A high-latitude biome composed of limited biodiversity. Because of the colder temperatures compared to lower latitudes, plants are limited to small bushes, grasses, mosses, and lichens and almost no trees. Mostly below freezing, the temperature in the summer in the tundra can go above zero and cause partial thawing of the upper layers above the permafrost (frozen soil and dead plant stuff) for only a few days or weeks per year. Lots of carbon is stored in the thick mats in tundra layers for hundreds and thousands of years and, if long term temperature of the earth's tundra increases then biological degradation can cause the release of carbon in the form of methane or carbon dioxide. A fire in 2007 in Alaska released large amounts of this type of stored carbon. It's realistic to assume that tundral fires might increase if the earth's temperature warms--warming that recent widely measured temperatures and atmospheric modelling show occurs more at higher latitudes rather than lower--and so tundral fires can increase overtime. This would be a positive feedback cycle since this released carbon would contribute to global warming.