Tree bark serves as a tree’s primary defense system, providing a physical barrier against external threats and regulating moisture loss. The inner layer of bark, known as the phloem, transports sugars and nutrients produced in the leaves down to the rest of the tree. Just beneath this transport tissue lies the vascular cambium, a thin layer of actively dividing cells that creates new wood and bark, making it the central engine for all healing and growth. Any injury that penetrates this cambium layer compromises the tree’s ability to seal the wound and maintain its circulatory system.
Assessing the Type and Severity of Bark Damage
The first step in treating an injured tree involves accurately assessing the depth and extent of the damage. Minor scrapes that only affect the rough outer bark are superficial and pose a lower risk. Major damage penetrates into the phloem and cambium layers, directly disrupting the flow of nutrients and moisture. You can determine if the cambium is affected by gently scratching the edge of the wound; a healthy, living cambium will show a thin, moist layer of green or white tissue.
The most severe type of injury is called girdling, where the bark and cambium are removed entirely around the circumference of the trunk. This circumferential damage severs the phloem pipeline, halting the transport of energy from the leaves to the roots. A fully girdled tree will eventually starve because its root system can no longer receive the necessary sugars to survive.
Immediate Wound Cleaning and Shaping
For common, non-girdling injuries, preparing the wound site correctly encourages the tree’s natural sealing process. Trees do not regenerate tissue like humans; instead, they seal off the damaged area by forming specialized growth called callus tissue around the wound edges. This process begins by removing all loose, ragged, or crushed bark from the perimeter of the injury.
Using a sharp, clean knife, carefully trace the wound into a smooth, elongated oval or football shape. The long axis of this shape should run vertically along the trunk, as callus tissue grows most quickly up and down the trunk. This smooth tracing, known as “wound tracing,” removes jagged edges that could hinder the uniform growth of the new protective tissue. Make shallow cuts, taking care not to expose more healthy cambium than necessary.
Modern arboriculture advises against applying commercial wound paints, tars, or sealants to the exposed wood. These materials have been proven to do more harm than good by trapping moisture and fungal spores against the wound site. Sealing the wound creates a damp environment that encourages decay and inhibits the tree’s defense mechanism, known as compartmentalization of decay in trees (CODIT). Leaving the cleaned and shaped wound exposed to the air allows the surface to dry, enabling the tree to naturally form its protective barrier and seal itself off from pathogens.
Specialized Repair for Girdled Trunks
When the trunk has been completely girdled, simple wound tracing is insufficient because the entire phloem pathway is severed. In this scenario, the specialized technique of bridge grafting may be necessary to save the tree. Bridge grafting uses several young, living branches, called scions, to physically bypass the damaged area and reconnect the cambium layers above and below the girdle. These scions are typically one-year-old growth, pencil-thick, and collected while dormant from a compatible tree species.
The scions are carefully inserted and secured beneath the healthy bark at the top and bottom edges of the wound, forming a living bridge over the girdle. When successful, the scions’ cambium layers fuse with the trunk’s cambium, re-establishing the flow of sugars to the root system. This procedure is complex, requiring precise cuts and timing, and is best performed in the early spring when the tree’s bark “slips” easily. Due to the high degree of skill required, a certified arborist should be consulted for bridge grafting.
If the girdling is complete and involves a large section of the trunk, or if the tree is a species with a low grafting success rate, survival may not be possible. The success of the graft depends entirely on the cambium layers making contact and fusing before the roots starve.
Long-Term Recovery and Preventive Measures
After the immediate repair, supporting the tree’s overall health maximizes its healing speed. A healthy tree allocates more energy toward compartmentalization and callus formation, allowing it to seal the wound faster. Ensure the tree receives adequate water, especially during dry periods, as drought stress significantly slows growth and repair processes. Avoid applying fertilizer directly near the wound, focusing instead on maintaining general tree vigor through proper cultural practices. Monitor the wound site for the growth of callus tissue, which appears as a thickened ridge rolling in from the edges.
To prevent future bark damage, implement simple physical barriers around the trunk. Mechanical damage from lawnmowers or string trimmers is a common cause of injury; placing a wide ring of mulch a few inches away from the trunk creates a protective buffer zone. For thin-barked, young trees, physical tree guards or wraps can prevent sun scald. Sun scald occurs when the bark rapidly heats up on sunny winter days and then freezes quickly at night, causing tissue death and cracking.