mean annual soil temperatures are around 6.7 degrees C. In the tropics, this region is typified by montane rain forest (above 3,000 ft) while at higher latitudes coniferous forests often dominate. Lowland layer: This lowest section of mountains varies distinctly across climates and is referred to by a wide range of names depending on the surrounding landscape. Colline zones are found in tropical regions and Encinal zones and desert grasslands are found in desert regions. Colline (tropics): Characterized by deciduous forests when in oceanic or moderately continental areas, and characterized by grassland in more continental regions. Extends from sea level to about 3,000 feet (roughly 900 m). Vegetation is abundant and dense. This zone is the typical base layer of tropical regions. Encinal (deserts): Characterized by open evergreen oak forests and most common in desert regions. Evaporation and soil moisture control limitation of which encinal environments can thrive. Desert grasslands lie below encinal zones. Very commonly found in the Southwestern United States. Desert grassland: Characterized by varying densities of low lying vegetation, grasslands zones cannot support trees due to extreme aridity. Some desert regions may support trees at base of mountains however, and thus distinct grasslands zones will not form in these areas. For detailed breakdowns of the characteristics of altitudinal zones found on different mountains, see List of life zones by region. Treeline The most decisive biogeographic and climatic boundary along elevation gradients is the climatic high-elevation treeline. The treeline separates the montane from the alpine zone and marks the potential for tree growth, irrespective of whether trees are present or not. So when trees had been cut or burnt, and thus, are absent from the treeline, it is still in place as defined by the treeline isotherm. At the tree line, tree growth is often sparse, stunted, and deformed by wind and cold krummholz (German for "crooked wood"). The tree line often appears well-defined, but it can be a more gradual transition. Trees grow shorter and often at lower densities as they approach tree line, above which they cease to exist. Animal zonation Animals also exhibit zonation patterns in concert with the vegetational zones described above. Invertebrates are more clearly defined into zones because they are typically less mobile than vertebrate species. Vertebrate animals often span across altitudinal zones according to the seasons and food availability. Typically animal species diversity and abundance decrease as a function of elevation