Types Of Biomes And Their Global Distribution
Introduction:
• Biome is a large naturally occurring community of flora and fauna occupying a major habitat, e.g. forest or tundra.
• Walter's climate zones are one of many biome classification systems. While the number of biomes recognized by these systems varies, and some emphasize biological characteristics over the physical environment, they all paint a similar picture of ecosystem variation across the globe.
• The World Wildlife Fund, for example, recognizes fourteen biomes instead of Walter's nine.
• Its system is designed to identify the world's major ecological regions, whose conservation would preserve the vast majority of the planet's ecosystem diversity.
• Any system's global distribution of biome types follows the same general patterns of temperature and precipitation across the globe.
Tropical Rain Forest Biome (Climate Zone I)
• Tropical rain forest climates (in Walter's equatorial climate zone) are always warm and receive at least 200 cm of precipitation per year, with no less than 10 cm in any given month.
• These conditions are prevalent in three key tropical regions.
• First, the Neotropical rain forest is made up of the Amazon and Orinoco basins in South America, as well as other areas in Central America and along Brazil's Atlantic coast.
• Second, the African rain forest encompasses an area extending from southernmost West Africa eastward through the Congo River basin (with an added area on the eastern side of the island of Madagascar).
• Third, the Indo-Malayan rain forest extends across Southeast Asia (Vietnam, Thailand, and the Malay Peninsula); the islands between Asia and Australia (the Philippines, Borneo, and New Guinea); and Australia's Queensland coast.
• The tropical rain forest climate has two rainfall peaks centered on the equinoxes, which correspond to the times when the intertropical convergence passes over the equator.
• Rain forest soils are typically oxisols that are old and weathered.
• They take on the reddish color of aluminum and iron oxides and retain nutrients poorly because they lack humus and clay.
• Rain forest vegetation is dominated by a continuous canopy of tall evergreen trees rising to 30–40 m, despite the low nutrient status of the soils. Occasional emergent trees can reach heights of 55 meters or more above the canopy.
• Even in a mostly evergreen rain forest, emergent trees face a lot of water stress due to their height and exposure, so they're often deciduous. Because so little light penetrates the canopy, tropical rain forests typically have several understory layers beneath the canopy, containing smaller trees, shrubs, and herbs, but these are usually sparse.
• The forest canopy is dominated by climbing lianas, or woody vines, and epiphytes, or plants that grow on the branches of other plants but are not rooted in the soil (also known as air plants).
• The diversity of species is greater here than anywhere else on the planet.
• Tropical rain forests outperform all other terrestrial biomes in terms of biological productivity per unit of area, and their standing biomass outperforms all other biomes except temperate rain forests.
• Plant litter decomposes quickly due to the constant high temperatures and abundant moisture, and the vegetation quickly absorbs the released nutrients.
• The rain forest's high productivity is aided by rapid nutrient cycling, but it also makes the ecosystem extremely vulnerable to disturbance.
• Many nutrients are carted off in logs or go up in smoke when tropical rain forests are cut and burned.
• The sensitive soils erode quickly, filling the streams with silt. The environment degrades quickly in many cases, and the landscape becomes unproductive.
Tropical Seasonal Forest/Savanna Biome (Climate Zone II)
• There is typically a pronounced dry season within the tropics, but beyond 10° from the equator (in Walter's tropical climate zone), which corresponds to winter at higher latitudes.
• Seasonal forests in this climate zone are dominated by deciduous trees, which shed their leaves during the dry season.
• The vegetation becomes shorter and thorns develop to protect leaves from grazing where the dry season is longer and more severe.
• In the rain shadows of mountain ranges or along coasts with cold ocean currents running alongside, the vegetation grades from dry forest to thorn forest and finally to true desert as the aridity increases.
• The soils are strongly lateritic and nutrient poor, as they are in more humid tropical environments. Savannas are grasslands with a few trees strewn about.
• They can be found throughout the dry tropics, particularly at moderate elevations in East Africa.
• The average annual rainfall is 90–150 cm, with the driest three or four months bringing less than 5 cm each.
• Fire and grazing undoubtedly play important roles in maintaining the savanna biome's character, especially in wetter areas, because grasses can withstand both influences better than other types of vegetation.
• Dry forest often develops when grazing and fire are controlled within a savanna habitat.
Subtropical Desert Biome (Climate Zone III)
• The term "desert" has a wide range of meanings.
• The dry areas of the Great Basin and central Asia are commonly referred to as deserts; the Gobi Desert is a well-known example.
• The climates of those "deserts," on the other hand, are part of Walter's continental climate zone, which is marked by low precipitation and cold winters. Cold deserts are the name given to these areas.
• Subtropical deserts, also known as hot deserts, grow at latitudes of 20°–30° north and south of the equator, in areas with high atmospheric pressure associated with Hadley cell descending air.
• Subtropical deserts have low rainfall (less than 25 cm per year), high temperatures, and long growing seasons. The soils of subtropical deserts (arid sols) are shallow, devoid of organic matter, and neutral in pH due to low rainfall.
• Calcium carbonate hardpans that are impermeable to water develop frequently at depths of a meter or less.
• In the subtropical deserts of the Americas, creosote bush (Larrea tridentata) replaces sagebrush in the cold deserts of the Great Basin. Succulent cacti, shrubs, and small trees such as mesquite (Prosopis) and paloverde thrive in wetter areas of this biome (Cercidium microphyllum).
• Summer rains fall on most subtropical deserts. Many herbaceous plants sprout from dormant seeds after summer rains, grow quickly, and reproduce before the soils dry out again. Many plants in subtropical deserts are sensitive to frost. In temperate arid lands, species diversity is usually much higher.
Woodland/Shrubland Biome (Climate Zone IV)
• The Mediterranean climate zone is found on the western sides of continental landmasses at 30°–40° north and south of the equator—and at slightly higher latitudes in Europe—where cold ocean currents and winds blowing from the continents dominate the climate.
• Mediterranean climates can be found in the Northern Hemisphere in southern Europe and southern California, and in the Southern Hemisphere in central Chile, the Cape region of South Africa, and southwestern Australia.
• Mild winter temperatures, winter rain, and summer drought characterize Mediterranean climates.
• Thick, evergreen, shrubby vegetation 1–3 m tall, with deep roots and drought-resistant foliage, thrives in these climates.
• Sclerophyllous (“hard-leaved”) vegetation refers to plants that grow in a Mediterranean climate and have small, durable leaves. In the woodland/shrub land biome, fires are common, and most plants have fire-resistant seeds or root crowns that resprout quickly after a fire.
Temperate Rain Forest Biome (Climate Zone V)
• Temperate climate zones have average annual temperatures between 5°c and 20°c
• Mild winters, heavy winter rains, and summer fog create conditions that support extremely tall evergreen forests in warm temperate climates near the Pacific coast in northwestern North America, as well as in southern Chile, New Zealand, and Tasmania.
• These forests are dominated by coast redwood (Sequoia sempervirens) to the south and Douglas-fir to the north in North America (Pseudotsuga spp.). These trees are usually 60–70 meters tall, but they can grow to be over 100 meters tall.
• The fossil record shows that these plant communities are very old, and that they are remnants of forests that were vastly more extensive during the Mesozoic era, as recently as 70 million years ago.
• In contrast to tropical rain forests, temperate rain forests typically support a small number of species.
Temperate Seasonal Forest Biome (Climate Zone VI)
• The temperate seasonal forest biome, also known as deciduous forest, occurs in moderate climates with winter freezing.
• This biome is primarily found in the eastern United States and southeastern Canada in North America, but it is also widespread in Europe and eastern Asia. Because the larger ratio of ocean surface to land moderates winter temperatures and prevents frost, this biome is underdeveloped in the Southern Hemisphere.
• The length of the growing season in this biome varies from 130 days at higher latitudes to 180 days at lower latitudes in the Northern Hemisphere.
• Water tends to move downward through soils and drain from the landscape as groundwater and surface streams and rivers when precipitation exceeds evaporation and transpiration.
• Soils are frequently podsolized, slightly acidic, moderately leached, and brown in color due to the presence of organic matter.
• The dominant plant growth form is deciduous trees.
• A layer of smaller trees and shrubs often grows beneath the dominant trees, as well as herbaceous plants on the forest floor.
• Many of these herbaceous plants mature and flower before the trees have fully leaf out in the spring.
• Warmer and drier parts of the temperate seasonal forest biome, especially where soils are sandy and nutrient poor, tend to develop needle-leaved pine forests.
• The pine forests of the Atlantic and Gulf coasts of the United States are the most important of these ecosystems in North America; pine forests also exist at higher elevations in the western United States. Because of the warm climate in the southeast, the soils are typically laqwtite and nutrient deficient. Because nutrients and water are scarce, evergreen, needle-leaved trees thrive.
• They resist desiccation and release nutrients slowly because their needles last for several years. Because dry soils are common, fires are common, and most species can withstand fire damage.
Temperate Grassland/Desert Biome (Climate Zone VII)
• In North America, grasslands grow in continental climate zones with annual rainfall ranging from 30 to 85 cm and cold winters. From north to south, the growing season lengthens from 120 to 300 days.
• Prairies are the common name for these grasslands. Steppes are large grasslands that can be found throughout Central Asia. Because rainfall is scarce, organic detritus does not decompose quickly, and the soils are rich in organic matter.
• Prairie soils, which belong to the mollisol group, are not heavily leached and tend to be rich in nutrients due to their low acidity.
• Grasslands dominate the vegetation, which can reach heights of over 2 m in the moister parts of the grasslands and less than 0.2 m in the drier parts.
• There are also a lot of forbs. In these grasslands, fire is a major factor, especially when the habitat dries out in the late summer. Most grassland species have either fire-resistant underground stems, or rhizomes, from which shoots resprout, or fire-resistant seeds.
• Grasslands transition to deserts when annual precipitation is between 25 and 50 cm, and winters are cold and summers are hot.
• The temperate desert biome covers the majority of the western United States' Great Basin.
• The dominant plant in the northern part of the region is sagebrush (Artemisia), whereas in the south, on slightly moister soils, widely spaced juniper and pion trees predominate, forming open woodlands less than 10 m tall with sparse grass coverings.
• During most of the year, evaporation and transpiration exceed precipitation in these temperate deserts, resulting in dry soils and little water percolating through them to form streams and rivers.
• Calcium carbonate leached from the soil's surface layers tends to build up at the depths to which water typically penetrates. Because the habitat produces little fuel, fires in temperate deserts are uncommon.
• However, because the plant community is low in productivity, grazing can put a lot of pressure on the vegetation and may even favor the persistence of shrubs that aren't good forage. Indeed, overgrazing has turned many dry grasslands around the world into deserts.
Boreal Forest Biome (Climate Zone VIII)
• Boreal and Polar climate zones have average temperatures below 5°c
• The boreal forest biome, also known as taiga, stretches across a broad belt centered at about 50°N in North America and about 60°N in Europe and Asia.
• Winters are harsh and the average annual temperature is below 5°C. Annual precipitation is typically between 40 and 100 cm, and due to low evaporation, soils remain moist for the majority of the growing season.
• The vegetation consists of dense, seemingly endless stands of 10–20 m tall evergreen needle-leaved trees, mostly spruces and firs, with dense, seemingly endless stands of 10–20 m tall evergreen needle-leaved trees.
• Plant litter decomposes slowly at low temperatures and accumulates at the soil surface, forming one of the world's largest organic carbon reservoirs. As a result of the high levels of organic acids produced by needle litter, the soils are acidic, highly podsolized, and generally low in fertility.
• Growing seasons are rarely more than 100 days long, and are frequently less than that. Temperatures in the winter can reach –60°C, so the vegetation is extremely frost-tolerant. The diversity of species is extremely low.
Tundra Biome (Climate Zone IX)
• The Arctic tundra is a treeless expanse underlain by permanently frozen soil, or permafrost, to the north of the boreal forest, in the polar climate zone.
• During the brief summer growing season, the soils thaw to a depth of 0.5–1 m. Although precipitation is typically less than 60 cm, in low-lying areas where permafrost prevents drainage, soils may remain saturated with water for the majority of the growing season.
• Because of their high organic matter content, soils are acidic and deficient in nutrients.
• Plants retain their foliage for years in this nutrient-deficient environment.
• The majority of the plants are dwarf, prostrate woody shrubs that grow low to the ground in the winter to gain protection from the snow and ice.
• Blowing ice crystals shear off anything protruding above the snow's surface.
• The tundra is a harsh environment for most of the year, but during the 24-hour summer days, the flurry of biological activity in the tundra attests to life's remarkable adaptability.
• Vegetation resembling that of the Arctic tundra can be found at high elevations in temperate latitudes, and even in the tropics, and even includes some of the same species or close relatives.
• Above the tree line, alpine tundra can be found in the Rocky Mountains of North America, the Alps of Europe, and especially on the Tibetan Plateau in central Asia.
• Despite their similarities, alpine and Arctic tundra have significant differences as well. In comparison to Arctic tundra, alpine tundra has warmer and longer growing seasons, more precipitation, milder winters, higher productivity, better-drained soils, and greater species diversity. Even so, trees' growth is limited by harsh winter conditions.