Class 7 Science Forests Our Lifeline Notes

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Revision Notes for Class 7 Science Chapter 17 Forests: Our Lifeline

Class 7 Science students should refer to the following concepts and notes for Chapter 17 Forests: Our Lifeline in Class 7. These exam notes for Class 7 Science will be very useful for upcoming class tests and examinations and help you to score good marks

Chapter 17 Forests: Our Lifeline Notes Class 7 Science

Forests: Our Lifeline

 

INTRODUCTION

 

About one-third of the world’s land surface is covered by forests. It is an eco-system (plants and animals) which is dominated by trees. Forest 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 invaluable wealth of a nation and a renewable natural resource.

 

In the last hundred years, utilization of forest products has gone up mainfold 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 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% which is an increase of 4.75% over ten years.

 

Forests provides 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.

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. from xerophytic scrub forest in the arid zones of Rajasthan to evergreen rainforests in the tropics as in the Western Ghats, Assam, North Bengal, Andamans and Conifers in the temperate regions. In between the extremes, there are many intermediate types of forests.

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

 

In North-East India, Sal is most common tree, while in South West 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 lie 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 levels 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 wind breaker. The canopy is the densest layer and home to unique flora and fauna.

 

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 have 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 of dead animals and plants thus converting them into dark coloured organic matter. This forms the humus.

 

Types of plants in the forest

A forest has annual, biennial and perinnial plants:

• The plant with a life span of one year are called annuals. They are herbs. 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 pernnials. These include shrubs and trees.

 

Forest Animals and their Habitats

Forests provide habitat for a large variety of wild animals. Boars, bisons, jackals, elephants live in deeper areas of the forest, where 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 type of birds occupy tree and tree twigs.

 

Interdependence of Plants and Animals in Forest

Animals, human beings and plants show fundamental dependence on each others. 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 nongreen 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 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 fire wood, plywood, pulp far 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 carbondioxide

Plants need carbon dioxide for manufacturing food by photosynthesis. Animals release carbondixoide into the atmosphere through respiration. Human activities like burning of wood, coal, petrol, gas, etc., also release carbon dioxide into 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 dropping of animals are acted upon by microbes. They reduce them to inorganic compounds. These compounds get percolated 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 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 environment and survival of all the animals including man.

1. Forests purify air and provide clean and fresh air for breathing to all living being

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 flood by holding water and not allowing its free flow.

5. Trees make the soil fertile by recycling the nutrient

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, beewax, 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 forest 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 forest

3. There will be a shortage and scarcity of forest product

4. Since forests play a major role in keeping the environment cool and regulating the rainfall, their absence will lead to reduced rainfall and increased temperature.

5. It will be very difficult to recharge the existing water resources without forest

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 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 do, it turns 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. Forest absorbs carbon dioxide from atmosphere by doing photosynthesis, a simple chemical reaction that produces energy in the presence of light by chlorophyll. The main substance for this reaction is carbon dioxide and water. A by-product of photosynthesis is, of course, oxygen. Over 40 percent 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 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 and causing the earth 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 little amount of oxygen, while a forest which contains thousands or even millions of trees can produce millions times more. Earth’s population is keep growing, and our technology is rapidly increasing. This situation leads to the rise of carbon dioxide concentration in the atomosphere. However, over the past half century, the Earth’s forests have been reduced greatly. The destruction about 14.6 million hectares of forest in the last dozen years makes the earth getting hotter. The average global temperature over the past 50 years has increased at fastest rate in recorded history. And the trend is accelerating, the 10 hottest years on record have all occurred since 1990. If this situation continues, scientists believe that the average global temperatures 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 wouldn’t be able to get clean oxygen and eventually die. In this case, we - all living creatures on earth - are the cells.

Food Chain and Food Web

The 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.

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 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 a ecosystem.