Imagine yourself looking out at a dramatic mountain vista, the jagged peaks of rock rising up well above the trees. The land above the tree line is known as the alpine, while the thick band of pine, spruce and fir that dominate the base of the high mountains is known as the sub-alpine. What causes this dramatic shift from thick forest to treeless tundra?
Let’s start by asking, “What does it take for trees to grow?” Like all green plants, trees capture energy from the sun through the process of photosynthesis. Some trees like lodgepole pines, pinyon pines, and aspens love direct sun. Other trees like spruce and fir are shade tolerant and can thrive in less sunny sites.
Plants and trees also require water that they draw up through their roots along with nutrients from the soil. In Colorado, levels of precipitation are largely determined by elevation, in a weather process known as orographic precipitation. Our weather generally comes from the west, and as the clouds reach the mountains they are forced up in elevation rapidly which cause them to cool, condensate and then precipitate.
The higher we go into the mountains, the more snow and rain falls throughout the year. Once well above tree line however, the winds tend to be strong and actually blow the ridges dry of snow. These areas have been wind scoured. Wind plays an important role in evaporation, especially near the ground’s surface where roots from plants and trees gather water. The winds in the highest elevations also blow much of the snow back down into the sub-alpine. This means that the sub-alpine accumulates the deepest snows. This is why ski areas in Colorado are always located in the sub-alpine, and rarely go very far above tree line.
Plants and trees also thrive in warm temperatures. While trees have adapted to the short growing season in our mountain environment, they require warm enough temperatures in the summertime to grow enough to compensate for getting beaten down by blowing snow and ice crystals in the winter. So the tree line is actually determined by average annual temperature.
We know that sunny south-facing slopes are warmer and drier than shady north-facing slopes, so this helps to explain why the tree line undulates on a mountainside. Favored sites that are less windy and receive warm sun enable the trees to grow higher, while the coldest and windiest areas will not let trees take hold.
I enjoy getting above the tree line whether it’s hiking in the summer, or snowshoeing or telemark skiing in the winter. I love the expansive views and the feeling of wide-open space moves me. It reminds me that I am a very small part of a very large world.