Characteristics (AQA A Level Geography)

• Deserts share a number of climatic characteristics
• Located in zones of high atmospheric pressure due to sinking, warm, dry air
• Arid regions have a large diurnal (daily) range in temperature (up to 30°C) due to a lack of cloud cover
  • Intense solar radiation heats the ground and warms the air above it, preventing the formation of clouds
  • At night, the heat escapes under the clear skies 
  • Distance from the coasts – land heats and cools faster inland, providing a greater range of temperature
  • Height above sea level – the higher the desert the cooler it will be
• Semi-arid deserts have seasonal rainfall
• The mean annual temperature of most desert margins is lower than in deserts between 10-20°C
• The diurnal range is less extreme (up to15°C)

Other climatic characteristics

• Deserts have strong winds and sandstorms
• Low latitude (sun overhead) provides intense convective activity, which triggers thunderstorms
• Proximity to cool ocean currents depresses temperatures and produces sea fog
• Albedo – salt-encrusted, dried-out lakes reflect insolation and are, therefore, cooler

• Due to harsh conditions, plants are sparse and small, however, variations in temperature, rainfall, geology and soils can lead to wide differences in species diversity
• Generally, low biomass – not much grows (500kg per hectare in deserts but rainforests = 250 000kg per hectare)
• Adapted to cope with drought and salt with limited nutrient cycling due to lack of water
• The amount of vegetation increases with distance from arid regions due to the increase in the availability of water
• Adaptation occurs gradually over time; it is when a species changes to suit the area in which it is living
• Once the species has adapted to its habitat, it can survive more easily
• There are a number of ways in which vegetation in arid and semi-arid areas has adapted

Hot Arid and Semi-Arid Plants and Adaptations

• Harsh conditions make food supply scarce
• To protect themselves, many plants have evolved spines e.g. prickly pear cactus  
• The creosote bush gives off toxic substances to deter other plants from growing nearby
• Some plants are parasitic e.g. orobanche
• Others contain poisonous latex - opium poppy exudes fresh latex from a cut 
• Australian deserts are dominated by spinifex grass and eucalyptus
• The Namib has euphorbia succulents
• Kokerboom or quiver tree has smooth branches covered with a thin layer of whitish powder that helps reflect away the hot sun rays
• Desert vegetation is generally sparse but grows in greater densities wherever there is more moisture, such as hollows, at the bottom of slopes, or in dry valleys – which can be moist under the surface
• There are fewer plants where the surfaces are impermeable or on shifting sand dunes

Nutrient cycle of desert regions - note that soil is the largest store of nutrients and transfer is quickest between biomass and litter

• Desert soils (aridisols) have a number of distinctive characteristics:
  • Many hot desert soils are red because of oxidation
  • Dry, shallow and coarse-textured soils
    • Little moisture reduces the chemical weathering of rocks, leading to thin/shallow soil and few soil horizons (layers) in the wetter arid margins
  • Rocky, sandy, or gravelly
    • The surfaces are impermeable due to beating rain on bare surfaces compacting them. Fine material breaks off and is washed or blown down into the pores of the soil, blocking them
    • Despite being compact, these surfaces are fragile and are easily damaged by human activity
    • Some soils have low clay and organic matter content as this has been leached to leave sandy gravelly soil, while others have layers of sticky clay or hard crusts called duricrusts
    • The commonest types of duricrusts are:
      • Calcrete, formed from calcium carbonate, found mainly in semi-arid regions
      • Gypcrete formed from calcium sulphate, is found mainly in arid regions. 
  • Little organic matter
    • • Little vegetation, so little humus or biomass in the soil, and the lack of rotting vegetation is often below 3% of total biomass
      • Duricrusts are a hard rock band at or below the surface that makes root development in plants difficult
  • Saline - one of the main products of soil formation is salinisation
    • Sodium salts in the soil cause the soil structure to collapse
    • High concentration of sodium salts causes deflocculation (dispersal) of the soil particles; in good soil, the particles stick together in small lumps, but in deflocculation the soil becomes impermeable and the air necessary for plant growth cannot penetrate
    • Salts prevent plants from absorbing water into their roots
    • Salts are poisonous to many plants

Typical soil profile of hot desert