Vegetation and Soils in Arid Environments (Cambridge International AS & A Level Geography 9696)
Objective
To understand how plants adapt to physical and physiological (saline) drought in arid regions, how these adaptations influence soil development, and how human activities modify these natural processes.
1. Climate of Arid Zones
- Definition of aridity: Potential evapotranspiration (PET) far exceeds precipitation (P).
Aridity Index (AI) = P / PET ( AI < 0.2 = hyper‑arid, 0.2 – 0.5 = arid )
- Köppen classification: BWh (hot desert) and BWk (cold desert).
- Typical climatic parameters (illustrative world deserts):
| Desert |
Mean Annual Temp (°C) |
Mean Annual Rainfall (mm) |
PET (mm yr⁻¹) |
AI |
| Sahara (North Africa) |
22–30 |
100–150 |
2 500 |
0.04–0.06 |
| Gobi (East Asia) |
–5–10 (cold desert) |
100–200 |
1 800 |
0.06–0.11 |
| Atacama (South America) |
12–18 |
≤ 15 |
1 500 |
≤ 0.01 |
- Seasonality: Rainfall is erratic, often occurring as isolated, high‑intensity storms; diurnal temperature ranges can exceed 30 °C, driving high PET.
- Spatial distribution: A world map of BWh/BWk climates (available in the Cambridge textbook) helps students visualise the global extent of arid zones – a key “scale” concept.
2. Physical Processes Shaping Arid Landscapes
- Aeolian processes – wind deflation, saltation and deposition create dunes, desert pavements and loess deposits.
Example: In the Sahara, average sand‑transport rates can reach 10 km yr⁻¹ during peak wind seasons.
- Flash flooding & sheet wash – short, intense rain events generate rapid surface runoff that erodes gullies and builds alluvial fans.
Quantitative illustration: A 20 mm h⁻¹ storm lasting 30 min in the Sonoran Desert can produce a peak discharge of > 150 m³ s⁻¹ in a narrow wadi.
- Physical weathering – thermal expansion‑contraction, salt‑crystallisation and freeze‑thaw dominate; chemical weathering is limited by low moisture.
- Salt‑crust formation – capillary rise of saline groundwater deposits surface salts, creating hard crusts that affect seed germination and infiltration.
3. Soils of Arid Environments
Arid soils are classified in the World Reference Base (WRB) and the FAO system. The five most common orders are summarised below, together with the five soil‑forming factors (parent material, climate, organisms, topography, time) that control their development.
| Soil Order (WRB/FAO) |
Diagnostic Horizons / Features |
Key Constraints for Plants |
| Regolith (Lithic Leptosol) |
Shallow depth (< 30 cm), fragmented parent material, minimal horizon development. |
Limited rooting depth, very low water‑holding capacity. |
| Calcisols (Calcareous soils) |
Accumulation of calcium carbonate (calcic or petrocalcic horizon), often a hardpan. |
High pH, low nutrient availability, restricted root penetration. |
| Gypsisols (Gypsum soils) |
Gypsum (CaSO₄·2H₂O) accumulation in B‑horizon; may harden when dry. |
Potentially toxic sulfate levels, very low organic matter. |
| Saline soils (Solonchaks, Halosols) |
Surface or subsurface salt crusts; electrical conductivity (EC) > 4 dS m⁻¹. |
Physiological drought – water present but osmotically unavailable. |
| Aridisols (FAO) / Aridic soils (USDA) |
Low organic carbon (< 0.5 %), coarse texture, high bulk density, often a B‑k horizon. |
Low water‑retention, high temperature fluctuations. |
Soil‑formation factors in arid zones
- Parent material: Predominantly coarse, quartz‑rich sands or carbonate/gypsum deposits.
- Climate: Low and erratic precipitation, high PET – limits chemical weathering and organic matter accumulation.
- Organisms: Sparse vegetation, specialised microbes (cyanobacteria in biological crusts) that modestly increase CEC at depth.
- Topography: Flat plains favour salt‑crust formation; depressions collect runoff and may develop deeper moisture zones.
- Time: Slow pedogenic development; many soils remain in an early stage of horizon differentiation.
Suggested diagram: A labelled vertical section of a typical Calcisol showing (from top to bottom) a surface salt crust, a thin organic‑rich A‑horizon, a calcic B‑horizon with a petrocalcic hardpan, and a deeper C‑horizon of weathered parent material.
4. Plant Adaptations to Physical Drought
Physical drought = shortage of usable water in the soil profile. Adaptations are grouped into structural, phenological and behavioural strategies. The table below links each adaptation to a measurable functional trait.
| Adaptation |
Functional Trait (Typical Range) |
How It Helps |
Example Species |
| Deep tap‑root |
Root depth 5–30 m; root‑to‑shoot ratio > 3 |
Accesses residual moisture stored in deeper horizons. |
Acacia tortilis (Sahelian fringe) |
| Extensive shallow lateral roots |
Lateral spread 5–10 m within top 30 cm; high root density. |
Rapid capture of brief rain pulses. |
Stipagrostis spp. (desert grasses) |
| Reduced leaf area (scale‑, needle‑, or absent leaves) |
Specific leaf area (SLA) < 5 mm² mg⁻¹; leaf area index (LAI) < 0.5. |
Minimises transpiration surface. |
Calligonum comosum (Central Asian dunes) |
| Vertical or V‑shaped leaf orientation |
Leaf angle 60°–90° from horizontal. |
Reduces solar radiation interception during peak heat. |
Various xerophytic shrubs. |
| Thick cuticle & epicuticular wax |
Cuticle thickness 10–20 µm; wax load > 30 µg cm⁻². |
Lowers cuticular transpiration. |
Aloe vera (succulent) |
| Trichomes / leaf hairs |
Hair density > 200 mm⁻². |
Reflects radiation, creates a still‑air boundary layer. |
Larrea tridentata (creosote bush) |
| Succulence (water‑storage tissues) |
Water content > 80 % of fresh weight; hydraulic capacitance > 5 mmol kg⁻¹ MPa⁻¹. |
Buffers short‑term water deficits. |
Welwitschia mirabilis (Namib Desert) |
| Phenological timing (rapid germination & short life cycle) |
Time from germination to seed set < 30 days after rain. |
Completes life cycle before the onset of severe drought. |
Annuals such as Stipagrostis pungens |
5. Plant Adaptations to Physiological (Saline) Drought
Physiological drought occurs when water is present but its water potential is too low for plant uptake, usually because of high salinity. Adaptations are often seen in halophytic communities.
- Osmotic adjustment – synthesis/accumulation of compatible solutes (proline, glycine betaine, soluble sugars) that lower cellular water potential and maintain turgor.
- Salt excretion mechanisms
- Specialised salt glands or bladder hairs on leaves/stems (e.g., Haloxylon ammodendron, Spartina maritima).
- Excreted salts form surface crusts; wind or rain later removes them.
- Ion compartmentalisation – sequestration of Na⁺ and Cl⁻ in vacuoles, keeping the cytoplasm relatively ion‑free.
- Reduced stomatal density & conductance – limits transpiration and passive salt influx.
- Energy trade‑offs – salt excretion and osmotic adjustment consume ATP, often reducing growth rates and photosynthetic efficiency.
- Phenological adaptation – growth and reproduction timed to periods of lower surface salinity, such as after a flushing rain that leaches salts deeper.
Halophyte community types and succession
- Coastal saline marshes – dominated by Spartina*, Salicornia*, Atriplex* spp. High tidal influence, frequent leaching.
- Inland saline flats (solonchaks) – pioneer Salicornia europaea, later replaced by Atriplex nummularia and woody halophytes (Haloxylon*) as soil structure improves.
- Successional trend: crust‑forming cyanobacteria → herbaceous halophytes → shrub/woody halophytes**, each stage enhancing organic matter and soil stability.
6. Representative Species and Their Key Adaptations
| Adaptation |
Mechanism |
Example Species (Habitat) |
| Deep taproot |
Roots > 5 m access residual moisture. |
Acacia tortilis – Sahelian savanna & desert fringe |
| Succulent stems |
Fleshy parenchyma stores water; low surface‑area‑to‑volume ratio. |
Welwitschia mirabilis – Namib Desert |
| Reduced leaf area |
Scale‑like leaves minimise transpiration; stems photosynthesise. |
Calligonum comosum – Central Asian dunes |
| Salt glands |
Epidermal structures excrete Na⁺/Cl⁻ onto leaf surface. |
Haloxylon ammodendron – Chinese Gobi |
| Osmotic adjustment |
Accumulation of proline & soluble sugars maintains turgor under low water potential. |
Salicornia europaea – Coastal saline flats |
7. Interaction Between Vegetation and Soil Development
Plant adaptations actively modify soil properties, creating feedback loops central to the “systems” key concept of the syllabus.
- Organic‑matter translocation – Deep‑rooted shrubs transport photosynthates to sub‑soil, increasing carbon inputs, cation‑exchange capacity (CEC) and microbial activity at depth.
- Bioturbation & surface stability – Dense root mats bind surface particles, reducing wind erosion and the formation of deflation hollows.
- Salinity regulation – Salt‑excreting plants deposit salts on leaf surfaces; wind or rain later removes them, locally lowering surface salinity and creating microsites for less‑tolerant species.
- Biological soil crust formation & breakdown – Litter and fine root debris promote cyanobacterial and lichen crusts that increase infiltration, protect against raindrop impact and add organic matter.
- Microhabitat creation – Shade and litter from shrubs raise local soil moisture, moderate temperature extremes, and facilitate recruitment of understory herbs and grasses.
8. Human Impacts and Management Strategies
- Over‑grazing – Removes protective vegetation, accelerates wind erosion, and promotes desertification.
- Unsustainable irrigation – Raises groundwater tables, leading to secondary salinisation of soils.
- Land‑clearing for mining or urban expansion – Destroys root networks, increases surface runoff and flash‑flood risk.
- Reclamation techniques
- Planting salt‑tolerant pioneer species (e.g., Salicornia*, Atriplex* spp.) to stabilise soils and extract salts.
- Construction of windbreaks and sand‑fences to reduce aeolian erosion.
- Application of gypsum or organic amendments to improve soil structure and leach excess salts.
9. Summary of Key Points
- Arid climates are characterised by low precipitation, high PET and an AI < 0.5.
- Physical processes (aeolian transport, flash floods, salt‑crust formation) dominate landscape development and control soil depth and texture.
- Typical arid soils are shallow, low in organic matter, often calcareous, gypsum‑rich or saline; their formation is governed by the five soil‑forming factors.
- Physical‑drought adaptations focus on water acquisition (deep tap‑roots, shallow lateral roots) and loss reduction (small leaves, thick cuticles, succulence, phenological timing). Functional‑trait values provide quantitative context for exam data tables.
- Physiological‑drought (saline) adaptations involve osmotic adjustment, salt excretion, ion compartmentalisation and reduced stomatal density, but they incur energetic trade‑offs.
- Vegetation–soil feedbacks include organic‑matter enrichment at depth, surface stabilisation, salinity regulation and microhabitat creation, all of which influence subsequent succession.
- Human activities can exacerbate drought stress; sustainable grazing, careful irrigation management and the use of salt‑tolerant reclamation species mitigate degradation.
10. Suggested Diagram
Cross‑section of a desert shrub (e.g., Acacia tortilis) illustrating:
- Deep taproot extending into a Calcisol horizon with a petrocalcic hardpan.
- Succulent stem showing thick cuticle, wax layer and trichomes.
- Leaf surface with salt glands and a surface salt crust.
- Soil profile: surface salt crust → thin organic A‑horizon → calcic B‑horizon → deeper C‑horizon of weathered parent material.
11. Example Examination Question
Explain how the deep taproot system of Acacia tortilis helps the species survive in an arid environment, and discuss two ways this adaptation influences the surrounding soil.
12. Further Reading (Suggested Topics)
- Water‑Use Efficiency (WUE) in xerophytic plants – WUE = A / E where A = photosynthetic rate, E = transpiration rate.
- Soil‑Plant‑Atmosphere Continuum (SPAC) – hydraulic conductance from deep soil to leaf.
- Long‑term impacts of desertification on soil carbon stocks and global climate feedbacks.
- Management of saline soils – leaching, gypsum amendment and phytoremediation.
- Halophyte succession on inland solonchaks – case studies from the Australian Outback and the Central Asian steppes.