Anne S. Writes: We are up in Alaska looking at the majestic mountains. Question is... why do trees grow only to a given elevation? Is it lack of oxygen, temp, or what???? Always wanted to know.
The short answer is temperature.
Trees will not grow beyond a certain elevation at a specific location if the climate is too harsh for survival. There are several factors that contribute to trees’ ability to grow and survive. As these factors vary in different locations, the elevation of the tree line also varies across the globe. For example, the tree line in the Teton Mountains is at 10,000 feet, while the tree line at Mt. Washington in New Hampshire is at 4,500 feet.
The primary factor that determines the tree line is temperature. According to plant scientists, plants cannot effectively build cells when the average growing-season temperature is lower than 44° F. Trees can withstand quite cold winters but need a long enough and warm enough growing season in order to build up sufficient energy reserves to grow, reproduce, and survive. The Teton Mountains have warmer and longer growing seasons than Mt. Washington has, accounting for the difference in tree line elevation between the two sites. Similarly, mountains near the equator have a much higher tree line elevation than mountains at higher latitudes due to higher temperatures in the tropics.
Other factors also influence the location of the tree line, including moisture, sunlight, wind, and soil. The tree line in the desert or on the slopes of Hawaiian volcanoes is often at relatively low elevations because the soil is too dry for tree growth. Trees often become smaller and smaller as you approach the tree line because smaller trees need less moisture and oxygen to survive than tall trees. The larger canopy of taller trees also shades the ground and makes it colder. Taller trees are also more exposed to chilling winds that damage tender growing buds.
As the planet warms, the tree line in the Canadian Arctic is much higher than it used to be due to warmer temperatures and greater precipitation. But the tree line may not move higher in other areas due to the presence of other factors such as fire or increased insect pest pressures.
National Geographic - Timberline
Northern Woodlands Magazine - Autumn 2008 issue
If you have a question for Pat email firstname.lastname@example.org! He looks forward to answering your questions!
Anna Lentz, artist, writer, and creativity coach who blogs about making a creative life connected with nature at Spring Bird.