The Heart of Barkness

Everything you always wanted to know about tree bark but were afraid to ask.

by Jack MacRae

In a roundabout way, tree bark has been a recurring factor in my livelihood. As a youth, I looked at bark with an entrepreneurial eye. My scheme was to set up a card table and sell pieces of bark to tourists on Washington Island, Wisconsin. Birch stationary was to be my cash cow; pinecones were an attempt at diversifying the inventory. I closed the stand after one day of zero sales and much sibling ridicule. As a young adult, I had a job where we built a few 18-foot birch bark canoes — obviously good work if you can get it. Currently as a naturalist, I am proficient in extolling the value of trees (generally) and bark (specifically) as I happily push the environmental agenda on new generations of school children.

I trust bark needs no introduction. It is one of the most conspicuous of all natural objects. Examples are literally all around us; chances are good that you are within 100 yards of bark as you read this. Further, bark — unlike the more celebrated leaf — is with us 12 months of the year.

There is, of course, the boring definition of bark. Most basically, bark is layers of complex plant tissue found on the outside of a tree. The living inner bark transports the nutrients a tree needs for growth. The rough, dry material we’re all familiar with — the outer bark — is filled with dead cells and air.

But just like the metal of some random black van isn’t all that interesting until you realize it’s bulletproof and it’s protecting Mr. T and the rest of the A-Team (for readers under 20, that was a reference to the 1980s), it’s what bark does that really gets the sap flowing.

Bark is the most protective tissue — plant or animal — in the natural world, a barrier to both freezing winter temperatures and the raging flames of a prairie fire. In a fire regime biome such as Chicago Wilderness, a plant’s ability to tolerate occasional scorching ultimately influences the entire prairie-savanna-woodland matrix, which in turn determines the food webs and animal life.

Hot prairie fires incinerate most woody plants. The premier exception is the bur oak, with thickly furrowed fire retardant bark that can’t even be ignited with a blowtorch (do not try this at home, or anywhere else for that matter). On the other hand, birch bark contains highly flammable organic molecules, such as cresol, phenol, and xylenol, that make it ignite very easily.

Tree bark is remarkably impervious to liquids. Water from outside the tree (rain, snow, lawn sprinklers) cannot get in, and fluids within the tree (sap) cannot get out. Such properties are why one layer of tree bark — the cork layer — is widely used as a bottle stopper. Bark insulates the tree from temperature extremes and protects the interior from disease and insect infestation.

Outer bark must not taste very good, as few large animals eat it on a regular basis. Rodents (beavers being an obvious exception), rabbits, and deer will eat inner bark on occasion, but typically only in late winter when more desired food is scarce. Humans eat bark. Our breakfast would not be the same without our liberal use of powdered inner bark from new shoots of the cinnamon tree.

The medicinal properties of certain barks have been recognized since ancient times. The well-documented effects of willow bark extract for treating fever, pain, and inflammation led to experiments with salicylic acid, an important ingredient in the first laboratory batches of aspirin. Tamoxifen, another well-known “bark medicine,” comes from the Pacific yew tree and is a leading weapon in the fight against breast and ovarian cancer.

Bark speaks in volumes, serving as a public record of a tree’s life. Many old trees are marred from a lifetime of battles with beetles, fire, fungus, woodpeckers, and pocketknives. These scars are on permanent display; even small wounds leave a mark that can last for years. Interestingly, due to a tree’s manner of growing, bark scars always stay at the same height no matter how tall the tree grows.

Bark also tells the story of the land. Naturalists can trace the outlines of old farmsteads by noticing the straight lines of Osage orange with its gold-tinged bark. Rivers and streams can be spotted at a distance by looking for the dark-barked alders and white barked sycamore. Clumps of thick-barked cottonwood can indicate damp ground.

Bark is what gives our trees and forests their identity and, literally, their standing. Using your mind as a bark search engine can provide a new insight into the woods. One astute person said having “bark eyes” is like being given super powers of perception where certain trees pop out of the landscape. Bark boring? Hardly.

Seeing the Forest for the trunks

Learning to identify trees in winter — that is, without leaves — is a fine cold-weather activity. With very little practice, one can learn to identify a half-dozen Chicago Wilderness trees by their bark. While some tree species can be challenging to ID, many have bark truly unlike any other.