Forest Preserve District of Cook County, Illinois
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Another Look at Bark
Nature Bulletin No. 630   February 25, 1961
Forest Preserve District of Cook County
Daniel Ryan, President
Roberts Mann, Conservation Editor
David H. Thompson, Senior Naturalist

Last year, in Bulletin No. 594, we explained how a tree grows and, each year, adds a new layer of wood and another of inner bark. Also how the bark protects a tree and is as essential to its life as your skin is to you. Many kinds can be readily identified by their bark.

A few species are able to stretch their bark as the annual rings push it farther and farther outward. Even when old and large, a beech retains a smooth silver-gray surface free from cracks and blotches. Although fluted so that the trunk commonly appears "muscular", the thin bluish- gray bark of the American hornbeam, miscalled blue beech, is equally smooth. In contrast, the outer bark of a sycamore has so little stretch that it peels off and exposes white patches of the more elastic inner bark.

On the trunks of most trees the bark splits and breaks or peels, always in a certain way, to form characteristic patterns. An elm has tight hark with shallow lengthwise ridges; an ash has crisscross ridges that form diamond-shaped patterns. A bur oak has dark, very thick, deeply furrowed bark. That of a white oak is pale gray with shallow fissures and scaly plates. The gray trunk of a hackberry is distinctively marked with warts and irregular ridges.

On a shagbark hickory the outer bark splits vertically into strips that become loose at the bottom and bend outward like shingles on an old cabin. On a hop hornbeam or ironwood the bark divides into narrow vertical strips that tend to curl at the ends until, like the red cedar, it has a "shredded" appearance. The silky white bark of a paper birch peels easily in horizontal sheets that reveal its copper-colored inner bark.

As stated in that earlier bulletin, the Indians used the bark of many trees for various purposes. The early colonists and pioneers adopted some of those and discovered others, such as the use of bark from a few kinds to supply the tannin for tanning leather. Gradually, all of those uses of bark from our native trees were discontinued. It became "useless".

However, during and since World War II, the thick shaggy bark of the California redwood was found to have valuable properties. It is fibrous, fire resistant, and durable. When shredded it is used in insulating materials and as a substitute for wool in fabrics; now it is being ground for use in floor cleaners and as a soil conditione.

From vast forests of Douglas fir in our northwestern states, huge mills annually convert millions of logs into lumber. Until recent years, about 30 percent of a log was wasted in slabs and edgings with bark on them, usable only as fuel. Then it was discovered that Douglas fir bark contains valuable waxes and useful fibers. Now, debarking is part of the mill operation. The slabs and edgings are converted into many products such as toys, broom handles, and box parts. The bark is used as a source of wax and tannin; in cork products, plastics, adhesives, flooring materials, undercoatings for automobile bodies; and as a soil conditioner.

One of the greatest and most rapidly expanding uses of timber today is to supply pulpwood for the manufacture of several hundred different kinds of paper, cardboard, carton materials, insulation boards, and many other products. All wood to be converted into pulp must be free from bark. Bark not only makes the pulp and paper dirty, it adds useless bulk because it contributes nothing to the strength or finish of the paper. Expensive machines and processes are now used to debark pulpwood logs at the mills or as they are cut in the forests.

Someday we will find uses for all bark.

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