Website Notes - Deserts


Biome – one of the major categories of the world’s distinctive plant assemblages; e.g. tundra, tropical rainforest, desert.


Some definitions of deserts take into account one or more characteristics of these lands in question:

          Productivity – Primary productivity is the rate at which energy is stored as organic matter by photosynthesis.

Net primary productivity is the rate at which plants store energy or organic matter, not already used up in respiration. We see it as plant growth, and this is what is available for consumption by heterotrophs.


Ecosystem type

Net primary productivity: g/m2/yr





Trop rainforest



Temperate evergrn forest









Extreme desert



Open ocean



Coral reefs







Cold deserts - The mean temperatures of the warmest months in cold deserts are generally less than 10d C (50d F), and less than 0d C (32d F) in winter.

Hot deserts - Mean temperatures of hot deserts is 30d C (86d F) in summer months.


Cold deserts make up 24% of all deserts, whereas hot deserts comprise 43% of all deserts.


Typical warm desert climate

1.     Scarce, variable, unpredictable precipitation.

2.     High air and soil surface temperatures.

3.     High evaporation rates from soil surfaces.

4.     Low relative humidity and high potential evapotranspiration.

5.     High solar irradiance, often with cloudless conditions

6.     Steady to strong gusty winds.


These features force perennial plants to survive in soils with limited available moisture, which thereby limits photosynthesis and productivity,and can place plant organs under lethal daytime thermal conditions if they do not have adaptations for coping with high temperatures.



1.     Extremely arid - < 70 mm (< 3”) per year: Sahara, Atacama, Namib.

2.     Arid (typical) – 70-150 mm per year: Mojave

3.     Semi-arid – 150-300 mm per year: Sonoran, Chihuahuan.

4.     > 500 mm per year can still be considered desert if the rain comes in a very restricted time frame. ?Single rainstorms can exceed annual averages, as in January 1995 in LasVegas: 100 mm of rain in one day (man/year is 100-200 mm: 4-8”).

5.     Fog is important to both the Atacama and Namib Deserts.


Heat and High Temperatures

1.     Contributing to intense daytime heat is aridity.

a.      Little atmospheric moisture to absorb or deflect sun’s rays.

b.     Much radiation reaches the desert surface, and warms it during the day.

c.     At night, heat is released as surface emits infrared radiation that escapes unhindered through the dry atmosphere, and into space. This results in large diurnal fluctuations in temperatures.

1.     Dry Tonopah, NV has a daily diurnal fluctuation of 34 F.

2.     Aridity and heat are closely related and positively feed back on each other.

a.      Heat increases evapotranspiration, and this promotes aridity.

b.     Aridity promotes increased penetration of solar radiation, which increases surface heating.



1.     Aridisols

a.      High pH (alkaline)

b.     Generally less well-developed than soils of adjacent lands.

c.     Lack of organic matter, including humus.

d.     Accumulation of salts because of low penetration of precipitation and high evaporation

e.      Caliche, an accumulation of calcium carbonate into rock-hard, water-impermeable, root-limiting layer. Can promote surface run-off of water, that otherwise might hydrate a parched soil. Occurs where evaporation exceeds precipitation (in areas with a carbonate substrate/parent rock material: limestone).

2.     Winds modify soil.

a.      Due to limited cover of protected vegetation, dry clay and silt particles are easily eroded from the soil surface. Sands may also move. What is left behind is often a surface layer of closely knitted stones, called desert pavement.

1.     Desert pavement may protect underlying soil from further erosion.

2.     Desert pavement may also prevent dispersed seeds from reaching soil beneath, and may therefore limit seed germination and establishment.

3.     Desert pavement may inhibit rain penetration into the soil depths, leading to water (sheet) run-off.

3.      Cryptogamic crusts – interlacing web of lichens, mosses, cyanobacterial filaments that hold soil particles in place, and fix nitrogen.

4.     Despite severe moisture deficits, water is an important geomorphological agent in desert regions.

a.      Water moves soil and rocks (alluvium) down mountain slopes towards washes and playas, creating what we call bajadas or alluvial fans.

b.     Thermal expansion of moisture trapped in rocks my cause disintegration, and even in hot deserts rock shattering can occur when surfaces moistened with dew freeze in the clear night air.

c.     Rain splash and sheet wash cause pronounced erosion during periods of heavy rainfall because there is little vegetation to protect the surface.


Causes of Aridity

1.     Continentality or (Distance form oceans) – most water in atmosphere was evaporated from the sea, and this water eventually precipitates on land. Land closer to the sea generally receives much of this moisture. As air moves inland, it gets depleted of moisture and precipitation drops.

2.     Coastal Cooling - Deserts may result if air is cooled, and then rewarmed, prior to reaching the region.

3.     Atmospheric high pressure zones: Hadley cells

4.     Rainshadow effect: adiabatic heating and cooling lapse rates



1.     Arid regions of the world occupy 16-35% of the Earth’s land surface. The wide range reflects various definitions based on climatic conditions, vegetation types, potential for food production, etc.

2.     Much of this land lies between 15 and 30 degrees latitude.


North America – most deserts in North America are semi-arid areas, with a few arid areas in central pockets.

a.      Sonoran Desert – summer and winter rain (bi-modal). Hot summers, mild winters. Low elevation. Plant types include saguaro (indicator species), and many other cacti.

b.     Chihuahuan Desert – summer rains, hot summers, cool winters, relatively high elevation. Aridity from Hadley cells, not rainshadow. Plants – Creosotebush, Acacia, Flourensia (tarbush).

c.     Mojave Desert – winter rain, hot summers, cool winters. Plants – Creosotebush, bursage, blackbrush, Joshua tree (indicator plant). Rainshadow. Unpredictable rains.

d.     Great Basin – precipitation in winter or as snow melt, hot summers, cold winters, high elevation. Plants – sagebrush. Rainshadow.


South America

a.      Atacama Desert – coastal area of Peru and Chile. Cool, coastal strip that receives most of its precipitation as fog. Precipitaion is about 0.04” per year. Driest coastal desert. Coastal cooling.

b.     Puna Desert – Northern Argentina. Cold, high elevation in the Andes. Rainshadow.

c.     Monte Desert – of Argentina, east of the Puna. Warmer desert than Puna. Rainshadow effect of the Andes.

d.     Patagonia Desert – cold, windy, rainshadow desert (Andes Mountains) that runs the length of Argentina, with average annual temperatures of 7 C. Rainshadow.


3.     Africa

a.      Sahara Desert – largest desert in the world, ca. 9 million km2, (between 3,000,000 and 3,500,000 square miles) from the Atlantic Ocean to the Red Sea (3,000 miles). Extends from North Africa through Egypt to the deserts of the Arabian Peninsula, eastward into Iran, Afghanistan, and Pakistan, terminating in the Thar Desert of northwest India.The core is hyper-arid. Temperatures of 55 C (131 F) in shade is possible. Sandy dunes (ergs), stony (regs) substrate dominates; vegetative cover very sparse, susceptibility to wind erosion is very high, water infiltration rates are good, depressions fill with saline water. Herbs, small shrubs,with larger shrubs and trees where moisture is abundant (oases). Succulents not common. Atmospheric high pressure zones.

b.     Namib Desert – coastal strip desert along the west coast of Africa, from southern Angola to South Africa. Very dry, but not too hot because of coastal fog. High humidity and fog (like Atacama), with less than 2” annual precipitation. Very little vegetation. Lichens on the leeward side of rocks, leafless,stem-succulent plants,and halophytes. Coastal cooling.

c.     Kalahari Desert – inland desert from the Namib; north of Botswana. Notable for its deep subsurface sands. Landscape dominated by gentle dunes. Summer precipitation in the northern Kalahari; summer precipitation in the southern Kalahari.

d.     Arabian Desert – includes the whole peninsula of Arabia (Jordan, Iraq, Israel, Syria, and part of Iran); an empty quarter in the southern part of the desert is too hot and dry even for desert nomads (similar to the Sahara). Arid or extremely arid. Most of the plants are annuals; most of the perennials are halophytes. Atmospheric high pressure zone.


4.     Asia

a.      Takla-Makan Desert – northwest China. Rainshadow from 12,000’ Tibetan plateau (high mountains), as well as continentality. Hot summers, cold winters, without any particular seasonality for scanty precipitation. A large portion is extremely arid. Vegetation sparse except along streams. Herbaceous vegetation where moisture is low, and scanty woody vegetation on larger, stabilized dunes.

b.     Gobi Desert – China, Mongolia, high plateau, <40C to 45 C; 50-100 mm precipitation per year;  mountain and basin desert, northwest of Takla-Makan, widespread sand dunes and gravels (gobi). Shrubs and semi-shrubs.


5.     Australia – 40% of Australia’s land mass is classified as desert (3.4 million square miles). No succulents, very few spiny plants. Prevalent desert plants are the perennial evergreen tussock grasses (spinifex) and small trees or shrubs belonging to the genus Acacia.

a.     Simpson Desert – stony desert in western Australia

b.     Great Sandy

c.     Great Victoria