The bark of a tree is far more than just a rough exterior; it is the complex, multi-layered armor that protects the living tissues beneath from weather, pests, and disease. This outer shell is composed of dead, corky cells that form a protective barrier against the environment. The familiar sight of a cracked or deeply furrowed trunk is a common observation in nearly every forest and park, indicating an ongoing process within the tree. These fissures are a natural, often necessary consequence of a tree’s fundamental biology and its continuous interaction with external forces.
The Primary Cause: Outward Growth and Rigid Structure
The most fundamental reason for a tree’s cracked appearance lies in the mechanics of its radial growth. Trees increase in girth through a layer of actively dividing cells called the vascular cambium, which produces new wood inward and new bark outward. The inner bark, or phloem, is living tissue that expands to accommodate this growth and transports sugars from the leaves to the rest of the tree.
The outer bark, known as the rhytidome, is made up of layers of dead cells that are relatively rigid and cannot stretch. As the trunk expands in diameter, the internal pressure from the growing wood and inner bark eventually exceeds the tensile strength of this inflexible outer layer. This forces the dead tissue to split and crack, resulting in the vertical furrows and patterns visible on the trunk. The cracking is essentially a relief mechanism that allows the trunk to continue its expansion and is a normal part of the tree’s development.
Environmental Factors That Shape the Cracks
While internal growth is the main driver of cracking, external stressors dictate the severity and specific form the cracks take. Rapid and intense temperature changes are a major contributing factor, often leading to distinct types of vertical splits.
A common example is the “frost crack,” which occurs when the wood beneath the bark contracts rapidly during a sudden drop in temperature, such as on a cold winter night following a warm, sunny day. This swift contraction stresses the wood and bark, causing a long, deep split that may expose the inner wood.
“Sunscald” happens when intense winter sunlight warms the bark on the south or southwest side of the trunk, causing the tissue to expand. When the sun sets, the tissue freezes and contracts quickly. This cycle can kill the inner bark and cause the outer layer to split or peel off in patches. Fluctuations in moisture also play a role, as a period of drought followed by heavy rain can cause a sudden surge in growth that puts rapid pressure on the outer bark, leading to splitting.
Distinguishing Normal Cracking from Tree Distress
A certain degree of cracking, especially the deep furrows or plated patterns, is genetically determined and completely healthy for a mature tree. Species like oaks often develop rugged furrows as a normal part of their aging process. When assessing a cracked trunk, it is important to recognize the species-specific pattern, which will not expose the healthy, underlying wood.
Cracking that signals distress often appears as sudden, vertical splits that expose the wood beneath, or as cracks accompanied by other symptoms. For instance, a crack that is weeping or oozing sap, or has sawdust-like material around its edges, may indicate an insect infestation or a fungal canker. Cracks resulting from immediate physical injury, such as a lightning strike or mechanical damage from equipment, will typically be jagged and localized, sometimes splitting down to the wood. While a healthy tree compartmentalizes these wounds, deep, unhealed splits provide easy entry points for disease organisms and pests.