Brian Paul, age 8, of Wichita, Kansas, for his question:

What causes a timberline?

The timber is trees, growing on the slopes of a mountain. If the mountain is very tall, they grow as high as a certain line    and no higher. This is the timberline, where the forest trees thin out and disappear. Tough little bushes and shrubs may climb higher up the slopes. But if the mountain is very high, they also disappear. And higher still the peak of the mighty mountain is crowned with snow.

This arrangement happens because plants grow wherever they can. Trees must have deep layers of moist soil to anchor their roots. High mountain peaks are rocky and wild winds blow away the dusty dirt. The ground is hard and bare. What's more, the higher up we go the colder it gets. Even in the tropics, the highest peaks are frozen solid, summer and winter. Trees cannot grow in this hard rock and frozen ground. But they grow just as far up the mountain as they can    and where they can grow no higher, they foam a timberline.