The timberline is the elevation on a mountain above which trees cannot grow. If you've ever hiked up a tall mountain, you might have noticed the trees getting shorter and more twisted as you climbed higher, until suddenly there were no trees at all, just low shrubs, grasses, and bare rock. That boundary where the forest ends is the timberline.

Trees can't grow above the timberline because conditions become too harsh. The air gets colder, winds blow harder, the growing season becomes too short, and the soil becomes thin or frozen. Different mountains have timberlines at different heights depending on their location: mountains near the equator have higher timberlines than mountains closer to the poles, where it's generally colder.

The timberline is also called the treeline or the tree line. Above it, you enter what's called the alpine zone. This treeless zone has its own special ecosystem of hardy plants and animals adapted to extreme conditions. Mountain goats, pikas, and marmots live above the timberline, along with tough little wildflowers that bloom during the brief summer.