Trees grow all over the world, in many different types of weather. But above certain elevations, trees just cant grow. Think of it like this: someone draws a horizontal line on a mountainside; above that line, there are no trees. This imaginary line on Earth is called the timberline, or the tree line. The timberline is usually a point where there isnt enough air, heat, or water to keep trees alive.

Although the timberline often seems abrupt from a distance, on the ground you can observe a gradual change from big, tall trees to stumpy ones. For example, trees at the timberline start to look more like low bushes than trees. Small trees need less moisture and less oxygen. The trees will get shorter and shorter until the weather is too harsh for any trees, large or small, to grow.

Sometimes, the timberline can be lowered by natural causes such as fire. Other times, the timberline can be lowered by human activity. Development can lower the timberline when land is used for commercial activity, agriculture, or housing. Even hiking, over a long period of time, can trample saplings or prevent new ones from taking root. This means trees stop growing a lot sooner than they would have otherwise, lowering the timberline.

Pollution can also lower the timberline. Trees need air, water, and soil to survive. If one of those elements is contaminated, entire groves of trees can die. The copper and nickel smelter on the Kola Peninsula in Russia is one of the largest producers of heavy metals in Europe. Trees in the area have large amounts of copper, nickel, lead, and sulfur. These heavy metals have prevented many trees from growing in the cold climate of the natural timberline. The timberline in northwest Russia is lower as a result.

Timberline can climb up as well as climb down. Due to the effects of global warming, the timberline in Canadas Arctic is much higher than it was in the past. Warmer temperatures and greater precipitation have improved growing conditions in the area. The provinces of Yukon and Labrador now have trees such as white spruce and balsam fir growing past what used to be the natural timberline.

Types of Timberlines

The alpine timberline marks the point where the elevation is too high, and usually too cold, for tree growth. The city of Vail, Colorado, is located near an alpine timberline in the Rocky Mountains. Trees along the Vail timberline include quaking aspen and lodgepole pine.

The desert timberline marks the point where the soil is too dry for tree growth. Youll find this kind of timberline at very low elevations, usually below 1,500 meters (5,000 feet). The desert timberline in the Sonoran Desert of the United States and Mexico features cactus as well as trees like palo verde.

A desert-alpine timberline is the point where the elevation is too high and the soil is too dry for tree growth. Some places, like the slopes of the Mauna Loa volcano in the U.S. state of Hawai'i, are very high up but have low rainfall and a lot of exposure to the sun. The conditions are too dry for tree growth.