Geography tropical rainforest tropical savana grasslands

Geography tropical rainforest tropical savana grasslands meaning definition and explanation

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Geography tropical rainforest tropical savana grasslands

VEGETATION TYPES

Tropical Broadleaf Evergreen Forest: The Tropical Rainforest

Introduction: The tropical rainforest is earth's most complex biome in terms of both structure and species diversity. It occurs under optimal growing conditions: abundant precipitation and year round warmth. There is no annual rhythm to the forest; rather each species has evolved its own flowering and fruiting seasons. Sunlight is a major limiting factor. A variety of strategies have been successful in the struggle to reach light or to adapt to the low intensity of light beneath the canopy.

Climate: Mean monthly temperatures are above 64 ° F; precipitation is often in excess of 100 inches a year. There is usually a brief season of reduced precipitation. In monsoonal areas, there is a real dry season, but that is more than compensated for with abundant precipitation the rest of the year.

Vegetation: A vertical stratification of three layer of trees is apparent.. These layers have been identified as A, B, and C layers:

A layer: the emergents. Widely spaced trees 100 to 120 feet tall and with umbrella-shaped canopies extend above the general canopy of the forest. Since they must contend with drying winds, they tend to have small leaves and some species are deciduous during the brief dry season.
B layer: a closed canopy of 80 foot trees. Light is readily available at the top of this layer, but greatly reduced below it.
C layer: a closed canopy of 60 foot trees. There is little air movement in this zone and consequently humidity is constantly high.
Shrub/sampling layer: Less than 3 percent of the light intercepted at the top of the forest canopy passes to this layer. Arrested growth is characteristic of young trees capable of a rapid surge of growth when a gap in canopy above them opens.
Ground layer: sparse plant growth. Less than 1 percent of the light that strikes the top of the forest penetrates to the forest floor. In such darkness few green plants grow. Moisture is also reduced by the canopy above: one third of the precipitation is intercepted before it reaches the ground.

Growth forms: Various growth forms represent strategies to reach sunlight:

Epiphytes: the so-called air plants grow on branches high in the trees, using the limbs merely for support and extracting moisture from the air and trapping the constant leaf-fall and wind-blown dust. Bromeliads (pineapple family) are especially abundant in the neotropics (tropical regions in the new world, the Americas); the orchid family is widely distributed in all three formations of the tropical rainforest. As demonstration of the relative aridity of exposed branches in the high canopy, epiphytic cacti also occur in the Americas.
Lianas: woody vines grow rapidly up the tree trunks when there is a temporary gap in the canopy and flower and fruit in the tree tops of the A and B layers. Many are deciduous.
Climbers: green-stemmed plants such as philodendron that remain in the ground levels. Many climbers, including the ancestors of the domesticated yams (Africa) and sweet potatoes (South America), store nutrients in roots and tubers.
Stranglers: these plants begin life as epiphytes in the canopy and send their roots downward to the forest floor. The fig family is well represented among stranglers.
Heterotrophs: non-photosynthetic plants can live on the forest floor.

Parasites derive their nutrients by tapping into the roots or stems of photosynthetic species. Rafflesia arnoldi, a root parasite of a liana, has the world's largest flower, more than three feet in diameter. It produces an odour similar to rotting flesh to attract pollinating insects.
Saprophytes derive their nutrients from decaying organic matter. Some orchids employ this strategy common to fungi and bacteria.

Common characteristics of tropical trees: Tropical species frequently possess one or more of the following attributes not seen in trees of higher latitudes.

Buttresses: many species have broad, woody flanges at the base of the trunk. Originally believed to help support the tree, now it is believed that the buttresses channel stem flow and it’s dissolved nutrients to the roots.
Large leaves are common among trees of the C layer. Young individuals of trees destined for the B and A layers may also have large trees. When the reach the canopy new leaves will be smaller. The large leaf surface helps intercept light in the sun-dappled lower strata of the forest.
Drip tips and waxy leaves: facilitate drainage of precipitation off the leaf to promote transpiration. They occur in the lower layers and among the saplings of species of the emergent layer (A layer). The waxy leaves aid rapid transpiration.
Common trees: hard woods as, rose wood, mahogany, ebony, greenheart, palm and rubber.

Other characteristics that distinguish tropical species of trees from those of temperate forests include

Exceptionally thin bark, often only 1-2 mm thick. Usually very smooth, although sometimes armed with spines or thorns.
Cauliflory, the development of flowers (and hence fruits) directly from the trunk, rather than at the tips of branches.
Large fleshy fruits attract birds, mammals, and even fish as dispersal agents.

Soil: Oxisols - infertile, deeply weathered and severely leached. Rapid bacterial decay prevents the accumulation of humus. The concentration of iron and aluminum oxides by the laterization process gives the oxisols a bright red colour and sometimes produces minable deposits (e.g., bauxite). On younger substrates, especially of volcanic origin, tropical soils may be quite fertile.

Subclimaxes: Distinct communities develop on floodplains. Jungles may line rivers where sunlight penetrates all layers of the forest. Where forests have long been cleared and laterites have developed to cause season waterlogging, tropical grasslands and palm savannas occur.

Fauna: Animal life is highly diverse. Common characteristics found among mammals and birds (and reptiles and amphibians, too) include adaptations to a dense forest life (for example, the prehensile tails of New World monkeys), bright colours and sharp patterns, loud vocalizations, and diets heavy on fruits.

Temperate Grasslands

Introduction: Temperate grasslands are composed of a rich mix of grasses and forbs and underlain by some of the world's most fertile soils. Since the development of the steel plough most have been converted to agricultural lands. It lies in the centre of continents approximately between latitudes 40 and 60 degrees north of the equator. The two main areas are the North America prairies and the Russian Steppes.

Climate: Semiarid, continental climates of the middle latitudes (Koeppen's BSk climate type) typically have between 10 and 20 inches of precipitation a year. Much of this falls as snow, serving as reservoir of moisture for the beginning of the growing season. Warm to hot summers are experienced, depending on latitude.

Vegetation: Perennial grasses and perennial bushes especially Compositae and Leguminosae--the sunflower and pea families, respectively are dominant growth forms. Two or more strata of grasses (erect grasses and recumbent species) are recognized in the more humid expressions of the biome.

Fire: Fire plays a big role in this biome, preserving biodiversity and keeping trees from overtaking the grasses. Lightning from large storms rolling over the grasslands ignites large grass-fires. These fires help certain plants by germinating seeds, clearing ground cover to allow rare plants a chance and by nourishing the soil with freshly burnt vegetation.

Drought: Drought plays a large role in keeping trees from taking over the grasslands. Some years receive less rain than others, just as certain seasons receive significantly less rain than others. Trees generally cannot stand the lack of water as easily as grasses and thus grasses remain dominant.

Grasses: Perennial grasses, with their growth buds at or just below the surface, are well-adapted to drought, fire, and cold. The tiller or narrow, upright stem reduces heat-gain in the hot summers; the intricate root systems trap moisture and nutrients. Two basic types are:

Turf- or sod-forming grasses, with rhizomes or underground stems from which new plants spring forth; associated with the more humid grasslands
Bunch grasses, without rhizomes, that reproduce by seed; associated with the drier parts of the biome.

Soils. Calcification is the dominant soil-forming process in semiarid regions. This process occurs where precipitation is either equal to, or slightly higher than, evapo-transpiration. Mild leaching, high organic content, and concentration of calcium carbonate in the B horizon typify the dark brown mollisols developed under the temperate grasslands. When this process works on a loess that itself is rich in calcium, the world's most fertile soils are created, the chernozems (A Russian term meaning black soil). Loess and hence chernozem underlie the eastern prairies of the US, the pampas of South America, and the steppes of Ukraine and Russia.

Fauna. The temperate grassland fauna is very low in diversity, especially in comparison with the tropical grasslands or savannas of Africa. In North America the dominant herbivores are bison and pronghorn. Rodent herbivores include the pocket gopher, ground squirrels, and the prairie dog. Carnivores include coyote, badger, and the federally endangered black-footed ferret, the last two members of the weasel family.

Tropical Savannas or Tropical Grasslands

Introduction: Tropical savannas or grasslands are associated with the tropical wet and dry climate type (Koeppen's Aw), but they are not generally considered to be a climatic climax. Instead, savannas develop in regions where the climax community should be some form of seasonal forest or woodland, but edaphic conditions or disturbances prevent the establishment of those species of trees associated with the climax community. Seasonal forests of the tropics are also widespread and vary along a latitudinal/moisture gradient between the tropical broadleaf evergreen forest of the equatorial zone and the deserts of the subtropics.

The word savanna stems from an Amerindian term for plains which became Hispanic after the Spanish Conquest.

The vegetation: Savannas are characterized by a continuous cover of perennial grasses, often 3 to 6 feet tall at maturity. They may or may not also have an open canopy of drought-resistant, fire-resistant, or browse-resistant trees, or they may have an open shrub layer. Distinction is made between tree or woodland savanna, park savanna, shrub savanna and grass savanna. Furthermore, savannas may be distinguished according to the dominant species in the tree layer: for example, palm savannas, pine savannas, and acacia savannas.

Climate: A tropical wet and dry climate predominates in areas covered by savanna growth. Mean monthly temperatures are at or above 64° F and annual precipitation averages between 30 and 50 inches. For at least five months of the year, during the dry season, less than 4 inches a month are received. The dry season is associated with the low sun period.

Soils. Soils vary according to bedrock and edaphic conditions. In general, however, laterization is the dominant soil-forming process and low fertility oxisols can be expected. Certain savannas are caused because of shallow or clayish soils. These savannas are called edaphic savannas. Most savanna soils however, are very porous and have only a very thin layer of organic matter. This allows the rains to drain quickly.

Savannas as subclimaxes.

Edaphic Subclimaxes.

Waterlogged conditions occur when the A-horizon of lateritic soils is exposed to the atmosphere. Alternating wet and dry seasons and baking by the sun create a brick-hard layer impermeable to water. This usually red hardpan is called a laterite (from the Latin for brick). During the rainy season, there is standing water above the hardpan for several months, preventing the establishment of most tree species. During the dry season, the laterite prevents penetration of roots, also inhibiting the growth of most trees. Several species of palms do tolerate these conditions and, along with grasses, occur above laterites.
Droughty substrates, such as quartz or volcanic sands, also inhibit the growth of most trees. The pine savannas of Central America are examples of savanna vegetation developed on droughty, low-nutrient conditions of quartz sands; the grass savanna of the Serengeti--with its herds of large mammals--is virtually treeless.
Low-nutrient soils. The cerrado of Brazil occupies a broad expanse of the Brazilian Highlands that, were it not for the low-nutrient level of the heavily-leached soils, would be occupied by a seasonal forest.

Fire subclimaxes. Two groups of plants that are pre-adapted to survive fire become dominant in areas where burning is frequent and periodic. Such fires have both natural and human origins. The savannas of South east Asia are generally considered to be man-made.

Palms have the advantage of being monocots: their vascular bundles are scattered throughout the stem so that scorching of the outermost layer of the trunk will not kill the plant. (Dicot trees, on the other hand, have their vascular bundles arranged around the outer, living part of their stems where they may be easily destroyed by fire.)
Perennial grasses have underground stems or rhizomes and so their growth nodes are protected by the soil during a ground fire. Trees and shrubs--with renewal buds above the surface--are selected against by fire and the balance tips toward the grasses.

Grazing subclimax. Large mammals such as the elephant open woodlands by debarking the trees and by knocking them over. This opens the woodland to grass invasion and attracts a variety of grazing animals, including zebras, wildebeest, and the diverse antelopes of the Ethiopian province. Grazers will both eat and trample tree seedlings, inhibiting the regrowth of the woodland. Only well-armed species of shrubs and trees can establish themselves in the clearings, leading to thickets of thorny acacias. Protected in the thicket, some acacias and other thorny trees will grow to mature specimens.

Overgrazing: if a grass savanna is overgrazed, patches of bare ground will be created. The grassland will not longer carry a ground fire and invasion by trees becomes possible. The bare ground will suffer from increased evaporation and a dry microhabitat quickly develops. Well-armed, drought-resistant species like the acacias tolerate both grazing and drought, so again an acacia savanna can become established.

Animals of the Tropical Savanna

Since savannas are spread around the globe there are many different types of animals that have become successful. For instance, in Australia one can find different types of kangaroos and wallabies that mingle with emus and frilled lizards. In South America roams the large rhea, which is the flightless equivalent of Africa's ostrich. Yet, it is the African animals that most people think about when they hear the word savanna.

The African savannas are home to many large herbivores. The common ones include several species of zebra, lions, giraffes, rhinos, elephants, hyenas, gazelles, and more.

The animals of the savanna play an important roll in maintaining the grasslands. Because the large herds eat the seeds and leaves from trees and trample the ground, grasslands are protected. A large population of elephants for instance can quickly turn a forest into a savanna as it breaks branches and topples trees.

Taiga or Boreal Forest or Coniferous Forest

Introduction: The taiga or boreal forest exists as a nearly continuous belt of coniferous trees across North America and Eurasia. Overlying formerly glaciated areas and areas of patchy permafrost on both continents, the forest is patterned by successional and subclimax plant communities sensitive to varying environmental conditions. Taiga is the Russian name for this forest which covers so much of that country. However, the term is used in North America as well.

Climate: The taiga corresponds with regions of subarctic and cold continental climate (Koeppen's Dfc, Dfd, and Dwd climate types). Long, severe winters (up to six months with mean temperatures below freezing) and short summers (50 to 100 frost-free days) are characteristic, as is a wide range of temperatures between the lows of winter and highs of summer. For example Russia, has recorded extremes of -30 degrees Celsius. Mean annual precipitation is 15 to 20 inches, but low evaporation rates make this a humid climate.

Vegetation: Needleleaf, coniferous (gymnosperm) trees are the dominant plants of the taiga biome. A very few species in four main genera are found: the evergreen spruce, fir, and pine, and the deciduous larch or tamarack. In North America, one or two species of fir and one or two species of spruce are dominant. Across Scandanavia and western Russia the Scots pine is a common component of the taiga.

Broadleaf deciduous trees and shrubs are members of early successional stages of both primary and secondary succession. Most common are alder, birch, and aspen.

Growth forms: The conical or needleleaf trees common to the taiga are adapted to the cold and the physiological drought of winter and to the short-growing season:

Conical shape - promotes shedding of snow and prevents loss of branches.
Needleleaf - narrowness reduces surface area through which water may be lost (transpired), especially during winter when the frozen ground prevents plants from replenishing their water supply. The needles of boreal conifers also have thick waxy coatings--a waterproof cuticle--in which stomata are sunken and protected from drying winds.
Evergreen habit - retention of foliage allows plants to photosynthesize as soon as temperatures permit in spring, rather than having to waste time in the short growing season merely growing leaves. [Note: Deciduous larch are dominant in areas underlain by nearly continuous permafrost and having a climate even too dry and cold for the waxy needles of spruce and fir.]
Dark colour - the dark green of spruce and fir needles helps the foliage absorb maximum heat from the sun and begin photosynthesis as early as possible.

Soil: Podzolization occurs as a result of the acid soil solution produced under needleleaf trees. This process is a more intense form of leaching. Rain percolating through the needleleaf litter becomes progressively more acidic and can reach a pH below 4.5. This dissolves a lot of bases in the soil can create a pan hard at lower levels in the profile. The main soil order associated with the taiga is podsols.

Fauna: Fur-bearing predators like the lynx and various members of the weasel family (e.g., wolverine, fisher, pine martin, mink, ermine, and sable) are perhaps most characteristic of the boreal forest proper. The mammalian herbivores on which they feed include the snowshoe or varying hare, red squirrel, lemmings, and voles.

Large herbivores are more closely associated with successional stages where there is more nutritious browse available and include elk or wapiti, known as red deer in Europe) and moose. The beaver, on which the early North American fur trade was based, is also a creature of early successional communities, indeed its dams along streams create such habitats.

Among birds, insect-eaters like the wood warblers are migratory and leave after the breeding season. Seed-eaters (e.g., finches and sparrows) and omnivores (e.g., ravens) tend to be year-round residents. During poor cone years, normal residents like the evening grosbeak, pine siskin, and red crossbill leave the taiga in winter and may be seen at bird feeders here in Virginia.

Temperate Broadleaf Deciduous Forest or Temperate Forest

Introduction: The Temperate Broadleaf Deciduous Forest (TBDF)--especially in eastern North America, where is remains most intact--is known for the turning of the colours of its leaves to brilliant reds, oranges, and golds in autumn. The shortening days of fall stimulate the plants to withdraw chlorophyll from their leaves, allowing a brief but beautiful display of other pigments before the leaves are shed completely and plants enter an extended period of dormancy.

Climate: Associated with warmer continental and humid subtropical climates. There is an approximately 6 month growing season. The 20 to 60 inches of precipitation is distributed evenly throughout the year. The non-growing season is due to temperature-induced drought during the cold winters.

Vegetation: oak, maple, beech, chestnut, hickory, elm, basswood or linden, walnut, and sweet gum. Different species of these occur on each continent.

Structure and Growthforms: Five layers are recognized:

a tree stratum, 60 -100 feet high, dominated regionally by various combinations of the species listed above;
a small tree or sapling layer, with not only younger specimens of the tall trees with species limited to this layer such as serviceberry or shadbush, sourwood, dogwood, and redbud;
a shrub layer often with members of the heath family such as rhododendron, azaleas, mountain laurel, and huckleberries;
an herb layer of perennial forbs that bloom primarily in early spring; and
a ground layer of lichens, clubmosses, and true mosses. Lichens and mosses also grow on the trunks of trees.

Lianas such as wild grape, poison ivy, and Virginia creeper climb the trees to flower and fruit high in the forest canopy. Trees commonly found are the Oaks, Elm, beech, sycamore, ash and chestnut. The temperate deciduous forest contains lesser amounts of species than the TRF. The forest floor, if the shrub layer is not too dense, is often covered in a thick undergrowth of brambles, grass and ferns. The floor has a reasonably thick leaf litter which is readily broken down by the numerous mixing agents living in the relatively warm soil.

Soil: Brown forest soils (alfisols, in the American soil taxonomy) develop under the TBDF. Broadleaf trees tend to be nutrient-demanding and their leaves bind the major nutrient bases. Thus the litter under this forest is not as acidic as under needleleaf trees and aluminum and iron are not mobilized from the A horizon. The autumn leaf fall provides for an abundant and rich humus which begins to decay rapidly in spring just as the growing season begins. The humus content gives both A and B horizons a brown colour. [Until John Deere's invention of the steel plough in the 1800s and the subsequent ability to break the prairie sod, the alfisols were considered the most fertile, most easily worked, and most easily cleared of northern hemisphere temperate zone soils. Many have been under continuous cultivation since.] Because of the amount of soil organisms, there is a lot of mixing of the layers in the soil profile. There is no hardpan, so the brown earths tend to be free-draining. There is considerable recycling as the deciduous trees take up many nutrients from the soil only to return them later through the fallen leaves. The soil in deeper and more fertile than the podsols and tree roots may penetrate and break up bedrock.

Fauna: Characteristic members of the fauna are either mast-eaters (nut and acorn feeders) or omnivores. Mammals show adaptations to an arboreal life; a few hibernate during the winter months.

North American herbivores include white-tail deer, grey squirrel, and chipmunk.
Omnivores include raccoon, opossum, skunk, and black bear.
Carnivores have been largely eliminated through the deliberate effort of humans but should include timber wolves, mountain lions, and bobcats. The coyote, native to the western grasslands and deserts, has recently dispersed east and taken over the niche of its departed cousin, the timber wolf.

Resident bird species also tend to be seed-eaters or omnivores. Many, like the several species of woodpeckers and the chickadees, are cavity-nesters. The loud, conspicuous blue jay is a major agent in the dispersal of oaks onto abandoned farmland and pastures. Migratory species tend to be insectivorous and include many so-called neotropical migrants, including warblers, wrens, thrushes, tanagers, and hummingbirds.

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