Tree bark provides a protective outer layer, shielding the living tissues beneath from various external threats. It defends against pests, diseases, extreme temperatures, and water loss, like human skin. While bark loss can sometimes signal a problem, it is also a natural process for many trees, indicating healthy growth and adaptation. Understanding the difference between normal shedding and distress is important.
Normal Bark Shedding
Many trees naturally shed bark as they grow. As a tree expands in girth, the older, outermost layers of bark may crack and detach to accommodate the increasing circumference of the trunk. This process, known as exfoliation, distinguishes certain species. Trees like sycamores, birches, crape myrtles, and some maples have distinctive peeling or flaking bark. This shedding reveals fresh, often lighter-colored bark underneath, indicating a healthy tree.
Shedding also serves as a self-cleaning mechanism for the tree. It removes superficial growths like moss or lichen that accumulate on the outer bark. Discarding these older layers maintains a more efficient and cleaner surface. This natural renewal ensures the bark performs its protective functions effectively.
Environmental and Stress Factors
Trees can lose bark due to environmental stressors. Sunscald, often called “southwest injury,” occurs when intense sunlight heats bark, particularly on thin-barked trees or newly exposed trunks. This warming activates dormant cells, which then die when temperatures drop suddenly, leading to discolored, sunken, cracking, and peeling bark. Sunscald typically affects the south or southwest side of the trunk, where sun exposure is most direct.
Rapid temperature fluctuations can also cause frost cracks, which are vertical splits in the bark and wood. These cracks form when the outer bark contracts more rapidly than the inner wood during sharp temperature drops, especially after a warm period. Frost cracks are frequently observed on the south or southwest side of the trunk and re-open over several winters. Drought stress, from prolonged insufficient water, weakens trees, leading to bark drying, cracking, and peeling. Poor site conditions like compacted soil, improper planting depth, or nutrient deficiencies also stress trees, making them more susceptible to bark damage.
Biological Agents
Insects and diseases can cause bark to loosen or fall off. Insect borers, such as the larvae of certain beetles and moths, tunnel under the bark, disrupting the flow of water and nutrients within the tree. Signs of borer infestation include sawdust-like material, oozing sap, small exit holes, or swollen, knotty, or cracked bark. Most borer species target trees that are already weakened by stress.
Fungal diseases, notably cankers, infect the bark and underlying wood, leading to localized dead tissue. These infections cause the bark to become sunken and discolored, eventually detaching. Examples include Cytospora, Hypoxylon, and Nectria cankers. Bacterial diseases, such as wetwood or slime flux, result from infections causing fermentation within the tree, leading to foul-smelling sap oozing from cracks or wounds. This ooze can discolor the bark and, in stressed trees, contribute to bark decay or branch dieback.
Physical Damage and Injury
Direct physical trauma can also result in bark loss, compromising a tree’s protective layers. Mechanical injuries are common, often from human activities near trees. Lawn mowers, string trimmers, construction equipment, or vehicles can impact the trunk, tearing or scraping away bark. Damage to the cambium layer, just beneath the bark, disrupts the tree’s internal transport system for water and nutrients. If the bark is removed around the entire circumference of the trunk (girdling), it can be fatal.
Animals can cause bark damage through gnawing or rubbing. Deer, rodents, and other wildlife may strip bark from trunks and branches, especially in winter when other food sources are scarce. Severe weather events, such as strong winds, heavy ice, or snowfall, can also damage bark. These forces can cause branches to break and fall, or even rip bark directly from the trunk.