Vegetation and soils: characteristics, adaptations, human impacts

Arid Environments – Vegetation and Soils

1. Characteristics of Arid‑zone Vegetation (Syllabus 10.1)

• Rainfall: < 250 mm yr⁻¹ (highly irregular).
• Growth forms: dwarf shrubs, low‑lying trees, succulents (aloes, cacti), short‑lived annuals.
• Cover pattern: patchy vegetation mosaics (shrub‑steppe, true desert, scattered grasses) interspersed with extensive bare ground.
• Biomass productivity: very low net primary productivity (≈ 50–150 g C m⁻² yr⁻¹).
• Succession: primary succession → pioneer grasses → shrubland (sub‑climax) → possible plagioclimax where grazing or fire maintains a non‑climax state (the syllabus expects the term “succession”).

2. Plant Adaptations to Aridity (Syllabus 10.2)

Adaptations are grouped into three categories. Water‑use efficiency (WUE) is the amount of carbon fixed per unit of water lost; higher WUE means a plant can grow with less water. For example, CAM plants can achieve a WUE of ≈ 3 g CO₂ g⁻¹ H₂O, whereas typical C₃ plants are ≈ 1 g CO₂ g⁻¹ H₂O.

Additional Physical Weathering Mechanism (Syllabus 10.3)

• Thermal‑stress cracking: repeated diurnal temperature extremes cause expansion‑contraction cycles that fracture rocks.
• Salt‑crystallisation weathering: evaporation leaves salts in pores; when they crystallise they expand, breaking rock fragments and contributing to soil particle generation.

3. Characteristics of Arid‑zone Soils (Syllabus 10.4)

• Very low organic matter (< 1 %); humus formation is limited.
• Coarse texture – high sand content, field capacity < 10 %, low water‑holding capacity.
• Alkaline (pH ≈ 8–9) because of calcium carbonate accumulation.
• Frequent saline or gypsum‑rich horizons (solonchaks, gypsic horizons).

Dominant Soil Orders & Diagnostic Horizons (Syllabus 10.5)

Soil Nutrient Cycling (Syllabus 10.6)

• Carbon: limited organic inputs → slow humus formation; most carbon stored as inorganic carbonates.
• Nitrogen: low mineralisation rates; biological N‑fixation by sparse leguminous shrubs is a crucial source.
• Phosphorus: largely bound in calcium phosphates; low solubility restricts plant availability.

Suggested teacher diagram: a simple flow‑chart linking low organic matter → limited mineralisation → reliance on N‑fixing plants → feedback on soil fertility.

4. Soil Formation Processes in Arid Environments (Syllabus 10.7)

1. Physical weathering: thermal expansion/contraction, thermal‑stress cracking, salt‑crystallisation, wind abrasion.
2. Limited chemical weathering: low moisture reduces hydrolysis, oxidation and leaching.
3. Aeolian deposition: wind‑blown silt (loess) can create thin, relatively fertile layers on desert margins.
4. Pedogenic carbonate & gypsum accumulation: precipitation of CaCO₃ (caliche) or CaSO₄ (gypsic) as water evaporates.
5. Salinisation: upward movement of saline groundwater concentrates salts in the surface horizon.
6. Time‑scale: noticeable soil development typically requires centuries to millennia in arid settings, underscoring the “change over time” key concept.

5. Human Impacts on Arid‑zone Vegetation (Syllabus 10.8)

• Over‑grazing removes protective plant cover → increased bare ground → desertification.
• Unsustainable groundwater extraction lowers water tables, stressing deep‑rooted shrubs.
• Land clearing for agriculture introduces invasive species that out‑compete native xerophytes.
• Mining, road building and urban expansion disturb biological crusts, accelerating wind erosion.
• Fire regime changes: fire introduced to manage livestock or for hunting can convert shrubland to a plagioclimax state, reducing woody cover and altering nutrient cycles.

6. Human Impacts on Arid‑zone Soils (Syllabus 10.8)

• Destruction of biological and physical crusts → loss of surface stability and nutrient fixation.
• Salinisation from irrigation with poor‑quality water; rising water tables bring salts to the surface.
• Compaction by heavy machinery reduces infiltration and root penetration.
• Dust storms mobilise fine particles, depleting topsoil of organic matter and nutrients.

7. Detailed Case Study – Sahelian Desertification (Niger) (Syllabus 10.9)

Cause: Recurrent droughts (1970s‑80s) combined with expanding livestock numbers and unsustainable cultivation on marginal lands.

Impact on vegetation: Conversion of savanna woodland to shrub‑steppe and bare patches; loss of > 30 % of woody biomass.

Impact on soils: Development of surface solonchak crusts, increased wind erosion, and a ≈ 40 % reduction in soil organic carbon.

Management interventions:

1. Community‑led “Fadama” water‑harvesting basins to capture runoff and support small‑scale irrigation.
2. Agro‑forestry programmes planting drought‑tolerant species (e.g., Acacia senegal) as windbreaks.
3. Rotational grazing schemes enforced through local herders’ cooperatives.

Evaluation of success (aligned with the Cambridge evaluation framework):

• Effectiveness: Soil organic matter increased by 15 % in pilot basins; vegetation cover rose from 35 % to 55 % over ten years.
• Sustainability: Community enforcement provides social robustness, but long‑term water availability remains vulnerable to climate variability.
• Economic feasibility: High initial construction costs; improved crop yields are beginning to offset expenses.
• Comparative note: Large‑scale irrigation schemes in the same region have caused severe salinisation, illustrating the importance of low‑leaching water use.

8. Management Strategies – Evaluation Framework (Syllabus 10.10)

When assessing any mitigation measure, consider the four criteria below.

Contrasting Example Strategies

9. Scale‑Reflection Box

10. Summary

Arid environments are characterised by:

• Sparse, highly adapted vegetation mosaics with low productivity and a recognised succession pathway (primary → pioneer → sub‑climax → possible plagioclimax).
• Plant adaptations (morphological, physiological, phenological) that raise water‑use efficiency; quantitative examples (CAM WUE ≈ 3 g CO₂ g⁻¹ H₂O).
• Soils that are low in organic matter, alkaline, often saline or gypsum‑rich, and develop extremely slowly (centuries‑to‑millennia).
• Physical weathering dominated by thermal‑stress cracking and salt‑crystallisation, with limited chemical weathering.
• Human pressures—over‑grazing, groundwater extraction, land clearing, mining, altered fire regimes—that destabilise vegetation and soils, leading to desertification.
• Management must combine ecological (native windbreaks, controlled grazing, restoration of biological crusts) and engineering (water‑harvesting, drip irrigation) approaches, evaluated against effectiveness, sustainability, socio‑economic feasibility and scalability.