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Tropical and subtropical desert climate, major climate type of the Köppen classification dominated in all months by the subtropical anticyclone (or subtropical high), with its descending air, elevated inversions, and clear skies. Such an atmospheric environment inhibits precipitation. Most of Earth's tropical, true desert ( BW) climates occur between 15° and 30° latitude, at the poleward end of the Hadley cell circulation ( see atmosphere). In the Köppen-Geiger-Pohl system, this climate is divided between the tropical desert (BWh) and subtropical desert (part of BWk) subtypes.

The most extreme arid areas also are far removed from sources of moisture-bearing winds in the interiors of continents and are best developed on the western sides of continents, where the subtropical anticyclone shows its most intense development. An exception to the general tendency for aridity to be associated with subsidence is in the so-called Horn of Africa region, where the dryness of Somalia is caused more by the orientation of the landmass in relation to the atmospheric circulation. Both the high- and low-sun monsoonal winds blow parallel to the coast, so that moisture-laden maritime air can penetrate over land only infrequently.

Köppen climate classification map M.C. Peel, B.L. Finlayson, and T.A. McMahon (2007), updated world map of the Köppen-Geiger climate classification, Hydrology and Earth System Sciences, 11, 1633-1644.

In most low-latitude deserts, cloud cover is uncommon (fewer than 30 days per year have clouds in some areas). Precipitation amounts are mostly in the range 0-25 cm (0-10 inches), although the unreliability of precipitation is more significant than the small totals. Average figures have little meaning; a location with a 10-year mean of 5 cm (2 inches), for example, might have received 50 cm (about 20 inches) in one year as a result of an unusual intrusion of moist air, followed by nine years with no measurable precipitation.

Temperatures are high, with monthly means in the range 21-32 °C (70-90 °F). Daily temperature variations are extreme. Ranges of 35 °C (63 °F) are not unknown when daytime maxima in excess of 40 °C (104 °F) are followed by a rapid nocturnal temperature drop brought about by the limited capacity of the dry, cloudless desert air to emit infrared radiation to the ground to offset radiation loss from the surface at night. The highest air temperatures recorded on Earth have been in the BWh regions; for example, in shaded, well-ventilated locations, Death Valley in the western United States has reached 57 °C (135 °F), while al-ʿAzīzīyah in Libya has had a recorded high of 58 °C (136 °F). Actual surface temperatures may reach 82 °C (180 °F) on dry sand under intense sunshine.

An interesting variant of tropical and subtropical deserts are the so-called West Coast Desert areas found on the western coastal margins of the regions discussed above (such as in the Sonoran Desert of North America, the Peru and Atacama deserts of South America, and the Sahara [Moroccan part] and Namib deserts of Africa). These areas are much cooler than their latitude would suggest (monthly mean temperatures of only 15-21 °C [59-70 °F]), and parts are classified as BWk in Köppen's scheme. The cooling results from airflow off adjacent coastal waters where upwelling of the ocean gives rise to cold currents. Deserts of this sort are subject to frequent fog and low-level clouds; yet they are extremely arid. Some parts of the Atacama Desert, for example, have recorded no precipitation for 20 years.

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Tropical and subtropical desert climate: Tropical and subtropical desert climate, major climate type of the Köppen classification dominated in all months by the subtropical anticyclone (or subtropical high), with its descending air, elevated inversions, and clear skies. Such an atmospheric environment inhibits precipitation. Most of

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Woodland/Grassland

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The chaparral biome is a type of temperate woodland biome that is dominated by more broad-leafed evergreen shrubs than by evergreen trees. The area in where the biome is found in Mediterranean...

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Temperate Rainforests are located mostly in South America, Parts of Asia and Australia, but we have one right here in the United States. In Washington, to be exact. The Olympic Peninsula is...

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The desert biome is an example of terrestrial (land) biome. Daytime temperatures are high in summer and low in winter in temperate desert biomes. The average temperature range in temperate desert...

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Polar ecosystem: Polar ecosystem, complex of living organisms in polar regions such as polar barrens and tundra. Polar barrens and tundra are found at high latitudes on land surfaces not covered by perpetual ice and snow.

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Coastal deserts are considered more hospitable compared to other desert types: hot and dry, cold and semi-arid. This is based on the range of plants and animals that coastal deserts can support with a reference to temperature levels and variations, rainfall experienced, and soil characteristics. Major coastal deserts are found along coastlines near large water bodies especially oceans. More often than not, they are also found between ranges of mountains. The close proximity to water bodies and mountain ranges confers to these deserts their features. Known coastal deserts are Namib Desert and Atacama Desert.

Characteristics of Coastal Deserts

Coastal deserts have moderately brief winter seasons whose temperatures are 5°C and below, with the lowest recorded temperature being -4°C. The summer temperatures range from 13°C to 24°C, with 35°C being the highest ever experienced in the coastal deserts. An average of 13cm of rainfall is known to occur annually. Despite being near water sources, these deserts remain notably dry because most precipitation occurs in the oceans leaving negligible amounts of rainfall to pour onto land. These deserts have more porous soils characterized by a moderate texture and a moderate salt content.

Flora and Fauna of Coastal Deserts

Animals in existence in the coastal desert climatic conditions are larger compared to other deserts. Mammals like coyotes are a common occupant of these deserts. Others are reptiles like snakes and lizards. These animals have adapted to these hostile conditions in various ways like being nocturnal and burrowing tendencies. The birds and reptiles supported in this habitat lay eggs that remain dormant until conditions are more bearable. The plants in this ecosystem have thick fleshy leaves and stems for storing nutrients and large root systems for tapping moisture. These adaptations help in survival in the harsh coastal desert climate for plants like black sage, salt bushes, and rice grass.

Namib Desert

Found in the African continent, this desert is considered one of the oldest on planet Earth owing to the fact that it has remained in its state of aridity for more than 50 million years. It is a 1,200 miles stretch of arid land in the South African region involving three countries: South Africa, Angola and Namibia. It is along the coastline of Atlantic Ocean and closely associated with the Great Escarpment on one side. Namib Desert is very dry, experiencing an average of as little as 10 mm of rainfall annually. This desert does not support any form of human life and common sites in it are sand dunes, gravel plains, and range of mountains.

Atacama Desert

This is the third driest desert in the world after the two polar deserts; Arctic and Antarctica. It is situated in the South American continent, along the Pacific Ocean with the bulk of the desert being in Chile. It is a 600 miles stretch, between Andes Mountains and a range of mountains on the Chilean border, making it a rain shadow. Its position between these mountains makes it harder for rainfall to form thus this desert receives approximately 1 mm of rainfall annually and virtually no life. Because it is very dry, this desert is occasionally used equipment for use in Mars by National Aeronautics and Space Administration.

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Coastal deserts are considered more hospitable compared to other desert types: hot and dry, cold and semi-arid. This is based on the range of plants and animals that coastal deserts can support with a reference to temperature levels and variations, rainfall experienced, and soil characteristics.

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, also spelled savannah, vegetation type that grows under hot, seasonally dry climatic conditions and is characterized by an open tree canopy (i.e., scattered trees) above a continuous tall grass understory (the vegetation layer between the forest canopy and the ground). The largest areas of savanna are found in Africa, South America, Australia, India, the Myanmar (Burma)- Thailand region in Asia, and Madagascar.

Origin

Savannas arose as rainfall progressively lessened in the edges of the tropics during the Cenozoic Era (66 million years ago to the present)-in particular, during the past 25 million years. Grasses, the dominant plants of savannas, appeared only about 50 million years ago, although it is possible that some savanna-like vegetation lacking grasses occurred earlier. The South American fossil record provides evidence of a well-developed vegetation, rich in grass and thought to be equivalent to modern savanna, being established by the early Miocene Epoch, about 20 million years ago.

Climates across the world became steadily cooler during that period. Lower ocean surface temperatures reduced water evaporation, which slowed the whole hydrologic cycle, with less cloud formation and precipitation. The vegetation of midlatitude regions, lying between the wet equatorial areas and the moist cool temperate zones, was affected substantially.

The main regions in which savannas emerged in response to that long-term climatic change-tropical America, Africa, South Asia, and Australia-were already separated from each other by ocean barriers by that time. Plant migration across those barriers was inhibited, and the details of the emergence of savannas on each continent varied. In each region different plant and animal species evolved to occupy the new seasonally dry habitats.

In temperate regions, savannas became much more widespread, at the expense of forests, during the long, cool, dry intervals-contemporaneous with the ice ages, or glacial intervals, of the Pleistocene Epoch (2.6 million to 11,700 years ago). Studies of fossilized pollen in sediments from sites in South America, Africa, and Australia provide strong support for this view.

When human beings (Homo sapiens) first appeared, in Africa, they initially occupied the savanna. Later, as they became more adept at modifying the environment to suit their needs, they spread to Asia, Australia, and the Americas. There their impact on the nature and development of savanna vegetation was superimposed on the natural pattern, adding to the variation seen among savanna types. The savannas of the world currently are undergoing another phase of change as modern expansion of the human population impinges on the vegetation and fauna.

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Savanna | African Grasslands, Wildlife & Climate

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Savanna, vegetation type that grows under hot, seasonally dry climatic conditions and is characterized by an open tree canopy (i.e., scattered trees) above a continuous tall grass understory (the vegetation layer between the forest canopy and the ground). The largest areas of savanna are found in

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North America - Temperate grasslands: The temperate grasslands, or prairies, form a belt between forest and desert, mainly on the Great Plains but also on the mid-slopes of the intermontane basins, above the salty desert flats.

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Aquatic Ecosystem

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Because they harbour great concentrations of biodiversity, coral reefs have been called the rain forests of the sea. With hundreds of species of corals and fishes frequently found on a single reef, metre for metre these undersea Ecosystems may even exceed tropical rain forests as the most species-rich places on Earth. Ironically, however, reefs have been far less studied--until now. In 1992 scientists at the seventh International Coral Reef Symposium in Guam raised a global alarm. They estimated that 10% of the world's coral reefs were effectively lost, an additional 30% were under immediate threat, and another 30% could be destroyed by 2050. To focus the world's attention on the plight of coral reefs, governments, organizations, and individuals around the world have recognized 1997 as the International Year of the Reef (IYOR).

Even the most concerned researchers agree that coral reefs are remarkably resilient, having withstood such massive forces as periodic temperature fluctuations, ice ages, volcanic eruptions, tropical storms, and flooding for about 35 million years. Recent evidence, however, suggests that while reefs have survived the assaults of nature, they may be succumbing to an unrelenting barrage of human-induced stresses. Steady increases in sediment runoff onto reefs; excessive mining of coral sand and rock for building; increases in pollution, particularly from agriculture and domestic sewage; and chronic overexploitation of reef resources have resulted in widespread reef damage and, in some cases, ecological collapse. The most endangered reefs are located along some of the world's most heavily populated coastlines--Asia, East Africa, and the Caribbean, including the Florida Keys ( see Map ).

Encyclopædia Britannica, Inc.

Among the most alarming threats to reefs is the increase in destructive fishing practices. The poison cyanide, for example, was being used routinely in the capture of fish for the live fish trade, a billion-dollar business that supplies seafood to Chinese restaurants in Hong Kong and nearby regions, as well as to Taiwan and Singapore. Fishermen liberally douse the reefs with cyanide in concentrations that temporarily stun large fish but indiscriminately kill smaller animals, including the corals themselves. The use of toxins, however, accounts for only part of the damage. With the help of compressed air, fishermen now can dive as deep as 40 m (130 ft), where they are able to flush dazed escapees from reef crevices by demolishing the fishes' refuges with crowbars.

Even more devastating is the wholesale harvesting of fish during critical spawning periods. Particularly targeted are coral trout and other groupers (family Serranidae) and the spectacular humphead wrasse ( Chelinus undulatus). Once or twice each year, fish from all parts of the reef congregate at a single location, usually a place swept by ocean currents so that their larvae are washed away from the reef's plankton eaters, including the corals, into the relative safety of the open ocean. Intent on spawning and therefore less cautious than normal, the fish are vulnerable to unscrupulous fishermen, who can wipe out all the breeding species on a reef within hours.

Studies conducted on reefs off the coasts of places as far apart as Jamaica, southern Japan, and Kenya have shown that eliminating these large fish can have disastrous consequences. Large fish serve as beneficial grazers, helping to check the growth of algae that threatens to smother the corals. In addition, without a diverse army of fish predators, populations of other marine organisms explode, including sea urchins that wreak havoc by grazing everything in their path, even juvenile corals, and the rampaging crown-of-thorns starfish ( Acanthaster planci), the scourge of coral colonies throughout the Indo-Pacific.

Coral reefs also may be suffering from less-direct, but no-less-damaging, forces. Many scientists suspect that rising ocean temperatures due to global warming are compounding the extent and severity of coral bleaching. Increases in temperature of only one or two degrees can stress corals, causing them to expel the symbiotic algae known as zooxanthellae that they harbour in their stomach cells. The algae not only give corals their brilliant colours (without them corals appear white or "bleached") but also supply them with the surplus energy and nutrients they need to build reefs. Satellite images, especially of the Caribbean and parts of the Pacific, show distinct correlations between large areas of bleaching and increased sea-surface temperatures. In time and under optimal conditions, reefs can recover. Scientists, however, have documented an alarming reef mortality.

Some researchers suspect that these rising temperatures and other stresses such as pollution may be causing corals to lose their resistance to once-minor ailments. In the last decade alone, scientists have documented about eight new diseases of corals. Even very large, century-old corals appear to be succumbing to these previously unknown maladies.

In 1993 concerned scientists realized that it would be necessary to involve the public, particularly communities dependent on reef resources, in rescuing the reefs. In 1994 small-island nations met in Barbados to discuss their concerns about reef destruction and rising sea levels. Many of these countries have few resources other than coral reefs; some are built on coral islands only a few metres above the surrounding reefs. Together they formed the International Coral Reef Initiative, a global effort that has drawn the world's attention to the plight of coral reefs and spawned numerous other monitoring and management initiatives, including the Global Coral Reef Monitoring Network. In one of the projects sponsored by the network, leading coral-reef scientists train personnel in government natural-resource agencies and universities in the basic methods of assessing the health of corals and reef-fish populations.

To highlight reef issues and challenges further, Robert Ginsburg of the University of Miami, Fla., proposed that 1997 be designated the International Year of the Reef. Among the hands-on projects launched under IYOR auspices was Reef Check, coordinated by Gregor Hodgson of the Hong Kong University of Science and Technology. Between June and August hundreds of scientists and recreational divers measured indicators of human activities on 300 reefs in 30 countries. This first global survey of the human impact on coral reefs provided valuable data on reef damage due to blast or cyanide fishing, overharvesting of marine organisms, and boat and anchor collisions.

Preliminary results from the Reef Check reconnaissance were announced at a press conference in Hong Kong on October 16. The report confirmed what recreational divers had been reporting for several years--coral reefs were being abused by people throughout the world. The data showed that although some pristine reefs were found, few were in excellent condition. Even the most remote reefs had been heavily fished for sharks, lobsters, giant clams, and, most troubling, large predator fish such as grouper and snapper. In many of the reefs surveyed in Southeast Asia, these marketable species were completely absent.

IYOR also targeted children by means of interactive educational activities. Coral reef scientists, for example, prepared teaching kits for primary and secondary schools. Children learned about coral reef values and problems through painting, writing, song, and dance competitions. Even a new computer game entitled Murder Under the Microscope was developed, in which players solved problems of reef degradation by enlisting the input of stakeholders in decision making and management of reef resources.

The World Bank helped promote coral reef education and awareness by producing and distributing an award-winning hour-long video, The Fragile Ring of Life. The documentary explored reef problems through the eyes of some of the millions of people whose livelihoods depend on ecologically healthy marine environments. Finding solutions will not be easy, but as Charles Birkeland of the University of Guam's Marine Laboratory observed, people "need to develop a new paradigm for the exploitation of coral reefs, a new perspective that might also be a framework for our management of the Earth as a whole."
Clive R. Wilkinson is coordinator of the Global Coral Reef Monitoring Network at the Australian Institute of Marine Science, Townsville, Queen., Australia.

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coral reefs: the forgotten rain forests of the sea

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Coral Reefs: The Forgotten Rain Forests of the Sea

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Because they harbour great concentrations of biodiversity, coral reefs have been called the rain forests of the sea. With hundreds of species of corals and fishes frequently found on a single reef, metre for metre these undersea Ecosystems may even exceed tropical rain forests as the most species-rich places on Earth.

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Aquatic Ecosystem

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Other articles where Mangrove forest is discussed: rainforest: Mangrove forests occur along estuaries and deltas on tropical coasts. Temperate rainforests filled with evergreen and laurel trees are lower and less dense than other kinds of rainforests because the climate is more equable, with a moderate temperature range and well-distributed annual rainfall.

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Deciduous forest | Definition, Climate, & Characteristics

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Deciduous forest, vegetation composed primarily of broad-leaved trees that shed all their leaves during one season. This biome is found primarily in three middle-latitude regions with a temperate climate characterized by a winter season and year-round precipitation.

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North America - The boreal forest: One of the greatest sweeps of forest in the world, the boreal forest (or taiga) extends in a vast and virtually unbroken sheet of green eastward from the Aleutian Islands through Alaska and northern Canada to the island of Newfoundland.

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taiga, also called boreal forest, biome (major life zone) of vegetation composed primarily of cone-bearing needle-leaved or scale-leaved evergreen trees, found in northern circumpolar forested regions characterized by long winters and moderate to high annual precipitation. The taiga, "land of the little sticks" in Russian, takes its name from the collective term for the northern forests of Russia, especially Siberia.

The taiga, which is also known as the boreal (meaning northern) forest region, occupies about 17 percent of Earth's land surface area in a circumpolar belt of the far Northern Hemisphere. Northward beyond this limit, the taiga merges into the circumpolar tundra. The taiga is characterized predominantly by a limited number of conifer species-i.e., pine ( Pinus), spruce ( Picea), larch ( Larix), fir ( Abies)-and to a lesser degree by some deciduous genera such as birch ( Betula) and poplar ( Populus). These trees reach the highest latitudes of any trees on Earth. Plants and animals in the taiga are adapted to short growing seasons of long days that vary from cool to warm. Winters are long and very cold, the days are short, and a persistent snowpack is the norm. The taiga biomes of North America and Eurasia display a number of similarities, even sharing some plant and animal species.

Origin

During the final period of maximum cold temperatures (23,000 to 16,500 years ago) in the latter part of the Pleistocene Ice Age (which ended 11,700 years ago), species that now constitute the taiga were displaced as far south as 30° N latitude by the continental glaciers of Europe, Asia, and North America and by the hyperarid and extremely cold environments of unglaciated Asia and North America. As the glaciers began to retreat gradually about 18,000 years ago, species of the taiga began to move northward in Europe and North America. In eastern and central North America the northward movement of the forest was relatively steady and gradual. An exception to this progression occurred about 9,000 years ago in western Canada, when white spruce spread rapidly northward across 2,000 km (1,240 miles) of newly deglaciated land in only 1,000 years. This rapid migration resulted from seed dispersal facilitated by strong northward winds caused by clockwise atmospheric circulation around the remnant ice cap of northern Quebec and the western part of Hudson Bay.

Because so much of Earth's water was bound up in ice at this time, sea levels were lower than they are today, and this allowed migrations of various terrestrial species to occur. Many areas that are now islands were then connected to the nearby mainland; e.g., the British Isles were linked to Europe. As the climate warmed during the last stages of the glacial period, but before the sea level rose to its current position, some plants and animals of the mainland European taiga ecosystem migrated to Britain. This biota exists today as part of the taiga in the Highlands of Scotland. The areas of lowland central Alaska, the central Yukon territory, and the Far East region of Russia, which had climates too arid to permit the formation of ice sheets, were connected by the Bering Land Bridge, across which many species migrated. As a result, today across Alaska a gradient in plant characteristics can be observed, ranging from typical North American forms in the east to those with Eurasian characteristics in the west.

Distribution

The taiga regions of North America and Eurasia are broad belts of vegetation that span their respective continents from Atlantic to Pacific coasts. In North America the taiga occupies much of Canada and Alaska. Although related transition forest types are present in the northern tier of the lower 48 United States, true taiga stops just north of the southern Canadian border. The vast taiga of Asia extends across Russia and southward into northeastern China and Mongolia. In Europe most of Finland, Sweden, and Norway are covered with taiga. A small, isolated area of boreal forest in the Scottish Highlands lacks some continental species but does contain the most widespread conifer of the Eurasian taiga, Scotch pine ( Pinus sylvestris).

The position of the taiga generally is controlled by the degree of warmth experienced during the growing season, the temperature of the soil, and the extreme minimum winter temperature. The taiga biome consists of three roughly parallel zones: closed-canopy forest, lichen woodland or sparse taiga, and forest-tundra. The closed-canopy forest is the southernmost portion of the taiga. It contains the greatest richness of species, the warmest soils, the highest productivity, and the longest growing season within the boreal zone. North of the closed-canopy forest is the lichen woodland-a smaller parallel zone of sparse forest or woodland in which tree crowns do not form a closed canopy. Lichen mats and tundralike vegetation make up a significant portion of the ground cover. To the north of the lichen woodland lies forest-tundra, which occurs along the northern edge of tree growth (tree line). Patches of trees consisting of only a few species dot restricted portions of the landscape, forming a complex mosaic with tundra. Many trees in the forest-tundra zone have never been known to produce viable seeds or have done so only sporadically. These trees were established during warmer climatic episodes from a few hundred to a few thousand years ago and have persisted since, usually by vegetative (asexual) reproduction. Forest fires in this zone remove trees, and, because of the lack of reproduction, only unburned patches of trees remain.

The closed-canopy forest, or southern taiga zone, on both continents is not distributed along a strictly east-west axis. At the western margin of Europe, the warming influence of the Gulf Stream allows the closed-canopy forest to grow at its northernmost location, generally between about 60° and 70° N. In western North America the Kuroshio and North Pacific currents likewise warm the climate and cause the northward deflection of the forest into Alaska and Yukon in Canada. On the eastern margin of the continents, the taiga is deflected southward to between about 50° and 60° N by the cold polar air masses that flow south along these coasts. This is the southernmost limit of the taiga, to the south of which, in humid eastern North America and Europe, lies a northern deciduous broad-leaved transition forest. In this forest small stands of boreal conifers are distributed on cooler or less-productive sites such as peaty wetlands. In the arid centre of both continents, the closed-canopy boreal forest is bordered to the south by a forest parkland of trees and grassland.

The central portions of Eurasia and North America are regions of flat or gently rolling topography. There, the northern and southern boundaries of the taiga are broad and gradual; they have fluctuated by as much as 200 km (125 miles) during the past few thousand years. A well-defined but complex boundary is formed between taiga and alpine tundra on the mountains of the Pacific edge in western North America and the Far East region of Russia. Generally, the taiga does not come into contact with the humid temperate or subpolar rainforest of coastal Alaska and British Columbia because of high mountain barriers, but some low-elevation regions have a transition zone often characterized by trees that are a hybrid of Sitka spruce ( Picea sitchensis) and white spruce ( P. glauca). In Norway and Scotland a variant form of the taiga occupies extremely humid environments.

Practically all the large river systems of the taiga of Siberia, including the Ob, Yenisey, and Lena rivers, are northward-flowing. The Ob in western Siberia forms a great lowland basin with a considerable percentage of the land surface covered with poorly drained peaty wetlands. In such situations within the taiga a closed-canopy forest is generally absent.

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Taiga | Plants, Animals, Climate, Location, & Facts

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Taiga, biome composed mainly of cone-bearing needle-leaved or scale-leaved evergreen trees, found in northern circumpolar regions typified by long winters and moderate to high annual precipitation. Taiga, 'land of the little sticks' in Russian, is named for the term for Russia's northern forests, especially Siberia.

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Origin

Tropical rainforests represent the oldest major vegetation type still present on the terrestrial Earth. Like all vegetation, however, that of the rainforest continues to evolve and change, so modern tropical rainforests are not identical with rainforests of the geologic past.

Tropical rainforests grow mainly in three regions: the Malesian botanical subkingdom, which extends from Myanmar (Burma) to Fiji and includes the whole of Thailand, Malaysia, Indonesia, the Philippines, Papua New Guinea, the Solomon Islands, and Vanuatu and parts of Indochina and tropical Australia; tropical South and Central America, especially the Amazon basin; and West and Central Africa ( see biogeographic region). Smaller areas of tropical rainforest occur elsewhere in the tropics wherever climate is suitable. The principal areas of tropical deciduous forest (or monsoon forests) are in India, the Myanmar- Vietnam-southern coastal China region, and eastern Brazil, with smaller areas in South and Central America north of the Equator, the West Indies, southeastern Africa, and northern Australia.

The flowering plants ( angiosperms) first evolved and diversified during the Cretaceous Period about 100 million years ago, during which time global climatic conditions were warmer and wetter than those of the present. The vegetation types that evolved were the first tropical rainforests, which blanketed most of Earth's land surfaces at that time. Only later-during the middle of the Paleogene Period, about 40 million years ago-did cooler, drier climates develop, leading to the development across large areas of other vegetation types.

It is no surprise, therefore, to find the greatest diversity of flowering plants today in the tropical rainforests where they first evolved. Of particular interest is the fact that the majority of flowering plants displaying the most primitive characteristics are found in rainforests (especially tropical rainforests) in parts of the Southern Hemisphere, particularly South America, northern Australia and adjacent regions of Southeast Asia, and some larger South Pacific islands. Of the 13 angiosperm families generally recognized as the most primitive, all but two- Magnoliaceae and Winteraceae-are overwhelmingly tropical in their present distribution. Three families-Illiciaceae, Magnoliaceae, and Schisandraceae-are found predominantly in Northern Hemisphere rainforests. Five families-Amborellaceae, Austrobaileyaceae, Degeneriaceae, Eupomatiaceae, and Himantandraceae-are restricted to rainforests in the tropical Australasian region. Members of the Winteraceae are shared between this latter region and South America, those of the Lactoridaceae grow only on the southeast Pacific islands of Juan Fernández, members of the Canellaceae are shared between South America and Africa, and two families- Annonaceae and Myristicaceae-generally occur in tropical regions. This has led some authorities to suggest that the original cradle of angiosperm evolution might lie in Gondwanaland, a supercontinent of the Southern Hemisphere thought to have existed in the Mesozoic Era (252 to 66 million years ago) and consisted of Africa, South America, Australia, peninsular India, and Antarctica. An alternative explanation for this geographic pattern is that in the Southern Hemisphere, especially on islands, there are more refugia-i.e., isolated areas whose climates remained unaltered while those of the surrounding areas changed, enabling archaic life-forms to persist.

The first angiosperms are thought to have been massive, woody plants appropriate for a rainforest habitat. Most of the smaller, more delicate plants that are so widespread in the world today evolved later, ultimately from tropical rainforest ancestors. While it is possible that even earlier forms existed that await discovery, the oldest angiosperm fossils- leaves, wood, fruits, and flowers derived from trees-support the view that the earliest angiosperms were rainforest trees. Further evidence comes from the growth forms of the most primitive surviving angiosperms: all 13 of the most primitive angiosperm families consist of woody plants, most of which are large trees.

As the world climate cooled in the middle of the Cenozoic, it also became drier. This is because cooler temperatures led to a reduction in the rate of evaporation of water from, in particular, the surface of the oceans, which led in turn to less cloud formation and less precipitation. The entire hydrologic cycle slowed, and tropical rainforests-which depend on both warmth and consistently high rainfall-became increasingly restricted to equatorial latitudes. Within those regions rainforests were limited further to coastal and hilly areas where abundant rain still fell at all seasons. In the middle latitudes of both hemispheres, belts of atmospheric high pressure developed. Within these belts, especially in continental interiors, deserts formed ( see desert: Origin). In regions lying between the wet tropics and the deserts, climatic zones developed in which rainfall adequate for luxuriant plant growth was experienced for only a part of the year. In these areas new plant forms evolved from tropical rainforest ancestors to cope with seasonally dry weather, forming tropical deciduous forests. In the drier and more fire-prone places, savannas and tropical grasslands developed.

Retreat of the rainforests was particularly rapid during the period beginning 5,000,000 years ago leading up to and including the Pleistocene Ice Ages, or glacial intervals, that occurred between 2,600,000 and 11,700 years ago. Climates fluctuated throughout this time, forcing vegetation in all parts of the world to repeatedly migrate, by seed dispersal, to reach areas of suitable climate. Not all plants were able to do this equally well because some had less-effective means of seed dispersal than others. Many extinctions resulted. During the most extreme periods (the glacial maxima, when climates were at their coldest and, in most places, also driest), the range of tropical rainforests shrank to its smallest extent, becoming restricted to relatively small refugia. Alternating intervals of climatic amelioration led to repeated range expansion, most recently from the close of the last glacial period about 10,000 years ago. Today large areas of tropical rainforest, such as Amazonia, have developed as a result of this relatively recent expansion. Within them it is possible to recognize "hot spots" of plant and animal diversity that have been interpreted as glacial refugia.

Tropical rainforests today represent a treasure trove of biological heritage, and they also serve as sinks for more than 50 percent of all atmospheric carbon dioxide absorbed by plants annually. They not only retain many primitive plant and animal species but also are communities that exhibit unparalleled biodiversity and a great variety of ecological interactions. The tropical rainforest of Africa was the habitat in which the ancestors of humans evolved, and it is where the nearest surviving human relatives- chimpanzees and gorillas-live still. Tropical rainforests supplied a rich variety of food and other resources to indigenous peoples, who, for the most part, exploited this bounty without degrading the vegetation or reducing its range to any significant degree. However, in some regions a long history of forest burning by the inhabitants is thought to have caused extensive replacement of tropical rainforest and tropical deciduous forest with savanna.

Not until the past century, however, has widespread destruction of tropical forests occurred. Regrettably, tropical rainforests and tropical deciduous forests are now being destroyed at a rapid rate in order to provide resources such as timber and to create land that can be used for other purposes, such as cattle grazing ( see deforestation). Today tropical forests, more than any other ecosystem, are experiencing habitat alteration and species extinction on a greater scale and at a more rapid pace than at any other time in their history-at least since the major extinction event (the K-T extinction) at the end of the Cretaceous Period, some 66 million years ago ( see Sidebar: Status of the World's Tropical Forests).

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Tropical rainforest | Definition, Characteristics, Location, Climate, Animals, Plants, & Facts

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Tropical rainforest, luxuriant forest found in wet tropical uplands and lowlands near the Equator. Tropical rainforests are dominated by broad-leaved trees that form a dense upper canopy and contain a wide array of vegetation and other life. Worldwide, they make up one of Earth's largest biomes (major life zones).

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Table of Contents

grassland, area in which the vegetation is dominated by a nearly continuous cover of grasses. Grasslands occur in environments conducive to the growth of this plant cover but not to that of taller plants, particularly trees and shrubs. The factors preventing establishment of such taller, woody vegetation are varied.

Grasslands are one of the most widespread of all the major vegetation types of the world. This is so, however, only because human manipulation of the land has significantly altered the natural vegetation, creating artificial grasslands of cereal crops, pastures, and other areas that require some form of repetitious, unnatural disturbance such as cultivation, heavy grazing, burning, or mowing to persist. This discussion, however, concentrates on natural and nearly natural grasslands.

Origin

The most extensive natural grasslands can be thought of as intermediates in an environmental gradient, with forests at one end and deserts at the other. Forests occupy the most favourable environments, where moisture is adequate for growth and survival of a tall, dense vegetation dominated by trees. Deserts are found where moisture is so lacking that a continuous, permanent vegetation cover cannot be maintained. Grasslands lie between these two extremes.

Like the savannas, deserts, and scrublands into which they commonly blend, grasslands arose during the period of cooling and drying of the global climate, which occurred during the Cenozoic Era (65.5 million years ago to the present). Indeed, the grass family itself (Poaceae or Gramineae) evolved only early in this era. The date of earliest appearance of grasslands varies from region to region. In several regions a succession of vegetation types can be recognized in the Cenozoic fossil record, as climate dried out progressively. For example, in central Australia during the past 50 million years tropical rainforest gave way successively to savanna, grassland, and, finally, desert. In some places expansion of grasslands to something approaching their modern extent occurred only during the extremely cold, dry intervals-called ice ages in north temperate regions-of the past two million years.

A dynamic balance commonly exists between grasslands and related vegetation types. Droughts, fires, or episodes of heavy grazing favour grassland at some times, and wet seasons and an absence of significant disturbances favour woody vegetation at others. Changes in the severity or frequency of these factors can cause a change from one vegetation type to another.

Other grassland types occur in places too cold for trees to grow-i.e., beyond the forest limits of high mountains or at high latitudes. A characteristic type of grassland in cool, moist parts of the Southern Hemisphere is tussock grassland, dominated by tussock or bunch grasses that develop pedestals of matted stems, giving the vegetation a lumpy appearance. Tussock grasslands occur at various latitudes. In the tropics they are found above the forest limit on some high mountains-e.g., in New Guinea and East Africa. At the higher latitudes of the Southern Ocean they form the main vegetation of subantarctic islands. They are also typical of the drier, colder parts of New Zealand and the southernmost regions of South America.

Not all natural grasslands, however, arise from climate-related circumstances. Woody plants may be prevented from growing in certain areas for other reasons, allowing grasses to dominate. One cause is seasonal flooding or waterlogging, which is responsible for the creation and maintenance of large grasslands in parts of the highly seasonal subtropics and in smaller areas of other regions. One of the best examples of a seasonally flooded subtropical grassland is the Pantanal in the Mato Grosso region of Brazil. Across an area of 140,000 square kilometres (54,000 square miles), dry grasslands prevail for half of each year and shallow wetlands for the other, with small forest patches restricted to low rises that do not flood during the wet season. In many other areas where climate is suitable for forest growth, very shallow or infertile soils may prevent tree growth and result in development of grassland.

The largest areas of natural grassland-those resulting from climatic dryness-can be classified into two broad categories: tropical grasslands, which generally lie between the belts of tropical forest and desert; and temperate grasslands, which generally lie between deserts and temperate forests. Tropical grasslands occur in the same regions as savannas, and the distinction between these two vegetation types is rather arbitrary, depending on whether there are few or many trees. Likewise, temperate grasslands may have a scattering of shrubs or trees that blurs their boundaries when they occur adjacent to scrublands or temperate forests.

Tropical grasslands are found mainly in the Sahel south of the Sahara, in East Africa, and in Australia. Temperate grasslands principally occur in North America, Argentina, and across a broad band from Ukraine to China, but in most of these regions they have been substantially altered by agricultural activities.

Many grasslands formerly supposed to be natural are now recognized as having once been forests that grew in a marginally dry climate. Early human disturbance is responsible for their transformation. For example, almost the entire extensive lowland grasslands of the eastern part of the South Island, New Zealand, are believed to have been created by forest-burning carried out by the Polynesians-the country's first colonists-during the eight centuries before European settlement in the 18th century.

Seminatural grasslands may occur where woody vegetation was once cleared for agricultural purposes that have since been abandoned; a return to the original vegetation is prevented by repeated burning or grazing. In wet tropical regions these types of grasslands may be very dense, such as those in East Africa that are dominated by elephant grass ( Pennisetum purpureum) or in New Guinea by pit-pit grass ( Miscanthus floridulus), both of which grow 3 metres (9.8 feet) tall.

All areas of grassland may owe something of their area and character to a long history of interaction with humans, particularly through the medium of fire.

Environment

Grassland climates are varied, but all large regions of natural grassland are generally hot, at least in summer, and dry, though not to the extent that deserts are. In general, tropical grasslands receive 500 to 1,500 millimetres (20 to 60 inches) of rain in an average year and in every season experience temperatures of about 15 to 35 °C (59 to 95 °F). The dry season may last as long as eight months. An excess of rainfall over evaporation, leading to ephemeral river flow, occurs only during the wet season. The tropical grassland climate overlaps very broadly with that of savanna. As previously stated, these vegetation types differ little from each other, a savanna being merely a grassland with scattered trees. Small changes in management and usage can convert one to the other.

Temperate grasslands are somewhat drier than tropical grasslands and also colder, at least for part of the year. Seasonal temperature variation may be slight in tropical grasslands but may vary by as much as 40 °C (72 °F) in temperate grassland areas. Mean annual rainfall in the North American grassland areas is 300 to 600 millimetres. Mean temperatures in January range from −18 °C (0 °F) in the north to 10 °C (50 °F) in the south, with corresponding values in July being 18 °C (64 °F) and 28 °C (82 °F). Mean annual temperature in the most northerly areas of the North American grassland zone is below 0 °C (32 °F).

Occurring as they do across a wide range of climatic and geologic conditions, grasslands are associated with many different types of soil. The grassland ecosystem itself influences soil formation, and this causes grassland soils to differ from other soils. The nature of grass litter and its pattern of decomposition commonly result in the development of a dark, organically rich upper soil layer that can reach 300 millimetres below the surface. This layer is absent from desert soils and is different from the surface layer of rotting leaf litter typical of forest soils. It is friable in structure and rich in plant nutrients. Lower soil layers are typically pale and yellowish, especially at depths close to two metres.

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grassland | Definition, Animals, Plants, Types, & Facts

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grassland, area in which the vegetation is dominated by a nearly continuous cover of grasses. Grasslands occur in environments conducive to the growth of this plant cover but not to that of taller plants, particularly trees and shrubs. The factors preventing establishment of such taller, woody vegetation are varied.

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tundra, a major zone of treeless level or rolling ground found in cold regions, mostly north of the Arctic Circle ( Arctic tundra) or above the timberline on high mountains (alpine tundra). Tundra is known for large stretches of bare ground and rock and for patchy mantles of low vegetation such as mosses, lichens, herbs, and small shrubs. This surface supports a meagre but unique variety of animals. The Finns called their treeless northern reaches the tunturi, but the concept of a vast frozen plain as a special ecological realm called tundra was developed by the Russians.

One constant factor shaping the tundra is alternate freezing and thawing of the ground. Along with the factors mentioned above, this freeze-thaw cycle sets the tundra apart from two ecosystems frequently found adjacent to it-the icy polar barrens on the one hand and the evergreen taiga on the other. Permafrost -perennially frozen ground-is a significant feature of the Arctic tundra; however, it does not typically occur in alpine regions.

Distribution

The global extent of the tundra biome is considerable, accounting for roughly 10 percent of Earth's surface. The southern limit of Arctic tundra follows the northern edge of the coniferous forest belt. In North America this line lies above latitude 60° N, while in Eurasia most of it occurs north of 70° N-except in eastern Siberia, where it extends southward to 60° N in Kamchatka. The northward bulge of forest in Eurasia is a result of the warmer summers that occur over that large contiguous landmass.

Alpine tundra covers approximately 3 percent of Earth's land surface, and it is mostly found in the Northern Hemisphere. This habitat can be found in mountainous areas worldwide, occurring at high elevations where temperatures are too low and winds are too strong for the growth of trees. The average elevation where alpine tundra occurs is generally higher near the Equator than at the poles.

Because nearly all of Antarctica is covered with ice, it lacks a well-developed tundra, though lichens, mosses, and at least three species of flowering plants occur in more favourable habitats there.

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Tundra | Definition, Climate, Animals, & Facts

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Tundra, a cold region of treeless level or rolling ground found mostly north of the Arctic Circle or above the timberline on mountains. Tundra is known for large stretches of bare ground and rock and for patchy mantles of low vegetation such as mosses, lichens, herbs, and small shrubs.

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Other articles where Aquatic ecosystem is discussed: coral reef: Tropical water conditions: Water conditions favourable to the growth of reefs exist in tropical or near-tropical surface waters. Regional differences may result from the presence or absence of upwelling currents of colder waters or from the varying relation of precipitation to evaporation.

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desert, any large, extremely dry area of land with sparse vegetation. It is one of Earth's major types of ecosystems, supporting a community of distinctive plants and animals specially adapted to the harsh environment. For a list of selected deserts of the world, .

Desert environments are so dry that they support only extremely sparse vegetation; trees are usually absent and, under normal climatic conditions, shrubs or herbaceous plants provide only very incomplete ground cover. Extreme aridity renders some deserts virtually devoid of plants; however, this barrenness is believed to be due in part to the effects of human disturbance, such as heavy grazing of cattle, on an already stressed environment.

According to some definitions, any environment that is almost completely free of plants is considered desert, including regions too cold to support vegetation-i.e., "frigid deserts." Other definitions use the term to apply only to hot and temperate deserts, a restriction followed in this account.

Origin

The desert environments of the present are, in geologic terms, relatively recent in origin. They represent the most extreme result of the progressive cooling and consequent aridification of global climates during the Cenozoic Era (65.5 million years ago to the present), which also led to the development of savannas and scrublands in the less arid regions near the tropical and temperate margins of the developing deserts. It has been suggested that many typical modern desert plant families, particularly those with an Asian centre of diversity such as the chenopod and tamarisk families, first appeared in the Miocene (23 to 5.3 million years ago), evolving in the salty, drying environment of the disappearing Tethys Sea along what is now the Mediterranean-Central Asian axis.

Deserts also probably existed much earlier, during former periods of global arid climate in the lee of mountain ranges that sheltered them from rain or in the centre of extensive continental regions. However, this would have been primarily before the evolution of angiosperms (flowering plants, the group to which most present-day plants, including those of deserts, belong). Only a few primitive plants, which may have been part of the ancient desert vegetation, occur in present-day deserts. One example is the bizarre conifer relative welwitschia in the Namib Desert of southwestern Africa. Welwitschia has only two leaves, which are leathery, straplike organs that emanate from the middle of a massive, mainly subterranean woody stem. These leaves grow perpetually from their bases and erode progressively at their ends. This desert also harbours several other plants and animals peculiarly adapted to the arid environment, suggesting that it might have a longer continuous history of arid conditions than most other deserts.

Desert floras and faunas initially evolved from ancestors in moister habitats, an evolution that occurred independently on each continent. However, a significant degree of commonality exists among the plant families that dominate different desert vegetations. This is due in part to intrinsic physiologic characteristics in some widespread desert families that preadapt the plants to an arid environment; it also is a result of plant migration occurring through chance seed dispersal among desert regions.

Such migration was particularly easy between northern and southern desert regions in Africa and in the Americas during intervals of drier climate that have occurred in the past two million years. This migration is reflected in close floristic similarities currently observed in these places. For example, the creosote bush ( Larrea tridentata), although now widespread and common in North American hot deserts, was probably a natural immigrant from South America as recently as the end of the last Ice Age about 11,700 years ago.

Migration between discrete desert regions also has been relatively easier for those plants adapted to survival in saline soils because such conditions occur not only in deserts but also in coastal habitats. Coasts can therefore provide migration corridors for salt-tolerant plants, and in some cases the drifting of buoyant seeds in ocean currents can provide a transport mechanism between coasts. For example, it is thought that the saltbush or chenopod family of plants reached Australia in this way, initially colonizing coastal habitats and later spreading into the inland deserts.

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Desert | Definition, Climate, Animals, Plants, & Types

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Desert, any large, extremely dry area of land with sparse vegetation. It is one of Earth's major types of ecosystems, supporting a community of plants and animals specially adapted to the harsh environment. In deserts, trees are usually absent, and shrubs or herbaceous plants provide only very incomplete ground cover.