These are the ecosystems having a predominance of trees that are interspersed with a large number of species of herbs, shrubs, climbers, lichens, algae and a wide variety of wild animals and birds. Forests are found in undisturbed areas receiving moderate to high rainfall and usually occur as stable climax communities.
Depending upon the prevailing climatic conditions forests can be of various types-
Tropical Rain Forests or Tropical Evergreen Forests:
- Tropical rainforests occur where the temperatures are warm throughout the year and precipitation occurs almost daily.
- The annual precipitation of a tropical rainforest is typically from 200 to 450 cm.
- It commonly occurs in areas with ancient, highly weathered, mineral poor soil. Little organic matter accumulates in such soil.
- Since the temperature is high, the organic litter is decomposed rapidly by bacteria, fungi, detritus-feeding ants and termites.
- The roots of the plants quickly absorb the nutrient minerals and hence are tied up in the vegetation rather than in the soil.
- It is found in Central America, the Amazon and Orinoco river basins of South America, the Congo river basin of Africa, the Malagasy Republic and South-East Asia. In India, tropical rain forests occur along the West Coast and in and around Assam.
- Tropical rainforests are very productive, excellent in species richness and variety. The trees are evergreen flowering plants. Their roots are shallow and concentrated near the surface.
- A fully developed tropical rainforest has at least three layers of vegetation-
- The topmost layer consists of the crowns of very tall trees, 50m or more in height, that are exposed to direct sunlight.
- The middle layer reaches a height of 30 to 40 m and forms a continuous canopy of leaves that lets in very little sunlight to support the bottom layer.
- Only 2% to 3% of sunlight reaches the bottom layer that consists of small plants that are adapted to darkness and seedlings of taller trees.
- Many types of epiphytes and woody climbers (lianas) grow on the trees to reach light for food making. The epiphytes are also called air plants. They have aerial roots to absorb water and trap falling leaves and other organic debris as a source of nutrients. Epiphytes include orchids, ferns and bromeliads.
- Due to abundant plant growth, a large number of animals live in tropical rain forests. They live both on the ground as well as at different levels on the trees. The upper storey has birds and bats, the middle storey has insects, lemur and monkeys, sloth, anteater, lizards and jungle cats. The ground fauna includes deer, tapir, antelope, forest goat, snakes, lizards, jaguar and leopard.
- Important plants of these forests are Rosewood, Ebony, Mahogany, Cinnamon, Nutmeg, Artocarpus, Rubber tree, Cloves, Fig, Alstonia, Cordia, Randia, Hopea etc.
Tropical Deciduous Forest:
- This forest biome is found in the West Indies, the Eastern region of Brazil, the Central Plateau of India, Indo-China and the North-Eastern projection of Australia.
- The climate of these forests is characterized by warm summers, cold winters and well-spaced rainfall amounting to about 100 to 200 cm.
- The sunlight is strong, and enough of it reaches the ground through the foliage to support vegetation.
- The soil is rich in minerals and humus as the climate is favourable for the reducers to decompose the fallen leaves.
- These forests are of short stature (10-20 m height) and show contrasting seasonal changes.
- The forest is lush green with dense foliage and a thick herbaceous layer, during the rainy season.
- But before the advent of summer, most trees shed off their leaves, turning the forest largely leafless along with dried up herbaceous layer.
- Many tree species possess thick barks to give protection against frequent fires.
- The dominant tree species in deciduous forests are Sal and Teak. The other useful species are Tendu, Chironji, Kair, Sagwan and Shisham.
Temperate Deciduous Forest:
- They are present in the temperate areas of both northern and southern hemispheres- parts of North America (Canada and USA), Europe, Asia, New Zealand and Australia.
- Seasonality (hot summers and cold winters) is characteristic of the temperate deciduous forest where precipitation ranges from about 75 to 150 cm annually.
- The soil of temperate deciduous forest consists of topsoil rich in organic materials and deep, clay-rich lower layers.
- As organic materials decay, mineral ions are released. Ions that are not absorbed by the roots of the living tree reach into the clay, where they may be retained.
- The forests predominantly contain broad-leaved, hardwood deciduous trees. Softwood evergreen conifers occur in a small number
- Common trees of the area are Oak, Beech, Birch, Elm, Poplar, Maple, Hickory, Chestnut, Magnolias etc.
- Shrubs, vines, herbs, grasses, ferns, mosses and lichens occur in abundance both on the ground as well as in epiphytic conditions.
Northern Coniferous Forests or Boreal Forest or Taiga:
- They occur in sub-arctic regions of Asia, Europe and North America just South of Tundra.
- The coniferous forest is also found in the southern hemisphere on the South Island of New Zealand.
- Rainfall is variable, being 10 to 35 cm in drier regions and over 100 cm in wetter regions.
- Snowfall also occurs, often heavy.
- Lakes and marshes are common in wetter regions.
- The climate is cold with an average temperature of 6°C in winter to 20°C in summer.
- Soil is typically acidic, mineral poor and characterized by a deep layer of partly decomposed, fine and spruce needles at the surface.
- Permafrost (permanently frozen ground) is patchy and, where found, is often deep under the soil.
- Taiga has numerous ponds and lakes in water-filled depressions that were dug in the ground by grinding ice sheets.
- Vegetation is predominantly evergreen and coniferous. Being evergreen, the conifers continue to manufacture food even in winter. In such plants photosynthesis continues even below -20°C.
- The common trees are Pine, Deodar, Larch, Spruce, Fir, Juniper and Yew. At places, broad-leaved plants like Birch, Aspen, Maple and other Populus species occur.
- Shrubs, herbs, ferns, mosses and vines are abundant.
- Sphagnum and Cotton Grass occur in marshes and bogs.
- The animal life of taiga consists of some larger species such as wolves, caribou, bears and moose. However, most mammals are small to medium-sized, including rodents, rabbits and fur-bearing predators such as lynx, sable and mink. Most species of birds are abundant in summer. Insects are abundant, but there are few amphibians and dentils, except in southern taiga.