Environmental Science

LESSON-6. Major Ecosystem Types

FOREST ECOSYSTEM

Forest ecosystems are formed predominantly by 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. As discussed above forests are found in undisturbed areas receiving moderate to high rainfall and usually occur as stable climax communities. In India forest occupies an area of 69.63 million ha in 2007-08 with 22.80 % of the geographical area whereas it was only 14.20 % with 40.48 million ha during independence. However, it remains same during the last one decade at 22.80 % but the scientists estimate that India should ideally have 33 % of its land under forests. Most of the ‘natural’ undisturbed forests are located mainly in the National Parks and Wildlife Sanctuaries in India.  The forests perform the following functions : i) Protective and ameliorative functions : protecting the watersheds, creating conducive conditions for rainfall, soil erosion control, nutrient conservation, atmospheric regulation, ii) productive functions : producing food, fodder, firewood, fibre, timber and also medicinal plants and apiculture, iii) Recreational and educational functions like eco-tourism and iv) development functions involving revenue and employment generation.

The forest ecosystem consists of two parts namely i) non-living or abiotic aspects and ii) living or biotic aspects. Vegetation in the forest areas differ according to climatic conditions like  amount of rainfall received, local temperature as well as its latitude, altitude and soil type. For each forest area there are specific plants and animals accordingly they can be of the following types:

(a) Tropical Rain Forests: This type of evergreen broad leaf forests are located near the equator region characterized by high temperature, high humidity and high rainfall, all of which favour the growth of trees. Here, the temperature is warm throughout the year and precipitation occurs almost daily. The annual precipitation of a tropical rainforest is typically from 2000 to 4500 mm. Throughout the year the climate remains uniform and the forests have rich biodiversity. An evergreen forest thus looks green throughout the year. Different forms of life occupy specialized areas (niches) within different layers and spaces of the ecosystem depending upon their needs for food, sunlight, water , nutrient etc. The trees overlap with each other to form a continuous canopy.

Different types and layers of plants and animals can be seen in the tropical rain forests. A fully developed rainforest has atleast three layers of vegetation. The topmost layer of the tallest broad-leaf evergreen trees of height 50 m or more are exposed to direct sunlight. Below which lies the canopy where top branches of shorter trees of height 30 to 40m form an umbrella like cover. Below this is present the understory of still smaller trees. A large variety of birds, insects and animals like monkeys have made their natural homes (habitats) in these forests.

The area beneath the thick trees usually receive very dim sunlight. They usually develop dark green leaves with high chlorophyll content so that they can use the diffused sunlight for photosynthesis . The shrub layer receives even less sunlight and the ground layer. Normally, the floor or ground of the forest area receives almost no sunlight and is a dark layer. The forest is rich in orchids, mosses, bromeliads and ferns. The barks of the trees are covered in moss. Interestingly, the flowers of forest trees are very large, colourful, fragrant and attractive which helps in pollination by insects, birds, bats etc. Most of the animals like bats, birds, insects etc. occupy the bright canopy layer while monkeys, toads, snakes, chameleons etc. keep on moving up and down in sunny and darker layers. Termites, fungi, mushrooms etc. grow on the ground layer. Warm temperature and high availability of moisture facilitate rapid breakdown (decomposition) of the dropped leaves, twigs etc thereby accumulating the organic wealth. These forests are found in Central and South America, Africa and South east Asis. The Silent Valley in Kerala is the only tropical rain forest lying in India which is the natural habitat for a wide variety of species.

(b) Tropical deciduous forests: They are found a little away from the equator in regions with a moderate amount of seasonal rainfall that lasts for only a few months in a year during monsoon and are warmer year round. A large part of the year remains dry and therefore different types of deciduous trees are found here, which lose their leaves during dry season. The deciduous trees shed their leaves during winter and hot summer months, thus having periods of leaf fall and canopy regrowth. A thick undergrowth of vegetation can be witnessed on the forest floor as light can penetrate easily onto the floor of the forest.

(c) Tropical scrub forests: They are found in areas where the dry season is even longer. Here there are small deciduous trees and shrubs.

(d) Temperate rain forests: They are found in temperate areas with adequate rainfall. The coniferous trees like pines, firs, redwoods etc are grown predominantly in these forests. They also consist of some evergreen broad leaf trees. Temperate rain forests have relatively nutrient poor soil, although its organic content may be high. Cool climate slows down the bacterial and fungal activities and hence the decomposition of fallen leaves, branches, trunks and twigs into nutrients takes longer time.

(e) Temperate deciduous forests: Hot summers and cold winters are characteristics of these forests. They are found in areas with moderate temperatures. The precipitation ranges from 750 to 1250 mm. There is a marked seasonality with long summers, cold winter and abundant rainfall  throughout the year. The major trees include broad leaf deciduous trees like oak, hickory, beech, poplar etc and they lose their foliage annually. Trees form a dense canopy that overlies samplings and shrubs.

(f) Evergreen coniferous forests (Boreal Forests): In these forests winters are long, cold and dry. Sunlight is available for a few hours only. In summer the temperature is mild, sun-shines for long hours but the season is quite short. These forests have tall stately trees with needle like leaves and downward sloping branches so that the snow can slip off the branches. The major trees include pines, spruce, fir, cedar etc. which have tiny, needle-shaped leaves having a waxy coating so that they can withstand severe cold and drought. The leaves of coniferous trees are looking like needles, fall on the forest floor and cover the nutrient poor soil. The soil is found to get frozen during winter when only few species can survive and soil is acidic and prevent other plants from/growing. Species diversity is rather low in these forests because the soil is thin and often not fertile.

GRASSLAND ECOSYSTEMS

A wide range of landscapes with grasslands are dominated by grass species but sometimes also has few trees and shrubs. Rainfall is average but erratic. Limited grazing helps to improve the net primary production of the grasslands but overgrazing leads to degradation of these grasslands resulting in desertification. Three types of grasslands are found to occur in different climatic regions:

(a) Tropical grasslands: They occur near the borders of tropical rain forests in regions of high average temperature and low to moderate rainfall. In Africa, these are typically known as Savannas, which have tall grasses with scattered shrubs and stunted trees. The Savannas have
a wide diversity of animals including zebras, giraffes, gazelle; antelopes etc. During dry season, fires are quite common. Termite mounds are very common here. The termites gather the detritus (dead organic matter) containing a lot of cellulose and build up a mound. On the top of the mound fungi are found to grow which feed upon this dead matter including cellulose and in turn release methane, a greenhouse gas.

Tropical savannas have a highly efficient system of photosynthesis. Most of the carbon assimilated by them in the form of carbohydrates is in the perennating bulbs, rhizomes, runners etc. which are present underground. Deliberate burning of these grasslands can release huge quantities of carbon dioxide, another green house gas, responsible for global warming.

(b) Temperate grasslands: They are usually found on flat, gentle sloped hills, winters are very cold but summers are hot and dry. Intense grazing and summer fires do not allow shrubs or trees to grow.

In United States and Canada these grasslands are known as prairies, in South America as Pampas, in Africa as Velds in central Europe and Asia they are known as Steppes.

Winds keep blowing and evaporation rate is very high. It also favours rapid fires in summer. The soils are quite fertile and therefore, very often these grasslands are cleared for agriculture.

(c) Polar grasslands (Arctic Tundra): They are found in arctic polar region where severe cold and strong, frigid winds along with ice and snow create too harsh a climate for trees to grow. In summers the sun-shines almost round the clock and hence several small annual plants
grow in the summer. The animals include arctic wolf, weasel, arctic fox, reindeer etc. A thick layer of ice remains frozen under the soil surface throughout the year and is known as permafrost. In summer, the tundra shows the appearance of shallow lakes, bogs etc. where
mosquitoes, different type of insects and migratory birds appear.

DESERT ECOSYSTEMS

Deserts have infrequent and unpredictable precipitation and are dry in nature. These ecosystems occur in regions where evaporation exceeds precipitation (rainfall, snow etc.). The precipitation is less than 25 cm per year. About 1I3rd of our world's land area is covered by deserts. Deserts are located in South west of America, north and south of Africa and also in China, India, France and Australia. The coastal deserts of South America and Africa are among the driest regions in the world. Deserts have little species diversity and consist of. drought resistant or drought avoiding plants. The deserts are very hot during day time and very cool during nights. Deserts are of three major types, based on climatic conditions:

(a) Tropical deserts like Sahara and Namib in Africa and Thar desert, Rajasthan, India are the driest of all with only a few species. Large sand dunes due to high wind erosion are very common.

(b) Temperate deserts like Mojave in Southern California where day time temperatures are very hot in summer but cool in winters.

(c) Cold deserts like the Gobi desert in China has cold winters and warm summers.

Desert plants and animals are having most typical adaptations for conservation of water. Many desert plants are found to have reduced, scaly leaves so as to cut down loss of water due to transpiration or have succulent leaves to store water. Many a times their stems get flattened and develop chlorophyll so that they can take up the function of photosynthesis. Some plants show very deep roots to tap the groundwater. Many plants have a waxy thick cuticle over the leaf to reduce loss of water through transpiration. Desert animals like insects and reptiles have thick outer coverings to minimize loss of water. They usually live inside burrows where humidity is better and heat is less. Desert soil is rich in nutrients but deficient in water. Due to low species diversity, shortage of water and slow growth rate, the desert-plant communities, if faced with a severe stress take a long time to recover.