms, animals, insects, and unconscientious people wielding carving knives all take their toll.
Healthy trees can withstand such minor onslaughts because they have enough energy in reserve to compartmentalize the wounds. Major injuries, however, can easily overwhelm the tree defensive system and result in massive infections of decay fungi that eventually lead to tree death. Decomposition is a natural process and part of a healthy forest so long as it remains at a low level. All trees eventually die and through decomposition become part of a humus layer and eventually soil that serves to support the next generation of trees.
In Delaware we have a native insect called the Southern Pine Beetle (SPB), Dendroctonus frontalis (Coleoptera: Scolitidae). This tiny forest pest is about the size of a grain of rice but is a tree killer. Loblolly pine (Pinus taeda) is the favored host. SPBs take advantage of a pine tree’s defensive strategy by delivering what can be described as a “death by a thousand cuts.” Female beetles bore into a tree under the bark and lay eggs along niches in a gallery within the cambium/phloem portion of the tree. Larvae hatch and begin feeding on this critical tree tissue. But this action alone is not what ultimately causes tree death. A single beetle has no significant effect on the tree.
But the SPB, like many other scolitid bark beetles, inoculates the tree with decay fungi spores when it tunnels through the bark. Fungal spores are carried by the beetle in a structure called a mycangium located on the beetle’s pronotum (“neck”). It is the fungus that then elicits the compartmentalization response in the tree in a bid to wall off the inoculation site.
A small number of inoculations are no problem for a healthy tree and the SPB population remains at bay. However, SPBs utilize an aggregating pheromone—a chemical signal to its cohorts—to call in nearby beetles for a mass attack on the tree. With literally thousands of beetle entry sites in the bark and a corresponding number of fungal inoculations, a tree’s defensive reaction is overwhelmed. The tree weakens, the beetle broods multiply (up to four generations a year in Delaware), and the tree dies.
What happens next is what really concerns foresters. Once the SPB population takes hold in a handful of weakened trees, they have the potential to emerge in massive numbers and attack and kill neighboring healthy trees.
Fortunately, this kind of eruptive outbreak does not occur very often and staff from the Delaware Forest Service monitor SPB activity through an annual aerial survey of the entire state. Potential outbreaks are prevented by removing the epicenter weakened brood trees, and sometimes a buffer around the area to quell any further spread.
Ed. note: Occasional guest contributor Mike Valenti has been Director of Abbotts Mill Nature Center since this past March. He comes to us from the position of Delaware State Forester for the Delaware Forest Service.