Deserts

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Deserts

• Climate
• Distribution and subtypes
• Landscape-substrate-soils
• Limiting factors
• Adaptations to aridity
• Adaptations to high temperatures

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Definitions

• Boundary defined by absolute precipitation (e.g.
  lt4 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)

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Budyko-Lettau dryness ratio
D R / (L x P)
where D dryness ratio R mean ann. net
radiationP mean ann. precipitation L
latent heat of vaporization of water
Original definition D gt 2.3 semi-desert
D 3.4 desert. UNESCO D 10
extreme desert
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Dryness ratio
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semiarid desert (D2.3)
(D10)
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Unpredictability areas with gt30 variability in
precipitation
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(No Transcript)
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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)

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Coastal fog deserts
E.g. Atacama desert Namib
desert
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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 0C are hot deserts (e.g.
Sonora, Sahara)
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cold desertsagebrush steppe (Idaho-eastern
Oregon)
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Hot desertsaguaro - ocotillo community,
Arizona
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Sonoran cacti saguaro(Carnegiea gigantea) range
controlled by exposure to freezing temperatures
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Sonoran plant ranges
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Thermal microclimates
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Desert landscape-substrate elements
upland / hillslope
pediment/bajada/reg
dunefield/ erg
deflation
erosion
playa - sabkha
solutes
deposition
gravelly soils
skeletal soils
sandy silts sands
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Uplands of southern Bajaa wilderness of thorn
and rock
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Desert upland habitatskeletal soils
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Uplands and bajada complex, Death Valley, CA
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Mesquite growing on gravelly fan deposits, Arizona
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Playa deposits
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Mobile substratesSaharan dunes
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Limiting factors
Five interlinked stresses for biological
community REGIONAL 1. permanent moisture
deficit 2. high surface temperatures during
day large diurnal variation 3. highly
irregular and variable moisture supply LOCAL 4.
mobile substrates and 5. saline substrates
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Limitation of primary productivity in arid
ecosystems (data from Tunisia)
500 400 300 200 100 0
skeletal soils of uplands
Mean ann. rainfall (mm)
gravelly soils of pediments and alluvial fans
0.01 0.1 1.0
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Net primary production (kg ha-1 yr-1)
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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
and 4. minimize water outflows
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Evasion tactics to minimize exposure to heat and
drought
1. Organism dormant for substantial part of
life-cyclee.g. ephemeral plants, some reptiles,
most insects persist through extended droughts as
seeds, eggs, or larvae (only the reproductive
forms remain). 2. Nocturnal or crepuscular
foraging (hottest parts of day spent in burrows
or shade). Is crassulacean acid metabolism an
equivalent tactic for succulent plants? CAM -
stomates open at night CO2 absorbed, assimilated
during day when stomates closed to minimize water
loss
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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 gt25mm 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.

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Sonoran desert in bloom
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Refuging tactics chuckwalla lizard
How much cooler is it at a depth of 20 cm?
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Refuging behaviour camels in shade
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Refuging by desert aquatic speciese.g. pupfish
in Death Valley
desert pupfish
winter summer winter
Salt Creek, Death Valley
live lay eggs hatch
/die
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Reducing heat load

• Low surface-area/volume ratio
• Reflective skin/bark(colour changes in lizards)
• Vertical shoot-body architecture

Fouquieria/Idria columnaris (the cirio of
central Baja)
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Reducing heat loada joshua tree (Yucca
brevifolia)in the Mojave desert
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Maximise food reserves in times of plenty

• camels hump
• berber sheep (fat reserves in tailvary from
  2-10 kg)
• pack rats/gerbils hoard seeds
• succulents store water.

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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)

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Shrub/tree root patterns, Arizona
Plant spacing determined by moisture availability
and rooting niche
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Water conservationexpandable storage organs and
palisade tissue in succulents
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Tissue protectionthorns and spines
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Minimize water loss
1. 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 (camels body temperature can vary
by 6C when animal is dehydrated). 3. Tolerate
dessication camel can withstand water loss 25
of body weight
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Reducing water loss microphylly, deciduousness,
photosynthetic bark and shoots
Pachycormus discolor
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Reducing water loss microphylly in ocotillo
(Fouquieria splendens)
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Evidence of climate changelake levels in the
Great Basin
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Evidence of climate change from pack-rat middens
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Pink desert yellowsavanna grassland
browndry forest NB Lake Mega-Chad (bigger than
Caspian Sea at present, and at least 40 m deep)
http//www.uni-mannheim.de/phygeo/8000BP.htm
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Evidence for late Holocene climate change in the
Sahara
Lake sediments and pollen
Rock drawings, Tassili
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Desertification in progress?Rainfall in the
Sahel zone of W Africa
Cause ? Effect desertification?
Protracted drought
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Desertification on savanna margins Rapps albedo
hypothesis
Settlement
Nomadism

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The evidence for Rapps model
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The elements of anthropogenic desertification