Quick Revision Notes & Formula Sheet

Class 7 Science - Chapter: Forests: Our Lifeline

(MP Board)

Definition: A forest is a large area covered chiefly with trees and undergrowth. It is a complex ecosystem comprising a wide variety of flora (plants) and fauna (animals).
Habitat (आवास): It provides food, shelter, and protection to numerous living organisms.
Canopy (छत्रक / वितान): The branches of tall trees look like a roof over the other plants in the forest. This is called a canopy.
Understorey (अस्तर): Different layers of vegetation below the canopy formed by shrubs, tall grasses, herbs, and small trees.

Crown (वृक्ष का मुकुट): The branchy part of a tree above the stem is known as the crown of the tree.
Decomposers (अपघटक): Micro-organisms (like bacteria and fungi) that convert dead plants and animals into humus.
Humus (धूसर मृदा / ह्यूमस): A dark, nutrient-rich layer of soil formed by the decomposition of dead organic matter. It makes the soil fertile.
Food Chain (आहार श्रृंखला): A linear network of links in a food web starting from producer organisms and ending at apex predator species (e.g., Grass $\rightarrow$ Deer $\rightarrow$ Tiger).
Food Web (आहार जाल): The interconnection of multiple food chains in an ecosystem.
Deforestation (वनोन्मूलन): Large-scale cutting down of forest trees for human activities.
Afforestation (वनीकरण): Planting new trees to increase the forest cover.

"Green Lungs" of the Earth: Forests release oxygen through photosynthesis and help animals respire. They maintain the balance of oxygen ($O_2$) and carbon dioxide ($CO_2$) in the atmosphere.
Water Cycle (जल चक्र): Forests help in bringing rainfall and maintain moisture in the atmosphere through transpiration.
Soil Binder: Roots of trees bind the soil tightly, preventing soil erosion (मृदा अपरदन) and floods.
Natural Sponge: Forests absorb heavy rainwater and allow it to seep into the ground, maintaining the groundwater table (भू-जल स्तर).
Source of Products: Forests provide timber, medicinal plants, gum, paper, honey, and sealing wax.

Autotrophs (स्वपोषी): Plants that make their own food using sunlight (Producers).
Heterotrophs (विषमपोषी): Animals that depend directly or indirectly on plants for food (Consumers - Herbivores and Carnivores).
Scavengers (मृताहारी / अपमार्जक): Animals like vultures, crows, jackals, and hyenas that eat dead animals.
Nutrient Recycling: $$\text{Dead Plants & Animals} \xrightarrow{\text{Decomposers}} \text{Humus} \xrightarrow{\text{Absorbed by}} \text{Living Plants}$$ This ensures that nutrients are continuously returned to the soil.

Why are forests called green lungs? Because plants produce oxygen during photosynthesis which is essential for all living beings.
What happens if forests disappear?
- Amount of $CO_2$ in air will increase, leading to global warming.
- Animals will lose their homes and food (loss of habitat).
- Soil will get washed away by rains, leading to floods and soil erosion.
Forest Floor: The dark, damp lowest layer of the forest where dead leaves, decaying matter, insects, and fungi are found.