Chapter 3 (Part 2): Factors of Locality — Edaphic & Biotic Factors

Chapter 3 – Factors of Locality

Part 2: Edaphic and Biotic Factors

Learning Overview

Soil properties, soil profile, soil water, humus, parent rock, and the influence of living organisms on forest vegetation.

This chapter explains how soil conditions and living organisms influence the growth, distribution, regeneration and development of forest vegetation.

Edaphic factors Soil profile Soil water Parent rock Biotic factors Forest ecology

1. Key Concepts and Definitions

Edaphic Factors

Edaphic factors are soil-related factors that influence the growth, distribution and development of plants.

Definitions of Soil

• Pedological definition: Soil is a natural body consisting of mineral and organic constituents, differentiated into horizons.
• Wilde: Forest soil is the product of interactions among parent material, climate, organisms, topography and time under forest conditions.
• Champion and Seth: Soil is a dynamic natural body that supports plant growth and undergoes continuous changes through physical, chemical and biological processes.

Other Important Terms

Term	Meaning
Catena	A sequence of soils occurring along a slope under the influence of topographic conditions.
Biotic factors	Influences of living organisms on vegetation and forest ecosystems.
Competition	Interaction among organisms for limited resources such as light, water, nutrients and space.
Parasite	An organism that obtains nourishment from another living organism (host).
Epiphyte	A plant growing on another plant for physical support without necessarily obtaining food from it.
Liana	A woody climbing plant that uses other plants or structures for support.
Habitat	The natural environment in which an organism lives.

2. Soil Formation and Soil Dynamics

Agencies of Soil Formation

Soil formation is influenced by active and passive agencies.

Active agencies

• Climate
• Biological organisms

Passive agencies

• Parent rock/material
• Topography
• Time

Memory trick – Soil formation:

Active = Climate + Organisms

Passive = Parent material + Topography + Time

Weathering of Rocks

Physical weathering

Breakdown of rocks into smaller fragments without changing their chemical composition.

Examples: temperature changes, frost action and mechanical disintegration.

Chemical weathering

Decomposition and alteration of minerals through chemical reactions.

Examples: oxidation, hydration, hydrolysis and carbonation.

Parent Rock and Species Distribution

Parent material influences soil texture, mineral composition, nutrient availability and vegetation distribution.

Parent rock / soil condition	Species association / observation
Quartzite	Chir pine is associated with quartzitic formations.
Mica schist	Blue pine is associated with mica schist.
Lime-rich rocks	Teak is associated with lime-rich geological formations.
Lateritic soils	Teak generally avoids lateritic soils; Xylia xylocarpa and Cleistanthus collinus are associated with lateritic conditions.
Black cotton soils	Teak is reported to avoid black cotton soils.
Deccan trap	Sal is reported absent from Deccan trap formations.
Quartzite and gneiss	Red Sanders is associated with quartzite and is reported not to occur on adjoining gneiss.

Memory trick – Parent rock associations:

• Chir – Quartzite
• Blue pine – Mica schist
• Teak – Lime-rich rocks
• Red Sanders – Quartzite

3. Soil Profile and Soil Horizons

A soil profile represents the vertical section of soil showing different horizons from the surface down to the underlying parent material or bedrock.

Soil Profile

↓

A₀₀ Horizon: Litter layer

↓

A₀ Horizon: Humus layer (F and H)

↓

A Horizon: Eluviation zone (A₁, A₂, A₃)

↓

B Horizon: Illuviation zone

↓

C Horizon: Weathered parent material

↓

D Horizon: Bedrock

Description of Soil Horizons

Horizon	Description
A₀₀	Surface litter layer consisting mainly of undecomposed plant material.
A₀	Organic layer containing humus; includes fermentation and humification layers (F and H).
A	Eluviation zone, where materials are removed by downward movement of water.
A₁	Mineral horizon mixed with organic matter.
A₂	Zone of maximum eluviation and depletion of certain materials.
A₃	Transitional horizon between A and B.
B	Illuviation zone, where materials transported from upper horizons accumulate.
C	Weathered parent material.
D	Underlying bedrock.
G	Gley horizon associated with waterlogging and reducing conditions.

Memory trick – Soil horizon sequence:

“All Active Boys Can Dance”

A₀₀ → A₀ → A → B → C → D

Hardpans

Hardpan	Major constituent / description
Ortstein	Iron- and organic-matter cemented layer.
Lateritic iron pan	Iron-rich hardened layer.
Silican pan	Hardened layer associated with silica accumulation.
Kankar pan	Calcium carbonate-rich hardened layer.

4. Soil Water and Soil Moisture

pF Value

pF value is the logarithm to base 10 of the soil-water suction head expressed in centimetres of water.

Formula: pF = log₁₀(h)

Where h = suction head in centimetres of water.

Classification of Soil Water According to pF

Soil water category	pF range	Availability to plants
Gravitational water	Below 2.7	Moves downward under gravity; generally not retained for long.
Capillary water	2.7–4.5	Main source of water available to plants.
Hygroscopic water	Above 4.5	Held tightly by soil particles and unavailable to plants.

Important: Capillary water is the principal source of water available to plants. The pF value at permanent wilting point is approximately 4.2.

Groundwater

• Groundwater can influence vegetation where the water table is relatively shallow.
• A water table at approximately 5–10 m may influence vegetation depending on rooting depth and soil conditions.

Available Water Capacity

• Wilting coefficient is generally lowest in sandy soils.
• Wilting coefficient is generally highest in clay soils.
• Available water is generally maximum in loamy soils.

Memory trick – Soil water:

• Sand: Lowest wilting coefficient.
• Clay: Highest wilting coefficient.
• Loam: Maximum available water.

5. Soil Texture and Soil Humus

Soil Texture Classification (ISSS)

Soil particle	Particle size (mm)
Clay	Less than 0.002
Silt	0.002–0.02
Fine sand	0.02–0.2
Coarse sand	0.2–2.0
Gravel	Greater than 2.0

Memory trick – Soil particle sizes:

Remember the increasing order:

Clay → Silt → Fine sand → Coarse sand → Gravel

Types of Humus

Humus type	Characteristics
Mull	Well-decomposed organic matter, generally associated with biologically active soils and good mixing of organic matter with mineral soil.
Mor	Raw or poorly decomposed organic matter, generally associated with acidic conditions and slower decomposition.
Acid/Sour humus	Acidic humus associated with slow decomposition and accumulation of organic matter.

Litter Production

Tropical rain forests

Approximately 25 tonnes/ha

Tropical deciduous forests

Approximately 10 tonnes/ha

Memory trick:

Rain forest = 25; Deciduous forest = 10 tonnes/ha.

6. Soil Chemical Properties

Soil pH and Tree Species

Species / condition	pH range / response
Sal	4.5–5.5
Teak	6.5–7.6
Alkali-tolerant species	Can tolerate comparatively alkaline soil conditions.

Soil pH and Microbial Activity

• Nitrifying organisms are inhibited at soil pH below 5.5.
• Damping-off disease is reported to be controlled under soil conditions of pH 5.5 or below.

Soil C:N Ratio

The carbon-to-nitrogen ratio influences the decomposition of organic matter and nitrogen availability.

Soil C:N Ratio

↓

≤ 17:1

Nitrification

Above 33:1

Nitrogen immobilisation

↓

Ideal C:N ratio: 10:1

C:N ratio	Effect
≤ 17:1	Favourable for nitrification.
Above 33:1	Nitrogen immobilisation.
10:1	Ideal ratio.

Memory trick – C:N ratio:

• 17 or less: Nitrification.
• Above 33: Immobilisation.
• 10:1: Ideal ratio.

Sal Seedling Mortality

Sal seedling mortality is associated with a soil-air CO₂/O₂ ratio of 2.8.

7. Biotic Factors Affecting Forest Vegetation

Biotic factors include the influence of plants, animals, insects, microorganisms and human activities on forest vegetation.

7.1 Parasitic Plants

Parasitic plants obtain some or all of their nourishment from host plants.

Type	Examples	Important feature
Total stem parasite	Cuscuta reflexa, Cassytha filiformis	Depend on the host for their nourishment.
Partial stem parasite	Loranthus longiflorus, Viscum, Arceuthobium minutissimum	Partially depend on the host.
Root hemiparasite	Sandalwood (Santalum album)	Root parasite that obtains nutrients through host-root connections.

Memory trick – Parasites:

• Total stem: Cuscuta and Cassytha.
• Partial stem: Loranthus, Viscum and Arceuthobium.
• Root: Sandalwood.

7.2 Weeds

Lantana camara

Invasive weed that competes with forest vegetation and may interfere with natural regeneration.

Eupatorium odoratum

Weed that can compete with forest vegetation and affect regeneration.

7.3 Insect Pests

Host tree	Insect pest	Type of damage
Sal	Hoplocerambyx spinicornis	Sal borer.
Teak	Hapalia machaeralis	Teak defoliator.
Teak	Hyblaea puera	Teak skeletonizer.
Teak / Meliaceae	Hypsipyla robusta	Shoot borer.

Memory trick – Teak insect pests:

“Machaeralis Defoliates, Puera Skeletonizes.”

• Hapalia machaeralis – Defoliator.
• Hyblaea puera – Skeletonizer.
• Hypsipyla robusta – Shoot borer.

7.4 Damage by Wild Animals

Animal	Damage / affected species
Monkeys and langurs	Pull out Chir pine seedlings.
Porcupines	Damage bamboo, Khair and Semal.
Bears	Strip bark from Deodar, Spruce and Blue pine.

Memory trick – Animal damage:

• Monkey/Langur: Chir seedlings.
• Porcupine: Bamboo, Khair and Semal.
• Bear: Bark stripping of Deodar, Spruce and Blue pine.

7.5 Forest Fire

Controlled fire

• Can be beneficial under suitable management.
• May help in prescribed burning and fuel management.

Uncontrolled fire

• Damages forest vegetation.
• Can destroy regeneration and soil organic matter.
• May cause ecological and economic losses.

7.6 Grazing and Browsing

• Light and controlled grazing may be beneficial under suitable conditions.
• Heavy grazing damages vegetation and soil.
• Goats and camels can cause considerable browsing damage.
• Goats are considered the most destructive browsing animals.

Important: Controlled grazing and controlled burning may be beneficial, whereas excessive grazing and uncontrolled fire are harmful to forest ecosystems.

8. Important Scientific Names

Common name / identification	Scientific name
Sal borer	Hoplocerambyx spinicornis
Teak defoliator	Hapalia machaeralis
Teak skeletonizer	Hyblaea puera
Shoot borer	Hypsipyla robusta
Root hemiparasite	Santalum album
Partial stem parasite	Arceuthobium minutissimum
Invasive weed	Lantana camara
Invasive weed	Eupatorium odoratum
Woody climber / liana	Bauhinia vahlii

9. Important One-Liners for Competitive Examinations

1. Chir pine is associated with quartzite.
2. Blue pine is associated with mica schist.
3. Teak is associated with lime-rich rocks.
4. Red Sanders is associated with quartzite.
5. Top height is based on the 250 largest trees per hectare.
6. pF value is the logarithm of soil-water suction head expressed in centimetres.
7. Capillary water occurs within the pF range of 2.7–4.5.
8. Sal seedling mortality is associated with a soil-air CO₂/O₂ ratio of 2.8.
9. Nitrifying organisms are inhibited at pH below 5.5.
10. Damping-off is controlled at soil pH 5.5 or below.
11. C:N ratio above 33:1 results in nitrogen immobilisation.
12. Ideal C:N ratio: 10:1.
13. Litter production in tropical rain forests is approximately 25 tonnes/ha.
14. Litter production in tropical deciduous forests is approximately 10 tonnes/ha.
15. Controlled burning may be beneficial under suitable management.
16. Goats are considered the most destructive browsing animals.

10. Quick Revision – Must Remember

Topic	Important fact
Soil formation	Active: climate and organisms; Passive: parent rock, topography and time.
Soil profile	A₀₀ → A₀ → A → B → C → D.
Eluviation	A horizon.
Illuviation	B horizon.
Weathered parent material	C horizon.
Capillary water	pF 2.7–4.5.
Available water	Maximum in loamy soils.
Clay particle	Less than 0.002 mm.
Sal soil pH	4.5–5.5.
Teak soil pH	6.5–7.6.
High C:N ratio	Above 33:1 – nitrogen immobilisation.
Sal borer	Hoplocerambyx spinicornis.
Teak defoliator	Hapalia machaeralis.
Teak skeletonizer	Hyblaea puera.
Sandalwood	Root hemiparasite.

11. Interactive Practice Quiz

15 questions. Tap an option to see the correct answer and explanation straight away, then go to the next question.

Chapter 3 – Factors of Locality | Part 2: Edaphic and Biotic Factors

Silviculture notesAll quick revision notes →

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