8.4 Biomes Found at All Latitudes
Describe the major characteristics of the biomes found at all latitudes and human impacts in each.
Three biomes occur across a wide range of latitudes. We cover the biomes in this section using a moisture gradient, from wet to dry, rather than latitude. We start with the montane forest biome, then move into the relatively dry tundra, and then conclude with the desert biome.
Montane Forest
The montane forest biome occurs where orographic lifting increases precipitation on the windward side of a mountain range (see Section 3.3). The vegetation of the montane forest biome is needle-leaved in the Northern Hemisphere and broad-leaved in the Southern Hemisphere. Because it is found at many latitudes, the montane forest is one of the most climatologically diverse biomes (Figure 8.21).
Montane forest. (A) Montane forest near Creel, Chihuahua, Mexico. (B) Climate diagram for Creel. Average annual temperatures vary greatly within the montane forest biome depending on elevation and latitude. Average annual precipitation also varies significantly by latitude, with some areas receiving over 250 cm (98 in). As a rule, precipitation is always sufficient to support trees. (C) Montane forest is found in North America from the highlands of Mexico to southern Alaska. Other significant montane forests are found in the Andes in South America, the eastern African highlands, the European Alps, and the mountains of Asia.
(A. © Sebastian Rauprich)
montane forest
A forest biome composed of needle-leaved trees in the Northern Hemisphere and broad-leaved trees in the Southern Hemisphere; found on windward sides of mountains with abundant precipitation.
The montane forests of the Northern Hemisphere are dominated by tree species in the pine family, such as spruce, larch, pine, and fir. The pine family does not occur in the Southern Hemisphere, and montane forests there are dominated by different tree species depending on the location. Evergreen broad-leaved eucalyptus trees, for example, dominate Australia’s montane forests. New Zealand’s montane forests are dominated by evergreen broad-leaved southern beeches and podocarps, as are Patagonia’s in southern South America.
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8.10
How old is the world’s oldest living tree and where is it?
The world’s oldest living tree, a bristlecone pine, is estimated to be 5,062 years old. It is growing in California’s White Mountains, at a location that is kept undisclosed to protect it from souvenir seekers.
The world’s oldest trees are also found in this biome. High elevations at midlatitudes have low temperatures for much of the year. These low temperatures and a short growing season cause many tree species to grow slowly and live for millennia (Figure 8.22).
Montane forests are home to ancient trees. (A) The longest-lived tree species is the bristlecone pine (Pinus longaeva). The oldest known individual is 5,062 years old. (B) The oldest giant sequoias (Sequoiadendron giganteum) are over 3,000 years old. They are the heaviest single-stemmed trees on the planet. The General Sherman tree, shown here, is 31.3 m (102 ft) in circumference and 83.8 m (275 ft) tall. It weighs almost 2,000 metric tons.
(Left, © Rob Blakers/Lonely Planet Images/Getty Images; right, Bruce Gervais)
Commercial logging is a significant factor in montane forests. Many of these forests have been subjected to clear-cutting: removal of all trees over a large area. The results are clearly visible from space, as Figure 8.23 shows.
Clear-cuts in the Pacific Northwest. The montane forests of western Washington and Oregon have few areas that have been spared clear-cutting practices. Clear-cuts are visible here in southern Washington as light patches. The distance across this image is approximately 23 km (14 mi).
(NASA)
Fire is also a powerful force in the montane forest biome, as we saw in Section 7.4, and many of its plants are adapted to fire. Anthropogenic fire suppression, and the resulting fuel buildup, can result in catastrophic fires that change the forest structure and species composition. To reverse this trend, forest managers are now using prescribed burns and selective cutting to reduce accumulations of fuel. Where these techniques are applied, they can be effective at addressing the problem.
Tundra
The tundra biome occurs at any altitude or latitude where it is too cold for trees to grow. There are two types of tundra: alpine tundra, found at high elevations, and northern tundra, found at high latitudes in the Northern Hemisphere. Alpine tundra is a cold, treeless high-elevation biome whose vegetation consists mainly of shrubs and herbaceous perennials. Alpine tundra occurs in high mountainous areas, including the Canadian Rockies, the Andes, and the Himalayas (see Figure 8.25). Northern tundra is a cold, treeless high-latitude biome, also dominated by herbaceous perennials. It is found north of the boreal forest throughout northern Eurasia and North America (Figure 8.24).
Tundra. (A) Purple saxifrage (Saxifraga oppositifolia) is typical of the low-growing herbaceous plants in the tundra biome. This plant is in full bloom near Resolute, Nunavut, Canada. (B) Climate diagram for Resolute. Average annual temperatures in tundra range from −15°C to −5°C (−5°F to 23°F). Average annual precipitation varies considerably with latitude, but is generally less than 100 cm (39 in). (C) Most of the tundra biome is found at high northern latitudes. High mountain regions also support tundra.
(A. © Wayne Lynch/All Canada Photos/Alamy)
tundra
A biome that occurs at any latitude where it is too cold for trees to grow.
alpine tundra
A cold, high-elevation treeless biome with herbaceous perennials.
northern tundra
A cold, treeless high-latitude biome with vegetation consisting of shrubs and herbaceous perennials; found north of the boreal forest throughout northern Eurasia and North America
The ecotone between the boreal forest and the northern tundra is called the northern tree line. Similarly, the upper limit of trees in mountains, defined by low temperatures, is called the alpine tree line (or timberline) (Figure 8.25).
Alpine tree line, Alberta, Canada. An alpine tree line (marked by the dotted line) can be seen clearly in this photo in Banff National Park, Alberta. Harsh conditions stunt tree growth and inhibit trees above this line.
(© Daniel Mosquin)
northern tree line
The northernmost limit of the boreal forest.
alpine tree line
The upper limit of trees in mountains, defined by low temperatures.
The northern tundra and alpine tundra at mid-latitudes have large annual temperature ranges. Alpine tundra within the tropics, called tropical alpine scrubland, has little annual temperature variation. Instead, it experiences diurnal temperature swings: Days are cool and nights below freezing every day of the year. Tropical alpine scrubland occurs above 3,300 m (10,560 ft) elevation and is found in Hawai‘i, the Andes of South America, the mountains of East Africa, and in the New Guinea highlands. It is dominated by herbaceous perennials with rosette growth forms. The similarity of the plants in these widely separated locations is an example of ecological equivalence, as Picture This explains.
Many northern tundra soils are permafrost soils, which are frozen just below the surface year-round (see Section 17.1). The vegetation structure of all tundra consists of a single layer of small shrubs and low herbs. Plants remain low to the ground, where it is warmer and where they can be covered by an insulating layer of snow in winter. There are few annual plants in the tundra because the growing season is too short and cold for most plants to complete their life cycle and set seed. The biomass of standing tundra vegetation is low, as is the diversity of species. Many of the same species grow throughout the tundra across broad geographic regions.
Picture This
All of Earth’s biomes reflect the response of the biota to regional climate. Through evolution, different, unrelated groups of plants and animals within similar climates begin to resemble one another. Thus, widely separated but similar climates produce similar plant and animal adaptations. This biome-level resemblance is called ecological equivalence.
Although tropical alpine scrublands in different regions are structurally alike, their plant species are genetically unrelated. Different groups of plants have converged on the structure that works best in the Köppen H climate zone in the tropics. Tropical high-elevation settings receive intense ultraviolet radiation. Many tropical alpine plants are adapted to reduce the effects of UV exposure. Many have evolved parabolic rosettes, a growth form in which all the leaves emerge from one location on the plant. Reflective silver hairs, also common in tropical alpine plants, reflect UV radiation to protect the plant from its harmful effects. Shown here are the Haleakalā silversword (Argyroxiphium sandwicense) of Hawai‘i (A), espeletia (Espeletia pycnophylla) of the Andes in Ecuador (B), and giant lobelia (Lobelia deckenii) on Mt. Kilimanjaro in Tanzania (C). (A. © Rich Reid/NationalGeographic/Getty Images; B. © Christian Heeb/ Prisma BildagenturAG/Alamy; C. Photo © Marjn van den Brink)
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8.11
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8.12
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An important feature of the northern tundra is the number of migratory animals it receives each summer. Although its summers are brief, they are productive because the daylight hours are very long. North of the Arctic Circle, the Sun does not set (see Section 2.1). As a result, biological productivity experiences a brief burst in summer. Many bird species use the northern tundra for summer breeding.
Because of the tundra’s remoteness and harsh conditions, the human footprint in this biome is limited. Human impacts are occurring mostly through road building and resource extraction. As shown in Figure 8.26, the northern tundra is increasingly being developed for its mineral resources, particularly petroleum and minerals.
Ekati diamond mine, Yellowknife, Northwest Territories, Canada. Canada is the second largest diamond producer in the world, after Russia. The Ekati Diamond Mine is North America’s first commercial diamond mine and one of the largest diamond mines in the world. Open-pit mines such as this one permanently alter the landscape. These pits are about 0.75 km (0.5 mi) across and several hundred feet deep.
(Jason Pineau/All Canada Photos/ Getty Images)
Oil and gas exploration on the North Slope of Alaska has had an impact on the tundra. Large reserves of natural gas on the Yamal Peninsula of Russia have been developed as well. Metals such as nickel, tungsten, and platinum are extracted from many northern tundra sites, particularly in northern Russia.
Climate change is having a growing impact on tundra permafrost soils. The Arctic is the fastest-warming region on Earth, and tundra ecosystems are rapidly changing as their permafrost soils thaw. Permafrost soils are rich in carbon from organic remains. As they thaw, they emit methane and carbon to the atmosphere, contributing to the greenhouse effect (see Section 17.1).
Desert
Desert is a biome with chronic moisture deficits and sparse, drought-adapted vegetation. The largest of the biomes in area, desert covers nearly 30% of Earth’s land surface and is found on every continent. Most deserts receive less than 25 cm (10 in) of annual precipitation. The single largest desert is the Sahara of northern Africa, covering almost 10 million km2 (3.9 million mi2) (Figure 8.27).
Desert. (A) The Sahara near Salah, Algeria. (B) Climate diagram for Salah. (C) The largest and driest deserts are found beneath the subtropical high, centered at about 30 degrees latitude north and south, but deserts are found at other latitudes as well.
(A. © Raouf Djaiz)
desert
A biome with chronic moisture deficits and drought-adapted vegetation.
The three broad groups of deserts are hot deserts (which are located beneath the subtropical high), rain-shadow deserts (on the leeward sides of mountain ranges), and cold deserts (found at high latitudes and high elevations).
Hot deserts, such as the Namib Desert in Namibia and the Sahara in Northern Africa, are caused by the descending air of the subtropical high in the vicinity of 30 degrees latitude north and south.
Rain-shadow deserts form on the leeward sides of mountain ranges due to adiabatic heating (see Section 3.3). The Gobi Desert, for example, is found throughout Mongolia in the rain shadow of the Himalayas, the Pamirs, and the Altai ranges. Its location in the continental interior, and its high elevation and resulting low temperatures, also contribute to the Gobi’s aridity.
Cold deserts are found at the poles and at high elevations. Polar regions are deserts because cold air has low water vapor content. The McMurdo dry valleys of Antarctica, for example, are exceedingly cold and arid.
In many deserts, a combination of factors causes aridity. For example, the Atacama Desert of Chile averages 0.4 cm (0.15 in) of precipitation per year and is among the driest locations on Earth. It lies in the rain shadow of the Andes and is under the influence of the subtropical high. In addition, a cold offshore current inhibits evaporation and rainfall from the Pacific Ocean.
In all deserts, plants and animals exhibit a wide range of physiological and behavioral responses to the scarcity of water, including deep roots to reach water and germination from seeds (see Section 7.2).
Deserts are largely off-limits to permanent human settlement because of the lack of available water. As a result, human impacts have been relatively light in this biome. There are places, however, where large populations live in the desert biome. In the United States, for example, Las Vegas, Nevada, and Phoenix, Arizona, are located in deserts. These cities import water, mostly from the Colorado River, or pump it from the ground. This water use has reduced stream flow and influenced the ecology of many streams and riparian (streamside) areas significantly. Other rivers that flow through deserts include the Indus River, which flows through the deserts of eastern Pakistan and western India, and the Nile River, which flows through the easternmost Sahara. These rivers support large populations along their courses. People have lived sustainably in these desert areas for millennia.
