Trees that shed bark can be reacting to a normal growth process or an abnormal disease or pest. Generally, when a tree sheds bark and new bark is visible underneath, the shedding is a developmental characteristic of the tree. Some of the fastest growing trees routinely shed bark in strips and patches. But, if a tree sheds its bark and bare wood is visible in the gap, the tree very likely has a serious problem and should be examined by an arborist to see if it can be saved.
The eucalyptus, a native of Australia, is famous for shedding its bark. Not all of the more than 700 species of eucalyptus are shedders, but a number of them are prolific at it. The University of California Berkeley points out, as a possible reason, that the grove of eucalyptus in its botanical gardens discards harmful bark-burrowing insects along with the old shaggy bark. Eucalyptus bark may be shed in long strings, unbroken strips, flaky and crumbling bits and curling sheets, depending on the particular tree.
The river birch has salmon-coloured bark, unlike its more evocative silvery white cousins. The bark peels in paper-thin layers and the bark colour darkens with the age of the tree. The salmon bark of the young trees deepens to reddish-brown, then brown and finally a very dark grey. The older bark is furrowed and scaly and may shed in flakes rather than flat or curled sheets. The reddish peeling bark is considered a desirable ornamental quality for landscaping. But it serves a practical purpose as well. The bark collects around the base of the tree, effectively choking out other growth so the birch's aggressive seed dispersal and colonising has no competition.
Sycamores and London Plane Trees
The American sycamore and the London plane tree are two related cultivars that shed bark constantly. The sycamore achieves the biggest trunk circumference of any North American tree. Early settlers used the tree, which hollows as it reaches extreme age, as shelter. A large hollow tree could hold more than a dozen people in its trunk. Today, slimmer London plane trees are ubiquitous in New York City and its parks, having edged out most native sycamores. The trunks of the trees are patches of white green and light brown that resemble camouflage -- the mottled appearance is due to continual bark shedding. Theories about why the tree sheds bark year round begin with a need to discard the tight, old bark as the tree grows. But other theories explain the shedding as a way to drop the burrowing insects and fungal diseases common in smooth-barked trees that grow near flood plains. Some scientists see London plane bark shedding as an impulse toward increased photosynthesis. Sycamores and London planes conduct photosynthesis via their trunk as well as leaves. The shedding bark exposes more of the trunk to sunlight.