By Elaine Mills, Extension Master Gardener
Photos by Elaine Mills and Virginia Tech
The shapes and sizes of leaves, or colors and arrangements of flowers, are plant features people often use to identify trees and shrubs, but woody plants can also be identified upon close examination of their bark. Variations in color and texture of bark, as well as other characteristics, provide helpful clues to a plant’s identity. Winter can be the perfect time to begin a study of bark, when it stands out against fallen leaves or snow.
Bark and Its Functions
Bark is the outermost covering of dead tissue (cork), which protects a woody plant against the effects of weather, such as temperature extremes, scorching by sun, and drying by wind. It deters invading insects, bacterial infection, or fungal spores, and damage by animals, which are attracted to the sugar-rich sap transported by the vascular phloem from the leaves to the rest of the plant. Finally, bark helps defend a tree against fire, mechanical injury, and parasitic plants.
The bark of each type of tree has evolved to adapt to the environment in which the species grows. For example, the thick bark of Sequoiadendron giganteum (giant redwood) contains no flammable pitch or resin, allowing the tree to survive intense fires. In contrast, some pine trees, such as Pinus contorta (lodgepole pine), produce flammable oils in their bark because the high temperature of fire releases seeds from their waxy cones, aiding reproduction. The volatile oils in Betula ssp. (birch) bark make it waterproof and resistant to decay.
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Other trees produce chemicals in their bark to make them unpalatable or to ward off competition from other tree species. The tannins of Quercus ssp. (oak) trees and acetogenins of Asimina triloba (pawpaw) are powerful natural insecticides. The oxalate crystals contained in the inner bark of conifers destroy the mouthparts of insects, and the bitter, almond-scented cyanogenic glycosides in the bark of Prunus serotina (black cherry) deter browsing animals such as beavers. The essential oils in bark shed by Betula nigra (river birch) inhibit the growth of neighboring trees.
Bark texture also provides protections. Smooth bark repels climbing plants. The exfoliation of bark in birches, for example, allows them to shed the lichen and moss that form on their trunks. The young green branches of trees are thought to be photosynthetic, but Populus tremuloides (quaking aspen) is unique among North American trees in that it can photosynthesize through its greenish-gray bark even after its leaves drop.
Types of Bark
Young trees often have smooth, unbroken bark, but will usually roughen as the trees age. Only a few locally native trees, such as Fagus grandifolia (American beech), retain smooth bark through their lifetimes, detering climbing plants and insects. Bark can take on other textures by peeling, cracking, or dividing, and can be characterized as papery, scaly, plated, furrowed, or fibrous. These bark textures are relatively uniform within a single tree species.
In some cases, a tree’s wood grows faster than the surrounding bark and this pressure may cause thin layers of protective outer cork to separate. When outer dead layers of bark peel away in curling, horizontal strips to reveal inner layers of bark, the texture is described as “papery.” Birch trees exhibit this kind of bark, and Betula nigra (river birch) has the most abundant thin, flaking layers. Platanus occidentalis (American sycamore) also sheds its bark, but it comes away in great, irregular, brittle sheets.
The bark of trees can separate into scales or larger plates. Black cherry has bark with scales that resemble burnt potato chips, and evergreens such as pines and spruces have bark with broad, flat plates. The bark of two native trees, Cornus florida (flowering dogwood) and Diospyros virginiana (persimmon), splits into square blocks, resembling alligator hide. Carya ovata (shagbark hickory) is considered “shaggy” because its long scales are attached in the middle, but separate from the tree at both ends.
The most common tree bark texture is furrowed, with ridges that are separated by fissures of various lengths and depths. Liquidambar styraciflua (sweetgum) has furrowed bark. The ridges are flat and narrow and the fissures are neither shallow nor deep. On the other hand, the furrows of Juglans nigra (black walnut) are dark and quite deep. The fissured bark of Fraxinus pennsylvanica and F. americana (green and white ash trees) has interlaced corky ridges that form distinct diamond patterns. The bark of Carpinus caroliniana (American hornbeam) has widely spaced, sinewy ridges that give it a muscular appearance. Finally, there are trees such as Juniperus virginiana (eastern redcedar), in which the bark furrows are themselves furrowed, creating a fibrous texture.
Other Bark Characteristics
Tree bark can be distinguished by other characteristics, such as color, smell, and the presence of special structures. While bark is generally thought of as muted gray, it can contain various chemical compounds which reflect and absorb different wavelengths of light, producing a range of colors from white or silver, to light gray, to different shades of orange or brown. For example, the tannins in oaks lend the trees a reddish-brown color like that of black tea. A tree’s color can change with age as its bark becomes thicker.
Closer study of trees also reveals the various scents in their inner bark. Some pines smell like turpentine. Others smell like vanilla or butterscotch. Betula alleghaniensis (yellow birch) smells like wintergreen, while Sassafras albidum (sassafras) has a cinnamon or spicy fragrance, similar to that of root beer. The essential oils that produce these fragrances have been collected and processed for human use in food, cosmetics, and perfume. Their antibacterial and antioxidant properties have been medically therapeutic for thousands of years.
Another characteristic of bark is lenticels, “breathing holes,” that allow the exchange of oxygen and carbon dioxide between the atmosphere and the living cells of woody plants. All trees have lenticels scattered over the bark of their twigs and trunks, but some are more visible than others. Knowing the distinctive shape of a tree’s lenticels can help in identification. While often lentil-shaped, these raised pores can also be round, oval, or diamond-shaped, or can appear as horizontal lines, as on the bark of cherry and birch trees. Disease-causing organisms can invade through these pores, but that rarely happens.
Bark and Wildlife
A number of common North American trees have evolved sharp, woody projections on their outer surfaces to deter hungry animals from eating plant material. These defenses can take several forms: thorns, spines, and prickles. The thorns of Gleditsia triacanthos (honey locust), Robinia pseudoacacia (black locust), and several species of hawthorns, including Crataegus crus-galli, are actually modified stem shoots containing vascular tissue. The spines of shrubs such as Berberis vulgaris (barberry) are similar to thorns but arise from leaf tissue. Prickles, such as those seen on the trunks of Aralia spinosa (devil’s walking stick), are direct extensions of the outer covering and have no vascular tissue connecting them to the main body of the plant. While most animals avoid woody species with these sharp structures, birds may choose such plants for protected nesting.
Inner bark is a primary food source for beavers, and is also consumed by porcupines, rabbits, voles, squirrels, and mice. Deer strip the bark off young trees and damage it by fraying their antlers to shed the velvet coating. Some species of bats roost beneath loose, peeling bark of shagbark hickory. Insects and spiders find homes in the deep fissures and crevices of oak and other bark, and are in turn foraged by birds. Finally, bark crevices form important habitats for epiphytic organisms such as ferns, orchids, fungi, mosses, algae, and lichens.
Learn more about bark in the three-part series “Are You Lichen the Bark?”, the two-part series “Healthy Bark, Healthy Tree”, and “In the Winter Garden.”
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