Development of plant communities: climatic climax, subclimax and plagioclimax

Cambridge International AS & A Level Geography (9696) – Revision Notes

How to Use These Notes

• Read each section for core knowledge (AO1).
• Practice the tables, diagrams and worked examples to develop quantitative skills (AO2).
• Answer the evaluation prompts to sharpen critical thinking (AO3).
• Cross‑reference the Skills Checklist with past‑paper questions to ensure you meet every assessment objective.

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Key Geographical Concepts (to be woven into every topic)

Concept	What It Means	Typical Exam Prompt
Scale	Spatial extent of a phenomenon – local, regional, national, global.	“Explain how the scale of a flood influences the choice of management strategy.”
Change over Time	Processes of development, degradation, or recovery; rates and trends.	“Analyse the change in forest cover in the Amazon between 2000 and 2020.”
Cause‑and‑Effect	Linking drivers (climate, human activity) to outcomes (land‑use change, hazard risk).	“Discuss the causes of desertification in the Sahel.”
Systems & Interdependence	Viewing environments as interacting components (e.g., water cycle, carbon cycle).	“Evaluate how deforestation affects the hydrological cycle in a tropical basin.”
Evaluation	Weighing advantages and disadvantages, considering sustainability, equity and uncertainty.	“Assess the effectiveness of community‑based forest management.”

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Geographical Skills & Resource Types

• Interpreting and constructing maps (topographic, thematic, GIS outputs).
• Reading and analysing graphs/charts (hydrographs, climate diagrams, population pyramids).
• Using satellite imagery to identify land‑use change.
• Working with field data – soil profiles, water‑quality tables, hazard‑mapping data.
• Developing flow diagrams and conceptual models.
• Applying mathematical models (logistic growth, water‑balance equations).
• Writing concise, evidence‑based case‑study paragraphs.

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1. Development of Plant Communities – Tropical Environments

1.1. Quick‑Recall of Key Concepts

• Scale: Typically regional to landscape (e.g., a 10 km² catchment or a 500 km² forest block).
• Change over Time: Successional stages from primary succession to climax, and possible reversals via disturbance.
• Cause‑and‑Effect: Climate ↔ soil ↔ disturbance → vegetation type.
• Evaluation: Compare biodiversity, carbon storage and resilience of climax vs. plagioclimax states.

1.2. Successional End‑Points

• Climatic climax – The stable, mature community that would develop under the prevailing climate and soil conditions if left undisturbed.
• Subclimax – A semi‑stable community maintained by regular, moderate natural disturbance (e.g., seasonal flooding, fire).
• Plagioclimax – A community kept in a non‑climatic state by continual anthropogenic disturbance (e.g., agriculture, logging, grazing).

1.3. Factors Controlling Succession

1. Climate – temperature, rainfall amount & seasonality.
2. Soil – depth, texture, nutrient status, pH, bulk density.
3. Disturbance regime – natural (cyclones, fire) or human‑induced.
4. Seed dispersal mechanisms & seed‑bank composition.
5. Biotic interactions – competition, facilitation, herbivory.

1.4. Climatic Climax Communities in the Tropics

Climatic Zone (Mean Annual Rainfall)	Typical Climax Vegetation	Dominant Soil Characteristics (Quantitative)
Evergreen Rainforest > 2000 mm yr⁻¹	Multi‑layered canopy; emergent Dipterocarps; abundant epiphytes & lianas.	Depth ≈ 30–40 m; pH 4.0–5.5; bulk density ≈ 0.9 g cm⁻³; highly leached laterites, base saturation < 20 %.
Seasonal (Monsoonal) Forest 1200–2000 mm yr⁻¹	Deciduous trees, lianas, shrub understory; leaf‑fall in dry season.	Depth ≈ 15–25 m; pH 5.0–6.0; bulk density ≈ 1.1 g cm⁻³; lateritic/ferrallitic, moderate Al³⁺.
Savanna 800–1200 mm yr⁻¹	Grasses with scattered fire‑adapted trees (Acacia, Baobab, Combretum).	Depth ≈ 5–15 m; pH 4.5–5.5; bulk density ≈ 1.3 g cm⁻³; shallow laterites, low organic matter.

1.5. Subclimax Communities (Natural Disturbance)

• Riverine floodplain forest – Inundation every 2–3 years selects flood‑tolerant species such as Ficus spp. and Myrtaceae. Soil remains relatively young and nutrient‑rich.
• Fire‑prone savanna patches – Moderate fire frequency (every 3–5 years) maintains a mix of grasses and fire‑resistant trees; fire‑feedback promotes grass dominance.
• Secondary forest after single clear‑cut – Edge effects persist for 10–15 years; pioneers (Macaranga, Alstonia) coexist with early shade‑tolerants (Shorea spp.).

1.6. Plagioclimax Communities (Anthropogenic Disturbance)

• Shifting‑cultivation fallows – Land is re‑cleared after 3–5 years, preventing succession beyond early secondary stage.
• Pastoral grasslands – Continuous livestock grazing suppresses tree seedlings, favouring grasses such as Panicum spp.
• Commercial plantations – Non‑native species (oil palm, rubber, teak) replace native structure, alter litter quality and nutrient cycling.

Typical impacts: reduced species richness, altered litter input, soil compaction, lower carbon storage, and changed fire regimes.

1.7. Successional Pathways – Flow Diagram (description)

Primary succession → Pioneer community → Subclimax → Climatic climax with a side arrow indicating diversion to Plagioclimax when persistent anthropogenic disturbance occurs. Feedback loops (e.g., fire‑feedback in savanna) are shown with double‑headed arrows.

1.8. Quantitative Modelling of Successional Rate

The increase in canopy cover (C) over time (t) in a secondary tropical forest can be modelled with a logistic function:

C(t) = Cmax / [1 + e-k(t‑t₀)]

• C_max – Maximum canopy cover at climatic climax (≈ 95 %).
• k – Intrinsic growth constant (typical values 0.2–0.4 yr⁻¹). Fires, soil fertility or invasive species can increase or decrease k.
• t₀ – Inflection point (years after disturbance when growth is fastest).

Worked Example

Given: C_max = 95 %, k = 0.30 yr⁻¹, t₀ = 12 yr. Calculate canopy cover after 10 years.

1. Insert values: C(10) = 95 / [1 + e-0.30(10‑12)]
2. Exponent: -0.30 × (‑2) = 0.60
3. e^0.60 ≈ 1.82
4. Denominator = 1 + 1.82 = 2.82
5. C(10) ≈ 95 / 2.82 ≈ 33.7 %

Interpretation: After 10 years the forest has reached roughly one‑third of its eventual canopy cover – a typical subclimax stage.

Evaluation Prompt (AO3)

• How would repeated low‑intensity fires alter the value of k and the time to reach C_max?
• What are the limitations of using a logistic model for forests on highly leached lateritic soils?
• Discuss the usefulness of the model for planning restoration projects in different disturbance regimes.

1.9. Management Implications

• Restoration should aim to re‑establish the climatic climax by protecting sites from both natural (fire, flood) and anthropogenic disturbances.
• Where subclimax or plagioclimax states are intentionally maintained (e.g., agroforestry, grazing), set realistic biodiversity and productivity targets and monitor k as an indicator of ecosystem health.
• Model climate‑change scenarios to assess whether a site may shift from rainforest climax to seasonal‑forest subclimax, informing long‑term land‑use planning.

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2. Paper 1 – Physical Geography (Core Topics)

2.1. Hydrology

Key‑Concept Reminder

• Scale: Catchment‑scale processes (10–10 000 km²) versus river‑reach analysis.
• Change over Time: Seasonal hydrograph variation, long‑term trends in discharge.
• Evaluation: Trade‑offs between structural and non‑structural flood mitigation.

Core Content

• Drainage‑basin concepts – catchment area, river order, slope, discharge, runoff coefficient.
• Hydrograph interpretation – baseflow, peak flow, lag time, rising limb, recession limb, flood‑peak estimation.
• Flood management
  • Structural: dams, levees, channelisation.
  • Non‑structural: early‑warning systems, flood‑plain zoning, community preparedness.

Case Study

Ganges‑Brahmaputra Basin (India & Bangladesh) – monsoonal flood peaks, levee breaches, and community‑based early‑warning systems using river‑stage gauges and mobile alerts.

Evaluation Prompt (AO3)

• Assess the sustainability of large dams in the G‑B basin considering sediment trapping, downstream water security and displacement.
• Compare the effectiveness of structural vs. non‑structural measures in reducing flood‑related loss of life.

2.2. Atmospheric Processes

Key‑Concept Reminder

• Scale: Global (Hadley cell) to regional (monsoon trough).
• Change over Time: Seasonal shifts, inter‑annual variability (ENSO), long‑term climate change.
• Evaluation: Reliability of climate models for regional planning.

Core Content

• Energy budget components – solar radiation, albedo, long‑wave radiation, sensible & latent heat fluxes.
• Tri‑cellular atmospheric model – Polar, Ferrel and Hadley cells; location of the Inter‑tropical Convergence Zone (ITCZ).
• Jet streams – subtropical and polar jets; role in mid‑latitude weather systems.
• Monsoons – seasonal reversal of winds, pressure gradient mechanisms, examples (South‑Asian, West‑African).
• Climate change – greenhouse‑gas effect, radiative forcing, projected temperature & precipitation trends for the 21st century.

Diagram Suggestion

Global energy‑budget diagram with arrows showing heat transport from equator to poles via Hadley, Ferrel and Polar cells, and overlay of subtropical jet streams.

Evaluation Prompt (AO3)

• Critically evaluate the usefulness of the three‑cell model for explaining extreme rainfall events in the tropics.
• Discuss the uncertainties associated with down‑scaling global climate model outputs for regional flood risk assessments.

2.3. Earth Processes – Hazards

Key‑Concept Reminder

• Scale: Local (landslide) to global (tectonic plate boundaries).
• Change over Time: Hazard frequency, long‑term risk trends.
• Evaluation: Effectiveness of prediction, risk identification and mitigation.

Core Content

2.3.1. Plate Tectonics

• Types of boundaries – divergent, convergent (subduction, continental‑collision), transform.
• Associated hazards – earthquakes, volcanic eruptions, tsunamis.

2.3.2. Mass‑Movement Hazards

• Types – landslides, rockfalls, debris flows, slumps.
• Factors of stability – slope angle, material type, water content, vegetation.
• Prediction & risk identification
  • Hazard mapping (GIS slope, lithology, rainfall thresholds).
  • Monitoring techniques – inclinometers, piezometers, remote‑sensing of deformation.
• Detailed example: 2023 Central Italy landslides triggered by prolonged heavy rain; early‑warning system based on rainfall‑threshold modelling reduced casualties.

2.3.3. Earthquake Hazards

• Magnitude scales (Richter, moment magnitude) and intensity scales (MMI).
• Seismic gap theory and probabilistic seismic hazard assessment.
• Mitigation – building codes, retrofitting, land‑use planning.
• Case study: 2015 Nepal earthquake – shallow focus, high ground shaking, post‑seismic landslides.

2.3.4. Volcanic Hazards

• Eruption types – Hawaiian, Strombolian, Vulcanian, Plinian, phreatic.
• Primary hazards – lava flows, pyroclastic density currents, ashfall, lahars.
• Monitoring – seismographs, gas emission sensors, satellite thermal imagery.
• Case study: 2022 Hunga Tonga‑Hunga Haʻapai eruption – atmospheric shock wave, global climate impact.

Evaluation Prompt (AO3)

• Compare the strengths and limitations of GIS‑based landslide susceptibility mapping versus field‑based monitoring.
• Assess the cost‑benefit of strict seismic building codes in low‑income, high‑hazard regions.

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3. Paper 2 – Human Geography (Core Topics)

3.1. Population

Key‑Concept Reminder

• Scale: Local city growth to global demographic transition.
• Change over Time: Shifts in fertility, mortality, migration patterns.
• Evaluation: Policies for sustainable population management.

Core Content

• Demographic Transition Model (DTM) – four (or five) stages, characteristic birth/death rates, population growth rates.
• Strengths: provides a framework for historical population change in many countries.
• Limitations: assumes linear progression, less applicable to LICs with high HIV/AIDS prevalence or to HICs experiencing ageing.
• Population distribution – density, concentration, dispersion; factors influencing patterns (climate, resources, history).
• Migration – push‑pull factors, types (internal, international, forced), demographic impacts.
• Policy responses – family‑planning programmes, immigration control, urban‑growth management.

Case Study

Nigeria’s rapid urban growth – Lagos megacity, informal settlements, challenges for housing, water, sanitation, and transport.

Evaluation Prompt (AO3)

• Evaluate the usefulness of the DTM for predicting future population trends in sub‑Saharan Africa.
• Discuss the social and economic trade‑offs of strict family‑planning policies in a low‑income context.

3.2. Water Resources

Key‑Concept Reminder

• Scale: Watershed to transboundary river basin.
• Change over Time: Seasonal variability, long‑term scarcity trends.
• Evaluation: Sustainability of management strategies.

Core Content

• Water‑cycle components – evaporation, transpiration, runoff, infiltration, groundwater recharge.
• Water scarcity – physical (lack of water) vs. economic (lack of infrastructure); demand‑supply gap.
• Management strategies
  • Integrated Water‑Resources Management (IWRM) – catchment‑wide planning, stakeholder participation.
  • Technological options – desalination, rainwater harvesting, water‑recycling.
  • Demand‑side measures – water‑pricing, leakage reduction.

Case Study

Mekong River basin – upstream dam construction (e.g., Xayaburi), downstream flow alteration, regional cooperation through the Mekong River Commission.

Evaluation Prompt (AO3)

• Assess the effectiveness of IWRM in the Mekong basin given differing national priorities.
• Analyse the environmental trade‑offs of large‑scale desalination plants in arid coastal regions.

3.3. Urban Areas

Key‑Concept Reminder

• Scale: Neighborhood to megacity.
• Change over Time: Urbanisation rates, spatial expansion, functional change.
• Evaluation: Sustainability of urban development patterns.

Core Content

• Urban models – Burgess concentric zone, Hoyt sector, Harris‑Ullman multiple nuclei.
• Urban functions – residential, commercial, industrial, services, recreation.
• Sustainable urban development
  • Housing density vs. green‑space provision.
  • Transport – public transit, non‑motorised travel, congestion mitigation.
  • Solid‑waste disposal – recycling, landfill, incineration.
  • Energy efficiency – low‑carbon buildings, district heating.

Case Study

Curitiba, Brazil – innovative Bus Rapid Transit (BRT) system, integrated land‑use planning, high green‑space ratio.

Evaluation Prompt (AO3)

• Weigh the benefits of high‑density housing for reducing transport emissions against the need for adequate green‑space for wellbeing.
• Critically evaluate the transferability of Curitiba’s BRT model to rapidly growing African cities.

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4. Paper 3 – Global Environments (Key Themes)

4.1. Tropical Environments

Covered in Section 1 (development of plant communities) plus:

• Deforestation drivers – logging, shifting cultivation, mining, infrastructure.
• Carbon cycle impacts – forest carbon stock, emissions from land‑use change, feedback to climate change.
• Conservation approaches – REDD+, protected‑area networks, community‑based forest management.

4.2. Coastal Environments

Key‑Concept Reminder

• Scale – from a single beach to continental shelves.
• Change over Time – sea‑level rise, coastal erosion/accretion cycles.
• Evaluation – cost‑effectiveness of hard vs. soft coastal defence.

Core Content

• Coastal processes – longshore drift, wave energy, tidal regimes, sediment transport.
• Landforms – beaches, dunes, cliffs, estuaries, deltas, barrier islands.
• Human impacts – reclamation, port development, tourism, pollution.
• Management – hard engineering (sea walls, groynes), soft engineering (beach nourishment, dune stabilisation), managed retreat.

Case Study

The Netherlands – Delta Works – combination of massive sea‑defences and adaptive water‑management to protect low‑lying land.

Evaluation Prompt (AO3)

• Discuss the long‑term sustainability of hard engineering solutions in the face of projected sea‑level rise.
• Compare the social implications of managed retreat with those of beach nourishment in a coastal tourism hotspot.

4.3. Hazardous Environments

Key‑Concept Reminder

• Scale – local (volcano) to global (climate‑related hazards).
• Change over Time – frequency and intensity trends under climate change.
• Evaluation – effectiveness of early‑warning systems and risk communication.

Core Content

• Geophysical hazards – earthquakes, volcanoes, tsunamis, landslides (see Section 2.3).
• Hydro‑meteorological hazards – floods, cyclones, droughts, heatwaves.
• Risk assessment – hazard, exposure, vulnerability; use of risk matrices.
• Prediction & early‑warning – monitoring networks, modelling, community alert protocols.

Case Study

Bangladesh Cyclone Sidr (2007) – early‑warning dissemination via radio and community volunteers reduced mortality compared with earlier cyclones.

Evaluation Prompt (AO3)

• Evaluate the role of community participation in improving the effectiveness of early‑warning systems for cyclones.
• Analyse the challenges of integrating climate‑change projections into long‑term hazard‑risk planning.

4.4. Arid Environments

Key‑Concept Reminder

• Scale – desert basin to regional arid zone.
• Change over Time – desertification, groundwater depletion.
• Evaluation – sustainability of water‑use practices.

Core Content

• Climate characteristics – low rainfall (< 250 mm yr⁻¹), high evapotranspiration, large diurnal temperature range.
• Soil and vegetation – thin, coarse soils; xerophytic plants (e.g., Acacia, Prosopis).
• Desertification drivers – overgrazing, unsustainable irrigation, climate variability.
• Management strategies
  • Afforestation/reforestation with drought‑tolerant species.
  • Water‑saving irrigation (drip, sprinkler, deficit irrigation).
  • Land‑use planning – rotational grazing, sand‑dune stabilisation.

Case Study

China’s “Great Green Wall” (Three‑North Shelterbelt) – large‑scale tree planting to combat desertification in the Gobi‑Desert fringe.

Evaluation Prompt (AO3)

• Assess the ecological and socio‑economic outcomes of the Three‑North Shelterbelt after two decades.
• Discuss the trade‑offs between water‑intensive afforestation and groundwater sustainability in arid zones.

4.5. Global Themes (Paper 4)

4.5.1. Climate‑Change Impacts

• Physical impacts – sea‑level rise, glacier retreat, changes in precipitation patterns.
• Human impacts – food security, health, migration, economic loss.
• Mitigation – renewable energy, carbon pricing, reforestation.
• Adaptation – coastal defence, climate‑resilient agriculture, early‑warning systems.

Evaluation Prompt

• Compare the cost‑effectiveness of mitigation versus adaptation strategies for small island developing states.
• Critically discuss the equity implications of global carbon‑trading schemes.

4.5.2. Trade, Aid & Tourism

• Globalisation – patterns of trade, terms of trade, balance of payments.
• Foreign aid – types (ODA, humanitarian, development), effectiveness debates.
• Tourism – mass tourism vs. ecotourism, economic benefits, environmental pressures.

Evaluation Prompt

• Evaluate the role of ecotourism in biodiversity conservation in a Caribbean island.
• Analyse the arguments for and against conditional aid tied to environmental performance.

4.5.3. Disease

• Transmission pathways – vector‑borne, water‑borne, airborne.
• Geographical distribution – influence of climate, urbanisation, health infrastructure.
• Control measures – vaccination, sanitation, vector control, health education.

Evaluation Prompt

• Discuss how climate change may alter the geographic range of malaria and the implications for public‑health planning.
• Assess the long‑term sustainability of mass vaccination programmes in low‑income countries.

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5. Quick‑Reference Summary Table

Topic	Core AO1 Content	Key AO2 Skill	Typical AO3 Prompt
Plant Communities	Climatic climax, subclimax, plagioclimax; factors controlling succession.	Logistic‑growth modelling; flow‑diagram construction.	Evaluate biodiversity and carbon‑stock differences between climax and plagioclimax.
Hydrology	Drainage‑basin concepts, hydrograph components, flood management.	Interpret hydrographs; calculate lag time.	Assess structural vs. non‑structural flood mitigation.
Atmospheric Processes	Energy budget, three‑cell model, monsoons, climate change.	Label global energy‑budget diagram; analyse climate‑model data.	Critique the three‑cell model for extreme tropical rainfall.
Earth‑process Hazards	Plate tectonics, landslides, earthquakes, volcanoes, risk mapping.	GIS hazard‑susceptibility mapping; calculate seismic intensity.	Compare hard‑engineering and community‑based mitigation.
Population	DTM, distribution, migration, policy responses.	Construct population pyramids; calculate growth rates.	Evaluate DTM applicability to sub‑Saharan Africa.
Water Resources	Water‑cycle, scarcity types, IWRM, case studies.	Analyse river‑flow graphs; calculate water‑use efficiency.	Assess IWRM Create an account or Login to take a Quiz 30 views 0 improvement suggestions Log in to suggest improvements to this note. Support e-Consult Kenya Your generous donation helps us continue providing free Cambridge IGCSE & A-Level resources, past papers, syllabus notes, revision questions, and high-quality online tutoring to students across Kenya. Powered by