Tree bark does not regenerate like human skin; instead, trees use a sophisticated method for isolating and sealing injured areas. When wounded, the damaged tissue is not repaired or replaced. The tree actively walls off the injury to prevent the spread of decay, a defense mechanism developed over millions of years to ensure survival after physical trauma. Understanding this sealing process is fundamental to providing proper care for an injured tree.
The Biological Function of Tree Bark
The structure of tree bark serves as a protective shield for the living tissues beneath. The outermost layer consists of dead cork cells that insulate the tree against temperature extremes, prevent moisture loss, and defend against pests and physical damage. This tough exterior is constantly renewed from the inside, but once the outer bark is scraped away, it is permanently lost in that location.
Beneath the outer layer lies the inner bark, known as the phloem, a network of living cells. The phloem’s primary function is to transport sugars and nutrients, produced during photosynthesis, down to the trunk and roots. If a wound penetrates the phloem layer all the way around the trunk—a condition called girdling—the tree’s vital pipeline is severed, which can be lethal.
The vascular cambium layer, located directly beneath the phloem, is the zone responsible for all secondary growth. This single layer of cells divides to produce new wood (xylem) toward the inside and new phloem toward the outside, increasing the tree’s girth annually. Damage that reaches and destroys the cambium prevents the tree from creating new wood and bark in that specific area, impacting its future growth and structural integrity.
Compartmentalization: How Trees Seal Wounds
Trees respond to injury through a process called sealing, known scientifically as Compartmentalization of Decay in Trees (CODIT). First described by plant pathologist Dr. Alex Shigo, CODIT is a defensive strategy to limit the spread of decay and infection caused by fungi and bacteria entering the wound. The tree forms chemical and physical barriers, often visualized as four walls, to isolate the injured wood.
The first three walls are internal barriers that attempt to contain the decay within the wood existing at the time of injury. Wall 1 is the weakest, formed by plugging the water-conducting tissues (xylem) above and below the wound to slow vertical decay spread. Walls 2 and 3 involve chemical changes and new cell formation to resist the inward and lateral spread of decay organisms.
The most effective barrier is Wall 4, also called the barrier zone, formed by the cambium layer generating a new layer of chemically-defended wood after the injury. This new growth seals off the decay from spreading into subsequently formed, healthy wood. This mechanism results in the formation of callus wood, which is new growth that rolls over the edges of the wound. The callus tissue progressively closes the wound over time, but the original damaged area remains walled off inside the tree.
Caring for a Tree with Damaged Bark
A tree’s ability to successfully compartmentalize damage depends heavily on its overall health and the severity of the wound. The most immediate concern is girdling, which occurs when a wound completely encircles the trunk. If the bark and cambium are damaged around more than 25% of the tree’s circumference, the prognosis is serious, and damage exceeding 50% often results in death.
For fresh wounds, the only recommended intervention is to carefully clean and trim away any loose or jagged bark with a sharp knife. This trimming should create a smooth, elongated oval shape around the wound, encouraging the cambium to form an even, rapid margin of callus tissue. The goal is to avoid leaving ragged edges that could slow the sealing process or trap moisture.
It is important to resist the urge to apply any type of wound dressing, tree paint, or sealing compound to the exposed area. Research indicates these substances do not help the tree and can actually hinder the natural compartmentalization process by trapping moisture and creating a favorable environment for decay. The tree’s best defense is its own biology, supported by keeping the tree vigorous through proper watering and nutrition.