Tree bark, the outermost layer of a tree, serves as a protective barrier. This covering shields the tree from physical damage, pests, diseases, and extreme weather conditions. It also helps minimize water loss, which is important for the tree’s survival. Despite these protective qualities, tree bark often develops cracks or fissures. This common observation prompts questions about the biological and environmental factors that contribute to this phenomenon.
The Mechanism of Bark Expansion
Trees increase in girth through secondary growth, driven by the vascular cambium. This layer of cells continuously produces new wood inward and new inner bark outward. As the trunk expands in diameter from this internal growth, the older, outermost layers of bark are pushed outward. Since these outer bark layers are largely composed of dead cells, they lack the elasticity to stretch and accommodate the increasing circumference. The rigid outer bark must crack and split to relieve the pressure from the expanding trunk.
Environmental Triggers
External environmental conditions also contribute to bark cracking by inducing stress. Rapid temperature fluctuations, especially during winter, can cause “frost cracks.” On warm, sunny days, the sun heats the tree’s bark and underlying wood, causing them to expand. When temperatures plummet at night, these tissues contract rapidly. This differential expansion and contraction creates internal stress that can lead to vertical cracks, frequently appearing on the south or southwest side of the trunk.
Intense sunlight can also cause bark damage known as “sunscald.” This typically occurs when bark, particularly on young or thin-barked trees, is exposed to strong sun, leading to the overheating and death of bark cells. The damaged tissue becomes brittle and prone to cracking. Additionally, severe drought conditions can lead to moisture loss within the tree, causing the bark to shrink and become brittle, making it more susceptible to cracking.
Species-Specific Patterns and Age
The specific patterns and extent of bark cracks vary significantly among different tree species. Some trees, like oaks, develop deep, rugged furrows, while others, such as birches, exhibit peeling or papery bark, and pines may have scaly patterns. These distinct bark characteristics are genetically determined, meaning each tree species has evolved a unique way for its bark to accommodate growth and respond to its environment.
A tree’s age also plays a role in the appearance of its bark. Young trees often have smooth, relatively thin bark. As a tree matures, its bark naturally becomes more deeply fissured, furrowed, or develops more pronounced patterns. The presence of cracked bark is frequently a normal and healthy sign of a mature tree.