The outer layer of a tree, known as bark, functions much like skin, providing a durable defense against the external world. Bark loss is common, but the underlying reason ranges widely, from a healthy, expected process to a symptom of a serious injury. Determining whether the shedding is natural or a sign of a problem requires understanding the bark’s function and the specific patterns of loss.
The Functions of Bark
Bark represents all tissues located outside the vascular cambium, serving as the tree’s shield. This protective layer insulates the tree from temperature extremes, fire, and physical impacts. The outer bark is composed of dead cork cells, which are largely impervious to water and prevent the tree from drying out.
Just beneath the outer covering lies the inner bark, which contains the phloem, a living tissue responsible for transporting sugars from the leaves down to the roots. The delicate cambium layer, which produces new phloem and new wood (xylem), is also protected by the bark. Damage to the bark exposes these tissues, creating an entry point for pests and pathogens.
Normal Bark Exfoliation and Growth
In many tree species, bark shedding is a natural and recurring phenomenon linked to healthy growth. This process, known as exfoliation, occurs because the rigid outer bark does not stretch as the trunk expands in girth. Species like river birch (Betula nigra), paperbark maple (Acer griseum), and sycamore (Platanus occidentalis) are characterized by their peeling or flaking bark.
The old, rigid outer layers tear and peel away in sheets, flakes, or plates, revealing the fresh inner bark beneath. On trees like oaks and pines, the bark splits vertically to form deep fissures as the trunk grows outward, creating a rugged texture. Shedding old bark also helps the tree eliminate parasites, moss, or fungi that have colonized the outer surface.
Harmful Factors Leading to Bark Loss
When bark loss is not natural, it is usually a result of harmful biotic or abiotic factors. Abiotic factors are non-living environmental or physical forces. Mechanical injury is a common cause, often resulting from lawnmowers or string trimmers striking the trunk base. This damage creates open wounds that expose inner tissues to infection.
Environmental stresses can also lead to bark failure. Sun scald occurs when thin-barked trees are exposed to rapid temperature changes, often on the south or west side of the trunk during winter. Frost cracking is another temperature-related injury where rapid drops in temperature cause the outer wood and bark to split. Animals, particularly deer, rabbits, and rodents, cause bark loss by feeding on the outer tissue or rubbing their antlers against the trunk.
Biotic factors involve living organisms, primarily insects and diseases, that attack the tissues beneath the bark. Wood-boring beetles, such as bark beetles and the emerald ash borer, tunnel into the phloem and cambium layers to lay eggs, disrupting the tree’s vascular system. Fungal and bacterial diseases often manifest as cankers, which are localized, sunken areas of dead bark and underlying tissue. These infections typically begin at a wound site and expand, killing the bark and causing it to slough off.
When to Worry About Missing Bark
The severity of bark loss depends on the depth of the wound and its extent around the trunk’s circumference. Shallow damage that only removes the dead outer bark is not a concern, as the living tissues beneath remain intact. Damage becomes serious when the wound penetrates the phloem and cambium layers, which are responsible for transporting sugars and generating new growth.
The most dangerous form of bark loss is called girdling, which occurs when a wound completely removes the phloem and cambium in a continuous ring around the entire trunk. This severs the downward flow of sugars to the roots, effectively starving the root system, which will eventually lead to the tree’s death. If the wound affects more than 50% of the trunk’s circumference or if the tree is thin-barked and the damage extends into the wood, professional assessment is recommended.