Slide 1

Deserts Climate Distribution and subtypes Landscape-substrate-soils Limiting factors Adaptations to aridity Adaptations to high temperatures Slide 2 Definitions Boundary defined by absolute precipitation (e.g. 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 Slide 4 Dryness ratio semiarid desert (D2.3) (D10) 8 7 Slide 5 Unpredictability: areas with >30% variability in precipitation Slide 6 Slide 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) Slide 8 Coastal fog deserts E.g. Atacama desert Namib desert Slide 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 0C are hot deserts (e.g. Sonora, Sahara) Slide 10 cold desert sagebrush steppe (Idaho-eastern Oregon) Slide 11 Hot desert: saguaro - ocotillo community, Arizona Slide 12 Sonoran cacti: saguaro (Carnegiea gigantea) range controlled by exposure to freezing temperatures Slide 13 Sonoran plant ranges Slide 14 Thermal microclimates Slide 15 Desert landscape-substrate elements upland / hillslope pediment/bajada/reg playa - sabkha dunefield/ erg erosion deposition deflation solutes skeletal soils gravelly soils sandy silts sands Slide 16 Uplands of southern Baja a wilderness of thorn and rock Slide 17 Desert upland habitat: skeletal soils Slide 18 Uplands and bajada complex, Death Valley, CA Slide 19 Mesquite growing on gravelly fan deposits, Arizona Slide 20 Playa deposits Slide 21 Mobile substrates: Saharan dunes Slide 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 Slide 23 Limitation of primary productivity in arid ecosystems (data from Tunisia) 0.01 0.1 1.0 10 500 400 300 200 100 0 Mean ann. rainfall (mm) Net primary production (kg ha -1 yr -1 ) skeletal soils of uplands gravelly soils of pediments and alluvial fans Slide 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 Slide 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; CO 2 absorbed, assimilated during day when stomates closed to minimize water loss] Slide 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]. Slide 27 Sonoran desert in bloom Slide 28 Refuging tactics: chuckwalla lizard How much cooler is it at a depth of 20 cm? Slide 29 Refuging behaviour: camels in shade Slide 30 Refuging by desert aquatic species e.g. pupfish in Death Valley flow salinity water temp. winter summer winter desert pupfish Salt Creek, Death Valley live lay eggs hatch /die Slide 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) Slide 32 Reducing heat load: a joshua tree ( Yucca brevifolia ) in the Mojave desert Slide 33 Maximise food reserves in times of plenty camels hump; berber sheep (fat reserves in tail vary from 2-10 kg); pack rats/gerbils hoard seeds; succulents store water. Slide 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) Slide 35 Shrub/tree root patterns, Arizona Plant spacing determined by moisture availability and rooting niche 40 Slide 36 Water conservation: expandable storage organs and palisade tissue in succulents Slide 37 Tissue protection: thorns and spines Slide 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 (camels body temperature can vary by 6C when animal is dehydrated). 3. Tolerate dessication: camel can withstand water loss = 25% of body weight Slide 39 Reducing water loss: microphylly, deciduousness, photosynthetic bark and shoots Pachycormus discolor Slide 40 Reducing water loss: microphylly in ocotillo (Fouquieria splendens) Slide 41 Evidence of climate change: lake levels in the Great Basin Slide 42 Evidence of climate change from pack-rat middens Slide 43 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) Slide 44 Evidence for late Holocene climate change in the Sahara Lake sediments and pollen Rock drawings, Tassili Slide 45 Desertification in progress? Rainfall in the Sahel zone of W Africa Protracted drought Cause: ? Effect: desertification? Slide 46 Desertification on savanna margins: Rapps albedo hypothesis Settlement Nomadism Grazing Vegetation Albedo Convectional rainfall +- Grazing Vegetation Albedo Convectional rainfall + + - - + -+ - Slide 47 The evidence for Rapps model Slide 48 The elements of anthropogenic desertification