As elevation increases, the climate becomes cooler, due to a decrease in the greenhouse effect. The characteristic flora and fauna in the mountains tend to strongly depend on elevation, because of the change in climate. This dependency causes life zones to form: bands of similar ecosystems at similar altitude.
One of the typical life zones on mountains is the montane forest: at moderate elevations, the rainfall and temperate climate encourages dense forests to grow. Holdridge defines the climate of montane forest as having a biotemperature of between 6 and 12 °C (43 and 54 °F), where biotemperature is the mean temperature considering temperatures below 0 °C (32 °F) to be 0 °C (32 °F). Above the elevation of the montane forest, the trees thin out in the subalpine zone, become twisted krummholz, and eventually fail to grow. Therefore, Montane forests often contain trees with twisted trunks. This phenomenon is observed due to the increase in the wind strength with the elevation.The elevation where trees fail to grow is called the tree line. The biotemperature of the subalpine zone is between 3 and 6 °C (37 and 43 °F).
Above the tree line the ecosystem is called the alpine zone or alpine tundra, dominated by grasses and low-growing shrubs. The biotemperature of the alpine zone is between 1.5 and 3 °C (34.7 and 37.4 °F). Many different plant species live in the alpine environment, including perennial grasses, sedges, forbs, cushion plants, mosses, and lichens. Alpine plants must adapt to the harsh conditions of the alpine environment, which include low temperatures, dryness, ultraviolet radiation, and a short growing season. Alpine plants display adaptations such as rosette structures, waxy surfaces, and hairy leaves. Because of the common characteristics of these zones, the World Wildlife Fund groups a set of related ecoregions into the "montane grassland and shrubland" biome.
Montane forests occur between the submontane zone and the subalpine zone. The elevation at which one habitat changes to another varies across the globe, particularly by latitude. The upper limit of montane forests, the forest line or timberline, is often marked by a change to hardier species that occur in less dense stands. For example, in the Sierra Nevada of California, the montane forest has dense stands of lodgepole pine and red fir, while the Sierra Nevada subalpine zone contains sparse stands of whitebark pine.