Class 7 Science Forests Our Lifeline Chapter Notes

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This CBSE Class 7 Science chapter notes page covers Forests: Our Lifeline. It explains forest types in India and their layers (canopy, understorey and forest floor), the classification of forest plants, how animals, humans and plants depend on each other, why forests must be preserved, food chains and food webs, forest products, and the measures taken to conserve and manage forests.

INTRODUCTION

About one-third of the world's land surface is covered by forests. A forest is an eco-system (plants and animals) which is dominated by trees. Forests provide the most useful renewable resources. They prevent soil erosion, replenish oxygen in the air and maintain the fertility of the soil.

A forest is a biotic community spread over a large tract of uncultivated land and is predominantly composed of trees, shrubs, herbs and climbers. Forests occupy a special place in the life of people. They are an invaluable wealth of a nation and a renewable natural resource.

In the last hundred years, utilization of forest products has gone up manifold to meet the growing demands for wood and timber. Excessive felling of forest trees because of setting up industries, building of roads, cities and for cultivation is very common. Deforestation poses a serious threat to our economy, quality of life and environment. However, the State of Forest Report (SFR-2009) gives an encouraging picture. Our forest and tree cover stands at 23.84 per cent, which is an increase of 4.75 per cent over ten years.

Forests provide a home for different animals, birds, reptiles, insects, worms and microbes. Some live on the floor, others on tree twigs. Some walk on the ground, some burrow and others fly.

[Figure: Fig.1 A Forest. Fig.2 Forest Floor. Two photographs of forest scenes]

FOREST TYPES IN INDIA

The distribution of natural forests in India is governed principally by rainfall. Rainfall varies from 12 cm to 125 cm annually in different parts of the country, resulting in a wide variety of vegetation. It ranges from xerophytic scrub forest in the arid zones of Rajasthan to evergreen rainforests in the tropics, as in the Western Ghats, Assam, North Bengal and Andamans, and conifers in the temperate regions. In between the extremes, there are many intermediate types of forests.

[Figure: Fig.3 Pie graph showing forest distribution in India. Non Forest 77.72; Very Dense Forests 2.54; Moderate Dense Forest 9.71; Open Forest 8.77; Scrub Forest 1.26]

[Figure: Fig.4 Four photographs: A green forest, Evergreen rain forest, Moist tropical forest, Xerophytic scrub forest]

Due to different climatic conditions, different forests show variation in the type of trees and animals inhabiting them.

In North-East India, Sal is the most common tree, while in South West India the common tree is Teak. In the arid zone, species such as Cacti and Acacias are dominant. The Himalayan mountains have coniferous as well as trees with broad leaves. Between the foothills of the Himalayas and the plains of river Ganga lies the great terai forest, in which Sal is a common species. The dense forest consists of distinct horizontal layers. These are:

Canopy

The canopy is the uppermost level of a forest, formed by the crowns of trees and shrubs. The crown of a tree is the branched part of the stem and leaves above the main trunk. Tree crowns are of different shapes and sizes. The canopy layer is formed by the largest trees. This layer is very crowded and blocks out most of the sunlight. It also acts as a wind breaker. The canopy is the densest layer and is home to unique flora and fauna.

[Figure: Fig.5 Canopy and understoreys in a forest. Labels: CANOPY LAYER, UNDERSTOREY LAYER, FOREST FLOOR]

Understorey

The understorey layer lies between the canopy and the forest floor. This layer is home to a number of birds, snakes and lizards, as well as predators such as jaguars and leopards. The leaves are much larger at this level. A wide range of insects are also found. Only about 5% of the sunlight shining on the forest reaches the understorey. Forest understorey also has higher humidity than the exposed areas.

Forest Floor

The forest floor layer receives only 2 per cent of sunlight. The forest floor has almost no vegetation because of the low sunlight penetration. Only plants adapted to low light can grow in this region. Many forms of fungi grow here, which help in decaying dead animals and plants, thus converting them into dark coloured organic matter. This forms the humus.

[Figure: Fig.6 Shapes of tree crowns. Pyramidal, Round, Columnar, Weeping, Broad, Oval, Layered, Vase, and one further crown shape]

Types of Plants in the Forest

A forest has annual, biennial and perennial plants:
The plants with a life span of one year are called annuals. They are herbs and their stem is soft.
The plants with a life span of two years are called biennials.
The plants that survive for several years are called perennials. These include shrubs and trees.

Forest Animals and Their Habitats

Forests provide habitat for a large variety of wild animals. Boars, bisons, jackals and elephants live in the deeper areas of the forest, where the forest is thicker and undisturbed. They live on the ground.
Snakes either wriggle on the ground or burrow in the soil.
The forest floor is occupied by spiders, beetles and grubs.
Monkeys, bats and various types of birds occupy the trees and tree twigs.

Interdependence of Plants and Animals in Forest

Animals, human beings and plants show fundamental dependence on each other. The relationship is mainly for food, oxygen, energy and shelter or protection.

Animals Obtain Food from Plants

Green plants are autotrophs. Using the process of photosynthesis, green plants capture solar energy from sunlight in energy-storing chemical compounds that make their food. Almost all the energy needed by living beings on the earth comes ultimately from the sun through green plants (known as producers).
Animals and non-green plants obtain nutrients from the green plants. They are called consumers. It means all animals depend on plants for food and energy.

Animals Obtain Oxygen from Plants

Plants release oxygen during photosynthesis as a by-product. This oxygen is utilised by animals during respiration for releasing energy.

Animals Get Shade and Protection from Sun and Rain

During hot summer days, wild animals sit and relax under the trees, and during the rainy season they hide themselves under the trees.

Human Beings Depend on Plants in Many Ways

Human beings get food, natural fibres like cotton and jute from plants.
They get a variety of wood products like firewood, plywood, pulp for paper, etc.
Many plants provide different types of medicines and spices.
Human beings also get non-wood products from plants, like lac, dyes, cane, bamboo, grasses, fodder, bidi leaves, gums, resin, rubber, etc.

Plants Purify Air

Plants remove carbon dioxide from the air and use it in photosynthesis. They release oxygen into the atmosphere during daytime. Also, by holding soil particles together they decrease the dust contamination of air. These particles form Suspended Particulate Matter (SPM) in the atmosphere. These activities of plants help purify air.

Plants Depend on Animals for Carbon Dioxide

Plants need carbon dioxide for manufacturing food by photosynthesis. Animals release carbon dioxide into the atmosphere through respiration. Human activities like burning of wood, coal, petrol, gas, etc., also release carbon dioxide into the air.

Plants Depend on Animals for Pollination and Seed Dispersal

Bees, wasps, butterflies and moths visit flowers for collecting nectar, and in this act they help in the transfer of pollen from one flower to another. For attracting these insects, plants have colourful and scented flowers. Some animals feed on fruits and deposit seeds at different places. Seeds of some plants cling to the body of animals and are carried away to new places. This helps in the dispersal of seeds.

Animals Provide Nutrients to Plants

The bodies of dead and decaying animals and the droppings of animals are acted upon by microbes. They reduce them to inorganic compounds. These compounds percolate into the soil and provide nutrients to the plants. This is called recycling of nutrients. Therefore, plants depend on animals for inorganic nutrients.

Living organisms cannot survive alone. They have to interact with each other and with their environment. Successful survival of a biological community depends upon the interaction of its members and a balance between their activities.

Why Should We Preserve Our Forest?

Forests are an important natural resource. They are essential for the preservation of the environment and the survival of all animals, including man.
1. Forests purify air and provide clean and fresh air for breathing to all living beings.
2. Forests regulate climate by reducing temperature and help in rainfall.
3. Trees bind the soil and control soil erosion.
4. Forests control and prevent floods by holding water and not allowing its free flow.
5. Trees make the soil fertile by recycling the nutrients.
6. Forests help in reducing pollution.
7. Forests provide animals with a place to live (shelter).
8. Forests provide us food, wood, rubber, gum, resin, non-edible oils, honey, beeswax, lac, bamboo, fuel, etc.
9. Some forests have been developed into recreational parks where people enjoy camping and trekking. Some forests have been made wildlife reserves where animals and plants are protected.
10. Some ancient tribes live in forests. The forests provide them everything they need, like food, shelter, water and medicines. Products obtained from forests are a source of steady income for tribals.

What Will Happen If Forests Disappear?

1. Disappearance of forests will severely disturb the ecosystem and the ecological balance in nature.
2. Wildlife may also not be able to survive in the absence of forests.
3. There will be a shortage and scarcity of forest products.
4. Since forests play a major role in keeping the environment cool and regulating rainfall, their absence will lead to reduced rainfall and increased temperature.
5. It will be very difficult to recharge the existing water resources without forests.
6. Unpredictable changes will take place in weather and climate patterns across the world.
7. Soil erosion will increase. This will adversely affect soil fertility and productivity.
8. Global warming will lead to flooding of low-lying areas, as ice caps and glaciers melt. This is already happening. Two islands in the Andamans have vanished under water.
9. Disappearance of forests will cause a drastic increase in air pollution, which will ultimately threaten the very existence of life on the earth.

Forests Are the Green Lungs

Forests are the lungs of the earth. Just like human lungs, they turn carbon dioxide into clean oxygen. Every living creature in this world needs oxygen. Without trees and forests, our earth would be running out of oxygen. Forests absorb carbon dioxide from the atmosphere by doing photosynthesis, a simple chemical reaction that produces energy in the presence of light by chlorophyll. The main substances for this reaction are carbon dioxide and water. A by-product of photosynthesis is, of course, oxygen. Over 40 per cent of the world's oxygen is produced from the forests. Another major function of the forest, directly connected to its function as the earth's lungs, is to keep the planet cool. It helps stop global warming, also known as the greenhouse effect.

As we already understand, trees are an important part of the carbon cycle, a process that keeps the atmospheric concentration of carbon dioxide under control. Carbon dioxide in the atmosphere traps heat, like a blanket, causing the earth's temperature to rise.

Destroying the forest is the same as destroying our own life-support system. A single tree in the park can only produce a small amount of oxygen, while a forest which contains thousands or even millions of trees can produce millions of times more. The earth's population keeps growing, and our technology is rapidly increasing. This situation leads to a rise in the carbon dioxide concentration in the atmosphere. Over the past half century, the earth's forests have been reduced greatly. The destruction of about 14.6 million hectares of forest in the last dozen years makes the earth hotter. The average global temperature over the past 50 years has increased at the fastest rate in recorded history. The trend is accelerating, and the 10 hottest years on record have all occurred since 1990. If this situation continues, scientists believe that the average global temperature could be 3 to 9 degrees higher by the end of the century.

We can take another point of view to understand the cause of forest destruction. Consider the earth as a living organism, and the forest as its lungs. If the lungs have been destroyed, all cells inside the organism would not be able to get clean oxygen and would eventually die. In this case, we, all living creatures on earth, are the cells.

Food Chain and Food Web

Nature, the organisms, are in constant interaction with the non-living part of the physical environment. The organisms include the green plants, which are the producers, and animals, which form the consumer category.

[Figure: Fig.7 A food chain. Green Plant (Producer), Rabbit (Primary consumer), Fox (Secondary consumer), Lion (Tertiary consumer)]

The non-living part of the environment includes factors like light, temperature, soil and water. Such factors are important to all living organisms for their survival. Thus, the interdependence of living organisms with the non-living part of the environment is important for a system to sustain. Such a system is referred to as an ecosystem.

Forests form a part of the ecosystem. These are self-sustaining and self-regulatory units of nature. The interactions among organisms in the forest ecosystem are depicted in the form of a food chain. A food chain involves transfer of energy through the process of eating and being eaten. All green plants trap solar energy and convert it into chemical energy, which is stored as carbohydrate. The plants further transfer their energy to animals after being eaten. The animals may in turn be eaten by other animals, and in this way energy is transferred at all levels. The transfer of energy through a series of organisms, each feeding on the preceding organism, is called a food chain. Many such food chains are found in forests and are interlinked to form a food web.

Fig. 8 A typical food web (organisms: owl, dragonfly, bird, mouse, mosquito, grasshopper, caterpillar, ladybird, bug, beetle, sunflower plant, grub; arrows point from the organism eaten to the organism that eats it, with the plant at the base and all the animals interlinked)

When plants and animals die, they form the food for the saprotrophs, referred to as decomposers. These play an important role in forests. They feed upon dead animals and plant tissues and convert them into a dark coloured substance called humus. Thus, the nutrients of dead plants and animals are released into the soil. These minerals are absorbed by the trees and plants inhabiting the forest for their growth and development.

Forest Products

Forest products play an important part in the socio-economic development of a country. Important forest products are timber, small timber, fuelwood, bamboo and many non-wood forest products like Tendu Patta, Harra, Sal seed, etc. Teak (Tectona grandis), Sal (Shorea robusta), Bija (Pterocarpus marsupium), Khair (Acacia catechu), Tinsa (Ougeinia oojeinensis), Salai (Boswellia serrata) and Saja (Terminalia) are the main tree species which are harvested annually from the forest. Timber and bamboo harvesting is carried out in a selective manner.

Other Products Obtained from Forests

Non-wood products provided by forests include food such as berries, mushrooms, edible plants, game and bushmeat, as well as fodder and medicinal plants. These products play a crucial role in meeting the subsistence needs of a large part of the world's population living in or near forests, and they provide them with income-generating opportunities. Non-wood forest products are collected for local household use or trade, though some find export markets.

Asia has traditionally used non-wood forest products and often includes them in official national accounts and international trade statistics, which is not generally the case in other regions. Asia accounts for the largest removals of non-wood forest products. With a share of 74%, China reports by far the world's largest removals of forest plant products for food, consisting mainly of oil seeds, nuts and bamboo shoots. Other countries with significant removal volumes for food are India, the Republic of Korea and Pakistan in Asia, the Czech Republic, Finland, Italy and Sweden in Europe, and Brazil in South America. China also accounts for 72% of removals in the category of exudates, such as tannin extract and raw lacquer. India accounts for half of reported removals of plant raw materials for medicinal and aromatic uses. India also has a 42% share of total removals of other plant products, such as tendu leaves, medicinal herbs and lac, followed by Brazil and Mexico.

Forest Conservation and Management

With the increase in human population, there is ever-rising demand for converting forests into residential and agricultural areas. As a result, the habitats of several species of animals and plants are being destroyed. Therefore, we need to reorient our activities so as to ensure the least possible damage to our forests. The following measures are to be followed to conserve our forests.
1. Large-scale felling or cutting of forest trees must be stopped.
2. When trees are cut, more trees should be planted in their place.
3. Overgrazing by cattle and animals should be stopped.
4. To develop new forests, more saplings should be planted every year during the rainy season.
5. Forests must be protected from insects, pests and infections by treating them with insecticides and pesticides.
6. Forest fires must be checked. People should avoid smoking or cooking in the forest area.
7. Various activities leading to soil erosion must be avoided.

By conserving air, land and water → The pollution of air, land and water should be controlled so that trees and vegetation can survive.

Conservation and management of forest resources aim at:
Increasing forest cover to 33% of the total land area.
Increasing the productivity of forests.
Providing fuelwood, fodder and other forest products to rural and tribal people.
Avoiding over-exploitation or misuse of forest resources.

The Indian Government introduced social forestry in 1970. Under this scheme, wastelands are used to grow trees.

The Chipko Movement was started in 1974 by Sunder Lal Bahuguna to reduce tree felling. This movement initially started in the Garhwal region. The villagers came forward and hugged the trees to prevent the contractors from felling them.