Tree bark functions much like skin, shielding the living tissues beneath from external threats. For most tree species, this bark thickens, develops deep fissures, and remains largely intact throughout the tree’s life. However, for a select group of trees, the shedding of the outer bark layer is a natural part of their growth cycle. This process, known as exfoliation, often creates striking visual textures and colors. Understanding the difference between healthy shedding and pathological loss is the first step in maintaining a healthy landscape.
The Biological Purpose of Bark Shedding
The primary reason many trees shed their bark is to accommodate the increasing girth of the trunk. Unlike the living tissues of the inner cambium layer, the outer bark is composed of dead cells that cannot stretch or expand indefinitely. As the tree grows outward from the center, the rigid outer layer splits and is pushed off, allowing the new, more elastic bark underneath to take its place. This continuous renewal is an adaptation to rapid growth rates or a naturally inflexible bark structure.
Shedding also serves as a defense mechanism against environmental threats. By regularly sloughing off the outermost layer, the tree actively removes surface infestations, such as moss, lichens, and scale insects, before they can become deeply established. This acts as a natural cleaning mechanism, preventing a buildup of organisms that could compromise the tree’s defenses. In some thin-barked species, the freshly exposed inner layer of bark contains chlorophyll, allowing the tree to continue a degree of photosynthesis during the winter months when leaves are absent.
Distinctive Tree Species Known for Natural Exfoliation
Trees that exhibit natural bark shedding can be grouped by the texture and pattern of their exfoliation, which is a key to their identification.
The American Sycamore (Platanus occidentalis) sheds its dark, rigid outer bark in large, irregular, plate-like patches. This process reveals a lighter, creamy white or pale green inner bark, creating a striking mottled pattern, particularly visible high up on the trunk where the bark is youngest.
In contrast, the River Birch (Betula nigra) and Paperbark Maple (Acer griseum) are known for their thin, papery peeling. River Birch exfoliates in thin, curly sheets that range from pinkish-brown to cinnamon-brown, often clinging loosely to the trunk and branches. Paperbark Maple sheds its outer layer in coppery orange-brown curls that typically remain partially attached, giving the trunk a shaggy, three-dimensional texture.
The Crape Myrtle (Lagerstroemia species) offers a different aesthetic, shedding thin flakes of bark to expose a smooth, polished underlayer. The resulting trunk is smooth and sinuous, displaying mottled patches of tan, gray, and cinnamon-brown that are especially prominent in the winter.
The Shagbark Hickory (Carya ovata) lives up to its name, with thick, long plates of bark that detach at both ends and curve away from the trunk. This gives the tree a coarse, shaggy appearance that is unique among exfoliating species.
Identifying Abnormal Bark Loss
Distinguishing healthy exfoliation from pathological bark loss requires close inspection of the exposed tissue and the peeling pattern. When bark loss is natural, the exposed layer underneath appears smooth, firm, and healthy, often a different but uniform color. Abnormal shedding is frequently accompanied by other signs of distress.
Sun scald or frost cracking causes bark loss typically limited to the south or southwest side of the trunk. This damage presents as a long, vertical crack that exposes dead wood or discolored tissue, resulting from rapid temperature fluctuations killing the cambium layer. Cankers are sunken, oval, or elongated lesions that may ooze resin or sap, indicating localized tissue death caused by fungal or bacterial pathogens.
Insect activity leaves telltale signs that differ significantly from natural shedding patterns. Flatheaded borers leave behind distinctive D-shaped exit holes in the bark, while bark beetles create tiny, round entry holes. The presence of fine, sawdust-like material, known as frass, packed into bark crevices or tunnels is a clear indicator of a borer infestation. Dark, foul-smelling, and frothy sap oozing from a crack, known as slime flux or wet wood, signifies an internal bacterial or fungal problem.