The bark of a maple tree is its primary protective layer, shielding the delicate living tissues underneath from injury, pests, and disease. This outer layer, composed of dead cells, safeguards the cambium, the thin layer responsible for outward growth and the transport of nutrients and water. When bark peels off a mature maple, it signals significant stress or severe damage, not normal growth. Peeling exposes the cambium and inner wood, leaving the tree vulnerable and interrupting the flow of resources. Understanding the specific cause of this bark loss is the first step in determining the tree’s prognosis and necessary intervention.
Physical Damage and Environmental Stress
Maple bark loss often results from external physical injury or harsh weather conditions. Mechanical injury is a frequent culprit, commonly caused by gardening tools like string trimmers or lawnmowers used too close to the trunk. These tools can easily nick or girdle the base of the tree, destroying the cambium layer in a specific area. This damage interrupts the flow of nutrients, causing the bark above the injury to eventually die and slough off.
Construction activities or accidental impacts from vehicles can create large wounds that expose the inner wood to the elements and pathogens. Maple trees are also susceptible to environmental stresses that lead to bark deterioration, particularly sunscald. Sunscald occurs primarily on the south or southwest side of the trunk, often during the late winter or early spring.
On sunny winter days, the dark bark absorbs heat, causing the underlying cells to warm and become active. When the sun sets or is blocked, the temperature drops rapidly, killing those active cells. This localized damage results in patches of dead, cracked bark that peel away in sheets.
Another weather-related issue is the formation of frost cracks, which are severe vertical splits in the trunk that often appear on the south side. These cracks develop during rapid temperature fluctuations when the outer wood contracts faster than the inner wood.
As the tree attempts to heal, it forms ridges of callus tissue around the crack. However, the repeated splitting can lead to bark detachment and deep wounds that become entry points for decay organisms. Protecting young or newly transplanted maples with tree guards or wrapping the trunk can help mitigate both mechanical and sunscald damage.
Pests and Pathogens
Biological agents actively target the living tissues beneath the maple’s bark, leading to widespread bark death and detachment. Fungal pathogens, which cause canker diseases, are a common source of this problem, often exploiting trees already weakened by drought, wounds, or environmental stress.
Fungal Cankers
Nectria canker invades through wounds, causing sunken, discolored patches on the bark that eventually crack and peel. These fungi sometimes produce distinct yellow to orange-red fruiting bodies on the dead bark, which is a key sign of infection. Another fungal issue is Botryosphaeria canker, which is worsened by drought conditions and kills the sapwood, cutting off water flow to the upper canopy. This infection causes branches to die back and often results in patches of dead bark on the main trunk. In both types of canker, the fungi actively kill the cambium layer, preventing the tree from healing the wound and causing the surrounding bark to shed.
Wood-Boring Insects
Wood-boring insects are highly destructive pests whose larval stage feeds directly on the cambium and phloem layers, effectively girdling the tree beneath the bark. The sugar maple borer (Glycobius speciosus), a type of long-horned beetle, is a native pest. Its larvae create meandering tunnels that disrupt the flow of sap, causing sections of bark to die and fall off. Signs of borer activity include exit holes in the bark and the presence of frass—a coarse, sawdust-like material—expelled from cracks or tunnels. The extensive tunneling can severely weaken the trunk, potentially causing it to snap at the site of infestation.
Sapsucker birds also create distinctive damage patterns that can lead to minor bark peeling. These birds drill small, organized rows of shallow holes into the bark to feed on the sap and insects attracted to it. While this usually does not threaten the tree’s life, the repeated wounding can cause the bark to crack and peel in the affected area, providing an entry point for other pathogens.
Assessing the Damage and Next Steps
When bark loss is observed, the first step involves a careful assessment of the wound’s depth and extent around the trunk. The tree’s chances of survival are directly related to the amount of functional, living cambium remaining. A general guideline suggests that if the bark loss completely encircles more than 50% of the tree’s circumference, the tree is severely threatened because the flow of sugars from the leaves to the roots is interrupted. If the damage is limited to 25% or less, the tree has a much better chance of compartmentalizing the wound and healing itself successfully over time.
Diagnosing the Cause
Examine the exposed wood for signs that can help diagnose the underlying issue. Look for evidence of insect frass or distinct, deep larval galleries etched into the sapwood, which points toward borer activity. Check for the presence of fungal fruiting bodies, which may appear as small, colorful bumps on the dead bark tissue, confirming a canker disease. If the exposed wood appears wet, discolored, or soft, it may indicate a secondary wood decay organism has invaded the site.
Supporting Recovery
Immediate care focuses on preventing further stress and supporting the tree’s natural defense mechanisms. Ensure the maple receives adequate deep watering, especially during dry periods, and apply a layer of mulch around the base to conserve soil moisture. While removing loose, detached bark can sometimes help the tree compartmentalize the wound, do not attempt to cut into or clean up the wound aggressively. Furthermore, never apply wound paint or tar to the exposed wood, as these practices can trap moisture and promote decay.
If the damage is extensive, caused by aggressive borers, or if the tree is leaning toward a structure, consulting a certified arborist is the most prudent next step. An arborist can provide an accurate diagnosis, assess the structural integrity, and determine if the tree requires professional treatment or removal for safety reasons.