The majority of the ground cover stays frozen throughout the year, with only the top 2 to 3 feet thawing out during the short, four-month spring and summer period. Tundra is found near the poles. Mountains are very complex areas. Drastic changes in elevation over a short distance impact rainfall, soil types, and temperature. Because the type of ecosystem found in an area is dependent on these abiotic factors, a mountain can contain several radically different types of ecosystems in a relatively small area.
Ecologists refer to mountains as areas of complex zonation because of the many ecosystems found.
For instance, temperate deciduous forest can be found at the base of a mountain. As the altitude climbs, and the climate becomes colder, the forest may change to a more rugged coniferous form. Once the altitude becomes too high and the temperature too cold, trees will no longer grow. This is referred to as the tree line or timberline. At this point, the ecosystem is similar to that found in the tundra.