Trees do not grow on mountain tops either because the situation is too exposed, or because the soil is too thin or too frozen to allow their roots to draw nourishment from the ground.
In most mountainous areas there is usually a clearly marked timberline, a boundary above which there is no tree growth. Sometimes the height of the timberline is dictated by local climatic or soil conditions, but as a general rule the boundary gets lower as the distance from the Equator increases. In the far north and south the cold is so intense that it is quite impossible for any trees to grow, and the timberline is therefore at sea level.
A range of mountains on or near the Equator, like the Ruwenzori range in Africa, can be divided into different belts of vegetation according to the types of trees growing at its base and at various heights up its slopes.
Similarly the belts of vegetation change according to the distance from the Equator. The first belt is the tropical or rain-forest region where it is hot and trees grow rapidly. Next comes a hot dry belt where few trees grow because there is so little rain. This is followed by the deciduous or warm and temperate belt, and by the coniferous belt, with very cold winters but fairly warm summers. Then comes the timberline, beyond which trees cannot grow, and finally the regions of permanent ice and snow, where no vegetation at all can live.