Geography (9696) | Tropical climates

🌍 Comprehensive Study Notes: Tropical Environments (9696 Advanced Physical Geography)

Hello Geographers! Welcome to the fascinating world of Tropical Environments. This is a crucial and often highly rewarding topic in Paper 3. The tropics are where you find the most vibrant climates and ecosystems on Earth, but also where geomorphological processes (landform changes) happen incredibly fast!

Don't worry if some of the terminology seems tough—we will break down the complex atmospheric processes and the unique landforms found here step-by-step. Focus on the processes (how things happen) and the links between climate, landforms, and ecosystems.

Section 7.1: Tropical Climates

Global Distribution and Key Influences

Tropical climates are found generally between the Tropics of Cancer (23.5°N) and Capricorn (23.5°S). We divide them into two main types based on rainfall:

• Humid Tropical (Tropical Rainforest): Found closest to the Equator (0°–10° N/S). Characterised by high, consistent rainfall throughout the year. (Example: Amazon Basin, Congo Basin).
• Seasonally Humid Tropical (Tropical Savanna): Found further away from the Equator (10°–25° N/S). Characterised by distinct wet and dry seasons. (Example: East Africa, Northern Australia).

The Role of Key Atmospheric Systems

The unique climate of the tropics is controlled by three major atmospheric mechanisms:

1. The Intertropical Convergence Zone (ITCZ)

The ITCZ is perhaps the most important climatic feature of the tropics.

• What is it? It's a low-pressure belt near the equator where the northeast and southeast trade winds meet (converge). Because the air here is warm and moist, it rises strongly (convection), leading to massive cloud formation and heavy, frequent rainfall.
• How does it move? The ITCZ follows the overhead sun (solar maximum). It migrates north in July and south in January.
• Impact:
  Humid Tropics: Areas directly under the ITCZ year-round (like the Amazon) experience consistent high rainfall.
  Seasonally Humid Tropics: Areas further away experience the ITCZ passing overhead twice (wet season) or once (monsoon areas) or avoiding it entirely (dry season). This migration creates the distinct wet/dry cycle of the savanna.

2. Subtropical Anticyclones (High Pressure)

• What are they? These are belts of high pressure (sinking, stable air) found around 30° N/S. Sinking air suppresses convection and prevents cloud formation.
• Role: When the ITCZ moves away in the winter hemisphere, the subtropical high-pressure belt dominates the seasonally humid tropical regions, causing the long, pronounced dry season.

3. Monsoons

The word 'monsoon' simply means a seasonal wind shift, but it’s critical for rainfall in areas like South Asia.

• Mechanism: Caused by the dramatic difference in heating between land and sea (land heats up and cools down much faster than water).
• Summer Monsoon (Wet): Land heats rapidly, creating low pressure. Moist air is drawn in from the ocean, leading to intense orographic and convectional rainfall.
• Winter Monsoon (Dry): Land cools rapidly, creating high pressure. Dry air blows out to sea.

Key Features of Temperature and Rainfall (7.1)

Temperature

• High Annual Temperatures: Temperatures are consistently high, usually between 20°C and 30°C, because the sun’s angle is high throughout the year.
• Small Annual Variation: The difference between the hottest and coldest month is very small (often less than 5°C). The temperature range in the humid tropics is smallest (e.g., Singapore).
• Large Diurnal Variation: In the seasonally humid tropics, the difference between daytime and nighttime temperature can be much larger than the annual variation (perhaps 10°C or more), especially in the dry season when there is little cloud cover to trap heat overnight.

Rainfall

• Humid Tropics: Total annual rainfall is very high (>2000 mm). Rainfall is heavy, frequent (often daily convectional storms), and well-distributed throughout the year.
• Seasonally Humid Tropics (Savanna): Total rainfall is lower (500–1500 mm). There is a sharp contrast between a high-rainfall wet season (when the ITCZ is overhead) and a prolonged low-rainfall dry season (when subtropical high pressure dominates).

Section 7.2: Landforms of Tropical Environments

Tropical environments have unique landforms because the combination of high heat and high rainfall accelerates chemical weathering processes far beyond what is seen in temperate zones.

1. Granite Landforms: The Importance of Deep Weathering

Granite is an intrusive igneous rock, naturally resistant, but in the tropics, it succumbs to intense chemical breakdown, mainly through hydrolysis.

• Hydrolysis Step-by-Step:
  1. Water (especially slightly acidic rainwater) reacts with minerals in the granite, particularly feldspar.
  2. This chemical reaction converts the hard minerals into soft clay materials (kaolin).
  3. The solid rock structure breaks down in situ (on the spot), creating a deep layer of rotten rock called regolith or saprolite (deep weathering profile).

The depth of this regolith can be enormous, sometimes reaching over 100 metres!

Characteristic Granite Landforms

• Tors: Outcrops of resistant granite blocks that survive the deep weathering. They usually stand proud on the surface after the softer surrounding saprolite has been washed away (stripped).
• Inselbergs: Isolated, steep-sided hills or mountains that rise abruptly from a gently sloping plain. They are essentially very large tors.
• Bornhardts: A specific type of inselberg that is dome-shaped, large, and smooth. They often form due to pressure release (dilatation), where the removal of overlying weathered material causes the granite to expand and fracture in curved sheets (exfoliation).

2. Limestone Landforms: Tropical Karst

Karst landscapes form when soluble rock (like limestone, made mainly of calcium carbonate, \( \text{CaCO}_3 \) ) is dissolved by acidic water.

In the tropics, the dissolution process is highly accelerated because of high temperatures and large amounts of biogenic CO\(_2\) (from decaying vegetation) in the soil and water. This results in spectacular "Tropical Karst" landforms.

Key Process: Carbonation

Rainwater absorbs CO\(_2\) from the atmosphere and soil, forming weak carbonic acid. This acid dissolves the limestone (solution).
\( \text{H}_2\text{O} + \text{CO}_2 \rightleftharpoons \text{H}_2\text{CO}_3 \) (Carbonic Acid)
\( \text{H}_2\text{CO}_3 + \text{CaCO}_3 \rightleftharpoons \text{Ca}(\text{HCO}_3)_2 \) (Calcium Bicarbonate - Soluble)

Types of Tropical Karst

• Cockpit Karst: Found in areas of intense, uniform dissolution (e.g., Puerto Rico, Jamaica). The landscape is dominated by numerous small, rounded hills (known as cones or kegelkarst) separated by star-shaped depressions called cockpits.
• Cone Karst (Kegelkarst): These are the steep, conical hills that make up cockpit karst.
• Tower Karst (Turmkarst): Forms when erosion and fluvial action widen the depressions between the cones, leaving the resistant limestone cones isolated as vertical, cylindrical towers with incredibly steep sides. (Example: Guilin, China or Ha Long Bay, Vietnam - though that is submergent karst).

Section 7.3: Tropical Ecosystems (Rainforest and Savanna)

We must examine the structure, processes, and soils of the two key tropical biomes: the Humid Tropical (Rainforest) Ecosystem and the Seasonally Humid Tropical (Savanna) Ecosystem.

Plant Communities

Plant communities develop over time towards a stable state based on the local climate and soil.

• Climax Community: This is the final, stable plant community that is in equilibrium with the climate and soil. In the humid tropics, the climax community is the Tropical Rainforest. In the seasonally humid tropics, it is the Savanna grassland/woodland.
• Subclimax Community: A community held below the true climax stage by local environmental factors (like poor drainage or exceptionally thin soil).
• Plagioclimax Community: A stable community maintained by human interference (e.g., repeated burning, grazing, or deforestation). The vast majority of modern savanna grasslands are considered plagioclimax, maintained by fire and grazing animals, preventing them from developing into dense woodland.

Nutrient Cycling and Soil Fertility

Tropical environments, especially rainforests, are famous for their high productivity despite having infertile soils. This is explained by the Nutrient Cycling Model (Gersmehl Diagram).

The Gersmehl Diagram (Energy Flows and Trophic Levels)

The Gersmehl model compares the size of three stores (Biomass, Litter, Soil) and the speed of three flows (Uptake, Fallput, Weathering/Leaching).

1. Rainforest Ecosystem (Humid Tropical)

• Stores: The largest store is Biomass (the living vegetation). The soil store is very small.
• Flows: All flows are extremely rapid.
  Fallput (dead leaves/material falling) is fast, and Decomposition is incredibly fast due to hot, wet conditions (bacteria/fungi thrive).
  Uptake (roots absorbing nutrients) is immediate.
• Soil Fertility: The soil is largely infertile because nutrients are cycled so quickly from biomass (leaves) back into biomass (roots) before they can leach away. If you remove the trees, the nutrient cycle breaks, and the soil fertility is quickly lost.
• Energy Flows & Trophic Levels: Energy flow is efficient and supports massive biodiversity. Rainforests contain many trophic levels (producers, primary consumers, secondary consumers, etc.) due to the high energy input (insolation) and high net primary productivity (NPP).

2. Savanna Ecosystem (Seasonally Humid Tropical)

• Stores: The Soil store and Litter store tend to be larger than in the rainforest, but the overall cycling speed is seasonal (slow in the dry season, fast in the wet season).
• Adaptation: Grasses and woody shrubs often store nutrients in roots or underground bulbs to survive the dry season and fires.

Tropical Soil Formation and Characteristics (7.3)

The intense weathering and leaching in the tropics create distinctive soils known globally as Oxisols or Latosols.

• Soil Forming Processes:
  1. Intense Weathering: Chemical weathering (hydrolysis) dominates, breaking down parent material rapidly.
  2. Leaching: Heavy rainfall washes soluble nutrients (like potassium, calcium, silica) downwards and out of the soil profile, leaving behind insoluble, heavy minerals.
  3. Laterisation: The intense leaching leaves behind iron and aluminium oxides, which gives the soil its characteristic red/brown colour. If this soil dries out and is exposed to the sun (often after deforestation), the iron oxides can harden irreversibly into a brick-like layer called laterite.

Soil Types and Profiles

• Oxisols / Latosols: These are the dominant soils of the humid tropics.
  • Colour: Red or Yellow (due to high iron/aluminium content).
  • Structure: Deep profiles (due to deep weathering) but very thin humus layer. They are often clay-rich but have poor structure and are prone to erosion.
  • Fertility: Low fertility (unless replenished immediately by decaying biomass) because most soluble nutrients have been leached away (eluviation).
• Tropical Red and Brown Earths: Found more commonly in the seasonally humid tropics. These soils can be slightly more fertile as the dry season slows down the leaching process, allowing some minerals to accumulate closer to the surface.

Section 7.4: Sustainable Management of Tropical Environments

Tropical environments face enormous pressure from human activity. The syllabus requires you to understand the threats and evaluate management solutions in either the rainforest or the savanna.

Threats and Exploitation

The main threats to tropical environments stem from rapid economic development, resource demand, and population pressure.

In the Rainforest Ecosystem (e.g., Amazon, Borneo)

• Deforestation for Agriculture: Large-scale clearance for cash crops (e.g., palm oil, soya) or cattle ranching. This is highly destructive because the soil is poor (Oxisols), leading to quick abandonment and erosion.
• Logging: Selective or clear-cut logging for hardwoods. This destroys the forest canopy, increases sunlight penetration, and disrupts the microclimate and nutrient cycle.
• Mining and Hydroelectric Power (HEP): Large infrastructure projects flood vast areas (Belo Monte Dam, Brazil) and create access roads, opening up previously remote areas to further exploitation.
• Threats to Biodiversity: Habitat loss leads to species extinction and disruption of ecological food webs.

In the Savanna Ecosystem (e.g., Sahel, East Africa)

• Overgrazing: Too many animals on the land, removing vegetation faster than it can regenerate. This exposes the soil.
• Overcultivation: Intensive farming without allowing fallow periods exhausts the limited soil nutrients.
• Fuelwood Collection: Removal of trees and shrubs for domestic fuel, removing protective cover and increasing soil erosion risk.
• Desertification: Human practices (overgrazing, deforestation) combined with climatic pressures (drought) degrade the land, leading to desert-like conditions spreading into the savanna.

Problems of Sustainable Management and Attempted Solutions

Sustainable management aims to meet current needs without compromising the ability of future generations to meet their own needs. Achieving this in the tropics is difficult due to conflicting interests (local livelihoods vs. global conservation).

When discussing management, you must evaluate the success or failure of the solutions based on social, economic, and environmental factors.

1. Conservation and Protection Strategies

• Designated Protected Areas: Creating National Parks or Biosphere Reserves (e.g., Monteverde Cloud Forest Reserve, Costa Rica).
  Evaluation: Effective ecologically, but often leads to social conflict with indigenous groups or local farmers displaced from the land. Enforcement can be difficult.
• Debt-for-Nature Swaps: High-income countries (HICs) forgive the debt of low-income countries (LICs) in exchange for them committing to conservation efforts (e.g., protecting rainforests).
  Evaluation: Good politically and environmentally, but the scale of debt is often far larger than the funds provided for the swap.

2. Economic and Livelihood Strategies

• Ecotourism: Using the natural environment (e.g., rainforest or safari parks) to generate income for conservation and local communities.
  Evaluation: Provides alternative income, reducing reliance on unsustainable activities like logging. However, it requires significant infrastructure investment and can cause environmental degradation if visitor numbers exceed carrying capacity.
• Agroforestry/Selective Logging: Integrating trees into farming systems or logging only small numbers of specific trees, allowing the forest structure to remain largely intact.
  Evaluation: Better than clear-cutting, but selective logging is still hard to monitor and can cause significant damage to surrounding trees during extraction.

3. Management of Desertification (Especially Savanna)

• Afforestation/Reforestation: Planting trees to bind the soil and reduce wind erosion (e.g., The Great Green Wall in the Sahel).
  Evaluation: Trees help restore soil structure and nutrient content, but requires sustained water supply and community buy-in. Success relies on selecting appropriate, drought-resistant species.
• Improved Farming Techniques: Using rotational grazing, terracing, and introducing drought-resistant crops to manage soil quality and reduce degradation risk.
  Evaluation: Requires education and support from governments, but leads to long-term soil health improvement, benefiting local resilience.