The bark of an oak tree is a complex, multi-layered defense system, acting as the tree’s armor against physical damage, disease, and environmental stress. This outer layer, composed of dead cells, protects the living tissues beneath, specifically the phloem (which transports sugars) and the cambium (the growth layer). When an oak’s bark splits, it is a visible sign that the tree is undergoing significant internal stress or mechanical injury, forcing the rigid outer layer to fracture. Identifying the cause is the first step toward determining the tree’s health and necessary intervention.
Distinguishing Natural Checking from Serious Splits
Not every fissure in a mature oak’s bark indicates a problem; some are a normal part of the tree’s growth process. As the trunk increases in diameter, the non-living outer bark must expand, resulting in minor, shallow vertical cracks known as “checking” or sloughing. This natural exfoliation leaves the living cambium layer intact and protected.
A serious split is a deep, vertical crack that penetrates through the bark and into the wood, or xylem, beneath. These fissures are often long and appear as an open wound, indicating the tree’s vascular system has been compromised. The depth determines the severity of the injury, as a deep crack creates an open pathway for moisture, insects, and fungal pathogens.
Abiotic Causes: Environmental Stressors
The most common causes of bark splitting are abiotic, meaning they result from non-living environmental factors, primarily involving rapid temperature fluctuations.
Sunscald
Sunscald is a type of winter injury typically occurring on the south or southwest side of the trunk. Dark bark absorbs intense sunlight on clear, cold days, temporarily activating the cambium cells beneath the surface. When the sun sets, the temperature of this activated tissue drops quickly, leading to cell death. This damage later causes the bark to become sunken, discolored, and ultimately split or peel away.
Frost Cracking
Frost cracking is related to sunscald but involves the internal wood structure. This occurs when a rapid temperature drop causes the water and wood tissue (xylem) inside the trunk to contract quickly. If the wood contains a pre-existing weakness, the internal stress results in a sudden, deep vertical crack, sometimes accompanied by an audible sound. This damage can recur over several winters, reopening the fissure and leading to a raised ridge of healing wood, known as a frost rib.
Moisture Stress
Extreme shifts in moisture availability can also cause splitting. A prolonged drought causes the tree’s tissues to dry out and contract, making the bark brittle. If heavy rain follows a severe drought, the tree may take up water too quickly, causing the trunk to expand rapidly and pressure the rigid outer bark until it cracks.
Biotic and Mechanical Sources of Injury
Splitting can be caused by living organisms or direct physical damage, which both create openings for secondary infections.
Cankers and Infections
Cankers are sunken, dead areas on the bark caused by fungal or bacterial infections that develop around a wound. As the tree attempts to compartmentalize the damage, the surrounding healthy tissue swells. This swelling can cause the bark to crack or peel at the edge of the canker. In some cases, pressure from internal sap and gas buildup, such as with bacterial infections like wetwood, can force the bark to split open to release the pressure.
Insect Infestations
Insect infestations, particularly from borers or bark beetles, can also lead to bark splitting. These pests tunnel into the phloem and cambium layers, disrupting the tree’s flow of nutrients and water. The tree’s defense mechanisms may also cause the bark to split open to expose the intruder.
Mechanical Damage
Direct mechanical damage from human activity is a common source of injury. Impacts from lawnmowers, string trimmers, or construction equipment create wounds that break the bark barrier. Even minor scrapes can damage the cambium layer, preventing the protective bark from reforming correctly and providing easy entry points for pathogens.
Treatment and Prevention Strategies
Treatment
For existing splits, focus on aiding the tree’s natural healing process. Experts advise against using commercial wound paints or sealants, as these can trap moisture and pathogens, hindering the tree’s ability to compartmentalize decay. Instead, gently remove any loose, ragged bark edges around the wound with a sharp, sterilized knife, a process sometimes called “tracing.” This smooths the perimeter, allowing the tree to form callus tissue, or “wound wood,” more effectively to close the split.
Prevention
Prevention is the most effective approach to protect oak trees from future splitting. To mitigate winter injury like sunscald, young trees with thin bark should have their trunks wrapped with a light-colored, reflective paper or commercial tree wrap before winter. This prevents the bark from overheating and freezing rapidly. Ensuring the tree has consistent moisture, especially during dry periods, helps maintain cell turgor, making tissues more resilient. Finally, establishing a mulch ring around the base and keeping lawn equipment away from the trunk prevents mechanical injuries.