Adaptation of plants to extreme temperatures

1. Introduction

Arid environments are characterised by very low and highly variable rainfall (< 250 mm yr⁻¹), extremely high potential evapotranspiration and large temperature fluctuations. Plants must cope with daytime temperatures that often exceed 40 °C and night‑time lows that can fall below 0 °C. Understanding the adaptations that enable survival under these thermal extremes is essential for interpreting vegetation patterns, soil development and the sustainability of human activities in deserts.

2. Arid Climate & Weather (Syllabus 10.1)

• Temperature range: diurnal swings of 20–30 °C are typical; seasonal ranges can reach 30–45 °C.
• Seasonal patterns: many deserts have a short summer rain season (e.g., monsoon‑like showers in the Sahel, North‑American monsoon) and a long dry season; others receive most rain in winter.
• Precipitation: < 250 mm yr⁻¹, highly erratic in space and time; most rain falls in brief, intense events.
• Potential evapotranspiration (PET): frequently > 2000 mm yr⁻¹, creating a large water deficit.
• Atmospheric stability: strong subsidence, clear skies and high solar radiation by day; rapid radiative cooling at night.

Suggested diagram: Typical arid climate graph (temperature & precipitation) and a world map showing the major deserts (Sahara, Arabian, Kalahari, Australian, Atacama, Sonoran).

3. Physical Landforms of Arid Zones (Syllabus 10.2)

• Dunes & ergs – wind‑blown sand accumulations; stabilised by grasses, lichens or crusts.
• Wadis & dry riverbeds – intermittent channels that flood during rare rain events.
• Playas & sabkhas – flat, often salt‑encrusted basins where water evaporates rapidly.
• Reg (desert pavement) – surface layer of closely packed pebbles that reduces heating.
• Calcretes & gypsic horizons – carbonate or gypsum accumulations formed by evaporation of groundwater.
• Pediments & alluvial fans – gently sloping rock surfaces at mountain bases where runoff spreads.
• Mesa & plateau – flat‑topped hills with steep sides, common in the American Southwest.
• Inselberg – isolated rock outcrops that create micro‑refugia.
• Badlands – heavily eroded, gullied terrain with little vegetation.
• Salt flats – extensive, level surfaces of evaporite minerals (e.g., Salar de Uyuni).

Suggested sketch: Cross‑section of a transverse dune showing windward slip face, lee side, a thin vegetative crust and a buried root system.

4. Soils in Arid Zones (Syllabus 10.3)

4.1 Dominant Soil Types

4.2 Soil Processes Influencing Plant Growth

• Salinisation – accumulation of soluble salts lowers osmotic potential; only salt‑tolerant species survive.
• Calcrete & gypsum horizon formation – hard layers impede root growth but store moisture in cracks.
• Low organic matter – rapid mineralisation and minimal inputs give low fertility; many plants form nitrogen‑fixing symbioses.
• Thermal conductivity – dark, coarse soils heat quickly, raising root‑zone temperatures.
• Solifluction / cryoturbation – seasonal freeze‑thaw cycles in high‑altitude deserts mix surface material, affecting seed‑bank stability.

5. Plant Adaptations to Extreme Temperatures (Syllabus 10.4)

Adaptations are grouped into morphological, physiological, phenological, root‑system and molecular / population strategies.

5.1 Morphological Adaptations

• Reduced leaf surface area – tiny leaves, spines or leaf‑less photosynthetic stems.
• Thick cuticles & waxy coatings – reflect solar radiation and limit water loss.
• Sunken stomata – recessed pores reduce exposure to hot, dry air currents.
• Compact growth forms – cushions, low rosettes or mat‑forming succulents lower wind exposure.
• Reflective surfaces – light‑coloured bark, silvery trichomes increase albedo.

5.2 Physiological Adaptations

• Crassulacean Acid Metabolism (CAM) – stomata open at night; CO₂ stored as malic acid; photosynthesis in daylight with closed stomata.
• Heat‑shock proteins (HSPs) – stabilise enzymes and membranes during temperature spikes.
• Osmotic adjustment – accumulation of compatible solutes (proline, sugars) lowers freezing point and maintains turgor.
• Antioxidant systems – enzymes such as superoxide dismutase scavenge reactive oxygen species generated by thermal stress.

5.3 Phenological Adaptations

• Seasonal leaf shedding – deciduous behaviour during the hottest or driest months.
• Rapid life cycles – many annuals complete germination, flowering and seed set within weeks after rain.
• Dormancy – seeds, buds or tubers remain dormant until temperature and moisture thresholds are met.
• Timing of growth – some species grow mainly at night or in the early morning when temperatures are lower.

5.4 Root‑System Adaptations

• Deep taproots – reach groundwater, provide thermal buffering and a stable water source.
• Extensive lateral & shallow roots – exploit brief surface moisture from flash floods.
• Root hairs with mucilage – increase surface area, retain a thin water film and buffer rapid temperature changes.
• Rhizomes or tuber storage – underground organs store water and carbohydrates, insulating them from extreme surface temperatures.

5.5 Molecular & Population Adaptations

• Antifreeze proteins & specialised dehydrins – bind ice crystals and protect cell membranes in species that experience sub‑zero night temperatures (e.g., Welwitschia mirabilis).
• Heat‑shock protein families (HSP70, HSP90) – up‑regulated during daytime peaks > 45 °C.
• Seed banks & longevity – long‑lived seeds remain viable for many years, ensuring recruitment after rare rain events.
• Clonal spread – ramet formation (e.g., in Acacia tortilis) allows rapid occupation of favourable microsites without reliance on seed germination.

6. Representative Desert Plants and Their Strategies (Syllabus 10.5)

7. The Role of Temperature in Soil Development

• Diurnal temperature swings cause repeated expansion‑contraction of soil particles, accelerating physical weathering and the formation of desert pavements.
• Deep‑rooted plants create bioturbation channels that enhance vertical heat transfer, moderating surface temperature extremes.
• Rapid mineralisation of short‑lived annuals releases nutrients but also speeds up carbonate and gypsum horizon development.
• Biological soil crusts (cyanobacteria, lichens) increase surface albedo, lowering soil temperature and reducing evaporation.

8. Human Activities & Pressures in Arid Environments (Syllabus 10.6)

• Water extraction – over‑pumping lowers water tables, threatening deep‑rooted species (e.g., Acacia, date palm).
• Over‑grazing – removal of protective vegetation destabilises dunes, destroys crusts and raises surface temperatures.
• Mining & quarrying – disturb calcrete and gypsum layers, alter local micro‑climates and generate dust that increases atmospheric temperature.
• Urbanisation & tourism – infrastructure and vehicle traffic create local heat islands and fragment habitats.
• Renewable‑energy installations (solar farms) – large reflective surfaces can modify ground temperature regimes and affect nearby vegetation.
• Agricultural expansion & irrigation – raises local water tables, encouraging invasive, water‑demanding species that out‑compete native drought‑adapted plants.
• Climate‑change‑driven desertification – rising mean temperatures and altered precipitation patterns expand arid zones, intensifying all of the above pressures.

9. Management & Sustainability (Syllabus 10.5)

• Water‑conservation techniques
  • Rainwater harvesting and runoff capture in wadis.
  • Drip‑irrigation to minimise evaporative loss, especially for cultivated oases.
• Restoration of vegetation
  • Planting native, deep‑rooted shrubs (e.g., Acacia) to stabilise dunes.
  • Seeding fast‑germinating annuals to rebuild surface crusts and reduce wind erosion.
• Protected areas & land‑use planning
  • Designating desert biosphere reserves to safeguard keystone species such as Welwitschia.
  • Regulating groundwater extraction through licensing and monitoring.
  • Guidelines for siting solar farms to avoid high‑value ecological zones.
• Desertification mitigation
  • Rotational grazing schemes to prevent over‑grazing.
  • Application of mulch or inoculation with biological soil crusts to increase surface albedo and retain moisture.

10. Summary of Adaptation Mechanisms

1. Minimise heat absorption – reduced leaf area, reflective surfaces, light‑coloured bark.
2. Control water loss – CAM photosynthesis, sunken stomata, thick cuticles, mucilaginous root hairs.
3. Protect cellular structures – heat‑shock proteins, antifreeze proteins, osmotic solutes, antioxidant enzymes.
4. Exploit temporal niches – dormancy, rapid life cycles, night‑time growth.
5. Secure water from depth or brief surface pulses – deep taproots, extensive shallow networks, fog capture.

11. Examination Checklist

• Explain how high daytime temperatures increase transpiration and the risk of tissue damage.
• Describe the CAM pathway, including the biochemical equation: CO₂ + H₂O → C₄H₈O₅ (malic acid) + O₂ (night) and the subsequent decarboxylation during daylight.
• Compare morphological adaptations of woody desert plants (e.g., acacias) with herbaceous annuals (e.g., desert sandwort).
• Assess the influence of root depth on soil temperature regimes and on the formation of calcrete or gypsum horizons.
• Discuss two human pressures on arid ecosystems and propose a management measure for each.
• Evaluate the role of biological soil crusts in moderating surface temperature and preventing erosion.