1. Deserts Climate Distribution and subtypes Landscape-substrate-soils
Limiting factors
Adaptations to aridity
Adaptations to high temperatures

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)

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”

4. Dryness ratio 8 7
semiarid desert
(D≥2.3) (D≥10)

5. Unpredictability: areas with >30% variability in precipitation

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)

8. “Coastal fog deserts”
E.g. Atacama desert Namib desert

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)

10. “cold desert” sagebrush steppe (Idaho-eastern Oregon)

11. “Hot desert”: saguaro - ocotillo community, Arizona

12. Sonoran cacti: saguaro (Carnegiea gigantea) range controlled by exposure to freezing temperatures

13. Sonoran plant ranges

14. Thermal microclimates

15. Desert landscape-substrate elements
upland /
hillslope
pediment/bajada/reg
dunefield/
erg
deflation
erosion
playa - sabkha
solutes
deposition
skeletal
soils
gravelly
soils
sandy silts sands

16. Uplands of southern Baja “a wilderness of thorn and rock”

17. Desert upland habitat: skeletal soils

18. Uplands and bajada complex, Death Valley, CA

19. Mesquite growing on gravelly fan deposits, Arizona

20. Playa deposits

21. Mobile substrates: Saharan dunes

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

23. Limitation of primary productivity in arid ecosystems (data from Tunisia)
500
400
300
200
100
skeletal soils of uplands
Mean ann. rainfall (mm)
gravelly soils
of pediments and
alluvial fans
Net primary production (kg ha-1 yr-1)

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

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]

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].

27. Sonoran desert in bloom

28. Refuging tactics: chuckwalla lizard
How much cooler
is it at a depth
of 20 cm?

29. Refuging behaviour: camels in shade

30. Refuging by desert aquatic species e.g. pupfish in Death Valley
desert pupfish
winter summer winter
flow
salinity
water temp.
Salt Creek, Death Valley
live lay eggs hatch
/die

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)

32. Reducing heat load: a joshua tree (Yucca brevifolia) in the Mojave desert

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.

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)

35. Shrub/tree root patterns, Arizona
Plant spacing determined by moisture availability and rooting niche 40

36. Water conservation: expandable storage organs and palisade tissue in succulents

37. Tissue protection: thorns and spines

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

39. Reducing water loss: microphylly, deciduousness, photosynthetic bark and shoots
Pachycormus
discolor

40. Reducing water loss: microphylly in ocotillo (Fouquieria splendens)

41. Evidence of climate change: lake levels in the Great Basin

42. Evidence of climate change from pack-rat middens

43. Pink = desert; yellow=savanna grassland; brown=dry forest
NB Lake Mega-Chad (bigger than Caspian Sea at present, and at least 40 m deep)

44. Evidence for late Holocene climate change in the Sahara
Lake sediments and pollen
Rock drawings, Tassili

45. Desertification in progress? Rainfall in the Sahel zone of W Africa
Cause: ?
Effect: desertification?
Protracted drought

46. Desertification on savanna margins: Rapp’s albedo hypothesis
Settlement Nomadism + -
Grazing
Vegetation
Albedo
Convectional
rainfall
Grazing
Vegetation
Albedo
Convectional
rainfall - + - + + - - +

47. The evidence for Rapp’s model

48. The elements of anthropogenic desertification