The correct option is (a).
Explanation
In a forest ecosystem, vegetation is arranged in vertical layers based on the height, growth habit, and light requirements of different plant species. This vertical arrangement is known as stratification.
Statement-wise Analysis:
- Statement 1 is Correct: Understoreys are formed due to the presence of trees and plants having crowns of different types and sizes. These variations allow vegetation to occupy different vertical levels, creating distinct layers beneath the main canopy.
- Statement 2 is Incorrect: Understoreys do not create a single uniform horizontal layer. Forests typically exhibit multiple layers of vegetation. Giant and tall trees constitute the top layer (canopy), followed by shrubs and tall grasses, while herbs form the lowest layer.
- Statement 3 is Incorrect: The composition of understoreys is not identical globally. It varies significantly depending on climatic conditions, soil type, rainfall, and the specific biome (e.g., the understorey of a tropical rainforest differs vastly from that of a temperate deciduous forest).
Key Takeaway:
Understoreys represent the vertical stratification in a forest ecosystem, formed by plants of varying heights and crown sizes, and their composition varies across different forest types.