Trees do not grow above the timberline due to a lack of air, heat, and water. The timberline is actually an area of gradual change from tall trees to short, to bushes and finally to no trees at all.
There are three distinct types of timberlines: the alpine timberline, the desert timberline and the desert-alpine timberline. At the alpine timberline, the elevation is too high for air to be well-oxygenated and temperatures are too cold to sustain tree growth. The desert timberline is a low elevation point where conditions are too dry for trees. The desert-alpine timberline occurs where the elevation is too high and too dry to nurture trees, even if temperatures are warm enough.