It's the second Tuesday of the month, which is when we hear from our commentators in the field.
In this month's installment of Field Notes, Scott Bowe of Kemp Station discusses how trees adapt with changes in elevation.
In my last Field Notes broadcast, I spoke about the amazing human body and how it adapts to working and playing at high elevations. Elevation has impact on other organisms besides humans. Let’s look at elevation and how trees adapt to this harsh environment.
Imagine a mountain. What changes can you see as you move from the lowlands up the mountain? One example is that tree type and tree abundance change. You move from deciduous forest in the lowlands to needled trees further up. Finally you reach a point called an alpine tree line above which trees can’t grow. There are a number of factors that impact tree growth with elevation. Major factors include temperature, water, soil, and sunlight to name a few.
Temperature is a major factor and it changes dramatically with elevation. As you increase in elevation, there is less air above you so the air pressure decreases. As the air pressure decreases, air molecules spread out and the temperature decreases. For every 1,000 feet you gain in elevation, the temperature drops about 5 degrees Fahrenheit. Colder temperatures at higher elevations often means more snowfall. Most needled trees employ a conical shape that allows them shed snow more efficiently, which prevents breaking under heavy snow loads.
Water is another major factor. As warm, moist air rises up the windward side of the mountain, the air temperature cools and loses its capacity to hold moisture. The resulting rainfall at these lower elevations can support deciduous forest development, such as the aspen forests you will see in Colorado. At higher elevations, needled trees are better suited for the temperature and moisture conditions. The needles smaller surface area and waxy coating help reduce water loss when compared to broadleaf trees. Above a certain elevation the rising air becomes too dry and cold discouraging tree growth. This is the alpine tree line I mentioned earlier. Even though rainfall may not be a significant factor for some mountains, atmospheric humidity can be very important. Both the overall levels of precipitation and humidity influence soil moisture, which will impact where trees can grow.
Other factors such as soil and sunlight play a role as well. If you imagine a mountain top, it doesn’t bring images of the deep black topsoil that you might find in the Mississippi River Valley. Mountain soil is normally rocky and thin. But there can be plenty of sun. Higher elevations receive much more intense radiation than at the base of a mountain. The earth’s atmosphere is filled with water vapor, particulate matter, and gases that filter the sun’s radiation before reaching the earth’s surface at lower elevations. High elevations do not benefit from this atmospheric filter. In fact, many high elevation plants have had to develop strategies to deal with the sun. Shrubs and grasses tend to thrive because of their small leaves and extensive root systems have adapted to deal with limited water and intense sunlight.
In addition to changes in plant communities by elevation, similar changes happen when we look at increasing latitudes across the globe. Recall that latitude measures distance from the equator where lines of latitude on the globe are parallel to the equator. One of the main drivers here is temperature. When we look at the angle of the sun on the equator, it is about 90 degrees and sunlight travels through the least amount of atmosphere to reach the earth’s surface. The angle of the sun to high latitudes, like the arctic, is around 25 degrees, so sunlight has much more of the earth’s atmosphere to pass through blocking more energy. Less energy means lower temperatures.
Mid latitudes like we have here in Wisconsin show an abundance of deciduous trees found in our temperate forests. In the higher latitudes of Canada, you will find a boreal forest with an abundance of needled trees. At even higher latitudes, you will find the arctic tree line (similar to the alpine tree line of a mountain), where trees do not grow.
Trees are amazing and adaptable organisms. Whether you are heading for the mountains or driving north into Canada, pay attention to the trees around you and how they have adapted to their environment.
For Field Notes, this is Scott Bowe from the University of Wisconsin-Madison’s Kemp Natural Resources Station.