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DESERT AN OVERVIEW
Climate
Distribution and subtypes
Landscape-substrate-soils
Limiting factors
Adaptations to aridit...
Definitions
• Boundary defined by absolute precipitation
(e.g. <4 inches or 100 mm mean annual
precipitation). Not an adeq...
Budyko-Lettau dryness ratio
D = R / (L x P)
where D = dryness ratio;
R = mean ann. net radiation;
P = mean ann. precipitat...
Dryness ratio
semiarid desert
(D≥2.3) (D≥10)
8
7
ARISE TRAINING &
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Unpredictability:
areas with >30% variability in
precipitation
ARISE TRAINING &
RESEARCH CENTER
ARISE TRAINING &
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Causes of regional aridity
• Persistent atmospheric subsidence associated with
the presence of sub-tropical anticyclones
s...
“Coastal fog deserts”
E.g. Atacama desert Namib desert
ARISE TRAINING &
RESEARCH CENTER
Temperature
Arid areas that are subject to sub-
freezing conditions (e.g. Gobi, Great
Basin) are “cold” deserts.
Areas whe...
“cold desert”
sagebrush steppe
(Idaho-eastern Oregon)
ARISE TRAINING &
RESEARCH CENTER
“Hot
desert”:
saguaro -
ocotillo
community,
Arizona
ARISE TRAINING &
RESEARCH CENTER
Sonoran cacti:
saguaro
(Carnegiea
gigantea)
range
controlled by
exposure to
freezing
temperatures
ARISE TRAINING &
RESEARC...
Sonoran plant ranges
ARISE TRAINING &
RESEARCH CENTER
Thermal microclimates
ARISE TRAINING &
RESEARCH CENTER
Desert landscape-substrate
elements
upland /
hillslope
pediment/bajada/reg
playa - sabkha
dunefield/
erg
deposition
deflat...
Uplands of southern Baja
“a wilderness of thorn and rock”
ARISE TRAINING &
RESEARCH CENTER
Desert upland habitat:
skeletal soils
ARISE TRAINING &
RESEARCH CENTER
Uplands and bajada complex,
Death Valley, CA
ARISE TRAINING &
RESEARCH CENTER
Mesquite growing on gravelly
fan deposits, Arizona
ARISE TRAINING &
RESEARCH CENTER
Playa deposits
ARISE TRAINING &
RESEARCH CENTER
Mobile substrates:
Saharan dunes
ARISE TRAINING &
RESEARCH CENTER
Limiting factors
Five interlinked “stresses” for biological community:
REGIONAL
1. “permanent‟ moisture deficit;
2. high s...
Limitation of primary productivity in
arid ecosystems (data from Tunisia)
0.01 0.1 1.0 10
500
400
300
200
100
0
Meanann.ra...
Forms of adaptation to
stresses in arid environments
Four strategies:
1. minimize heat intake or maximize heat
outflows;
2...
Evasion tactics to minimize
exposure to heat and drought
1. Organism dormant for substantial part of life-cycle:
e.g. ephe...
Desert ephemeral flora
• Large seedbanks (esp. in sites protected from wind,
e.g. around base of bushes; 100,000 seeds m-2...
Sonoran desert in bloom
ARISE TRAINING &
RESEARCH CENTER
Refuging tactics:
chuckwalla lizard
How much cooler
is it at a depth
of 20 cm?
ARISE TRAINING &
RESEARCH CENTER
Refuging behaviour:
camels in shade
ARISE TRAINING &
RESEARCH CENTER
Refuging by desert aquatic species
e.g. pupfish in Death Valley
flow
salinity
water temp.
winter summer winter
desert pupf...
Reducing
heat load
• Low surface-
area/volume ratio;
• Reflective skin/bark
(colour changes in
lizards)
• Vertical shoot-b...
Reducing heat
load:
a joshua tree
(Yucca
brevifolia)
in the Mojave
desert
ARISE TRAINING &
RESEARCH CENTER
Maximise food reserves in
times of plenty
• camel‟s hump;
• berber sheep
(fat reserves in tail
vary from 2-10 kg);
• pack ...
Maximise water inflows
• extensive lateral (cacti) or vertical
(mesquite) roots;
• rapid root growth after rains
• beetles...
Shrub/tree root
patterns, Arizona
Plant spacing
determined
by moisture
availability
and rooting
niche
40 ARISE TRAINING &
...
Water conservation:
expandable storage organs and
palisade tissue in succulents
ARISE TRAINING &
RESEARCH CENTER
Tissue protection:
thorns and spines
ARISE TRAINING &
RESEARCH CENTER
Minimize water loss
1. Transpiration reduced in desert plants by
microphylly, deciduousness, sunken stomata, waxy or
pubes...
Reducing water loss:
microphylly, deciduousness,
photosynthetic bark and shoots
Pachycormus
discolor
ARISE TRAINING &
RESE...
Reducing water loss: microphylly in ocotillo
(Fouquieria splendens)
ARISE TRAINING &
RESEARCH CENTER
Evidence of
climate
change:
lake levels in
the Great
Basin
ARISE TRAINING &
RESEARCH CENTER
Evidence
of climate
change
from
pack-rat
middens
ARISE TRAINING &
RESEARCH CENTER
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
http://www.uni-mannheim.de/phygeo/8000BP.htm
Pink...
Evidence for late
Holocene climate
change in the
Sahara
Lake sediments and pollen
Rock drawings, TassiliARISE TRAINING &
R...
Desertification in progress?
Rainfall in the Sahel zone of W Africa
Protracted drought
Cause: ?
Effect: desertification?
A...
Desertification on savanna margins:
Rapp‟s albedo hypothesis
Settlement Nomadism
Grazing
Vegetation
Albedo
Convectional
ra...
The evidence
for Rapp‟s
model
ARISE TRAINING &
RESEARCH CENTER
The elements
of
anthropogenic
desertification
ARISE TRAINING &
RESEARCH CENTER
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Deserts an overview

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DESERT AN OVERVIEW - MORE INSIGHTS ABOUT OUR NATURE - DAY TIME TOO HOT NIGHT TIME TOO COOL!!!

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Deserts an overview

  1. 1. DESERT AN OVERVIEW Climate Distribution and subtypes Landscape-substrate-soils Limiting factors Adaptations to aridity Adaptations to high temperatures ARISE TRAINING & RESEARCH CENTER
  2. 2. 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) ARISE TRAINING & RESEARCH CENTER
  3. 3. Budyko-Lettau dryness ratio D = R / (L x P) where D = dryness ratio; R = mean ann. net radiation; P = mean ann. precipitation; L = latent heat of vaporization of water Original definition: D > 2.3 = semi-desert; D ≥ 3.4 = desert. UNESCO: D ≥ 10 = “extreme desert” ARISE TRAINING & RESEARCH CENTER
  4. 4. Dryness ratio semiarid desert (D≥2.3) (D≥10) 8 7 ARISE TRAINING & RESEARCH CENTER
  5. 5. Unpredictability: areas with >30% variability in precipitation ARISE TRAINING & RESEARCH CENTER
  6. 6. ARISE TRAINING & RESEARCH CENTER
  7. 7. 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) ARISE TRAINING & RESEARCH CENTER
  8. 8. “Coastal fog deserts” E.g. Atacama desert Namib desert ARISE TRAINING & RESEARCH CENTER
  9. 9. 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) ARISE TRAINING & RESEARCH CENTER
  10. 10. “cold desert” sagebrush steppe (Idaho-eastern Oregon) ARISE TRAINING & RESEARCH CENTER
  11. 11. “Hot desert”: saguaro - ocotillo community, Arizona ARISE TRAINING & RESEARCH CENTER
  12. 12. Sonoran cacti: saguaro (Carnegiea gigantea) range controlled by exposure to freezing temperatures ARISE TRAINING & RESEARCH CENTER
  13. 13. Sonoran plant ranges ARISE TRAINING & RESEARCH CENTER
  14. 14. Thermal microclimates ARISE TRAINING & RESEARCH CENTER
  15. 15. Desert landscape-substrate elements upland / hillslope pediment/bajada/reg playa - sabkha dunefield/ erg deposition deflation solutes skeletal soils gravelly soils sandy silts sands ARISE TRAINING & RESEARCH CENTER
  16. 16. Uplands of southern Baja “a wilderness of thorn and rock” ARISE TRAINING & RESEARCH CENTER
  17. 17. Desert upland habitat: skeletal soils ARISE TRAINING & RESEARCH CENTER
  18. 18. Uplands and bajada complex, Death Valley, CA ARISE TRAINING & RESEARCH CENTER
  19. 19. Mesquite growing on gravelly fan deposits, Arizona ARISE TRAINING & RESEARCH CENTER
  20. 20. Playa deposits ARISE TRAINING & RESEARCH CENTER
  21. 21. Mobile substrates: Saharan dunes ARISE TRAINING & RESEARCH CENTER
  22. 22. 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 ARISE TRAINING & RESEARCH CENTER
  23. 23. Limitation of primary productivity in arid ecosystems (data from Tunisia) 0.01 0.1 1.0 10 500 400 300 200 100 0 Meanann.rainfall(mm) Net primary production (kg ha-1 yr-1) skeletal soils of uplands gravelly soils of pediments and alluvial fans ARISE TRAINING & RESEARCH CENTER
  24. 24. 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 ARISE TRAINING & RESEARCH CENTER
  25. 25. Evasion tactics to minimize exposure to heat and drought 1. Organism dormant for substantial part of life-cycle: e.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] ARISE TRAINING & RESEARCH CENTER
  26. 26. 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]. ARISE TRAINING & RESEARCH CENTER
  27. 27. Sonoran desert in bloom ARISE TRAINING & RESEARCH CENTER
  28. 28. Refuging tactics: chuckwalla lizard How much cooler is it at a depth of 20 cm? ARISE TRAINING & RESEARCH CENTER
  29. 29. Refuging behaviour: camels in shade ARISE TRAINING & RESEARCH CENTER
  30. 30. 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 ARISE TRAINING & RESEARCH CENTER
  31. 31. 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)ARISE TRAINING & RESEARCH CENTER
  32. 32. Reducing heat load: a joshua tree (Yucca brevifolia) in the Mojave desert ARISE TRAINING & RESEARCH CENTER
  33. 33. Maximise food reserves in times of plenty • camel‟s hump; • berber sheep (fat reserves in tail vary from 2-10 kg); • pack rats/gerbils hoard seeds; • succulents store water. ARISE TRAINING & RESEARCH CENTER
  34. 34. 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) ARISE TRAINING & RESEARCH CENTER
  35. 35. Shrub/tree root patterns, Arizona Plant spacing determined by moisture availability and rooting niche 40 ARISE TRAINING & RESEARCH CENTER
  36. 36. Water conservation: expandable storage organs and palisade tissue in succulents ARISE TRAINING & RESEARCH CENTER
  37. 37. Tissue protection: thorns and spines ARISE TRAINING & RESEARCH CENTER
  38. 38. 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 (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 ARISE TRAINING & RESEARCH CENTER
  39. 39. Reducing water loss: microphylly, deciduousness, photosynthetic bark and shoots Pachycormus discolor ARISE TRAINING & RESEARCH CENTER
  40. 40. Reducing water loss: microphylly in ocotillo (Fouquieria splendens) ARISE TRAINING & RESEARCH CENTER
  41. 41. Evidence of climate change: lake levels in the Great Basin ARISE TRAINING & RESEARCH CENTER
  42. 42. Evidence of climate change from pack-rat middens ARISE TRAINING & RESEARCH CENTER
  43. 43. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. http://www.uni-mannheim.de/phygeo/8000BP.htm Pink = desert; yellow=savanna grassland; brown=dry forest NB Lake Mega-Chad (bigger than Caspian Sea at present, and at least 40 m deep) ARISE TRAINING & RESEARCH CENTER
  44. 44. Evidence for late Holocene climate change in the Sahara Lake sediments and pollen Rock drawings, TassiliARISE TRAINING & RESEARCH CENTER
  45. 45. Desertification in progress? Rainfall in the Sahel zone of W Africa Protracted drought Cause: ? Effect: desertification? ARISE TRAINING & RESEARCH CENTER
  46. 46. Desertification on savanna margins: Rapp‟s albedo hypothesis Settlement Nomadism Grazing Vegetation Albedo Convectional rainfall + - Grazing Vegetation Albedo Convectional rainfall + + - - + - + - ARISE TRAINING & RESEARCH CENTER
  47. 47. The evidence for Rapp‟s model ARISE TRAINING & RESEARCH CENTER
  48. 48. The elements of anthropogenic desertification ARISE TRAINING & RESEARCH CENTER

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