If you are ever wondering about the natural history in your area, talk to the person who has lived in that area the longest. In my case, it is my graduate program’s registrar who has lived and worked in the Tetons since the year I was born. She had attended my group’s presentation on our research about frost cracking last week, and upon making a purchase from her at the school store a few days later, I learned some new information about the frost cracking phenomenon. She told me that some Native American plains  tribes of the American West called December (or January or February, depending on your sources)  the “Moon of the Popping Trees” due to the phenomenon now commonly called “frost cracking.” I got through an intensive several day research project on this phenomenon and of course didn’t come across that fascinating tidbit of information. Sometimes the best knowledge is hiding in unexpected places…

Frost cracking is a phenomenon that happens when a few circumstances are present: cold, clear, calm winter days + colder, clear, calm winter nights. When trees, especially those that are growing in open areas, are exposed to the sun’s rays throughout the day, they warm up and expand. However, when the sun sets and the temperatures drop dramatically, the tree starts to cool down and contract from the outside in. That temperature fluctuation causes uneven shrinking of the tree. If the outside shrinks faster than the inside and can no longer fit around the inside of the tree, a sound reminiscent of a gunshot occurs as the tree cracks: a frost crack. Obviously, having a giant crack is not conducive to winter survival as it opens the tree up to disease and makes it more likely to crack in that same place again.

So how do trees survive in northern climates that typically have that perfect equation of sunny cold days and colder clear nights that leads to frost cracking? Let’s consider a few northern trees.

First, let’s consider the evergreens.

Evergreens in Winter

These trees tend to have boughs that shade their trunks and usually grow in dense groves, thus reducing the amount of sunlight they absorb throughout the day and reducing the temperature fluctuation they might experience in a 24-hour period. I would guess that the structure of their wood resists insolation (absorption of solar radiation) too.

Now, the poplars (or “poples” as some Mainers call them or aspens as they are called in the west–I’m talking about Populus tremuloides).

DSC_1679

These trees do not have protective boughs or leaves that shade their trunks in the winter, but they do have smooth, light-colored bark. As you have probably experienced on a hot sunny day, wearing lighter colors helps reflect the sun’s rays whereas wearing darker colors absorbs that solar radiation. Poplars/aspens take advantage of this fact by reflecting as much solar radiation as they can throughout the day so that their temperature fluctuation in a 24-hour period is minimized.

How are birches (Betula sp.) different?

Betula allegheniensis
Yellow Birch, Betula allegheniensis

Birches, like poplars/aspens, do not have protective boughs or leaves to shade their trunks in the winter, but they do have light-colored bark and one other key adaptation: peeling bark. While their lighter-colored bark helps reduce insolation, their peeling bark helps shed anything that might start growing on their bark. These plants that grow on other plants, called epiphytes (i.e. mosses and lichens–yes, I know lichens are technically an algae-fungi combination organism, but you get my point), are usually darker in color and could increase that absorption of the sun’s rays. By maintaining peeling bark, it is hypothesized that birches reduce epiphytic growth and thus reduce insolation and the subsequent temperature fluctuation that could lead to frost cracking.

For more information about frost cracking (and natural history in general), check out Reading the Forested Landscape by Tom Wessels. It’s an engaging and informative read!

Advertisements