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Factors determining the distribution of major life-zones

differential heating of the Earth's surface by the Sun,
which results from:

  • the shape of the Earth
  • the tilted axis and rotation of the Earth (seasonality)
  • distribution of water and land and the differences in their physical properties.

    The first two would result in continuous belts of different biomes parallel with the Equator. The third factor disrupts this pattern.

    Circulation of air on the surface of the Earth.The circulation is modified by the rotation of the Earth around its axis (Coriolis effect)

    Aerial photograph of the Earth taken by the Apollo crew. Clouds are visible above much of the Congo basin, where rainforest occurs, whereas the sky is clear above the desert regions due to descending air masses.

    tundra -- taiga -- temperate deciduous forest -- grasslands -- desert -- savanna --tropical rainforests

    The distribution of different biomes on the Earth.

    Climograph of North American biomes.


    Tundra occurs between the taiga and the permanently frozen polar regions. The annual precipitation is usually less than 250 mm, and water is not available for living organisms most of the time. During summer months (mostly July and August), the upper layer of the soil thaws, but half a meter underground the soil remains frozen. As a result of the permafrost, short summers and very long cold winters, no trees occur in the tundra. Bogs, ponds and herbaceous plants (and billions of mosquitoes) characterize the landscape, and even the few woody plants (Dwarf Birch, Arctic Willow) are dwarf.

    tundra vegetation in Alaska

    Tundra in Asia

    Reindeer in the tundra at Kolyma

    Tundra bogs in Sweden

    Life in the tundra


    Taiga or coniferous forest zone occupies a wide belt between the tundra and the temperate forests on the American and Eurasian continents. It is characterized by short cool summers (but longer than in the tundra) and long, dry, cold winters. The precipitation is only 300-500 mm annually, and most of it is in the form of snow. The snow usually melts before the upper layers of the soil thaws, and thus much of the water is drained into streams and rivers instead of soaking into the ground. The forest trees are typically different species of firs, spruces and larches mixed with birch toward the south. The soil is grayish with a shallow humus layer, poor in nutrients and acidic with a permanently frozen layer at various depth.

    Taiga forest in Canada

    Taiga with lakes in the Alaskan wilderness

    The taiga in western Siberia

    Mountain coniferous forest in the Yu Long Xue Mountains, NW Yunnan, China

    Taiga forest close-up

    Taiga forest in Finland

    Life in the taiga

    Temperate deciduous forest

    Temperate or broadleaved deciduous forests receive 600-2500 mm precipitation annually, which is more or less evenly distributed throughout the year. Where the annual precipitation exceeds 1500 mm, the forest are referred to as temperate rainforests (such forests occur in the Cascade Range in the Northwestern US.) The winters are much milder and shorter than those in the taigawith only one or two month below freezing. Characterized by many species of oak, ash, beech, maple and elm, this biome is also rich in understory shrubs and herbaceous plants that are particularly dominant in the spring. The vegetation exhibits seasonal changes with growth mostly occurring during spring and summer. The soil has a well developed and thick humus layer which is usually brown; however, due to the large amount of precipitation, the soil is typically acidic.

    Forest in the Great Smoky Mountains

    North American beech-maple forest

    Interior of a European beech forest

    Temperate Deciduous Forest in China

    False-beech forest in New Zealand

    Life in the temperate forests


    Temperate grasslands usually occur between the broadleaved forest and desert biomes. They receive 250-600 mm rainfall during mostly the summer months. The winters are cold and dry. Due to slow decomposition, the soil accumulates huge amounts of humus, whose black layer may be more than a meter thick (hence the name of the soil: black earth, black land, chernozem). The little precipitation does not support trees, but grasses and herbs are present in great abundance. Depending on the amount of rainfall, there are varieties of grasslands such as tallgrass, mixedgrass or shortgrass prairies and steppes. In the latter, drought tolerant grasses (such as feathergrass and buffalograss) and annual plants predominate. Grasslands in North America are called prairie, in Eurasia they are called steppe (in Hungary it is the puszta), and in South America pampa. Grasslands support a large number of herbivores from antelopes, horses and bisons to mice, groundsquirrels and gophers.

    Tall-grass prairie in Kansas

    Short-grass prairie in South Dakota

    Short-grass prairie in Texas

    Life on the prairie

    Mixed-grass steppe in Southern Russia

    The protected mixed-grass steppe at Homutovo, SE Ukraine

    The Hungarian puszta

    Short-grass steppe in Mongolia

    Short-grass alkali-steppe in Manchuria, NE China

    Life on the Eurasian steppe

    South American pampa


    Deserts are places where there is less than 250 mm annual precipitation. The precipitation is unpredictable; there are long periods of drought lasting for eight to ten years. Warm deserts experience very mild winters with few freezing days (Sonoran, Chihuahuan, Saharan deserts), whereas cold deserts have an extended period of winter with termperatures well below freezing (Taklimakan, Gobi). Plant cover is sparse, and characterized by drought tolerant brushes, succulent and annual plants. Deserts are particularly rich in reptiles (snakes and lizards) and rodents (gerbils, jerboas and kangaroo rats), and most of the animals are nocturnal (active at night). The soil is very poor in nutrients, and the humus layer is absent or only weakly developed.

    The Mojave Desert

    Death Valley

    The Chihuahuan Desert in Texas

    The Great Basin Desert in Nevada

    The Sonoran Desert in Mexico

    The Atacama in Chile

    Life in the American deserts

    The Kara Kum desert in Uzbekistan

    Desert sand dunes in Kazakhstan

    The Gobi Desert in Mongolia

    The Taklimakan Desert in Western China

    Desert in the Turfan Basin, China

    The Thar Desert in India

    Life in the Asian deserts

    The Namib Desert in Africa

    The Sahara in Morocco

    The Skeleton Coast in Namibia

    Life in the African Deserts

    Australian deserts


    Toward the warmer climates, the intermediate biome between deserts and the tropical forests is the savanna and thorn bush. There is 900-1500 mm of rain each year, with a pronounced wet or rainy, and an almost equally long dry season. The temperatures are high year long, which results in droughts during the dry season, when most plants die or dry out. Trees are typically drought tolerant (baobab, acacia). Soils are relatively nutrient poor due to leaching out during the rainy seasons. Trees are widely spaced, and the majority of the vegetation is different species of grasses. Fire is important in the maintenance of the savanna, and many plant species are fire tolerant, or even require fire for growth and germination.

    East African savanna

    The Ngorongoro Crater

    The Serengeti

    Savanna near the Okawango Delta, Botswana

    Savanna in South Africa

    Savanna on Madagascar

    Life on the African savanna

    Savanna in India

    Savanna in Brazil

    Tropical rainforests

    Tropical rainforests occur close to the Equator, where temperature fluctuates only little, and the precipitation is more or less evenly distributed throughout a year. The annual rainfall is between 2000-5000 mm (up to 17 feet!), and as a consequence, the humidity is very high. Due to the excess rainfall and rapid decomposition, the soils are very devoid of nutrients. Most of the organic matter is in the form of litter (dead material) or in living organisms. The lowland rainforests (or jungle) are located near the sea level, whereas at higher elevations, montane rainforests or cloud forests develop. The vegetation is lush with many layers in the canopy that support an incredible diversity of life.

    Tropical rainforest in northern Costa Rica

    Tropical rainforest in southeast Costa Rica

    Montane rainforest in Costa Rica

    Amazonian rainforest

    Rainforest in Eastern Brazil

    Life in the American rainforests

    Tropical rainforest in Malaysia

    Lowland rainforest in Sumatra

    Montane rainforest on Mount Kinabalu, Kalimantan

    Tropical rainforest on New Guinea

    Life in the Asian rainforests

    Tropical lowland rainforest in Central Africa

    Ndoki, "the last place on Earth"

    Montane rainforest in the Rwenzori Mountains, Uganda

    Lowland rainforest in Madagascar

    Montane rainforest in eastern Madagascar

    Life in the African rainforests

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