Welcome to the Tropical Ecosystems Deep Dive!

Hi there! This chapter might seem overwhelming because of the biology elements, but don’t worry. We’re going to break down the two main tropical giants – the eternally wet Rainforests and the seasonally dry Savannas – focusing on their structure, how they manage nutrients, and the secrets hidden in their soils.
Understanding these ecosystems is vital because they are home to the vast majority of the world’s biodiversity and play a huge role in global climate regulation. Let’s get started!

Section 1: Tropical Climate Overview (The Foundation)

While Section 7.1 covers climate in detail, remember that the environment dictates the ecosystem. The key difference between the two environments we study here is the seasonality of rainfall.

1. Humid Tropical (Rainforest) Environments
These are governed by the constant presence of the Intertropical Convergence Zone (ITCZ). They experience consistently high temperatures (around 25–30°C) and exceptionally high rainfall (over 2000 mm annually), with little to no dry season.

2. Seasonally Humid Tropical (Savanna) Environments
These areas experience the ITCZ moving seasonally. When the ITCZ is overhead, they have a wet season. When it moves poleward, they fall under the influence of the high-pressure Subtropical Anticyclones, resulting in a distinct, lengthy dry season.

Quick Takeaway: Constant wetness = Rainforest. Wet and dry seasons = Savanna.

Section 2: Plant Communities and Vegetation Characteristics (7.3)

What is a Plant Community?

A plant community refers to the different groups of plants living together in an area. Geographers classify these communities based on how they develop over time, a process called succession.

1. Climax Vegetation

  • This is the stable, mature plant community that develops in an area given the existing climate and soil conditions.
  • It represents the theoretical 'end point' of succession where the community is in balance with the environment.
  • Example: The dense, multi-layered Tropical Rainforest is the climax vegetation for the humid tropics.

2. Subclimax Vegetation

  • This community exists just before the true climax stage but is usually held back by some environmental factor, often natural like fire or flooding, or soil limitations.

3. Plagioclimax Vegetation (The Human Influence)

  • Don't panic about the jargon! Plagioclimax simply means a community that is maintained in a specific state due to human activity, preventing it from ever reaching its natural climax.
  • Analogy: If the natural climax is a full-grown forest, a farmer repeatedly chopping trees or setting fire to grassland keeps the ecosystem in a plagioclimax state (like a perpetually mowed lawn).
  • Example: Most Savanna grasslands are considered plagioclimax ecosystems because repeated burning (to encourage fresh grass for grazing) stops trees from fully developing into dry forest.

Vegetation Characteristics

A. Humid Tropical Rainforest (Evergreen and Highly Layered)

  • High Biodiversity: Extreme variety of plant and animal life.
  • Vertical Stratification: The forest is structured into distinct layers to compete for sunlight.
    • Emergent Layer: Tallest trees (up to 50m) catching maximum sun.
    • Canopy Layer: Continuous, dense roof (25-35m) where most life exists.
    • Under-Canopy Layer: Shorter trees and shade-tolerant species.
    • Shrub/Ground Layer: Sparse due to lack of light, except along rivers or clearings.
  • Adaptations: Trees have shallow roots (nutrients are near the surface), smooth bark, and large, dark green leaves (to maximise photosynthesis in low light).
  • Lianas: Woody vines that climb trees to reach sunlight.
  • Epiphytes: Plants (like orchids) that grow harmlessly on other plants, getting nutrients from air and rain.

B. Seasonally Humid Tropical Savanna (Grasses and Scattered Trees)

  • Dominant Vegetation: Tall grasses (e.g., Elephant Grass) and scattered deciduous trees and shrubs.
  • Adaptations: Plants must cope with the long, intense dry season.
    • Deciduous Trees: Shed leaves during the dry season to reduce water loss (transpiration).
    • Deep Roots: Trees develop deep taproots to reach the water table.
    • Fire Resistance: Grasses grow quickly and often have underground buds to survive seasonal fires.
    • Waxy Leaves/Thorns: Reduced leaf size or thorns to minimise moisture loss.

Key Takeaway: Rainforests focus on layers (vertical competition for light); Savannas focus on survival (adapting to drought and fire).

Section 3: The Nutrient Cycle, Energy Flows, and Trophic Levels (7.3)

This is where we study how energy and materials move through the ecosystem.

1. Nutrient Cycling: The Gersmehl Model

The Gersmehl Diagram is essential. It illustrates the stores of nutrients (Biomass, Litter, Soil) and the flows between them (Uptake, Decomposition, Weathering, Leaching).

The Three Stores:

  1. Biomass (B): Nutrients locked up in living plants (leaves, roots, trunks).
  2. Litter (L): Nutrients in dead organic matter (fallen leaves, dead animals) lying on the ground.
  3. Soil (S): Nutrients dissolved in the soil water or held by soil particles.

The Flows (Transfers):

  • Uptake (L → B): Plants absorb nutrients from the soil.
  • Litterfall/Death (B → L): When plants die or drop leaves, nutrients move to the litter layer.
  • Decomposition (L → S): Bacteria and fungi break down litter, releasing nutrients into the soil.
Comparing the Nutrient Cycles

A. Humid Tropical Rainforest Cycle

Think of the rainforest ecosystem like a highly efficient recycling machine that never stops.

  • Dominant Store: Biomass (B) is the largest store. (Most nutrients are in the plants themselves).
  • Rapid Flows: The Decomposition (L → S) and Uptake (S → B) flows are incredibly fast due to high heat and moisture, leading to rapid breakdown of litter and immediate nutrient absorption by shallow roots.
  • Small Store: The Soil (S) store is very small because nutrients are washed out quickly by heavy rain (leaching).
  • Result: When a rainforest is cleared, the nutrients are lost rapidly through leaching, which is why rainforest soils quickly become unproductive for agriculture.

B. Seasonally Humid Tropical Savanna Cycle

The Savanna cycle is slower and suffers from a stop-start rhythm.

  • Dominant Store: Soil (S) and Biomass (B) are more equally sized compared to the rainforest.
  • Seasonality:
    • During the wet season, the cycle speeds up (fast decomposition and growth).
    • During the dry season, the cycle virtually stops, causing litter to accumulate (slow decomposition).
  • Fire Impact: Seasonal burning transfers nutrients from the Biomass/Litter to the Soil instantly (as ash), but these nutrients are quickly lost by wind or leaching during the first rains.

Memory Aid:
RBS: Rainforest = Biomass dominant.
SSB: Savanna = Soil and Biomass more balanced.

2. Energy Flows and Trophic Levels

Energy Flow is the movement of energy (usually measured in Joules or calories) through the ecosystem, typically starting with the sun.

  • Plants (producers) capture solar energy through photosynthesis.
  • The total amount of energy captured is the Gross Primary Productivity (GPP).
  • The energy remaining after the plant uses some for itself (respiration) is the Net Primary Productivity (NPP). Rainforests have exceptionally high NPP due to year-round growing conditions.

Trophic Levels describe the position an organism occupies in the food chain:

  1. Producers: Plants (e.g., trees, grasses).
  2. Primary Consumers: Herbivores (e.g., monkeys, giraffes).
  3. Secondary Consumers: Small carnivores (e.g., jaguars, wild dogs).
  4. Tertiary Consumers: Apex predators.

The 10% Rule: Energy is lost at each step up the trophic ladder (often only about 10% is successfully transferred), limiting the number of levels an ecosystem can support.

Key Takeaway: Rainforests are efficient but fragile (nutrients are stored high up). Savannas are resilient but slower (due to the dry season).

Section 4: Soil Formation and Soil Types (7.3)

Tropical soils are often counter-intuitively poor, especially given the lush vegetation.

1. Soil Forming Processes (Pedogenesis)

Soil formation is heavily influenced by the tropical climate (hot and wet).

  • Intense Chemical Weathering: High temperatures accelerate chemical reactions (hydrolysis, oxidation), leading to very deep weathering profiles (regolith).
  • Leaching: Heavy rainfall causes huge water surpluses, moving soluble minerals downwards out of the A horizon (topsoil) and into the B horizon (subsoil), or even washing them out of the soil profile entirely. This process removes key nutrients like calcium and potassium.

2. Soil Types and Profile Characteristics

The high rates of weathering and leaching create specific soil types known broadly as Oxisols (or Latosols).

Characteristics of Oxisols/Latosols (Rainforest Soils)

  • Colour: Often red or yellow. This is due to the oxidation of iron and aluminium compounds (literally rust).
  • Acidity: Very acidic (low pH) because the alkaline nutrients (like calcium) have been leached out, leaving behind resistant, acidic compounds.
  • Fertility: Generally poor and low fertility. The soil mainly consists of inert clays (kaolinite) and residual oxides.
  • Structure: Prone to laterisation (hardening) if exposed to the sun after deforestation. This makes it difficult for roots to penetrate.
  • Humus: Humus (decomposed organic matter) layer is thin due to rapid decomposition and uptake.

Profile Structure (Typical Oxisol)

  • O Horizon (Litter): Very thin or non-existent, due to rapid decomposition.
  • A Horizon (Topsoil): Very thin layer of mixed mineral and organic matter. This is the productive layer, but it is shallow.
  • B Horizon (Subsoil): Thick, red or mottled layer. This is where the iron and aluminium oxides accumulate (the zone of maximum deposition).
  • C Horizon: Deeply weathered parent material (regolith).

Tropical Red and Brown Earths (Savanna Soils)

These soils are found in the seasonally humid zones and tend to show more variability.

  • Better Fertility: Because rainfall is seasonal, the degree of leaching is less severe than in the constant rainforests.
  • Structure: Often slightly deeper and richer in nutrients than true Oxisols, especially in areas with parent rock that weathers into fertile clay.
  • Processes: During the dry season, upward capillary movement of water can sometimes bring dissolved minerals back up towards the surface (though not as extreme as the salinisation seen in hot arid environments).

Did you know?
The red soil colour is not an indicator of fertility. It just tells us that the iron in the soil has been heavily rusted (oxidised).

Common Mistake to Avoid:
Students often assume rainforest soil is fertile because the forest is so lush. Remember the rule: The Biomass is rich, but the Soil is poor.

Key Takeaway: Tropical soils (Oxisols) are deep, acidic, and red due to intense chemical weathering and leaching caused by high heat and rainfall.