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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!!!

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


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  • 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. 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. 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. Dryness ratio semiarid desert (D≥2.3) (D≥10) 8 7 ARISE TRAINING & RESEARCH CENTER
  • 5. Unpredictability: areas with >30% variability in precipitation ARISE TRAINING & RESEARCH CENTER
  • 6. ARISE TRAINING & RESEARCH CENTER
  • 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. “Coastal fog deserts” E.g. Atacama desert Namib desert ARISE TRAINING & RESEARCH CENTER
  • 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. “cold desert” sagebrush steppe (Idaho-eastern Oregon) ARISE TRAINING & RESEARCH CENTER
  • 11. “Hot desert”: saguaro - ocotillo community, Arizona ARISE TRAINING & RESEARCH CENTER
  • 12. Sonoran cacti: saguaro (Carnegiea gigantea) range controlled by exposure to freezing temperatures ARISE TRAINING & RESEARCH CENTER
  • 13. Sonoran plant ranges ARISE TRAINING & RESEARCH CENTER
  • 14. Thermal microclimates ARISE TRAINING & RESEARCH CENTER
  • 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. Uplands of southern Baja “a wilderness of thorn and rock” ARISE TRAINING & RESEARCH CENTER
  • 17. Desert upland habitat: skeletal soils ARISE TRAINING & RESEARCH CENTER
  • 18. Uplands and bajada complex, Death Valley, CA ARISE TRAINING & RESEARCH CENTER
  • 19. Mesquite growing on gravelly fan deposits, Arizona ARISE TRAINING & RESEARCH CENTER
  • 20. Playa deposits ARISE TRAINING & RESEARCH CENTER
  • 21. Mobile substrates: Saharan dunes ARISE TRAINING & RESEARCH CENTER
  • 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. 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. 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. 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. 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. Sonoran desert in bloom ARISE TRAINING & RESEARCH CENTER
  • 28. Refuging tactics: chuckwalla lizard How much cooler is it at a depth of 20 cm? ARISE TRAINING & RESEARCH CENTER
  • 29. Refuging behaviour: camels in shade ARISE TRAINING & RESEARCH CENTER
  • 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. 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. Reducing heat load: a joshua tree (Yucca brevifolia) in the Mojave desert ARISE TRAINING & RESEARCH CENTER
  • 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. 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. Shrub/tree root patterns, Arizona Plant spacing determined by moisture availability and rooting niche 40 ARISE TRAINING & RESEARCH CENTER
  • 36. Water conservation: expandable storage organs and palisade tissue in succulents ARISE TRAINING & RESEARCH CENTER
  • 37. Tissue protection: thorns and spines ARISE TRAINING & RESEARCH CENTER
  • 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. Reducing water loss: microphylly, deciduousness, photosynthetic bark and shoots Pachycormus discolor ARISE TRAINING & RESEARCH CENTER
  • 40. Reducing water loss: microphylly in ocotillo (Fouquieria splendens) ARISE TRAINING & RESEARCH CENTER
  • 41. Evidence of climate change: lake levels in the Great Basin ARISE TRAINING & RESEARCH CENTER
  • 42. Evidence of climate change from pack-rat middens ARISE TRAINING & RESEARCH CENTER
  • 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. Evidence for late Holocene climate change in the Sahara Lake sediments and pollen Rock drawings, TassiliARISE TRAINING & RESEARCH CENTER
  • 45. Desertification in progress? Rainfall in the Sahel zone of W Africa Protracted drought Cause: ? Effect: desertification? ARISE TRAINING & RESEARCH CENTER
  • 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. The evidence for Rapp‟s model ARISE TRAINING & RESEARCH CENTER
  • 48. The elements of anthropogenic desertification ARISE TRAINING & RESEARCH CENTER

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