🌴 A-Level Geography Study Notes: Tropical Environments 🌴
Hello Geographers! Welcome to one of the most dynamic and fascinating sections of Advanced Physical Geography: Tropical Environments. These regions, close to the Equator, are defined by intense heat, unique landforms, and incredible biodiversity, but they are also facing major challenges.
This chapter is crucial for Paper 3, giving you the tools to understand the processes that shape the Earth's most complex physical systems. Don't worry if concepts like 'karst' or 'Gersmehl' seem alien—we will break them down into simple, memorable chunks!
7.1 Tropical Climates: Hot, Humid, and Dynamic
The tropical zone lies generally between the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S). The climate here is primarily controlled by global pressure belts.
Key Climatic Drivers (The Big Three)
The distribution of humid tropical (e.g., rainforest) and seasonally humid tropical (e.g., savanna) environments is fundamentally controlled by three major systems:
1. The Intertropical Convergence Zone (ITCZ):
- What it is: A belt of low pressure found near the Equator where the trade winds (from the Northern and Southern hemispheres) meet.
- Why it matters: Because it’s a low-pressure zone, air rises, cools, and condenses, leading to intense convectional rainfall. It is the engine of tropical rainfall.
- Movement: The ITCZ follows the thermal equator (the line of maximum heating) and shifts seasonally north and south. This shift is what creates the distinct wet and dry seasons in the Seasonally Humid Tropics (Savanna).
2. Subtropical Anticyclones (STAs):
- What they are: Belts of high pressure found roughly around 20° to 35° latitude (both north and south).
- Why they matter: High pressure means air is sinking, compressing, and warming (inhibiting rainfall). When the STAs shift towards the Equator during their respective winter/dry seasons, they bring stable, dry conditions to the seasonally humid regions.
3. Monsoons:
- What they are: Large-scale seasonal shifts in wind direction, often resulting in huge seasonal differences in precipitation.
- Classic Example: The Indian Monsoon. During summer, land heats faster than the ocean, creating a strong low-pressure centre over land, which pulls moisture-laden air from the ocean, causing heavy rainfall.
Key Features of Temperature and Rainfall
We generally differentiate between two main types of tropical climate based on their annual and diurnal (daily) variation:
| Feature | Humid Tropical (Rainforest, e.g., Amazon) | Seasonally Humid Tropical (Savanna, e.g., Serengeti) |
| Temperature | High (25-30°C) and constant throughout the year. Annual range is very low (less than 5°C). | High, but the annual temperature range is slightly larger (around 5-10°C). Hottest temperatures occur just before the wet season. |
| Rainfall | High annual total (often >2000 mm). Distributed evenly throughout the year (no true dry season). | Seasonal: Distinct wet season (when ITCZ is overhead) and a distinct dry season (when STAs dominate). |
| Diurnal Variation | The diurnal range (difference between day and night temp) is usually greater than the annual range. Day is hot, night is cooler. | The diurnal range is generally greater in the dry season due to clear skies, allowing for rapid cooling at night. |
Quick Takeaway 7.1: The ITCZ brings rain, the Subtropical Anticyclones bring dryness. Their seasonal movement creates the Savanna's wet/dry cycle, while the equatorial regions (rainforests) remain under the ITCZ influence year-round.
7.2 Landforms of Tropical Environments
Tropical landforms are shaped primarily by intense chemical weathering enabled by high temperatures and high moisture. This combination accelerates rock decay massively.
Prerequisite Concept: Deep Weathering Profiles
Because of the intense chemical activity, rock decay can penetrate deep into the ground, sometimes up to 100 meters. This layer of chemically altered, soft rock is called the regolith or saprolite (literally "rotten rock"). The boundary between the fresh rock and the weathered rock is the weathering front.
Analogy: Imagine a sugar cube (fresh rock). In a temperate climate, it dissolves slowly. In the humid tropics (hot water), it dissolves very quickly and deeply.
Landforms formed on Granite
Granite is a crystalline igneous rock. Its joint patterns determine how weathering attacks it.
- Tors: Outcrops of resistant, fresh granite rising above the surrounding deeply weathered regolith. They often form in two stages: deep chemical weathering underground along joint lines, followed by removal (denudation) of the soft weathered material (the saprolite) during a period of climate change or erosion.
- Inselbergs: Literally 'island mountains'. They are isolated, steep-sided hills or mounds rising abruptly from a gently sloping plain (a pediment). They represent highly resistant rock masses that were less affected by deep weathering than the surrounding rock.
- Bornhardts: A specific type of inselberg; large, dome-shaped hills with smooth, bare rock surfaces. They usually form when overlying rock is stripped away, allowing pressure release (dilatation) to cause sheeting and exfoliation on the dome surface. (Example: Sugarloaf Mountain, Rio de Janeiro.)
Landforms formed on Limestone: Tropical Karst
Limestone (Calcium Carbonate, CaCO3) dissolves rapidly in tropical climates because the high temperatures and abundant rainfall lead to high concentrations of carbon dioxide (\(\text{CO}_2\)) in the soil water, increasing its acidity.
Tropical karst is characterized by its dramatic cone, tower, and cockpit shapes:
- Cockpit Karst: Characterised by numerous dome-shaped hills (cones) separated by deep, star-shaped depressions (cockpits). It looks like an egg-carton landscape. Found in regions with thick limestone and high rainfall (e.g., Puerto Rico).
- Cone Karst (Kegelkarst): The individual hills are steep-sided cones. These cones are the remnants of limestone left between solution depressions.
- Tower Karst (Turmkarst): Extremely steep or vertical-sided limestone hills that rise dramatically from a flat plain. These towers form when a river or erosion removes the weathered material around the base of the cone karst, leaving isolated pillars. (Example: Ha Long Bay, Vietnam or Guilin, China.)
Did you know? The speed of rock decay in the tropics is much faster than in temperate zones. Scientists estimate chemical weathering rates can be 5-10 times higher near the equator!
Quick Takeaway 7.2: Granite landforms (Tors, Inselbergs) are products of deep weathering followed by erosion. Limestone landforms (Karst) are shaped by rapid chemical solution, forming Cockpits and Towers.
7.3 Tropical Ecosystems: Rainforest and Savanna
The Humid Tropical (Rainforest) and Seasonally Humid Tropical (Savanna) environments host distinct ecosystems defined by their plant communities, nutrient cycles, and soil characteristics.
Plant Communities and Succession
Succession describes the predictable changes in species structure over time.
- Climax Community: The final, stable stage of succession where the vegetation is in equilibrium with the climate and soil (e.g., the mature Tropical Rainforest).
- Subclimax Community: A stage in succession that is maintained by environmental factors (like poor drainage or fire) and cannot reach the true climax stage.
- Plagioclimax Community: The stable, persistent stage maintained by human interference. For example, if a rainforest is cleared and the land is continually grazed or burned, the vegetation will remain as grass or scrub, prevented from returning to its climax forest state.
- Memory Aid: Plagioclimax = People interfering.
Nutrient Cycling: The Gersmehl Diagram
The Gersmehl diagram illustrates how nutrients are stored and moved within an ecosystem (stores: Biomass, Soil, Litter; flows: input, uptake, decomposition, leaching, output).
In the Humid Tropical Rainforest, the nutrient cycle is incredibly fast and tightly closed:
- Biomass Store: Massive. Most nutrients (over 70%) are stored in the living plants (trees, leaves, roots).
- Litter Store: Small. High temperatures and humidity lead to ultra-rapid decomposition by bacteria and fungi.
- Soil Store: Small. As soon as organic matter decomposes, the nutrients are instantly taken up by the dense root mat (uptake flow), preventing them from being washed away (leaching).
The implication: If you cut down the rainforest, you remove the biggest store of nutrients (biomass). The remaining soil store is tiny and quickly exhausted, making long-term farming difficult.
In the Seasonally Humid Savanna, the cycle is slower and less tight:
- Nutrients are more evenly distributed, with significant stores in the soil and litter due to the long dry season, which slows down decomposition.
- Fire is a common natural and human factor. Burning releases nutrients quickly but also results in loss to the atmosphere.
Tropical Soils (Soil Formation and Characteristics)
The intense climate drives specific soil-forming processes:
- Laterisation (or Ferrallitisation): The extreme leaching (washing out) of soluble base minerals (like calcium, potassium) and silica, leaving behind insoluble compounds of Iron and Aluminium.
- Soil Type: Oxisols / Latosols.
- Profile Characteristics: These soils are deep, highly weathered, and typically appear bright red or yellowish-red (due to the high iron oxide content). Despite their depth, they have poor fertility because all the key plant nutrients have been leached away. They often form a hard, impermeable crust (called hardpan or plinthite) when exposed to the sun (a major problem after deforestation).
Trophic Levels and Energy Flows:
Trophic levels describe feeding positions (producers, primary consumers, secondary consumers, etc.). Tropical ecosystems, particularly rainforests, have extremely high biomass productivity (the rate at which new organic matter is generated). The energy flow is efficient, supporting high biodiversity.
Quick Takeaway 7.3: Rainforest nutrients are in the Biomass (fast, tight cycle). Tropical soils (Oxisols) are old, leached, and infertile due to laterisation. Human intervention creates Plagioclimax communities.
7.4 Sustainable Management of Tropical Environments
Tropical environments, especially rainforests and savannas, face severe threats from human exploitation. Sustainable management aims to balance resource use with ecological protection.
Threats to Tropical Ecosystems (Exploitation)
Threats apply to both rainforests (HR) and savannas (SHR):
- Deforestation (HR): Driven by commercial logging, cattle ranching, and large-scale cash crops (e.g., palm oil). This destroys biodiversity and accelerates soil erosion and leaching.
- Mining and Resource Extraction (HR/SHR): Requires clearing large areas, contaminating water sources (e.g., mercury from gold mining), and generating vast amounts of waste.
- Unsustainable Agriculture (SHR): Overgrazing and unsuitable farming methods lead to degradation of soils and vegetation, increasing the risk of desertification (the process of land degradation in arid/semi-arid areas).
- Climate Change: Changing rainfall patterns threaten to increase the dry season severity in savannas and increase drought frequency in rainforests.
Problems of Sustainable Management
Management attempts often face immense challenges:
- Economic Pressure: Developing nations often prioritize short-term economic gains (e.g., selling timber) over long-term sustainability.
- Conflict of Interests: Conflict between indigenous groups (who rely on the forest) and external actors (governments, TNCs) wanting to exploit resources.
- Governance Issues: Lack of effective enforcement against illegal logging or poaching.
Attempted Solutions and Evaluation (Case Study Preparation)
For your essay/case study component, you must evaluate solutions in either a rainforest or savanna context.
A. Managing the Rainforest Ecosystem (HR)
- Solution 1: Protected Areas and Reserves
- Description: Establishing national parks or biological corridors (e.g., Tambopata National Reserve, Peru).
- Evaluation: Success depends entirely on funding and policing. "Paper parks" (parks established legally but not protected practically) are common. They often cause conflict with local populations excluded from traditional lands.
- Solution 2: Ecotourism
- Description: Tourism focused on conservation, employing locals and providing economic alternatives to logging (e.g., Costa Rica).
- Evaluation: Highly successful in providing alternative income and giving the forest economic value standing up. However, high visitor numbers can cause physical damage (erosion, noise pollution) if not strictly controlled.
- Solution 3: REDD+ Schemes
- Description: Reducing Emissions from Deforestation and Forest Degradation. Wealthy countries pay poorer countries to keep their forests standing to sequester carbon.
- Evaluation: Provides large-scale international financing, addressing the root economic causes of destruction. Challenges include ensuring the money reaches local communities and verifying that carbon savings are real.
B. Managing the Savanna Ecosystem (SHR)
- Solution 1: Controlled Grazing and Rangeland Management
- Description: Rotational grazing schemes to prevent livestock overstocking and allow grass recovery, preventing degradation and desertification.
- Evaluation: Effective in restoring soil health and increasing productivity, but requires cooperation from nomadic pastoralists and may conflict with traditional land use practices.
- Solution 2: Conservation and Wildlife Tourism
- Description: Establishing large national parks (e.g., Kruger National Park, South Africa) focusing on protecting large mammals, generating revenue from safari tourism.
- Evaluation: Provides massive income for conservation and local employment. Challenges include human-wildlife conflict (animals destroying crops) and leakage (tourism revenue leaving the country via foreign operators).
- Solution 3: Afforestation/Agroforestry
- Description: Planting trees, often drought-resistant native species, to stabilize soils and act as windbreaks, mitigating desertification.
- Evaluation: Trees improve soil structure and microclimate. However, initial seedling survival rates are often low due to dry conditions, and requires significant community involvement and water resources.
Quick Takeaway 7.4: Sustainable management requires tackling exploitation threats (logging, overgrazing) by providing economic alternatives (ecotourism, REDD) and applying appropriate physical controls (rotational grazing, protected areas). Evaluation must cover success *and* failure factors (e.g., funding, local conflict).
⭐ Final Review Checklist ⭐
Can you confidently explain these core concepts?
- The seasonal shift of the ITCZ and its impact on rainfall.
- How deep weathering creates tors and inselbergs.
- The difference between cockpit and tower karst.
- Why the Gersmehl diagram shows a massive biomass store in rainforests.
- The meaning of Plagioclimax (human interference).
- Why Oxisols/Latosols are infertile despite being red and deep.