Transcript File
Option 7: Soils and Biome Biome: Desert Biome Notice of Use Restrictions Certain materials in this presentation are included under the Fair Use exemption of the U.S. Copyright Law and/or under the Fair Dealing exemption of the Ireland Copyright and Related Rights Act, 2000 Materials are included in accordance with the [U.S.] multimedia fair use guidelines; and Materials are restricted from further use. © EDMAN YOST, J. (1999), Copyright Chaos - An Educator's Guide to Copyright Law and “Fair Use”, Intel Teach to the Future CD Biomes of the World Biomes • A biome is a major world region that is characterised by similarities in climate, soil, vegetation and animal life • All these aspects of a biome are interrelated What You Need to Know!! • The factors that influence the development of biomes • The characteristics of the desert biome • How humans alter biomes Influencing Factors • Climate: perhaps most important • Determines vegetation and animal life that can live in a region • Main influences are temperature and precipitation Influencing Factors • Soils: influence type of vegetation • Soils are in turn influenced by climate as well as parent rock • Vegetation: depends on soil and climate, influences plant and animal species in region • Animal Life: must adapt to conditions of climate and vegetation • Human impacts have modified biomes on a huge scale Desert Biome Desert Biome • Desert: ‘an arid region that is characterised by little or no rainfall, in which vegetation is scarce or absent, unless it is specially adapted’ Desert Biome • Availability of water: determines plant and animal life in the desert • Annual precip depends on geographical location 1cm-35cm • Precip in short intervals, unpredictable year by year • Strong tendency for evaporation Desert Biome • Temperature: deserts can be hot like the Sahara where summer temps hit 50 degrees • Or they can be cold like the Gobi Desert where winter temps hit –40 degrees • Location: Coastal eg. Sonora Desert • Interior of continents eg. Great Basin Desert and Australian Desert Desert Biome • Make-up: contrary to belief, deserts are not entirely covered in sand • Mainly regions of boulders, gravel or bare rock • Only about 10% of deserts are sandy • Productivity: least productive of the 9 biomes • Potentially fertile soils in some, irrigation Desert Biome • Adaptation: all deserts have some plant and animal life that have adapted • Population: some deserts are populated either seasonally by nomadic tribes or permanently • 3 categories: extremely arid, arid & semiarid Desert Biome • Extremely arid: at least 12 consecutive months with no precip • Arid: annual precip less than 250mm • Semi-arid: annual precip between 250-500mm Distribution of Deserts Distribution of Deserts • Majority on western margins or in the interiors of continental masses • Low-latitude deserts: 15-30degrees N and S of the equator. Sahara, Arabian and Kalahari deserts, California’s Sonora desert • Mid-latitude deserts: 30-40degrees N and S of the equator, continental interiors. The Great Basin and Mojave deserts of USA Distribution of Deserts • The dry regions of the world cover nearly 30% of the earth’s surface • No other biome covers so large an area Origin of Deserts • Most located in high pressure belts 1530degrees N and S equator • Air masses are heated by equator • Angle of the sun is almost perpendicular • Air rises, convectional rain occurs • Equatorial air masses travel away from equator, begin to descend at about 30degrees N and S Origin of Deserts • Compression warms the descending air to form belts of high pressure • On reaching the earth’s surface, the air is returned towards the equator as part of the trade winds system • Moisture is sucked from the surface because the air masses are increasing in temperature and able to hold more water Origin of Deserts • These conditions and an absence of surface water rule out cloud formation and precipitation • Clear skies, maximum sunshine and drought result • Rain-shadow effect often involved also • Coastal mountain ranges create a barrier to the movement of moist sea air, forcing it to rise Origin of Deserts • As it rises it expands and cools • Water it contains falls as rain on the windward side of the mountains • When air reaches inland or leeward side of the mountains, it has lost all moisture and can not provide rain • Rain-shadow effect, land beneath rainshadow becomes rain-shadow desert Rain Shadow Effect Origin of Deserts • Coastal deserts, western margins of continents • Cold ocean currents play role • Moisture laden winds blow in from ocean, must pass over cold currents • Currents absorb heat from air, reduce ability to hold water • Air masses lose water over ocean Origin of Deserts • Precipitation and fog • Contact with warmer land made, air is heated again and is able to retain what moisture it has left • Sucks up more moisture from land • Coastal region becomes desert • Atacama desert, only true rainless desert (Humboldt Current) Origin of Deserts • Continental interiors • As air masses move across a continent , lose moisture by precip, even in absence of mountains • When these masses reach interior, have become quite dry, land beneath receives little rain • Continentality (central Asia, Gobi desert) North American Deserts • • • • Chihuahua Desert Sonora Desert Mojave Desert Great Basin • Sandwiched between 2 mountain ranges: Rockies to east and Sierra Nevada to west • Cover an area 12 times size of Ireland North American Deserts North American Deserts • Chihuahua, Sonora and Mojave are hot deserts • Great Basin considered a cold desert, due to its more northerly latitude as well as its higher altitude North American Deserts • Climate: rainfall often low and unpredictable, comes in short bursts between long rainless periods • When rain falls there is a rapid run-off • Evaporation rates regularly exceed rainfall rates • Sometimes rain starts falling and evaporates before hitting the ground • Low level of infiltration and high rate of evaporation minimise its effectiveness for vegetation growth North American Deserts • Climate: rainfall mainly seasonal • 3 hot desert regions receive either winter or summer precip • Their annual rainfall varies between 150mm and 300mm • Cold, Great Basin, receives 300mm of precip annually, occurs throughout the year, winter precip comes as snow North American Deserts • Temperatures: characterised by their extremes • Annual range between 20 and 30degrees • Diurnal (day/night) range is often greater than 30degrees, leading to the description that ‘night is the winter of the desert’ Geofacts • The greater part of the Sonora Desert is in Arizona, Arizona means ‘arid zone’ • The all-time maximum temperature in the US was 57degrees, recorded in Death Valley in the Mojave Desert North American Deserts • Temperatures: daytime temps high often reaching 45degrees • In low latitudes the sun is high in the sky, shining almost vertically on the ground • Rays are concentrated over small areas and give great heat • Lack of cloud cover and absence of vegetation means very little heat is lost and most is absorbed by the bare rock or stony surface North American Deserts • In contrast, night-time temps are much lower, often falling below freezing in winter • Heat built up during the day lost after nightfall as there is no cloud cover or vegetation to help retain heat • Also, dry air cools very quickly Desert Soils Desert Soils • Aridisols: Latin word for ‘dry’ are the dominant soils of desert and semidesert regions • Range from sandy and fine-textured to gravelly and coarse-textured • Soils were eroded and washed down from mountain areas during occasional torrential downpours over 1000s years Desert Soils • Coarse textured soils are found on the lower slopes of mountains, well-drained due to coarse texture • Fine soils have been washed into the basin areas, giving them deep soil cover • If winds are strong, these fine particles are blown elsewhere leaving coarser particles of soil behind Desert Soils • Aridisols tend to be poorly developed • Low level of precip means that there is little chemical weathering • Although they have a high mineral matter the surface has little organic matter • This limits the soil building properties of micro-organisms that might convert organic matter into humus • Aridisols have poorly developed horizons and may be without a noticeable A horizon • Light grey colour Desert Soils • Abundant nutrients, only need water to be productive • Rapid growth of plants after a downpour • But, with so little organic matter, they are unable to retain all the moisture that falls, and there is intense evaporation Desert Soil • In arid conditions, moisture moves up through the soil by capillary action • This brings dissolved minerals to the surface • Salinisation • Calcification