Deserts

ClimateDistribution and subtypesLandscape-substrate-soilsLimiting factorsAdaptations to aridityAdaptations to high temperatures

Definitions

• Boundary defined by absolute precipitation (e.g. <4 inches or 100 mm mean annual precipitation). Not an adequate definition.

• Boundary defined by intensity of moisture deficit (e.g. ratio of available precipitation to evaporative demand; see, for example, the Budyko-Lettau dryness ratio)

Causes of regional aridity

• Persistent atmospheric subsidence associated with the presence of sub-tropical anticyclones suppresses convectional activity (e.g. Sahara, Kalahari, Australia)

• Absence of humid airstreams (e.g. Gobi)• Localized subsidence in rain-shadow areas (e.g.

Great Basin)• Absence of cyclonic disturbances (e.g. Sonora)• Inhibition of convectional activity by cold

coastal currents (e.g. Atacama, Namib)

Temperature

Arid areas that are subject to sub-freezing conditions (e.g. Gobi, Great Basin) are “cold” deserts.

Areas where air temperatures seldom or never fall below 0°C are “hot” deserts (e.g. Sonora, Sahara)

Limitation of primary productivity in arid ecosystems (data from Tunisia)

0.01 0.1 1.0 10

500

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300

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100

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Mean

an

n.

rain

fall (

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Net primary production (kg ha-1 yr-1)

skeletal soils of uplands

gravelly soils of pediments and alluvial fans

Forms of adaptation to stresses in arid environments

Four strategies:1. minimize heat intake or maximize heat

outflows;2. maximize food reserves in times of

plenty;3. maximize water inflows; and4. minimize water outflows

Desert ephemeral flora

• Large seedbanks (esp. in sites protected from wind, e.g. around base of bushes; 100,000 seeds m-2);

• Long seed viability in dry soils;• Rapid germination if rainfall sufficient (signalled

by leaching of inhibitors in seed coats [e.g. only rains >25mm in Arizona produce germination] or scarification of thick seed coat in flash floods)

• Short time [6-8 weeks] to seed-set;• Some species heteroblastic [produce seeds with

varying germination requirements].

Refuging tactics: chuckwalla lizard

How much cooler is it at a depth

of 20 cm?

Refuging behaviour: camels in shade

Refuging by desert aquatic species

e.g. pupfish in Death Valley

flow salinity water temp.

winter summer winter

desert pupfish

Salt Creek, Death Valleylive lay eggs hatch /die

Maximise water inflows• extensive lateral (cacti) or vertical

(mesquite) roots;• rapid root growth after rains • beetles in Namib desert stand on hind

legs to catch fog droplets on raised abdomen;

• mice in Arizona often feed on low-protein herbage with high water content

• camels can drink 100 L of water in 10 min!

• practice opportunistic migration to water and food sources (desert locusts, nomadic pastoralists)

Minimize water loss1. Transpiration reduced in desert plants by

microphylly, deciduousness, sunken stomata, waxy or pubescent leaves.

2. Water loss in desert fauna reduced by dry faeces, low urine prodcution, low dilution of uric acid, adaptive hyperthermia (camel’s body temperature can vary by 6°C when animal is dehydrated).

3. Tolerate dessication: camel can withstand water loss = 25% of body weight

Evidence of climate

change from

pack-rat middens

Evidence for late Holocene

climate change in the

SaharaLake sediments and pollen

Rock drawings, Tassili

Desertification in progress?Rainfall in the Sahel zone of W

Africa

Protracted droughtCause: ?Effect: desertification?

Desertification on savanna margins: Rapp’s albedo

hypothesisSettlement Nomadism

Grazing

Vegetation

Albedo

Convectionalrainfall

+ -

Grazing

Vegetation

Albedo

Convectionalrainfall

+

+

-

-

+

- +

-

The evidence for

Rapp’s model