The forest biomes include a complex assemblage of different kinds of biotic communities. Optimum conditions of temperature and ground moisture responsible for the growth of trees contribute greatly to the establishment of forest communities. In addition, 50 mm rainfall is a pre-requisite for the growth of trees. The nature of soil, wind and air currents determines the distribution (abundance or sparseness) of forest vegetation. Normally ecologists recognize among forest communities such features as their evergreen nature, whether deciduous or in deciduous, as well as their shape, whether broad-leaved as in temperate forests or more needles-like as in the conifers.
On the basis of these features the forest biomes of the world have been classified into following biomes—coniferous forest, tropical forest, and temperate forest. All these forest biomes are generally arranged on a gradient from north to south or from high altitude to lower altitude.
Cold regions with high rainfall and strongly seasonal climates with long winters and fairly short summers are characterized by boreal conifer forest which is transcontinental. For example, adjacent to the tundra region either at latitude or high altitude is the northern coniferous or boreal forest, which stretches across both North America and Eurasia just south of the tundra (i.e., Canada, Sweden, Finland, Siberia and Mussoorie).
The term taiga is applied to the northern range of coniferous forests. This is characterized by evergreen plant species such as spruce [Picea, etc.), spruce (Picea glauca), fir (Abies balsamea) and pine trees (Pinus resinosa, Pinus strobus) and by animals such as snow shoe hare, the lynx, the wolf, bears, red fox, porcupines, squirrels, amphibians like Hyla and Rana, etc. Large size is common, both in the trees, which range up to some 40m in height, and in vertebrates, which include the giants of several groups of animals, such as moose, caribou, elk, grizzly bear, wolverine, beaver, and several large species of birds. Species diversity is low, and pure stands of trees and shrubs are common, as are outbreaks of defoliating insects of several sorts (e.g., bark beetles, saw flies, geometrid moths, etc.).
Understory trees are uncommon as a result of the continual low light penetration. Among common understory associates are orchids and ericaceous shrubs like the blue berry. The tabloid mosses and lichens being very rich understory vegetation. Boreal forest soils are thin podozols, and are rather poor both because the weathering of rock proceeds slowly in the cold environments, and because the litter derived from conifer needle is broken down very slowly and is not particularly rich in nutrients. These soils are acidic and mineral deficient, the result of the movement of a large amount of water through the soil; in the absence of a significant counter upward movement of evaporation, soluble essential nutrients like calcium, nitrogen, and potassium are leached sometimes beyond the reach of roots thereby leaving no alkaline- oriented cations to encounter the organic acids of the accumulating litter (Kormondy, 1976).
The productivity and community stability of a boreal coniferous forest are lower than those of any other biome.
Temperate deciduous forest:
The temperate forest biomes are characterized by a moderate climate and broad-leaved deciduous trees, which shed their leaves in fall, are bare over winter and grow new foliage in the spring. These forests are the characteristics of North America, Europe, Eastern Asia, Chile, part of Australia and Japan, with a cold winter and a annual rainfall of 75—150 cm and a temperature of 10—20°C.
In these biomes the precipitation may be fairly uniform throughout the year. In India, at elevations of 9009—12,000 feet in Himalayas occur temperate vegetation including pines, fir, yew and juniper trees with an undergrowth of scrubby rhododendrons. Soils of temperate forests are podozolic and fairly deep. Trees are quite tall—about 40—50m in height and their leaves are thin and broad. The predominant genera of this biome are maple (Acer), beech (Fagus), oak (Quercus), hickory (Carya), basswood (Tilia), Chestnut (Castnea), cottonvvood (Fopulas), sycamore (Plafanus), elm (Ulmus) and willow (Salix). In some locations, coniferous vegetation may be quite predominant and that includes white pine (Pinus strubos), hemlock (Tsuga canadensis) and red cedar (Jimi- perus virginianus). There are not many epiphytes or lianas save for some species of mosses, algae and lichens growing on tree trunks, and a few vines, notably Vitis, the grape. The understory of shrubs and herbs in the deciduous forest is typically well-developed and richly diversified, with a considerable portion of the photosynthesis and flowering attuned to the short days of the spring season, prior to the leafing out of and consequent shading by the tree canopy.
Accordingly, there are often two separate herb assemblages. One consists of spring flowers, which bloom before the trees have expanded their leaves and are gone by summer, and the other is adapted to the low light levels of the forest floor and lasts into the fall. The animals originally present in temperate forests are deer, bears, squirrels, gray foxes, bobcats, and wild turkey and wood peckers. Other common animals of this region are invertebrates like earthworms, snails, millipedes, Coleoptera and Orthoptera and vertebrates like amphibians such as newts, salamanders, toads, and cricket frogs; reptiles such as turtles, lizards and snakes ; mammals such as raccoon opossum, pigs, mountain lion, etc.
, and birds like horned owl, hawks, etc. All these animals and plants show a profound seasonality; some may even hibernate throughout the winter. The range of animal size and adaptations is wide; the largest animals include such forms as the deer and black bear. The dominant carnivores are large, including the wolf and mountain lion, although smaller carnivores such as fox and skunk are also common. Diversity of fauna is lower than in any of the rain forests and a few species seems clearly to be dominant.
Temperate evergreen woodland:
Many parts of the world have a Mediterranean-type climate with warm, dry summers and cool, moist winters. These are commonly inhabited by low evergreen trees with small hard needles or slightly broader leaves. The most important area of tropical evergreen woodland in North America is the ‘Chaparral’ of the Pacific Coast, the Mediterranean ‘maquis’, Spanish ‘encinar’ and ‘melle scrab’ on Australia’s South Coast are the same type of community.
In such woodland, trees are essentially lacking, although shrubs may range upto 3—4 m in height. Species diversity is roughly intermediate between that of a temperate deciduous forest and drier grassland. Fire is an important factor in this ecosystem, and the adaptations of the plants enable them to regenerate quickly after being burned. The characteristic animals of temperate evergreen woodland or chaparral are mule deer, brush rabbits, wood rats, chipmunks, lizard’s wem-tits and brown towhees.
Small-hooved cursorial ungulates are the dominant herbivores. Saltatorial (jumping) animals and many fast moving ungulates are also common in this fauna.
The temperate rainforest is a colder ecosystem than any other rainforest. Such a forest has a definite seasonality, with both temperatures and rainfall varying throughout the year. Rainfall is high, but fog may be very heavy and actually more important as a source of water than rainfall. The diversity is much lower, both in plants’ and animals, in comparison to warmer rainforests, yet it remains still higher than other temperate forest types. The dominant trees (canopies) are coast redwood (Sequoia sempervirens) of the Pacific coast of North America and the alpine ash (Eucalyptus regnans) of Australia and Tasmania, both of which reach more than 100m in height. Epiphytes and lianas are common but are not abundant like those of other rainforests.
The animals of temperate rain forests are similar to those of deciduous forests, but show a somewhat higher diversity.
Tropical rainforests occur near the equator in Central and South America, Central and Western Africa’ (Congo, Zambesi river), Southeast Asia (parts of India and Malaysia), Malaya, Borneo, New Guinea and Northwest Australia. Tropical rainforests are among the most diverse communities on earth. Both temperature and humidity are high and constant. The annual rainfall which exceeds 200 to 225 cm is generally evenly distributed throughout the year.
The flora is highly diversified: a square mile may contain 300 different species of trees, a diversity unparalleled in any other biome. The extremely dense vegetation of the tropical rainforests remains vertically stratified with tall trees often covered with vines, creepers, lianas, epiphytic orchids and bromeliads. Under the tall trees is a continuous evergreen carpet, the canopy layer, some 25 to 35 meters tall. The lowest layer is an understory of trees, shrubs, herbs like ferns and palms, all of which become dense where there is a break in the canopy. Nearly all plants are evergreen, and those that do lose their leaves entirely do so at irregular intervals with no apparent regard to the climatic regime.
The leaves of most plants are of moderate size, leathery and dark green in color. Their roots are often shallow and have swollen bases or flying buttresses. Soils of tropical rainforests are red latosols, and they may be exceedingly thick. The high rate of leaching makes these soils virtually useless for agriculture purposes, but if they are left undisturbed the extremely rapid cycling of nutrients within the litter layer (due to decomposition) can compensate for the natural poverty of the soil. It is the nature of the soil, both its potential for high leaching as well as its chemical composition that promotes a rock-like quality when exposed to air, that largely has prevented western style agriculture from being applied to the tropical forests (Kormondy, 1976).
Invertebrate density and abundance are very high in tropical rainforests, but while vertebrates are diverse, they are not as abundant as in many other communities. The common invertebrates of these forests are worms, snails, millipedes, centipedes, scorpions, isopods, spiders, insects, planarians and leeches. Among insects, heteropterans, orthopterans, blattids, mantids, plasmids, bees, termites and ants are most common. The common vertebrates of tropical rainforests are the arboreal amphibian Rhacophorus malabaricus; aquatic reptiles, chameleons, agamas, geckoes, and many species of snakes; many species of birds, social birds being predominant; and a variety of mammals.
Nocturnal and arboreal habits are most common in many mammals such as insectivores, leopards, jungle cats, ant-eaters, giant flying squirrels, monkeys and sloths. But in New Guinea and Northern Queensland, where monkeys are absent, there are arboreal kangaroos (Dendrolapis sp.) despite the fact that the basic anatomy of the kangaroo is not particularly well-suited to arboreal life. Further, in the foot hills of the forest zone of peninsular India covered with dense tropical vegetation, we have the tiger (Panthera tigris), the elephant (Elephas maximus), samber deer (Rusa unicolor), muntjac (Muntiacus mtmtjak), the gaur (Bibos gaurus), the chital or spotted deer (Axis axis), and the swamp deer (Rucervus duraucelli) as the major ground dwelling mammals.
Tropical seasonal forests:
Tropical seasonal forests occur m region whose total annual rainfall is very high, but segregated into pronounced wet and dry periods Tropical seasonal forests are found in Southeast Asia, Central and South America, Northern Australia, Western Africa and the tropical islands of the pacific as well as India and Southeast Asia. In exceedingly wet tropical seasonal forests, commonly known as monsoon forests, the annual precipitation may be several times that of the tropical rainforests. Trees may reach heights over 40 m, but are more commonly 20 —30m high stratification is of a relatively simple type with a single understory tree layer, canopy is deciduous and understory is evergreen. Teak is often a major large tree in the best-known tropical seasonal forests, those of India (Central India) and Southeast Asia. Bamboo is also an important climax shrub in these areas although in other areas it is important only in earlier stages of the succession.
In regions of fairly high rainfall but where temperature differences between winter and summer are less marked, as in Florida (USA), the broad-leaved evergreen subtropical biome is found. The vegetation includes mahogany, gumbo limbo, bays, palms, oaks, magnolias, tamarinds, all laden with epiphytes (of pineapple and orchid families), ferns, vines and strangler fig (Ficus aureus). This stratification is simpler, with only one understory tree horizon. All these plants tend to be evergreen, but may lose their leaves during the dry season. Animal life of subtropical forest is very similar to that of tropical rainforests.