Deserts
Deserts are areas of low rainfall and high evaporation rates and cover about ¼ of the Earth’s surface. Contrary to common perceptions, most deserts are not sandy, nor are they populated with succulent cacti. The deserts of the world tend to be dominated by perennial grasses and annuals.
Climate
Arid climates that fluctuate temperatures throughout the day and the seasons
Rainfall
Precipitation is the abiotic factor that mostly defines this biome
Very little rainfall
Rainfall tends to come all at once in a short period, causing temporary flooding
Less than 25 cm/yr
Temperature
Extreme fluctuations in temperature between months and even day to night
Desert can occupy average annual extremes: -10° to 30°C
Hot deserts occupying the lower righthand portion of the graph
Cold deserts occupy the lower middle portion of the graph
Notice that the desert biomes occupies a broad temperature range at the bottom of the Whittaker graph (right)
Above: A Whittaker Graph showing where deserts occupy this abiotic space, based on precipitation and temperature
Types
Hot & dry deserts
Seasons are generally warm throughout the year and very hot in the summer. The winters usually bring little rainfall.
Annual average temperatures: 20-25° C
e.g. Chihuahuan, Sonoran, Mojave, and Great Basin of the United States, as well as the Sahara and Arabian deserts
Semiarid deserts
Summers are moderately long and dry, and like hot deserts, the winters normally bring low concentrations of rainfall
Summer temperatures usually average between 21-27° C, and evening temperatures are cool, at around 10° C
Condensation of dew caused by night cooling may equal or exceed the rainfall received by some deserts
e.g. Utah, Montana and Great Basin
Coastal deserts
Deserts found along the edge of oceans, but receive little precipitation due to wind patterns
e.g. Atacama desert in Chile, and the Namibian desert
Cold deserts
These areas have cold winters with snowfall and large rainfalls widely during the winter and occasionally over the summer
The mean winter temperature is -2-4° C, and the mean summer temperature is 21-26° C
Cold deserts and tundras can sometimes be viewed as synonymous, but subtle differences can be noted
Tundras are usually covered in small plants and have many small streams and ponds during the melt of summer.
A cold desert will have very few streams or ponds, and very few plants.
e.g. Greenland and the Nearctic realm
Soil
Abundant nutrients, but water is limited for absorption
Most deserts are not sandy, except the Saharan and Arabian deserts
n.b. "desertification" is a process by which fertile topsoils are damaged by over-farming or over-grazing, resulting in increasingly dry and infertile land (e.g. Dust Bowl in 1930s Midwest; modern impacts on the Sahel of Africa)
Regional Expressions
southwest North America, Namibia, southern Argentina, north and south Africa, middle east, central Australia, mountains of Asia
Flora
Desert flora exhibit many adaptations to this biome
Some adaptations to store water and prevent evaporation, but most desert plants (>85%) do not show structural adaptations to conserve water. (Gibson 1998)
Annuals
Short life spans, appearing after seasonal rains
Seeds lie dormant for a majority of the year
Geophytes
Plants that have underground storage organs, with ephemeral above-ground portions for photosynthesis and reproduction
e.g. bulbs, corms, rhizomes, and tubers
Grasses
Well adapted to dry conditions with bulliform cells and C4 photosynthesis
Deciduous shrubs
Similar to temperate deciduous forest, shrubs in the desert drop leaves before the dry season begins
Evergreen shrubs
Some shrubs, such as creosote bush (Larrea tridentata)
Phreatophytes
Plants with deep taproots, to access the water table
Succulent perennials
Less than 15% of plants in deserts are cacti (Gibson 1996)
In the United States, only the Sonoran Desert has abundant cacti, including the Saguaro cactus (Carnegiea gigantea), which is endemic to that desert.
Worldwide, only in the Karroo of South Africa and the Atacama desert of South America, possess a large percentage of stem-succulent plants in the landscape.
Above: Woolly tidestromia (Tidestromia lanuginosa) in the Sonoran desert
Above: the saguaro cactus (Carnegiea gigantea), which is native to the Sonoran desert of North America
Evolution of the biome
It is assumed that arid land, with desert-like conditions, has been on Earth since early in its history, similar to a Mars-like landscape
Due to limited water, deserts do not preserve evidence well, and therefore fossil evidence for deserts is limited
Evidence for plants living in desert biomes appears in the Permian, but may have occurred earlier
Additional Resources
In the World’s Driest Desert, Ancient Wisdom Blooms Eternal (NY Times 3Nov2023)
6,000 years ago the Sahara Desert was a tropical grassland (Texas A&M 2016; Nature GeoScience 2016)
The Mighty Saguaro (In Defense of Plants, 2018)
What survives int he Atacama Desert? (NY Times 10May2023)