desert
TRANSCRIPT
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
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Mean
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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 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 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
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Grazing
Vegetation
Albedo
Convectionalrainfall
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