Take Me Outside: To learn more about the trees around you, take a look at their bark
any of nature’s treasures have been buried by recent snow. Yet trees offer opportunities for exploration regardless of snow depth or season.
Tree bark has many functions and upon close examination varies considerably from one species to another. The variations represent different strategies for adapting to environmental conditions. Bark protects the inner tissue from insects, fire and disease, but it also provides a way to survive contrasting winter temperatures.
When the sun shines on a dark-colored tree, the bark temperature can reach about 70 degrees, even on a cold winter day. At nightfall, the contrast between that warmth and the air temperature can be dramatic, causing the bark to crack. One way to avoid frost cracks is by having built-in cracks and grooves, which are found on many species of northern trees. These act like fins on a radiator to dissipate heat and keep the bark from getting too warm. Another adaptation is to have light-colored bark which reflects sunlight rather than absorbing it. The paper birch and quaking aspen have white or cream colored bark for this reason. Not coincidentally, these two species have the most northern range of any deciduous tree in our area.
Bark is more than just the skin of the tree, and there are many layers that serve distinct functions. Beneath the bark we see (technically called the “outer bark”) is the “inner bark” or phloem. The phloem transports food from the leaves to other parts of the tree. Underneath that layer is the very thin cambium, where the growth rings are produced. This layer connects the roots of the tree to the very tips of the branches. If the cambium is severed all the way around the tree, the tree will die. Therefore, protecting this layer is vital to the health and growth of a tree. Beneath the cambium is the xylem or sapwood, which carries nutrients from the soil through the roots and into the leaves.
Amazingly, each tree has its own way of protecting these layers and the vital functions they perform. The distinct adaptations create bark variations that can help us identify trees.
As mentioned above, the white or paper birch has a white, papery bark that easily peels off, but don’t be tempted. The gray birch also has a white bark, but it is chalky and does not peel. All of the birches, as well as cherries and other fruit trees have thin, horizontal lines called lenticels. These are composed of small pores that allow for gas exchange between the trunk and the air.
The quaking or trembling aspen has a smooth, creamy colored bark that may have a greenish tinge to it. The green color comes from chlorophyll, the same substance that makes leaves green. Therefore the aspen can photosynthesize through its bark, even in the winter when there are no leaves on the tree. The American beech also has a lighter bark. Its smooth, blue-gray bark with occasional irregular dark blotches is very distinctive. Unfortunately it is often seen as a clean slate on which to carve initials. This obviously is not good for the tree.
Some trees, like red and sugar maple may have smooth bark on the young trees but as they mature the bark texture changes. Red maples develop dark gray, scaly plates as the trees age. Sugar maples have long ash-gray, flaky scales.
Though gray and brown are common colors of bark, the red oak actually has reddish furrows between the flat topped ridges that protect its trunk.
Botanists may tell you that winter tree identification is better done with bud and twig examination, but that’s no reason to ignore the bark. So, take a new look at trees that you pass and notice the features of their bark. Looking is only one way to explore the bark. Making rubbings, especially with children, is a lot of fun and a great way to notice the differences. Put a piece of white paper against the bark and use the side of a dark colored crayon to rub back and forth across the paper. Make note of where the tree is and be sure to visit it again in the spring. Then you’ll be able to examine the leaves, continue your exploration and deepen your acquaintance with your local trees.