Bark that peeled off a paper birch tree in Manistee. Photo by Josh Shields.
By Josh Shields, Ph.D.
When you hear the words “tree bark,” what is the first image that comes to mind? Some of you may envision the thick, rough bark of oak trees or mature eastern white pine trees. Perhaps you think of the smooth, gray bark of American beech or the beautiful, camouflage-like bark of American sycamore. For many, the white, paper-like bark of paper birch comes to mind.
But what, exactly, is tree bark, and why is it important?
Tree bark is actually part of a larger complex of layers that form the trunk of a tree.
The visible part of the bark that many of us call tree bark is known as the outer bark layer. This outermost layer of the trunk helps buffer the tree against biological factors such as insects and fungi that seek to colonize and sometimes harm the tree, environmental factors such as changes in air temperature, and physical factors such as mechanical injuries (e.g., accidentally nicking the tree with a lawnmower or weed whip).
Immediately inside the outer bark layer is a layer called the phloem, also referred to as the inner bark. Phloem is the main layer that trees use to transport sugar made by the leaves or needles to other parts of the tree, such as branches and roots. This layer is essentially a conduit for the tree’s food.
The next layer ⸺moving inward, toward the center of the trunk ⸺ is the cambium. The cambium is a small layer (its thickness is only one cell) that is responsible for the diameter growth of the tree. The cambium produces cells in the phloem layer to the outside of itself and cells in the xylem layer (also known as sapwood) to the inside of itself.
The xylem/sapwood layer has a very critical function, which is to transport water, minerals and nutrients from the roots to the leaves. It is the xylem/sapwood layer that is most visible when you look at the cross-section of a tree trunk. The part of the cross-section that looks like wood (because it is!) is the xylem/sapwood layer. It is also in the xylem/sapwood layer where annual growth rings are visible.
In this part of the world, annual growth rings can be used to determine the age of trees, since each ring, as the name indicates, represents one year of growth. To discern the separation between growth rings, look for the contrasting light and dark coloration. The lighter coloration is called “earlywood” because it develops in the spring, and the darker portion is called “latewood” because it develops in the summer. Together, the earlywood and latewood form one annual growth ring. The oldest rings are found in the center of the trunk and the newest rings are found toward the bark layers. Annual growth rings are very easy to see in some species of trees, such as pines, spruce and oak. It can be a bit more challenging to see these rings in species such as maple.
At the very center of the tree is a layer called the heartwood, which typically does not develop until the tree gets a little older. Heartwood provides the tree with structural support but does not carry water, minerals and nutrients like the xylem/sapwood. Heartwood can be easily differentiated from the xylem/sapwood because of its solid, dark color.
By now, I hope you are fascinated with the interesting layers that form the tree trunk, including the bark! All these components of the tree trunk are important for keeping a tree alive and vigorous. Excessive damage to these components can lead to a tree’s demise. For example, removing the outer bark and phloem around the entire circumference of a tree is called “girdling” and results in tree death.
Having said that, it is important to understand that some forms of death are ecologically beneficial, whereas others are not. One example of an ecologically beneficial form of tree death is when a tree dies due to old age and subsequent infestation by native insects and fungi. This results in the tree becoming valuable habitat as a dead standing tree (a “snag”) and as a log when it falls to the ground, where it eventually becomes part of the soil.
Other examples of ecologically beneficial forms of tree death are when windstorms take down trees or when timber harvesting is done in a sustainable way. Both result in the release of growing space, so that young trees can thrive where the blown-down or cut-down trees once did. There are also instances when living trees are purposely girdled to provide standing dead trees (“snags”) as habitat for wildlife, such as mammals and birds, that utilize tree cavities.
Conversely, sometimes trees are killed prematurely, when the goal was actually to keep those trees alive for as long as possible. A common example of this is when thin-barked tree species, such as paper birch, are girdled because the bark was peeled off, or when excessive carving takes place on the thin bark of American beech. It is important to refrain from doing this to thin-barked tree species, or any trees, when the goal is to keep those trees alive, whether growing in public spaces, such as parks or campgrounds, or in private backyards.
For more information, contact Josh Shields, forester and wildlife biologist with the Manistee and Mason-Lake Conservation District offices, at 231-889-9666 (Office Phone), 989-220-9236 (Mobile Phone), or via email at joshua.shields@macd.org.
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