Vegetation characteristics of rainforest ecosystems and savanna ecosystems

Vegetation, Soils and Ecosystems in Tropical Environments

Learning Objective

Identify, describe and evaluate the key vegetation characteristics, soil processes and human‑environment interactions of tropical rainforest and savanna ecosystems (Cambridge IGCSE/A‑Level Geography – Topic 7).

Key Geographical Concepts (AO3)

• Scale: local (site‑specific processes), regional (biome‑wide patterns) and global (distribution and climate belts).
• Change over time: seasonal cycles, inter‑annual variability (e.g., ENSO), long‑term climate change, land‑use change and successional dynamics.
• Cause‑effect relationships: climate ↔ vegetation structure ↔ soil properties ↔ disturbance regimes.
• Systems thinking: feedback loops (e.g., fire ↔ grass dominance in savannas) and interactions between biophysical and human systems.

1. Rainforest Ecosystems

1.1 Climate & Global Distribution (AO2)

• Latitude: mainly 5° N–5° S (humid tropical belt).
• Rainfall: > 2000 mm yr⁻¹, usually > 200 mm month⁻¹ throughout the year; no marked dry season.
• Temperature: mean annual 22–27 °C; diurnal range typically < 5 °C; seasonal temperature variation is minimal.
• Monsoon influence: slight seasonal peaks in rainfall where the Inter‑tropical Convergence Zone (ITCZ) stalls for a few weeks, but precipitation remains above the 200 mm month⁻¹ threshold.
• Scale note: locally, micro‑climates vary with elevation; regionally, the rainforest belt follows the equatorial low‑pressure zone; globally, it contributes to the Earth’s heat‑budget and carbon cycle.

1.2 Vegetation Structure & Layers (AO1)

1. Emergent layer – tallest trees 45–60 m (e.g., kapok Ceiba pentandra).
2. Canopy layer – continuous cover 30–45 m; site of > 90 % photosynthesis.
3. Understorey – shade‑tolerant shrubs, small trees, lianas.
4. Forest floor – thin herbaceous layer, abundant leaf litter, fungi and detritivores.

1.3 Leaf & Root Adaptations (AO1)

• Leaves: large, broad, drip‑tips to shed water; high chlorophyll content for low‑light conditions; thin cuticle to maximise gas exchange.
• Roots: shallow, widespread fibrous systems exploiting the nutrient‑rich A‑horizon; extensive mycorrhizal associations (mainly arbuscular) to improve phosphorus uptake.

1.4 Soils, Soil Profile & Water Relations (AO2)

• Dominant orders: Oxisols and Ultisols – highly weathered, acidic, low in base cations.
• Typical profile:
  • O‑horizon: thick litter layer, rapid decomposition.
  • A‑horizon: thin, dark, high organic matter, low CEC.
  • B‑horizon: accumulation of iron/aluminium oxides (laterite).
  • C‑horizon: weathered parent material, very low fertility.
• Water relations: high water table; soils remain moist year‑round, limiting deep root development.
• Fertility: intrinsically low; productivity sustained by rapid nutrient recycling (see 1.5).

1.5 Nutrient Cycling & Rapid Decomposition (AO2)

• Warm (≈ 27 °C) and humid conditions accelerate microbial activity.
• Leaf litter is thin, low in lignin, and high in nitrogen – favouring fast decomposition (often < 2 months).
• Most nutrients are immobilised in the thin A‑horizon; when litter falls, nutrients are quickly re‑absorbed by roots and mycorrhizae.
• Disturbance (e.g., canopy gaps) releases a pulse of nutrients, supporting rapid pioneer growth.

1.6 Disturbance Regime (AO2)

• Natural fire frequency is very low because of high humidity.
• Wind‑throw, large‑tree falls and landslides create canopy gaps – the “gap‑phase” succession that maintains species diversity.
• Human disturbances (selective logging, clear‑cutting) remove the protective canopy, expose soil to erosion and interrupt the rapid nutrient cycle.

1.7 Climate‑Change Impacts (AO3)

• Rising temperatures (+1–2 °C by 2050) may increase evapotranspiration, stressing shallow‑rooted species.
• Changes in ITCZ dynamics could lengthen short dry spells, raising fire risk in marginal rainforest zones.
• Elevated CO₂ may enhance growth, but nutrient limitation (especially phosphorus) could constrain the response.

1.8 Scale & Systems Summary (AO3)

At the local scale, gap formation drives micro‑succession; at the regional scale, rainfall gradients determine forest density; at the global scale, rainforests act as a carbon sink. The system is characterised by a feedback loop: high productivity → rapid litter fall → fast nutrient recycling → sustained canopy cover.

2. Savanna Ecosystems

2.1 Climate & Global Distribution (AO2)

• Latitude: roughly 10°–20° N and 10°–20° S (tropical grassland belt).
• Rainfall: 500–1500 mm yr⁻¹, concentrated in a wet season of 3–5 months; dry season lasts 3–7 months with < 50 mm month⁻¹.
• Temperature: mean annual 24–28 °C; seasonal range up to 12 °C (warmer in summer, cooler in winter); diurnal range often 10–15 °C.
• Seasonal ITCZ migration: produces a pronounced wet‑dry contrast.
• Inter‑annual variability: ENSO events can shift the onset and length of the wet season, causing severe droughts or anomalously wet years.
• Scale note: locally, soil moisture pockets create “green islands”; regionally, the savanna belt aligns with the subtropical high‑pressure cell; globally, savannas store large amounts of carbon in soils.

2.2 Vegetation Structure & Layers (AO1)

1. Grass layer – dominant C₄ grasses (e.g., Andropogon spp.), high photosynthetic efficiency under high light and temperature.
2. Shrub layer – fire‑resistant woody shrubs, often with lignotubers that resprout after burning.
3. Scattered tree layer – drought‑ and fire‑tolerant species (e.g., Acacia, Baobab Adansonia), typically > 30 m apart.

2.3 Leaf & Root Adaptations (AO1)

• Leaves: small, thick, waxy or sclerophyllous; many species are deciduous, shedding leaves during the dry season to minimise water loss.
• Roots: deep taproots (up to 20 m) accessing groundwater; extensive lateral roots enable rapid post‑fire regrowth.

2.4 Soils, Soil Profile & Water Relations (AO2)

• Dominant orders: Alfisols and Inceptisols – moderately developed, higher base saturation than rainforest soils.
• Typical profile:
  • O‑horizon: thin litter, often burnt.
  • A‑horizon: dark, moderate organic matter, good structure.
  • B‑horizon: accumulation of clay, iron‑oxyhydroxides; higher CEC than Oxisols.
  • C‑horizon: less weathered parent material.
• Water relations: deep water table; soils retain moisture in the B‑horizon, allowing deep roots to survive the dry season.
• Fertility: generally higher than rainforest soils; however, prolonged drought or over‑grazing can reduce organic matter and nitrogen availability.
• Mycorrhizal links: arbuscular mycorrhizae improve phosphorus uptake, especially important during the dry season when soil P is limiting.

2.5 Nutrient Cycling & Decomposition (AO2)

• Warm temperatures and seasonal moisture promote moderate microbial activity; decomposition is slower than in rainforests because litter is more lignified.
• Fire converts a portion of surface organic matter to ash, temporarily increasing soil pH and releasing nutrients (especially calcium and phosphorus) – a key part of the savanna nutrient cycle.

2.6 Fire Regime & Successional Dynamics (AO2)

• Fire frequency: low‑intensity fires every 1–5 years are typical.
• Effects of fire: removes accumulated grass litter, stimulates fresh grass growth, prevents woody encroachment, and creates a mosaic of successional stages.
• Feedback loop (systems thinking): abundant grasses provide fine fuel → fire maintains grass dominance → fire suppresses tree establishment → grasses proliferate.

2.7 Climate‑Change Impacts (AO3)

• Projected temperature rise (+2 °C by 2050) may lengthen the dry season, increasing fire frequency and intensity.
• Changes in precipitation patterns could shift some savannas toward either more arid shrubland or, conversely, toward woody encroachment if fire is suppressed.
• Elevated CO₂ may enhance C₄ grass productivity, potentially altering competition with C₃ shrubs.

2.8 Scale & Systems Summary (AO3)

Locally, fire creates a patchwork of grass‑dominant and tree‑dominant microsites; regionally, rainfall gradients control the tree‑grass balance; globally, savannas influence atmospheric dust and carbon fluxes. The system is driven by a fire‑grass feedback loop that is modified by human land‑use.

3. Human‑Environment Interactions

3.1 Rainforest Pressures & Management (AO3)

• Logging & timber extraction: canopy removal reduces litter input, disrupts nutrient cycling, increases erosion; selective logging can be mitigated by reduced‑impact techniques.
• Shifting cultivation (slash‑and‑burn): ash provides a short‑term nutrient boost, but after 2–3 years soil fertility collapses, leading to forest fragmentation.
• Mining & infrastructure: soil compaction, heavy‑metal contamination, edge effects that increase wind‑throw and invasive species.
• Conservation responses:
  • Protected areas (e.g., Yasuni National Park) – have reduced deforestation rates locally but face enforcement challenges.
  • REDD+ (Reducing Emissions from Deforestation and Forest Degradation) – offers financial incentives, yet success depends on clear land‑tenure and monitoring.
  • Community‑based forest management – integrates traditional knowledge, improves compliance, but scaling up remains difficult.
• Evaluation: while protected areas curb large‑scale clearing, illegal logging persists where enforcement is weak; REDD+ shows promise but outcomes vary widely across countries.

3.2 Savanna Pressures & Management (AO3)

• Cattle ranching & over‑grazing: reduces grass cover, compacts soil, accelerates desertification; loss of ground cover also heightens erosion during the wet season.
• Fire suppression: allows woody encroachment, altering the grass‑tree balance and reducing habitat for grass‑dependent herbivores.
• Agricultural expansion: conversion to cropland lowers biodiversity, fragments fire‑dependent landscapes, and can increase pesticide runoff.
• Conservation responses:
  • Integrated fire‑management plans – combine early‑dry‑season controlled burns with community monitoring; have reduced uncontrolled wildfires by ~30 % in the Serengeti.
  • Wildlife corridors (e.g., Serengeti‑Mara) – maintain seasonal migration routes, improve genetic flow, and support tourism revenues.
  • Sustainable pastoral practices – rotational grazing and water‑point management maintain grass productivity.
• Evaluation: fire‑management improves ecosystem resilience but requires continuous funding and local buy‑in; pastoral reforms can sustain livelihoods but may conflict with land‑tenure rights.

4. Representative Case Studies (AO3)

Ecosystem	Location & Size	Key Physical & Biological Features	Management Actions & Evaluation
Amazon Basin Rainforest	South America; ≈ 5.5 million km²	Highest global plant diversity (≈ 40 % of all tropical species). Oxisol soils, low natural fertility, rapid litter turnover. Low fire frequency; disturbances mainly from logging and road building.	Protected areas: Yasuni NP, Tambopata Reserve – reduced deforestation from 1.5 %/yr (1990s) to < 0.5 %/yr (2020s) locally. REDD+ pilots: Brazil’s Amazon Fund – generated US$1.2 billion in carbon credits, yet leakage to adjacent unprotected zones observed. Evaluation: success hinges on strong governance, monitoring via satellite, and inclusion of indigenous rights; where enforcement is weak, illegal logging persists.
Serengeti‑Mara Savanna	Eastern Africa; ≈ 30 000 km²	Seasonal migration of > 1.5 million wildebeest and zebras. Alfisol soils with moderate fertility; deep water table. Fire every 1–3 years maintains grass dominance.	Integrated fire‑management: community‑led early‑dry‑season burns – lowered uncontrolled fire incidents by ~30 % and enhanced post‑fire grass regrowth. Wildlife corridors: cross‑border agreements (Tanzania‑Kenya) preserve migration routes, supporting tourism revenue of US$200 m/yr. Evaluation: effective where local communities receive tangible benefits; however, expanding agricultural settlements on the periphery threaten buffer zones.

5. Comparative Summary

Feature	Rainforest	Savanna
Annual Rainfall	> 2000 mm, evenly distributed	500–1500 mm, strong wet‑dry contrast
Mean Annual Temperature	22–27 °C; diurnal range < 5 °C	24–28 °C; seasonal range up to 12 °C; diurnal 10–15 °C
Geographic Belt	5° N–5° S (humid tropical belt)	10°–20° N & 10°–20° S (tropical grassland belt)
Vegetation Structure	Four‑layered forest (emergent‑canopy‑understorey‑floor)	Grass → shrub → scattered drought‑tolerant trees
Leaf Adaptations	Large, broad, drip‑tips; high chlorophyll	Small, thick, waxy or deciduous
Root System	Shallow, fibrous, top‑soil focused	Deep taproots (≤ 20 m) + lateral roots
Soil Orders & Profile	Oxisols/Ultisols; thin A‑horizon, lateritic B‑horizon	Alfisols/Inceptisols; moderate A‑horizon, clay‑rich B‑horizon
Soil Fertility & Nutrient Cycling	Low intrinsic fertility; rapid litter decomposition	Higher intrinsic fertility; fire‑generated nutrient pulses; slower decomposition
Disturbance Regime	Low fire; wind‑throw, gap‑phase succession	Frequent low‑intensity fire; fire‑driven succession
Major Human Pressures	Logging, shifting cultivation, mining, road building	Cattle grazing, fire suppression, agricultural conversion
Key Climate‑Change Risks	Longer dry spells → fire risk; nutrient imbalance	Extended dry season → more intense fires; woody encroachment if fire suppressed

6. Key Points to Remember (Revision Checklist)

1. Rainforest vegetation is adapted to constant high moisture, low light and nutrient‑poor soils; savanna vegetation is adapted to seasonal water scarcity and regular fire.
2. Rainforest soils rely on rapid nutrient recycling; savanna soils are intrinsically more fertile but depend on fire‑induced nutrient pulses.
3. Fire is a central ecological driver in savannas (grass‑fire feedback); it is rare in rainforests where gap formation dominates disturbance.
4. Human activities (logging, agriculture, grazing, fire management) can upset the natural balance, leading to biodiversity loss, soil degradation and altered fire regimes.
5. When answering exam questions, always link: climate → vegetation structure → soil processes → disturbance regime**, and evaluate management strategies using specific case‑study evidence, explicitly referencing scale and systems thinking.