Figure 1.23 Biomes in relation to rainfall and temperature. The variety of environmental conditions experienced in terrestrial biomes can be described in terms of their annual rainfall and mean annual temperatures.

      Source: After Woodward & Lomas (2004).

      tundra

      Tundra occurs around the Arctic Circle, beyond the tree line. Small areas also occur on sub‐Antarctic islands in the southern hemisphere. ‘Alpine’ tundra is found under similar conditions but at high altitude. The environment is characterised by the presence of permafrost – water permanently frozen in the soil – while liquid water is present for only short periods of the year. The typical flora includes lichens, mosses, grasses, sedges and dwarf trees. Insects are extremely seasonal in their activity, and the native bird and mammal fauna is enriched by species that migrate from warmer latitudes in the summer. In the colder areas, grasses and sedges disappear, leaving nothing rooted in the permafrost. Ultimately, vegetation that consists only of lichens and mosses gives way, in its turn, to the polar desert. The number of species of higher plants (i.e. excluding mosses and lichens) decreases from the Low Arctic (around 600 species in North America) to the High Arctic (north of 83°, e.g. around 100 species in Greenland and Ellesmere Island). In contrast, the flora of Antarctica contains only two native species of vascular plant and some lichens and mosses that support a few small invertebrates. The biological productivity and diversity of Antarctica are concentrated at the coast and depend almost entirely on resources harvested from the sea.

      taiga

Taiga or northern coniferous forest occupies a broad belt across North America and Eurasia. Liquid water is unavailable for much of the winter, and plants and many of the animals have a conspicuous winter dormancy in which metabolism is very slow. Generally, the tree flora is very limited. In areas with less severe winters, the forests may be dominated by pines (Pinus species, which are all evergreens) and deciduous trees such as larch (Larix), birch (Betula) or aspens (Populus), often as mixtures of species. Farther north, these species give way to single‐species forests of spruce (Picea) covering immense areas. The overriding environmental constraint in northern spruce forests is the presence of permafrost, creating drought except when the sun warms the surface. The root system of spruce can develop in the superficial soil layer, from which the trees derive all their water during the short growing season.

      temperate forests

      Temperate forests range from the mixed conifer and broad‐leaved forests of much of North America and northern Central Europe (where there may be 6 months of freezing temperatures), to the moist, dripping forests of broad‐leaved evergreen trees found at the biome’s low‐latitude limits in, for example, Florida and New Zealand. In most temperate forests, however, there are periods of the year when liquid water is in short supply, because potential evaporation exceeds the sum of precipitation and water available from the soil. Deciduous trees, which dominate in most temperate forests, lose their leaves in the autumn and become dormant. On the forest floor, diverse floras of perennial herbs often occur, particularly those that grow quickly in the spring before the new tree foliage has developed. Temperate forests also provide food resources for animals that are usually very seasonal in their occurrence. Many of the birds of temperate forests are migrants that return in spring but spend the remainder of the year in warmer biomes.

      grassland

      Grassland occupies the drier parts of temperate and tropical regions. Temperate grassland has many local names: the steppes of Asia, the prairies of North America, the pampas of South America and the veldt of South Africa. Tropical grassland or savanna is the name applied to tropical vegetation ranging from pure grassland to some trees with much grass. Almost all of these temperate and tropical grasslands experience seasonal drought, but the role of climate in determining their vegetation is almost completely overridden by the effects of grazing animals that limit the species present to those that can recover from frequent defoliation. In the savanna, fire is also a common hazard in the dry season and, like grazing animals, it tips the balance in the vegetation against trees and towards grassland. Nonetheless, there is typically a seasonal glut of food, alternating with shortage, and as a consequence the larger grazing animals suffer extreme famine (and mortality) in drier years. A seasonal abundance of seeds and insects supports large populations of migrating birds, but only a few species can find sufficiently reliable resources to be resident year‐round.

      Many of these natural grasslands have been cultivated and replaced by arable annual ‘grasslands’ of wheat, oats, barley, rye and corn. Such annual grasses of temperate regions, together with rice in the tropics, provide the staple food of human populations worldwide. At the drier margins of the biome, many of the grasslands are ‘managed’ for meat or milk production, sometimes requiring a nomadic human lifestyle. The natural populations of grazing animals have been driven back in favour of cattle, sheep and goats. Of all the biomes, this is the one most coveted, used and transformed by humans.

      chaparral

      Chaparral or maquis