A tree’s bark is its interface with the outside world, and understanding it offers a reliable method for identification, especially when leaves have fallen. A tree’s outer layer is composed of dead tissue that forms a protective shield. This layer insulates the tree from temperature extremes, prevents moisture loss, and deters insects. Beneath the outer layer is the inner bark, a living tissue called the phloem, which transports sugars from the leaves to the rest of the tree. The distinct appearances of bark among species result from how the tree grows and sheds these layers, making it an accessible feature for identification.
Fundamental Bark Characteristics
The texture of bark is one of its most telling qualities, ranging from completely smooth to deeply textured. Some species maintain a smooth surface into maturity, while rough bark can be fibrous, with a stringy feel, or scaly, breaking off in small flakes. Other barks are described as warty for their scattered, bumpy protrusions, or corky, with thick, soft ridges.
Color is another characteristic, though it can be influenced by environmental factors. Bark colors range from the white of a birch to the nearly black shade of a black cherry. These colors are produced by chemical compounds in the bark; for example, tannins in oaks contribute to a reddish-brown hue. The perceived color is often altered by lichens, moss, or algae, which can add patches of gray, green, or orange.
Distinct patterns on the bark surface provide further clues for identification. Ridges and furrows are common, appearing as alternating crests and valleys that can be interlacing or broken and irregular. Fissures are cracks in the bark, while plates or blocks are larger, flattened sections that fit together. Lenticels, which are pores for gas exchange, can be visible as small horizontal or vertical lines or dots on the surface.
The way a tree sheds its bark is also a useful identifier. Some barks peel away in thin, papery sheets, which can be horizontal or vertical. Others exfoliate in larger strips or plates, revealing a new layer of bark underneath. Bark thickness also varies greatly, from the paper-thin covering of some birches to the thick, fire-resistant bark of a Giant Sequoia.
Recognizable Bark Patterns and Tree Examples
One of the most straightforward patterns is smooth bark. The American Beech (Fagus grandifolia) is well-known for its thin, smooth, light-gray bark that persists throughout its life, often likened to the skin of an elephant.
A shaggy or peeling appearance is characteristic of several trees. The Shagbark Hickory (Carya ovata) develops long, woody plates that curve away from the trunk, giving it a shaggy look. The River Birch (Betula nigra) has papery, peeling bark that exfoliates in layers of salmon-pink, brown, and cream. The American Sycamore (Platanus occidentalis) also has exfoliating bark, with large patches of brown and gray flaking off to reveal a smooth, whitish-green inner bark.
Deeply furrowed bark is common on many mature trees, especially oaks. The Northern Red Oak (Quercus rubra) has broad, flat-topped ridges separated by shallow furrows, often with a hint of red in the fissures. The Black Walnut (Juglans nigra) features a dark, chocolate-brown bark with a deep, diamond-shaped interlocking ridge pattern. White Ash (Fraxinus americana) also has interlacing ridges, but they are tighter and more regularly diamond-shaped than those of many oaks.
Other trees are identified by bark that forms distinct blocks or plates. The Flowering Dogwood (Cornus florida) has bark that breaks into small, squarish blocks, resembling alligator hide. Mature Black Cherry (Prunus serotina) trees have a dark, scaly bark that looks like burnt potato chips. The blocky bark of the Common Persimmon (Diospyros virginiana) is dark and deeply grooved into thick, square plates.
A few species are noted by their warty or corky bark. The Common Hackberry (Celtis occidentalis) is a prime example, featuring irregular, projecting, wart-like growths and corky ridges over a smoother gray bark. The Amur Cork Tree (Phellodendron amurense) is another example, known for its spongy, deeply ridged bark that can be pressed with a finger.
Variables Influencing Bark Appearance
The appearance of bark on a single species is not static; it is shaped by several variables. A primary factor is the tree’s age. Many species, like maples and cherries, start with smooth, thin bark in their youth. As the tree grows, the trunk expands, and this outer layer cannot stretch, causing it to crack, furrow, and develop the rougher texture of a mature specimen.
Environmental conditions also play a large part in shaping bark. A tree growing in full sun may have bark that is lighter in color or more prone to cracking than one of the same species in a shaded forest. Wind exposure can also affect texture over time. Geographic location influences these factors, leading to regional variations in appearance.
A tree’s health and any physical damage are reflected in its bark. Diseases can cause cankers, which are dead sections of bark that appear as sunken or swollen areas. Insect infestations may leave behind networks of tunnels or holes. Injuries from storms, animals, or human activity can create scars or cause the tree to ooze resin as a defense mechanism.
The rate at which a tree grows can influence how its bark develops. A fast-growing tree might have bark that appears more stretched, with wider and shallower furrows. In contrast, a slow-growing tree may develop a tighter, more compact bark pattern. These variables mean that identification requires looking at the overall pattern rather than a single, perfect feature.
Methods for Bark Examination
Effective bark identification involves a methodical approach. It is beneficial to examine the entire tree, as bark can differ between the main trunk and the upper branches. The bark at the base of a tree is often thicker and more deeply furrowed than the bark higher up, so a holistic view provides a more complete picture.
Looking at different sides of the same tree can reveal variations caused by environmental exposure. The side of the tree that faces the sun or prevailing winds might have a different texture or color than the more protected side. Gently touching the bark can also provide information about its texture—whether it is fibrous, brittle, hard, or soft.
For a closer look at fine details, a hand lens or magnifying glass is a useful tool. It allows for detailed inspection of lenticels, small scales, or the fine structure of ridges, which can help distinguish between similar species. Taking photographs of the bark from different distances and angles is also a practical step for later comparison with field guides.
Finally, consider the context in which the tree is growing. Pay attention to the surrounding environment and the tree’s overall shape and branching structure. Using a comprehensive field guide allows you to cross-reference your bark observations with other clues, such as lingering seeds, buds, or the tree’s habitat, to confirm an identification.