substrate that has been frozen for a minimum of 2 years. There are two types of permafrost: discontinuous and continuous. Discontinuous permafrost is found in areas where the mean annual air temperature is only slightly below freezing (); this forms in sheltered locations. In areas where the mean annual soil surface temperature is below , continuous permafrost forms. This is not limited to sheltered areas and ranges from a few inches below the surface to over deep. The top layer is called the active layer. It thaws in the summer and is critical to plant life. Biomes Moisture and temperature are major physical drivers of natural ecosystems. The more arid and colder conditions found at higher northern latitudes (and high elevations elsewhere) support tundra and boreal forests. The water in this region is generally frozen and evaporation rates are very low. Species diversity, nutrient availability, precipitation, and average temperatures increase as you move from the tundra to boreal forests and then to deciduous temperate ecosystems, which are found south of these Arctic biomes. Tundra Tundra is found from 55 ° to 80° N latitude in North America, Eurasia and Greenland. It can be found at lower latitudes at high elevations as well. The average temperature is ; during the summer it is less than . Average precipitation ranges from , and the permafrost is thick. Plant species supported by tundra have small leaves, are short (74 mm to <5 m), tend to be deciduous, and have a high ratio of roots to shoots. They are composed mainly of perennial forbs, dwarf shrubs, grasses, lichens, and mosses. Boreal In comparison to tundra, boreal forest has a longer and warmer growing season and supports larger species diversity, an increase in canopy height, vegetation density, and biomass. Boreal conditions can be found across northern North America and Eurasia. The boreal forests in the interior of the continents grow on top of permafrost due to very cold winters (see drunken trees), while much of the boreal forest has patchy permafrost or lack permafrost completely. The short (3–4 month) growing season in boreal forests is sustained by greater levels of rainfall (between per year) than the tundra receives; This biome is dominated by closed canopy forests of evergreen conifers, especially spruces, fir, pine and tamarack with some diffuse-porous hardwoods. Shrubs, herbs, ferns, mosses, and lichens are also important species. Stand-replacing crown fires are very important