Vegetation, soils and ecosystems: characteristics, nutrient cycling

Tropical Environments (Cambridge AS & A Level Geography – Syllabus 7)

1. Tropical Climates

The tropical zone lies between 0° and 23.5° N/S. High solar radiation gives a small annual temperature range (≈ 25–28 °C) and the amount and seasonality of rainfall are controlled by the Inter‑tropical Convergence Zone (ITCZ), subtropical anticyclones, monsoon systems and ENSO.

1.1 Global distribution of the main tropical climate types (Cambridge 7.1)

• Af – Tropical rainforest (humid) climate – > 2000 mm yr⁻¹, no dry season.
  Typical regions: Amazon Basin (South America), Congo Basin (Central Africa), Malay‑Indonesian Archipelago (Southeast Asia).
• Aw – Tropical savanna (seasonally humid) climate – 500–1500 mm yr⁻¹, dry season > 3 months.
  Typical regions: West Africa (e.g., Sudanian zone), northern Australia, parts of Brazil (Cerrado).
• Am – Tropical monsoon climate – 1200–2000 mm yr⁻¹, short dry season (4–6 months).
  Typical regions: Southern India, northern Myanmar, parts of the Amazon foothills.

1.2 Seasonal shift of the ITCZ

• During the boreal summer the ITCZ moves northward to ≈ 10° N, bringing heavy rain to the northern tropics.
• In the boreal winter it shifts southward to ≈ 10° S, delivering the main rainfall to the southern tropics.
• The latitudinal oscillation of the ITCZ therefore creates the marked wet and dry seasons of Aw/Am climates.

1.3 ENSO influence

• El Niño – suppresses convection over the western Pacific and eastern Amazon, reducing rainfall and increasing fire risk.
• La Niña – enhances convection, leading to above‑average precipitation in the same regions.

2. Weathering Processes, Landforms & Mass‑Wasting (Cambridge 7.2)

High temperature and abundant moisture accelerate chemical weathering, while steep relief on lateritic plateaus promotes mass‑movement.

2.1 Weathering processes and the landforms they generate

Dominant Weathering Process	Typical Resulting Landform(s)
Hydrolysis (chemical)	Lateritic hard‑pans, ferrallitic soils; rounded inselbergs where silicates are leached.
Oxidation (chemical)	Red‑coloured laterites, iron‑rich plateaus, rust‑stained granite tors.
Solution (chemical, especially in carbonates)	Karst features – dolines, tower karst, cockpit karst.
Mechanical (physical) weathering	Bornhardts, tors, and the exfoliation of granite domes.

2.2 Mass‑wasting in tropical settings

• Steep lateritic plateaus and volcanic slopes are prone to landslides, debris flows and soil creep.
• Intense rainfall can trigger rapid slope failure, especially where vegetation cover has been removed (e.g., after logging or shifting cultivation).

2.3 Representative landforms

• Granite: Bornhardts, tors, inselbergs.
• Limestone (karst): Dolines (sinkholes), tower karst, cockpit karst.
• Lateritic plateaus & ferrallitic soils: Iron‑ and aluminium‑rich hardpans, red‑coloured laterites.

3. Tropical Vegetation, Soils and Ecosystems

3.1 Vegetation types and the three climatic climax concepts (Cambridge 7.3)

Cambridge distinguishes:

• Climatic climax – vegetation that would exist under the prevailing climate if undisturbed (e.g., rainforest on Oxisols).
• Sub‑climax – vegetation that establishes after a moderate disturbance (e.g., seasonal forest after a short dry season).
• Plagioclimax – vegetation maintained by continuous human activity (e.g., plantation cocoa, shade‑grown coffee).

Vegetation Type	Climate & Rainfall	Structure & Key Adaptations	Typical Soil Association
Evergreen Tropical Rainforest	Af – > 2000 mm, no dry season	5‑layered (emergent, canopy, understory, shrub, forest floor); buttressed trunks, drip‑tips, shallow extensive roots	Oxisols (Ferralsols) – highly leached, acidic, low CEC
Tropical Seasonal (Monsoon) Forest	Aw/Am – 1200–2000 mm, 4–6 month dry season	Deciduous canopy, lower leaf‑area index than rainforests, light‑rich understory	Ultisols (Red Earths) – moderate leaching, clay‑rich, acidic
Tropical Savanna (Woodland)	Aw – 500–1500 mm, dry season > 3 months	Grassland with fire‑adapted trees (acacias, baobabs); deep roots, thick bark, resprouting ability	Inceptisols & shallow Ultisols; often over lateritic hardpan
Mangrove & Swamp Forest	Tidal, warm, high humidity	Stilt roots, pneumatophores, viviparous seedlings	Hydromorphic, often saline or water‑logged soils

3.2 Tropical soils (FAO classification)

Soil Type (FAO)	Typical Environment	Key Physical & Chemical Features	Management Recommendations
Oxisols (Ferralsols)	Old, stable rainforests; lateritic plateaus	Very low CEC, high Fe‑/Al‑oxides, pH < 5, thin organic layer	Liming; regular addition of well‑decomposed organic matter; rock‑phosphate or highly soluble P sources
Ultisols (Red Earths)	Seasonal forests, savannas	Moderately leached, clay‑rich, low base saturation, acidic	Calcium amendment (lime); balanced NPK; conservation tillage to protect structure
Inceptisols (Yellow Earths)	Young volcanic/alluvial deposits, lower slopes	Relatively high organic matter, moderate CEC, less leached	Maintain organic inputs; avoid over‑grazing; contour farming on slopes
Entisols (Sandy & alluvial)	River floodplains, coastal dunes	Very shallow development, high drainage, low nutrient‑holding capacity	Frequent organic mulching; raised beds; careful irrigation management

3.3 Ecosystem characteristics & soil‑vegetation feedback

• Primary productivity is among the highest on Earth (≈ 2–3 kg C m⁻² yr⁻¹ in rainforests) because light and water are abundant.
• Fast nutrient turnover: > 90 % of nutrients are stored in living biomass and the litter layer; only a small fraction resides in the mineral soil.
• Feedback loop – dense canopy reduces soil temperature, limiting further chemical weathering; abundant litter maintains a thin, nutrient‑rich organic horizon that supports the climatic climax vegetation.
• Disturbance regimes (fire in savannas, windthrow in forests, flooding in mangroves) reset the cycle and create sub‑climax or plagioclimax stages.

4. Nutrient Cycling in Tropical Environments

4.1 Fast (surface) cycle

Operates mainly in the litter layer, humus and the upper 10 cm of soil.

• Rapid decomposition by fungi, bacteria and detritivores (turnover time: weeks–months).
• Mineralisation of N, P, K and immediate uptake by shallow roots.
• High leaching potential, especially for soluble phosphorus; Fe‑ and Al‑oxides can adsorb P, reducing its availability.

4.2 Slow (deep) cycle

Involves weathering of parent material and long‑term storage of nutrients in mineral forms.

• Oxidation of Fe‑ and Al‑oxides binds phosphorus; silicate weathering releases Ca, Mg over centuries to millennia.
• Contributes little to immediate plant nutrition in highly weathered soils but is crucial for landscape evolution and long‑term fertility.

4.3 Factors controlling nutrient cycling (Cambridge 7.4)

1. Climate – high temperature and moisture accelerate decomposition.
2. Soil texture – clay particles adsorb cations, reducing leaching; sandy soils accelerate loss.
3. Vegetation type – evergreen rainforests retain nutrients in biomass; deciduous savannas return nutrients via leaf fall.
4. Disturbance – fire releases nutrients instantly but can volatilise N and cause P loss; logging removes biomass and disrupts the fast cycle.

4.4 Suggested diagram (for revision)

Fast and slow nutrient cycles in a tropical rainforest, showing litter layer, mineral soil, root uptake, leaching pathways and deep weathering processes.

5. Changes, Pressures and Management

5.1 Main pressures on tropical environments (Cambridge 7.5)

• Population growth & land‑use change – shifting cultivation, cattle ranching, oil‑palm and rubber plantations.
• Resource exploitation – selective & clear‑cut logging, bauxite/tin mining, peat extraction.
• Climate change – altered rainfall patterns, more frequent droughts and fires, sea‑level rise affecting mangroves.
• Fire‑regime alteration – higher fire frequency in savannas and at cleared forest edges.

5.2 Detailed example – The Amazon Basin

Location & importance: ~6 million km² of humid tropical rainforest; stores ~90–100 Gt of carbon.

Key pressures:

• Deforestation for cattle pasture (≈ 75 % of forest loss) and soy cultivation.
• Selective logging that creates canopy gaps, accelerating edge effects.
• Road construction (e.g., BR‑163, Trans‑Amazonian) that fragments habitat and facilitates further settlement.
• Climate‑induced droughts (2005, 2010, 2020) that increase fire incidence.

Consequences for nutrient cycling:

• Removal of the litter layer collapses the fast cycle; nutrients are rapidly lost by leaching.
• Soil compaction from heavy machinery reduces infiltration, increasing surface runoff and erosion.
• Fragmented forests have higher soil temperatures, altering microbial communities and decreasing organic‑matter turnover.

Management strategies (Amazon Cooperation Treaty Organization):

1. Establish and enforce protected areas covering > 30 % of the basin to preserve climatic climax rainforest.
2. Promote sustainable forest‑product certification (e.g., FSC) to curb illegal logging.
3. Implement agroforestry and shade‑grown cocoa/coffee to maintain sub‑climax vegetation and protect soil.
4. Restore degraded lands with mixed native species and organic amendments to rebuild the fast nutrient cycle.

5.3 General management recommendations for tropical environments

• Maintain continuous canopy cover to protect the litter layer and minimise leaching.
• Apply fire‑management plans in savannas (controlled burns, firebreaks) to balance nutrient release with loss.
• Use lime and organic matter to raise pH and improve CEC on Oxisols and Ultisols.
• Adopt contour farming, terracing and reduced‑tillage on sloping Inceptisols to prevent erosion and mass‑wasting.
• Protect karst aquifers from contamination by limiting fertilizer runoff on limestone terrains.

6. Summary Table (Cambridge 7.1‑7.5)

Component	Key Features (Cambridge syllabus)	Implications for Management
Climate	ITCZ‑driven rainfall, monsoons, ENSO variability; temperature 25–28 °C	Predict seasonal water availability; plan planting/harvest cycles; anticipate fire risk during El Niño.
Landforms	Bornhardts, tors, inselbergs (granite); karst towers, dolines (limestone); lateritic plateaus; mass‑wasting on steep laterites	Consider soil depth and stability for infrastructure; protect karst aquifers; avoid disturbing slopes prone to landslides.
Vegetation	Climatic climax (rainforest on Oxisols), sub‑climax (seasonal forest), plagioclimax (plantations)	Conserve climax vegetation to retain nutrient cycles; manage plagioclimax areas with mixed species and organic inputs.
Soils	Oxisols – low fertility, acidic; Ultisols – moderate fertility; Inceptisols – higher organic matter	Apply lime, organic amendments, and appropriate fertiliser regimes; avoid over‑application of P on Oxisols.
Nutrient Cycling	Fast surface cycle dominates; slow deep cycle limited by extreme weathering	Protect litter layer; minimise soil disturbance; use fire wisely in savannas.
Human Pressures	Population growth, agriculture, logging, mining, climate change	Implement sustainable land‑use policies; promote agroforestry; enforce protected‑area networks.

7. Key Points for Examination (AO 1‑3)

• Explain why tropical soils are often infertile despite high primary productivity (intense leaching, nutrients locked in biomass and organic horizons).
• Compare the structure, adaptations and nutrient‑cycling strategies of rainforest, seasonal forest and savanna vegetation.
• Describe the fast (surface) nutrient cycle, its importance for tropical ecosystems, and the main factors that accelerate or hinder it.
• Discuss how human activities (clearing for agriculture, logging, road building) disrupt the fast cycle and lead to long‑term soil degradation.
• Use a detailed specific example (e.g., Amazon Basin, Indonesian peatlands) to illustrate the interaction between climate, vegetation, soils and anthropogenic pressures.