Bark, the protective outer layer of a tree, is composed of dead tissue that shields against environmental threats and physical damage. The cambium layer constantly renews this tissue from within, adding new phloem and cork cells to the tree’s circumference. While most bark slowly fissures and thickens, many species use exfoliation, deliberately shedding the outer layer. This healthy, regular function allows trees to maintain defenses and accommodate growth.
The Biological Purpose of Bark Shedding
Bark shedding primarily accommodates the increasing girth of the trunk, a process known as secondary growth. Since the rigid outer bark cannot stretch indefinitely, it must be shed to make room for new layers forming underneath. If the bark did not exfoliate, the tension would cause deep fissures that could compromise the tree’s structural integrity and protective barrier. The shedding process prevents this damaging pressure buildup.
Shedding also serves as a sanitation mechanism, helping the tree rid itself of accumulating parasites and biological threats. The outer bark can host organisms like lichens, mosses, fungal spores, and insect eggs. By sloughing off this exterior layer, the tree physically removes these hitchhikers before they can penetrate the inner tissues. For urban trees, this process also helps remove atmospheric pollutants adhered to the surface.
Common Trees Known for Exfoliating Bark
Many landscape species are recognized for the dramatic appearance of their exfoliating bark, which often creates striking color contrasts.
- The American Sycamore (Platanus occidentalis) sheds large, irregular plates of dark, mottled bark. This reveals the creamy white, pale green, or tan inner bark beneath, creating a characteristic camouflage pattern particularly visible in winter.
- River Birch (Betula nigra) and Paper Birch (Betula papyrifera) are known for unique horizontal peeling, where thin, papery outer layers curl away from the trunk. The River Birch displays salmon-pink to reddish-brown patches, while the Paper Birch reveals a lighter, sometimes tan or pinkish-white underlayer.
- The Paperbark Maple (Acer griseum) is valued for its cinnamon-brown bark that peels into delicate, tissue-thin curls that remain partially attached to the trunk.
- The Shagbark Hickory (Carya ovata) develops long, thick plates of gray bark that separate from the trunk on both ends, remaining attached only in the middle, giving the tree a distinctly “shaggy” appearance.
- In warmer climates, the Crapemyrtle (Lagerstroemia indica) is prized for its smooth, polished trunk that results from the outer bark flaking off in summer, exposing a mottled underlayer of tan, gray, and brown.
Different Patterns of Bark Renewal
The appearance of exfoliating bark is determined by how the tree’s cork cambium, the tissue responsible for creating new bark, functions. Species like the birches utilize a continuous, thin layer of cork cambium that forms concentric rings. When the tree expands, these thin, older layers easily separate and peel off in delicate, horizontal sheets or strips.
Other trees, like the Sycamore, form new cork cambium in discontinuous, deeper layers. This results in the outer bark fracturing into thick, angular plates or scales. As the trunk diameter increases, pressure causes these isolated sections to detach in large, irregular patches, revealing the smooth, newly formed bark underneath. The pattern of bark renewal is a fixed genetic trait specific to its species.
When Bark Shedding Signals a Problem
While natural exfoliation signals a healthy, growing tree, shedding accompanied by other symptoms can indicate distress or disease. Healthy shedding is localized, reveals firm tissue underneath, and occurs during a specific growth phase. Premature or excessive bark loss that exposes bare wood or dark, mushy tissue is a serious warning sign.
Signs of trouble include fungal cankers, which are sunken, discolored lesions that weaken the bark and interrupt nutrient flow. Insect borers, such as bark beetles, tunnel beneath the surface, disrupting the vascular system and causing bark to detach in irregular patches, often accompanied by sawdust or oozing sap. If peeling is confined to a specific area and the exposed layer is discolored, shows decay, or exhibits insect galleries, the tree is likely struggling with a stressor or pathogen.