Bark for All Seasons

By Michael Wojtech Gazette Contributing Writer

As tree buds unfurl in spring, they cast sweet hues of yellow, pale green and red in the canopies of our local forests. Soon after, this new growth matures into a kaleidoscope of dense greens. This is the season when deciduous trees seem to come back to life, when they transform from a blur of browns and grays and resume their individual identities. But the source of these browns and grays—bark—harbors more distinguishing characteristics, more beauty and has been more active during the past six leafless months than you might imagine.

Even a small patch of woods displays myriad bark characteristics, including strips that appear woven like a basket (bitternut hickory), scales that look like burnt cornflakes (black cherry) and smooth-topped ridges that follow up the trunk like ski tracks (northern red oak). You can see the splendid contrast of these features best on early mornings and late afternoons, when light catches them just right.

One tree species stands out from the rest. With its gleaming white trunk and curly strips of bark, paper birch is one of the easiest species in the area to recognize. But why is its bark white? And what purpose do peeling strips of bark serve?

One lesser-known function of bark is to generate energy. Scrape away the outer bark on a twig or young branch and you will reveal a thin, surprisingly green layer that can perform, to a lesser degree, the same photosynthesis as leaves. Levels of energy production are highest on this newer growth, where up to half  of available sunlight can penetrate the outer bark and reach this green layer. Deciduous trees may appear completely dormant in their leafless state. However, bark photosynthesis has its heyday in spring and fall, when leaves do not shade the bark. Some species can even produce energy on freezing winter days, when direct sunlight warms their bark.

On most species in this region, the mature bark thickens over time and blocks the sunlight needed for photosynthesis. Paper birch bark, though, remains thin enough for photosynthesis to function—even on the trunk. As those curly strips of bark continually peel away, they help keep the bark from growing thicker.

When strips of bark do curl away from the trunk on paper birch they also take with them epiphytes, such as mosses and lichens, that may have been growing there. These epiphytes block sunlight for bark photosynthesis and, if dark colored, absorb heat and increase temperature fluctuations.

This peeling process also helps keep lenticels from becoming blocked. Lenticels are breathing pores through the protective outer bark that are vital to bark photosynthesis. On paper birch bark they are clearly visible as thin dark horizontal lines. On most other species in this region lenticels become hidden in cracks and furrows as trees mature and the bark thickens.

So why is bark photosynthesis important? By providing energy for regular cell maintenance in the trunk and branches, it helps keep trees healthy. It also helps fuel recovery from partial or full defoliation due to insect infestations, storms or severe drought. Supplemental energy from bark photosynthesis helps paper birch survive in some challenging ecological niches—it grows at high elevations, for example, and is one of the most northerly-sited hardwood species in North American.

Bark also serves to protect trees from sunlight. It may seem contradictory that this vital engine of photosynthesis is a potential threat to trees. But rapid temperature change can lead to significant bark damage; in winter, direct sunlight can warm the south and southwest-facing sides of a tree’s trunk to over 70°F. When the sun sets and temperatures plummet, the cooling and drying bark can crack as it contracts. If the heating is intense and long enough, portions of inner bark can be activated from winter dormancy; a subsequent drop in temperature, even from a passing cloud, can refreeze and kill sections of bark. These wounds, referred to as sunscald, provide pathways for insects, fungi and other harmful invaders.

The whiteness of paper birch bark helps make up for its lack of insulating thickness. By reflecting heat, the bark helps regulate temperature fluctuations and reduces bark damage. This function is especially important at high latitudes and elevations—places where temperature fluctuations are at their most extreme.

As you start to notice bark you’ll find additional species with curly, peeling bark, like yellow birch, and see lenticels in many different shapes, colors and sizes. These and other characteristics will begin to emerge from what may have previously appeared as just a haze of dun colors. So celebrate the returning leaves, but remember that your appreciation for bark can grow and last throughout the year. Paper birch is a great species to start with.

Michael Wojtech is a naturalist, freelance writer and teacher. His essay is adapted from his book Bark: A Field Guide to Trees of the Northeast.

Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 525 South Pleasant St., Amherst, appears every other week. For more information, call 413-256-6006, or write to us.

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