Biomes of the Earth-grasslands

Michael Allaby

Illustrations byRichard Garratt

BIOMES OF THE EARTH

GRASSLANDS

Grasslands

Copyright 2006 by Michael Allaby

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ISBN-13: 978-0-8160-5323-0ISBN-10: 0-8160-5323-5

Library of Congress Cataloging-in-Publication DataAllaby, Michael.

Grasslands / author, Michael Allaby ; illustrations by Richard Garratt.p. cm.(Biomes of the Earth)

Includes bibliographical references and index.ISBN 0-8160-5323-51. Grassland ecologyJuvenile literature. 2. GrasslandsJuvenile literature. I. Garratt,

Richard, ill. II. Title. III. Series.QH541.5.P7A38 2006577.4dc22 2005005615

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From Richard Garratt: To Chantal, who has lightened my darkness.

Preface ixAcknowledgments xiiiIntroduction: What are grasslands? xv

CHAPTER 1

GEOGRAPHY OF GRASSLANDS 1Where grasslands occur 1Temperate grasslands and tropical grasslands 3Prairie 4Steppe 7Pampa 10Veld 12Tropical grasslands of South America 14Savanna 17Australian grasslands 20Upland grasslands 22

CHAPTER 2

GEOLOGY OF GRASSLANDS 26Movement of continents 26

Alfred Lothar Wegener and continental drift 28How mountains rise and wear away 31Grassland soils 37

How soils are classified 39Laterite 40

Water and grasslands 41

CHAPTER 3

GRASSLAND CLIMATES 45Why there are seasons 45Continental and maritime climates 48

How climates are classified 49

CONTENTS

Dry seasons and rainy seasons 51The Dust Bowl 55Monsoons 57

Adiabatic cooling and warming 59El Nio 61Convection and storms 64

Lapse rates and stability 66Tornadoes 67

CHAPTER 4

HISTORY OF GRASSLANDS 71Evolution of grasslands 71Grasslands and past climate changes 73How forest can change into grassland 76The transformation of New Zealand 78

CHAPTER 5

LIFE ON THE GRASSLANDS 81What is grass? 81How grasses work 84

Photosynthesis 85Prairie grasses 89Pampas grasses 92Steppe grasses 94Savanna grasses 95Grassland trees and shrubs 99Grassland herbs 100Grassland insects 103Mongooses, prairie dogs, marmots, ground

squirrels, and pocket gophers 110Snakes and lizards 114Grazing animals 119Hunters of the grasslands 124

Hunter and prey: The evolutionary arms race 130Grassland birds 130Coping with drought 136Coping with heat and cold 139

Hibernation 141

CHAPTER 6

GRASSLAND ECOLOGY 143How the plant eaters help the grass 143Food chains and food webs 144Ecological pyramids 147Do predators control their prey? 151How herding provides safety in numbers 153Mammal migrations 155

CHAPTER 7

PEOPLES OF THE GRASSLANDS 158Peoples of the prairie 158Homesteaders and the way the prairie was

transformed 160Indians and gauchos: The peoples of the pampa 165Farmers of the pampa 167Peoples of the African savanna 169Traditional life on the steppe of Central Asia 171Genghis Khan, the Golden Horde, and

Mogul emperors 173Aboriginal peoples of the Australian grasslands 177

CHAPTER 8

USES FOR GRASSLAND 179Cereal farming 179

The origin of cereals 180Cattle ranching 182

The origin of cattle 184Sheep farms of Australia and New Zealand 186

The origin of sheep 189Upland sheep farming 190Forestry 191Biofuel production 194

Biofuels and the greenhouse effect 195

CHAPTER 9

BIODIVERSITY AND GRASSLANDS 198What is biodiversity? 198

Why it matters 199Protecting grassland species 201

The Biodiversity Convention 202

CHAPTER 10

THREATS TO GRASSLAND 205Conversion to farmland 205Conversion to forest 207Overgrazing and soil erosion 209Climate change 211

The greenhouse effect 212Expansion of towns and roads 214

CHAPTER 11

MANAGING THE GRASSLANDS 218Lessons from the Dust Bowl 218Ranching on equatorial grasslands 220Farming tropical grasslands 222

Dry farming 224Grassland restoration and conservation 225

CONCLUSION 228What future for the grasslands? 228

SI units and conversions 231Soil classification: Orders of the soil taxonomy 235Glossary 239Bibliography and further reading 253Index 257

IX

Earth is a remarkable planet. There is nowhere else in oursolar system where life can survive in such a great diversity offorms. As far as we can currently tell, our planet is unique.Isolated in the barren emptiness of space, here on Earth weare surrounded by a remarkable range of living things, fromthe bacteria that inhabit the soil to the great whales thatmigrate through the oceans, from the giant redwood trees ofthe Pacific forests to the mosses that grow on urban side-walks. In a desolate universe, Earth teems with life in a bewil-dering variety of forms.

One of the most exciting things about the Earth is the richpattern of plant and animal communities that exists over itssurface. The hot, wet conditions of the equatorial regionssupport dense rain forests with tall canopies occupied by awealth of animals, some of which may never touch theground. The cold, bleak conditions of the polar regions, onthe other hand, sustain a much lower variety of species ofplants and animals, but those that do survive under suchharsh conditions have remarkable adaptations to their test-ing environment. Between these two extremes lie manyother types of complex communities, each well suited to theparticular conditions of climate prevailing in its region.Scientists call these communities biomes.

The different biomes of the world have much in commonwith one another. Each has a plant component, which isresponsible for trapping the energy of the Sun and making itavailable to the other members of the community. Each hasgrazing animals, both large and small, that take advantage ofthe store of energy found within the bodies of plants. Thencome the predators, ranging from tiny spiders that feed uponeven smaller insects to tigers, eagles, and polar bears that sur-vive by preying upon large animals. All of these living things

PREFACE

form a complicated network of feeding interactions, and, atthe base of the system, microbes in the soil are ready to con-sume the energy-rich plant litter or dead animal flesh thatremains. The biome, then, is an integrated unit within whicheach species plays its particular role.

This set of books aims to outline the main features of eachof the Earths major biomes. The biomes covered include thetundra habitats of polar regions and high mountains, thetaiga (boreal forest) and temperate forests of somewhatwarmer lands, the grasslands of the prairies and the tropicalsavanna, the deserts of the worlds most arid locations, andthe tropical forests of the equatorial regions. The wetlands ofthe world, together with river and lake habitats, do not lieneatly in climatic zones over the surface of the Earth but arescattered over the land. And the oceans are an exception toevery rule. Massive in their extent, they form an intercon-necting body of water extending down into unexploreddepths, gently moved by global currents.

Humans have had an immense impact on the environ-ment of the Earth over the past 10,000 years since the last IceAge. There is no biome that remains unaffected by the pres-ence of the human species. Indeed, we have created our ownbiome in the form of agricultural and urban lands, wherepeople dwell in greatest densities. The farms and cities of theEarth have their own distinctive climates and natural history,so they can be regarded as a kind of artificial biome that peo-ple have created, and they are considered as a separate biomein this set.

Each biome is the subject of a separate volume. Each richlyillustrated book describes the global distribution, the climate,the rocks and soils, the plants and animals, the history, andthe environmental problems found within each biome.Together, the set provides students with a sound basis forunderstanding the wealth of the Earths biodiversity, the fac-tors that influence it, and the future dangers that face theplanet and our species.

Is there any practical value in studying the biomes of theEarth? Perhaps the most compelling reason to understandthe way in which biomes function is to enable us to conservetheir rich biological resources. The worlds productivity is the

X GRASSLANDS

basis of the human food supply. The worlds biodiversityholds a wealth of unknown treasures, sources of drugs andmedicines that will help to improve the quality of life. Aboveall, the worlds biomes are a constant source of wonder,excitement, recreation, and inspiration that feed not onlyour bodies but also our minds and spirits. These books aim toprovide the information about biomes that readers need inorder to understand their function, draw upon theirresources, and, most of all, enjoy their diversity.

PREFACE XI

XIII

Richard Garratt drew all of the diagrams and maps thatappear in this book. Richard and I have been working togeth-er for many years in a collaboration that succeeds becauseRichard has a genius for translating the weird electronicsquiggles I send him into clear, simple artwork of the highestquality. As always, I am grateful to him for all his hard work.I also wish to thank Elizabeth Oakes for her fine work as aphoto researcher.

I must thank Frank K. Darmstadt, Executive Editor, atChelsea House. Frank shaped this series of books and guidedthem through all the stages of their development. Hisencouragement, patience, and good humor have beenimmensely valuable.

I am especially grateful to Dorothy Cummings, project edi-tor. Her close attention to detail sharpened explanations thathad been vague, corrected my mistakes and inconsistencies,and identified places where I repeated myself. And occasion-ally Dorothy was able to perform the most important serviceof all: She intervened in time to stop me making a fool ofmyself. No author could ask for more. This is a much betterbook than it would have been without her hard work anddedication.

Michael AllabyTighnabruaich

ArgyllScotland

www.michaelallaby.com

ACKNOWLEDGMENTS

XV

What are grasslands?A wave moves over the landscape as the rustling grass bowsbefore the wind and then rises again. Another wave followsthe first, and then another, and the eye follows the ripplingmotion across the plain until each wave disappears in thehaze that obscures the distant horizon. Overhead the cloud-less sky is the palest blue, and immensely large. On a moon-less night the sky blazes with the light from countless mil-lions of stars.

Grass is the predominant vegetation, but it is mixed with abewildering variety of other herbs. Scattered shrubs andsmall belts of trees lining the river courses relieve what mightotherwise be a monotonous scene. Birds fly overhead, forag-ing for insects or seeds, and high above them the hawks circleslowly, alert to the tiniest flicker of movement that betraysthe presence of a small animal. Mice and ground squirrelslive on and below the ground, feeding on the vegetation andever watchful for snakes as well as birds of prey. Herds of big-ger animals graze the pasture and browse the leaves of shrubsand the lower branches of the trees.

This is one of the worlds great grasslands, which liebetween the temperate forests and the semiarid lands border-ing the desert, where meager seasonal rains allow a sparse,coarse pasture to survive. Grasslands once covered more than1 million square miles (2.59 million km2) in North America,extending from Illinois to Colorado and from Alberta toTexas.

It is a place of changing colors. In winter the northerngrasslands are white, hidden beneath a covering of snow.Southern grasslands see little or no snow, and there the land remains brown through the winter. In spring both aregreen, as the returning warmth and moisture from the rain

INTRODUCTION

XVI GRASSLANDS

or melting snow stimulate new plant growth. As the hot, drysummer progresses, the plants wilt and then turn yellow andbrown.

Grasslands occur in the deep interior of continents, farfrom the ocean, where rainfall is generally low. Temperategrasslands receive an average 1240 inches (3051,016 mm)of rain a year and tropical grasslands 2560 inches(6351,524 mm). Rainfall is not distributed evenly throughthe year, however. In temperate regions there is a dry seasonthat begins in the late summer, fall, or winter and continuesuntil spring. In the Tropics the dry season begins in earlysummer and most of the rain falls in the winter.

Although grasses are the most abundant plants in all grass-land, there are many types of grasses, and grasslands are notall the same. Tropical grasslands lie between the edge of thesubtropical deserts and the tropical forests. They are oftencalled savanna (or savannah) grasslands, a name that seems tohave originated in the Caribbean region and to have enteredEnglish from zavana, which was the Spanish form of the orig-inal name. Savanna grasslands extend across Africa as twobelts, one on each side of the equator. Grasslands of this typealso occur in South America, southern Asia, and Australia.They are usually dominated by grasses five to six feet (1.51.8m) tall, interspersed with thorn trees that are mostly lessthan 33 feet (10 m) tall and have distinctive flat tops.

Savanna grasslands have regional names. They are calledllanos in Venezuela and Colombia, and campo cerrado andcampo sujo are two different varieties of savanna found inBrazil. The Australian grasslands are identified not by theirgrasses, but by the species of acacia trees that grow amongthe grasses: brigalow scrub and mulga scrub.

Temperate grasslands occur between the subtropical de-serts and the temperate forests or the coniferous forestknown as taiga. Where rainfall is relatively high, the grassesare about five feet (1.5 m) tall. These are the typical grassesof North American prairie and they occur on the eastern sideof the continent. Shorter grassesless than 18 inches (45cm) tallgrow on the drier western side, in the plains,although nowadays the two types of grassland are usuallycalled tallgrass and short-grass prairie. Palouse prairie, also

called bunchgrass prairie, grows in Washington State andBritish Columbia.

In South America the temperate grassland is called pampa.The Eurasian grasslands stretch from Hungary in the west,across Russia, and as far as Mongolia and China. They arecalled puszta in Hungary and steppe in Russia. There are twomain types of steppe. Meadow steppe occurs in the north,where the climate is relatively moist, allowing many flower-ing herbs to flourish. Dry steppe occurs in the south and sup-ports fewer herbs. Southern African grasslands are called veldor grassveld.

Climate determines the growing season for plants any-where in the world, and grasslands are no exception. In tem-perate regions plants are able to grow when the temperatureis above freezing. The growing season in the prairie lasts fromabout 150 days in the palouse prairie to 270 days in Texas. Inthe Tropics, where the temperature is warm enough for plantgrowth throughout the year, rainfall is the crucial factor.

A type of vegetation that covers a very large area in conti-nents around the world is called a biome, and geographersdivide the entire world into a series of biomes. The species ofplants vary from one part of a biome to another; what mat-ters is that the predominant plants are all similar. They mightbe temperate trees, for example, or tropical trees, or grasses,but regardless of the species, North American grassland ismuch more similar to South American, Eurasian, African, orAustralian grassland than any of these is to forest.

All biomes are important to us, but the grassland biome isespecially important. Most of our staple foods, includingwheat, rice, barley, rye, corn (maize), and sugarcane, aregrasses, and wheat, barley, and rye were first cultivated onthe grasslands of what are now Turkey and Iraq. Our cerealand livestock farms are like artificial grassland and in manyplaces the lands they occupy were natural grasslands untilfarmers cleared away the wild grasses to grow cultivatedgrasses.

Some areas of natural grassland are not suitable for farm-ing, usually because they experience severe and prolongeddrought from time to time, but many make good farmland.This means they are valuable, and over the centuries most of

INTRODUCTION XVII

the natural grassland has been converted into farmland.When the grassland is plowed, the plants that grow amongthe grasses are destroyed along with the grasses themselves,and many of the animals that find food and shelter amongthe plants die or move away. If we wish the members of thenatural grassland community to survive, we must preserveareas of the grassland itself.

XVIII GRASSLANDS

Where grasslands occurGrasslands once covered more than 40 percent of the Earthsland surface. Grasses are geographically more widespreadthan any other group of plants, and grasslands compose oneof the most extensive types of vegetation on the planet. Asthe map shows, they are found on every continent exceptAntarctica.

Although the map shows the areas where grassland is thenatural vegetation, in fact large areas of grassland have beenconverted to farmland. Where the rainfall is sufficient to sup-port farming, the abundant sunshine, level plains, and fertilesoils are ideal for growing wheat and corn. These crops arealso grasses, so the conversion is from one type of grasslandto another. Agricultural grassland may also consist of pasturegrasses that have been sown to feed livestock.

Grasses thrive in climates that are too dry for most trees.They tolerate grazing by animals (see How grasses work onpages 8590), but they cannot survive in deep shade.Consequently natural grasslands occupy those parts of theworld that are neither desert, which is too dry, nor forest,which is too shady.

The map divides grasslands into two typestemperategrassland and tropical savanna grassland. Temperate grass-land occurs in regions with a temperate climate and is mostwidespread in North America, eastern Europe, Turkey andnorthern Iran, central Asia, and part of southern China.There is much less land in the Southern Hemisphere to thesouth of the tropic of Capricorn, and that is why southerntemperate grasslands are confined to part of Argentina and asmall area of Peru and South Africa.

Savanna grassland covers a large part of Africa, extendingfrom the southern edge of the Sahara to the southeastern tip

GEOGRAPHYOF GRASSLANDS

CHAPTER 1

1

of the continent, broken only by areas of tropical forest. Thereare also large areas of savanna in South America and Australia.

The climatic conditions that favor grasslands also occur inmountainous regions. Temperature decreases with altitude,and there is a level beyond which the mountain climate istoo cold for trees. At this heightand it varies according tothe latitude (see Upland grasslands on pages 2225)thegrowing season is so short that the needles of coniferous treeshave insufficient time to mature before they are killed byfrost. Trees cannot survive, but grasses and many floweringherbs are able to do so. Between the upper limit for treegrowth, called the tree line, and the region of bare rock andpermanent snow, there are mountain grasslands, more oftenknown as alpine meadows. Their composition varies fromplace to place, and in the Tropics there are also alpine savan-nas. Alpine grasslands occur in mountain ranges throughoutthe world.

2 GRASSLANDS

tropic of Cancer

tropic of Capricorn

equator

major temperate grassland

tropical savanna grassland

Distribution of temperateand tropical grasslands.

Grasslands tend tooccupy the interior of

continents, where the rainfall is sparse

because of the distancefrom the ocean.

GEOGRAPHY OF GRASSLANDS 3

Grasses are quick to invade land that has been cleared ofother types of vegetation. Ordinarily these grasses will disap-pear if taller plants, such as shrubs and trees, become estab-lished and shade the ground. There are places, however,where this has not been allowed to occur, resulting in succes-sional grasslands that are maintained by burning or grazing,both of which destroy shrub and tree seedlings. Successionalsavanna grassland has replaced tropical forest in many areas.Farmers clear an area of forest, farm it for a few years, andthen abandon it to allow the soil to recover fertility. If theyreturn to the site before the forest has had time to reclaim it,over a number of these cycles grasses may replace the trees.Such farming has not only extended the boundary of thesavanna grassland into what was formerly forest; it has alsoproduced areas of grassland inside the forest.

Temperate grasslands and tropical grasslandsTemperate and tropical grasslands are distinguished by theirclimate. They occur inland and are confined to particular lat-itudes, because climate characteristics largely depend on dis-tance from the equator and from the ocean (see the sidebarHow climates are classified on page 49).

Different climates suit different types of plant. Althoughgrass is the most obvious and abundant vegetation in anygrassland, there are many species of grass and those found inone type of grassland could not survive in another. All grass-es share certain characteristics (see What is grass? on pages8284) that make them resemble each other more closelythan they resemble any other type of plant, such as a tree.But the great variety among grasses gives each kind of grass-land a distinctive appearance, and there is a great contrastbetween temperate and tropical grasslands.

Temperate grasslands consist of grasses and herbs thatextend across vast plains with occasional low, rolling hills.Trees grow only beside riversand rivers are often manymiles apart. Although there are also areas of tropical grass-land that contain only grasses and herbs, woody plantsshrubs and treesgrow in most savanna grasslands. Thenumber of woody plants varies from place to place, but

because of them much of the savanna is open woodland orparkland, in which scattered shrubs and trees are surroundedby grass. Depending on the number of woody plants, tropicalgrasslands are divided into three categories: savanna wood-land, savanna parkland, and savanna grassland. In savannawoodland the tree canopy covers more than 50 percent ofthe ground; in savanna parkland it covers between 20 per-cent and 50 percent; and in savanna grassland it covers lessthan 20 percent.

The weather is always warm everywhere in the savanna.The average temperature is higher than 64F (18C) through-out the year, although occasionally it can fall as low as 40F(4C). It rises to 80100F (2738C) in summer. The threeto five months of winter are not cold; they are dry (see Dryseasons and rainy seasons on pages 5155). During the rainyseason the amount of rainfall ranges from about two inches(50 mm) near the desert edge to 60 inches (1,525 mm) closeto the edge of the tropical rain forest. Where rainfall is heavy,it often causes flooding. Savanna woodland occurs in thewetter areas and savanna grassland prevails where the rainfallis low.

Temperate grassland also grows in a fairly dry climatewhere prolonged drought occurs from time to time.Droughts occasionally last for 10 years (see The Dust Bowlon pages 5557). Annual rainfall on the North Americangrasslands averages about 40 inches (1,016 mm) in the east,decreasing to about 10 inches (250 mm) in the west, and itoccurs mainly in winter. As well as being dry, summers arehot, with temperatures rising to 100F (38C). Winters arecold, with daytime temperatures falling to about 10F (12C)or sometimes much lower. The lowest temperature everrecorded in Winnipeg, Manitoba, was 54F (48C), andSaskatoon, Saskatchewan, has recorded 55F (48C). Highsummer temperatures and the incessant wind mean that therate of evaporation is high, making the soil very dry.

PrairieThe first Europeans to gaze upon the grasslands of NorthAmerica were French. They approached from the east and

4 GRASSLANDS


saw before them an ocean of waving grass extending acrossthe level plain as far as the horizon. As they moved throughit and the horizon receded, the grassland continued tostretch for as far as they could see. It seemed endless. TheFrench called it la prairie, the meadow.

Nowadays people often call the whole of the NorthAmerican grassland prairie, but as the map shows, there areseveral distinct types of grassland. Originally only the tall-grass grassland was called prairie. The short-grass grasslandwas known simply as the plains, and the area of overlap

Gulf of Mexico

tallgrass prairie

mixed-grass prairie

short-grass prairie

bunchgrass prairie

Hudson Bay

PACIFIC OCEAN

Prairie. There are several types of prairie.Tallgrass prairie occurs in the east, where theclimate is moister than it is farther west, whereshort-grass prairie isfound. Mixed-grassprairie forms a wideboundary between thetwo and contains bothtall and short grasses.Bunchgrass and desertgrassland are found tothe west and south ofthe short-grass prairie.

between the two was called mixed prairie. Bunchgrassprairie, or palouse prairie, occurs to the west of the RockyMountains from Colorado to Oregon and northward toBritish Columbia, and in part of California. Tallgrass andshort-grass areas are defined by the height of the grasseswhen they are in flower. All of the plants die down in winter,and although most of them are perennialliving for manyyearsgrowth above ground starts afresh each year.

Big bluestem (Andropogon gerardii) is the most typical grassof the tallgrass prairie. It flowers in late summer, when itreaches an average height of five feet (1.5 m). In some placesbig bluestem can reach a height of 10 feet (3 m). Other tall-grass species include sand bluestem (A. halli), Indian grass(Sorghastrum nutans), slough grass (Spartina pectinata), andswitchgrass (Panicum virgatum). All of these grow 3.310 feet(13 m) tall. Growing among the grasses there is a wide rangeof herbs, called forbs.

Tallgrass prairie grows in the east, where the annual rain-fall averages about 34 inches (864 mm), with rather morefalling in summer than in winter. Formerly, in spring, afterthe thin covering of snow had melted but before the temper-ature had risen high enough to evaporate water near the sur-face, the native prairie was very wet in some places.Nowadays the ground is much drier. The land has beendrained and plowed, and today it grows wheat and corn.Most of the tallgrass prairie has disappeared.

Short-grass prairiethe plains grasslandgrows to thewest of longitude 100W, where the climate is drier. GreatFalls, Montana, for example, has an average annual rainfallof 15.3 inches (388 mm). Farmers can grow row crops onlywhere they are able to irrigate their fields, and the area ismainly given over to cattle ranching. The low rainfall alsomeans that native plants cannot grow very tall. Most of thegrassland plants are less than eight inches (20 cm) high.These include buffalo grass (Buchloe dactyloides), galleta grass(Hilaria jamesii), needle-and-thread grass (Stipa comata), vari-ous grama grasses (Bouteloua species), and penn (orPennsylvania) sedge (Carex pensylvanica). Grasses survive bet-ter than most other plants in this climate, so there are fewerforbs in short-grass prairie than there are in tallgrass prairie.

6 GRASSLANDS


Between the tallgrass and short-grass grasslands, the mixedprairie consists of grasses of both types. Tall grasses grow inthe wetter areas and short grasses in the drier placesalthough most of the area is now farmed, growing corn andsoybeans. The natural grassland plants grow up to aboutthree feet (90 cm) tall. They include little bluestem grass(Schizachyrium scoparium), needle grasses (Stipa species), drop-seed grasses (Sporobolus species), June grass (Koeleria pyramida-ta), wheatgrass (Elytrigia species), and wild rye (Elymuscanadensis). Wild rye is not related to cultivated rye.

Palouse prairie occurs around the Palouse River, inWashington State, extending eastward and northward fromthere. It is dominated by bunchgrasses. Most prairie grasseshave roots that form a continuous mat below the surface andproduce new shoots at intervals from stems that grow hori-zontally. Bunch or tussock grasses produce new shoots fromthe base of the main stem. Consequently they grow in dense,but separate, clumps or bunches. Bluebunch wheatgrass(Elytrigia spicata) is a typical grass of the palouse prairie. Thereare many forbs, and sagebrush (Artemisia tridentata) is wide-spread.

The bunchgrass prairie in California has long vanished.Travelers who saw it in the middle of the 19th centurydescribed an area 400 miles (640 km) long that in spring wasa mass of yellow and purple flowers. Purple needlegrass (Stipapulchra) was the predominant grass.

SteppeFrom the Danube in the west to Mongolia and China, 3,500miles (9,000 km) to the east, and from the edge of the Saharaand the Arabian Desert in the south to the taiga in the north,the grasslands of Europe and Asia are the most extensive inthe world. They are called puszta in Hungary. Farther eastthey are known by their more familiar Russian namesteppe.The map shows their original extent, but large areas havebeen converted to farming, including almost all of the pusz-ta, and forests have been planted in the north.

The name steppe is sometimes used to describe temperategrasslands of all kinds, including the prairie, pampa, veld,

and Australian grasslands. But there are differences amongthese types, and strictly speaking steppe refers to the Eurasiangrasslands. These are open, grass-covered plains, which aretreeless except where the steppe merges into the northernforests. The climate is dry, with 1220 inches (300500 mm)of rain a year in most places, and drought is common. All thesteppe plants are able to survive drought. Taller plants growin the wetter areas, and shorter plants grow where theground is dry most of the time.

Many grasses thrive in this vast area, but feather grassesgrow throughout the steppe and are the most typical grassesof the drier southern steppe. As their name suggests, theyproduce long, feathery flowers, and toward the end of Maythese dominate the landscape, swaying gracefully in thewind. The feather grasses belong to the genus Stipa, and thisLatin name may be related to the word steppe. Oat grasses(Avena species), fescues (Festuca species), and sedges (Carexspecies) are also widespread. Viviparous bluegrass (Poa bul-bosa), goose onions (Gagea species), and tulips (Tulipa

8 GRASSLANDS

the steppes

temperate grassland

Steppe. The steppegrasslands extend

from eastern Europe all the way to western

China, a distance ofapproximately 3,500

miles (9,000 km).


species) grow in the south of the region, near the Black Seaand in Kazakhstan, in places where the climate is warm andrelatively moist. Wormwoods, also called sagebrushes (Arte-misia species), grow in the Central Asian steppe. The steppe isrich in plants. There are up to 73 plant species in everysquare yard of the surface (80/m2) in some parts of the north-ern steppe.

The steppe lie between latitudes 40N and 50N, andRussian scientists have divided them into four types of grass-land. The most northerly steppe borders the taiga. It is calledforest steppe. Trees grow scattered across the landscape,becoming more frequent in the north until the steppemerges into the coniferous forest.

Just south of the forest-steppe the meadow steppe has a rela-tively moist climate. Sedges (Carex species) and feather grass-es (Stipa species) grow there, and parts of the meadow-steppeare used to graze livestock.

Dry-steppe lies to the south of the meadow-steppe. Therethe ground is dry most of the time because the annual rain-fall is less than the amount of water that could evaporate in ayear. Shorter feather grasses grow there, such as Ukrainianfeather grass (Stipa ucrainica); S. capillata, known locally astyrsa; and S. pennata, as well as fescues (Festuca species).Continuing southward, the climate becomes progressivelydrier as the steppe merges into the subtropical desert.

Although the climate is moister in the north, all four typesof steppe have a generally similar climate. The winter is cold,with temperatures below freezing for at least four monthsand snow cover that is more than four inches (10 cm) deep.Irgiz, Kazakhstan, at latitude 48.37N, experiences typicalsteppe temperatures. The average temperature remains be-low freezing from November through March. Summers arewarm, with an average temperature of 73F (23C) from Junethrough August. Irgiz is rather dry, however, with an averageannual rainfall of only seven inches (180 mm). Rainfall isdistributed evenly through the year, but on average occursonly 11 days between the beginning of May and the end of August. These are the months with the longest hours of sunshine, and the little rain that does fall evaporatesquickly.

By August the grasses are brown and wilting everywhere,and the steppe remains brown and dry until the followingspring. Droughts are common in late summer and autumn.

PampaIn Quechua, the Native American language spoken overmuch of western South America, a terrace or open space iscalled a bamba. Spanish settlers changed the word to pampaand used la pampa to describe a large plain. English speakersoften use the plural form, calling the grassland pampas, butthe singular, pampa, is more correct. The pampa covers a totalarea of about 290,000 square miles (751,000 km2), mostly inArgentina but extending into Uruguay. It is the home of thegaucho, the South American cowboy, and today most of thepampa area is used to ranch cattle and sheep. Very few areasof the original pampa remain.

The pampa extends from the Colorado River in the south tothe forested depression called the Gran Chaco in the north,and from the Atlantic coast in the east to the foothills of theAndes in the west. Lying approximately between latitudes27S and 38S, the pampa has a warm climate. Temperaturesrange between about 8590F (2932C) in summer and5759F (1415C) in winter. Winter nights are cold, howev-er, with temperatures often falling below freezing.

Highlandssierrasin the northwest, including the largeSan Luis and Crdoba sierras, and some hills in the southbreak up what otherwise is fairly level plain that slopes gen-tly from a little above sea level to an elevation of 2,320 feet(708 m) in the Andean foothills of Mendoza Province. Themap shows the extent and location of the pampa.

There are two distinct types of pampa. On the western sidethe sterile pampa is a dry plain that in places gives way tosandy desert. There are rivers in this part of the pampa carry-ing water that is too salty to drink and areas where the soil issalty. The vegetation is sparse, with scattered thornbushesand small trees, including chanal or chaar (Geoffroea decorti-cans), a low, thorny shrub that is fed to livestock and pro-duces edible fruit pods. Rainfall is also low in the southernpampa. Victorica, in San Luis Province in the west, has an

10 GRASSLANDS


annual rainfall of about 22 inches (559 mm), and BahaBlanca in the south receives an average of 20 inches (500mm). Most of the rain falls in summer.

The eastern pampa receives an average 39 inches (1,000mm) of rain a year and its soils are fertile. Buenos Aires hasan average 37 inches (940 mm) of rain a year. This part ofthe pampa is the most productive agricultural land inArgentina.

Pampas

BRAZIL

CHILE

BOLIVIA

PARAGUAY

URUGUAY

ARGENTINA

PACIFIC OCEAN

ATLANTIC OCEAN

temperate grassland

Pampas. Pampagrasslands occur insouthern Argentina.

Its most famous native grass is pampas grass, which isgrown ornamentally in many parts of the world but hasbecome a very invasive weed in other regions, includingCalifornia, Hawaii, and New Zealand. Also known as silverpampas grass and Uruguayan pampas grass (Cortaderia sell-oana), this grass grows in dense tussocks eight to 12 feet(2.43.7 m) tall and produces large, feathery flowers. Overlarge areas of the pampa, silver pampas grass crowds out allother plants, except for the herbs that grow between its tus-socks. It is used to make paper, but it is of little use for pas-ture. More nutritious pasture grasses and herbs have beensown to replace most of it.

There are few trees in the eastern pampa, because they can-not tolerate the frequent winter fires that are fanned by thestrong, perpetual winds. The grasses recover quickly from fireand benefit from the nutrients in the wood ash that the rainsoon washes down to their roots.

The one woody plant that does withstand fire is called theombu (Phytolacca dioica). It grows to a height of 4060 feet(1218 m) and its girththe trunk circumference measuredabout four feet (1.2 m) above ground levelcan reach 4050feet (1215 m). The plant often produces many trunks grow-ing side by side. Technically this structure makes it a shrubrather than a tree, despite its height. The ombus trunk isspongy because it contains tissues that store water, making itfire-resistant. Its ability to store water also helps it survivedrought.

Gauchos nicknamed these plants lighthouses, becausethey are visible from afar and the umbrella-shaped top offersshade on a hot, sunny day. Ombus are planted as shade treesin places with a suitable climate, such as Southern California.

VeldGrasslands known as the veld or grassveld cover the highplateau that occupies the eastern side of South Africa. Veld isthe Afrikaans word for field. The elevation varies fromapproximately sea level to more than 9,000 feet (2,745 m).The veld extends over most of the Eastern Cape, Free State,and Eastern Transvaal, as well as parts of KwaZulu-Natal,

12 GRASSLANDS


North West, and Northern Province. It covers all of Lesothoand part of Swaziland. The map shows the area and locationof the South African veld. The largest area, called theHighveld, covers most of the Free State. To the north of theHighveld the land rises into the WitwatersrandAfrikaansfor ridge of white watersand beyond that is the Bushveld,a region of dry savanna-type grassland. The Cape middle veldlies to the west of the Highveld.

The veld is also divided into sweet and sour types.Sweet veld occurs mainly on the western side of the country,where the annual rainfall is less than about 25 inches (635mm). Grasses found there have a low fiber content and retaintheir nutrients through the winter, so they are palatable tolivestock. The grasses of the sour veld grow on the wetter, east-ern side of the region. They are fibrous and lose their nutri-ents when they die down in winter. Winter is the dry season,although there is some rain in every month of the year.

Red oat grass (Themeda triandra) is the most widespreadspecies. It grows to a height of 1236 inches (3090 cm) and

Western Cape

Northern Cape

Northern

North West

Free State

KwaZulu-Natal

Gauteng

Mpumalanga

Eastern Cape

LESOTHO

BOTSWANANAMIBIA MOZAMBIQUE

REPUBLIC OF SOUTH AFRICA

SWAZILAND

ATLANTIC OCEAN

INDIAN OCEAN

temperate grassland

Veld. The veld grasslandof southern Africa occurson the eastern side ofSouth Africa and coversLesotho and the westernhalf of Swaziland.

is of poor nutritional value and readily overgrazed. Wherered oat grass has been overgrazed, love grass (Eragrostisspecies) often takes its place. Bristle grass, also called wiregrass, Ngongoni three-awn grass, and Gongoni steekgras(Aristida junciformis), and Bermuda grass (Cynodon dactylon)are also widespread. Bristle grass grows to a height of 2036inches (5090 cm) and Bermuda grass to four to 15 inches(1040 cm). Bluegrass (Festuca species) grows on the higherground, where the climate is cooler.

The veld has been farmed for many years. It supplies mostof South Africas dairy, beef, and wool products, and largeareas have been converted to cropland. Corn (maize) is themost important crop, but sorghum, wheat, and sunflowersare also grown. Part of the veld is highly urbanized andindustrial. The Witwatersrand is an important mining andindustrial region, and the cities of Johannesburg and Pretoriaare located there.

Tropical grasslands of South AmericaMost of us think of tropical South America lying to the east ofthe Andes as a land of forests growing in a hot, wet climate.South America is the continent of rain forests. As the mapshows, however, that picture is incomplete. Almost half oftropical South America is not forest at all, but open grassland.

The northern grasslands, covering about 125,000 squaremiles (323,750 km2) in Venezuela, are known as the llanos,and those occupying 100,000 square miles (260,000 km2) inColombia are the llanos orientales, the eastern llanos. Thellanos grasslands cover plains that are bounded by the moun-tains of the Cordillera Mrida in the north and by theOrinoco and Guaviare Rivers in the south. Together the grass-lands of the two countries compose the llanos orinoquia, theOrinoco llanos. This is the home of the llaneros, the skilledhorsemen who are the equivalent of the gauchos of thepampa, far to the south. In 1548 cattle were introduced tothe llanos, and it is an important ranching and stockbreedingregion.

The llanos lie in a large basin that formed millions of yearsago between the Guiana Highlands in the east and the Andes

14 GRASSLANDS


in the west and then filled with sediment. Later the basinsubsided in some places, creating the present landscape ofalmost completely level plains interrupted by flat-toppedhills called mesas. The grasslands are probably no more than10,000 years old. The llanos is a complex landscape, and sci-entists studying it divide it into seven regions.

Grasses are the predominant plants, and over large areas ofthe llanos Trachypogon is the most widespread grass genus.Trachypogon grasses grow in tussocks four to 12 inches (1030cm) apart to a height of more than six feet (1.8 m). Betweenthe tussocks there are herbs up to about three feet (1 m) tall.Several other grasses grow on the llanos, and there are scat-tered trees. These include various palm trees; the manteco orgolden spoon (Byrsonima crassifolia), which produces edible

BRAZIL

PARAGUAY

ARGENTINACHILE

BOLIVIA

PERU

ECUADOR

COLOMBIA

VENEZUELA

GUYANA

SURINAME

FRENCH GUIANA

PACIFIC OCEAN

ATLANTIC OCEAN

savanna and other tropical grassland

Tropical grasslands ofSouth America. Thegrasslands occupy thedrier regions, beyond the edges of the forests.The largest area is in Brazil.

fruits; the chaparro or rough-leaf tree, with leaves that areused for polishing metal (Curatella americana); and thealcornoque or sucupira (Bowdichia virgilioides), the bark ofwhich is used to treat tuberculosis and rheumatism. Denseforests grow beside the rivers.

There are two seasons on the llanos. Winters, lasting fromDecember through April, are dry, with less than two inches(50 mm) of rain a month. Most of the rain falls during therainy season, from April through November, and large areasare flooded between June and October. The total annual rain-fall averages 3047 inches (7601,200 mm) in the northeast,4763 inches (1,2001,600 mm) in the center, and 98 inches(2,500 mm) in the southwest. The average temperature is80F (27C) throughout the region, with no more than about3.6F (2C) difference between the warmest and coolestmonths.

South of the forests grasslands originally covered580,000770,000 square miles (1.52 million km2) of centralBrazil. That area, approximately equal to the combined areasof France, Germany, Italy, Spain, and the United Kingdom,amounts to about one-fifth of the total area of Brazil. Todayabout 40 percent of the original area has been converted toagriculture and grows crops such as soybeans.

These grasslands, called cerrado, include varying numbersof trees and are intersected by forests along the river valleys.The cerrado also includes the worlds largest area of conti-nental wetlands, known as pantanal. The cerrado is dividedinto four distinct types. Campo limpo is open grassland withno trees. Campo sujo, literally dirty field, is grassland withtrees. Cerrado sensu strictu is grassland with trees and areas ofwoodland, and cerrado is woodland. This diversity of condi-tions supports an estimated 6,600 species of plants, includingmore than 1,000 species of trees, as well as 110 species ofmammalsmost of them rodentsand 400 species of birds.There are approximately 500 species of grasses and almost asmany orchid species. The most common grasses are perenni-al tussock grasses, such as Trachypogon spicatus, which growsto about 2436 inches (6090 cm) tall.

The cerrado climate is tropical. It is hot throughout theyear, with temperatures averaging 72F (22C) in the south of

16 GRASSLANDS


the region and 81F (27C) in the north. Annual rainfall aver-ages 4771 inches (1,2001,800 mm), about 90 percent ofwhich falls during the Southern Hemisphere summer,between October and March. Over two-thirds of the cerradothe summer dry season lasts for five or six months and insome months there is no rain at all.

SavannaThe name savanna or savannah was first given to the tropicalgrasslands of the Caribbean islands and Central and SouthAmerica (see What are grasslands? on pages xvxviii). Theterm is now applied to any tropical grassland with a winterdry season lasting three to five months, a rainy season insummer, and temperatures that never fall below 64F (18C).But today the word savanna is especially associated withAfricaperhaps because animals of the African savanna have featured in so many nature programs on television. This savanna is home to such nature program favorites aslions, cheetahs, zebras, gazelles, elephants, wildebeests, andmeerkats.

It is also vast. As the map shows, the northern boundary ofthe African savanna extends from the southern edge of theSahara, along a line from Senegal and Guinea-Bissau, to theNile River. Southward the grassland continues to the edge ofthe tropical rain forest in West Africa and to the Ubangi (orOubangi) River and Lake Victoria in Central and East Africa.South of the equator the savanna covers the region boundedby the southern edge of the tropical forests, at about latitude7S, to latitude 32S in South Africa, excluding the Namiband Kalahari deserts. In all, it occupies approximately 5 mil-lion square miles (13 million km2). That is almost half of thetotal area of Africaand considerably more than the entirearea of the United States (3.675 million square miles [9.52million km2]).

Much of the area is classed as derived savanna. This meansthat it was once forest, but people cleared most of the treeslong ago. Grasses invaded the cleared area, followed by herdsof grazing animals that nibbled and trampled tree seedlings,preventing the return of the forest. Fires occur naturally dur-

ing the dry season, and they also prevent the forest fromreturning. Instead there are grasses. The tall grasses grow inthe moister areas, where the annual rainfall is 2440 inches(6001,000 mm), as it is along the edge of the tropical forests.Elephant grass (Hyparrhenia species) grows 1013 feet (34 m)tall. One species, H. filipendula, is used to make paper, and it

18 GRASSLANDS

SOUTH AFRICA

NAMIBIA

BOTSWANA

ZIMBABWE

ZAMBIA MOZAMBIQUE

MADAGASCAR

ANGOLA

UGANDASOMALIA

ETHIOPIA

SUDAN

CHADNIGER

NIGERIA

C.A.R.

CAMEROON

GABON

CONGO

MALIMAURITANIA

SENEGAL

WESTERNSAHARA

ALGERIA

MOROCCOTUNISIA

ERITREA

DJIBOUTI

RWANDABURUNDI

SWAZILANDLESOTHO

LIBYA EGYPT

BURKINAGUINEA

GUINEA

BENIN

SIERRALEONE

DEMOCRATICREPUBLICOF CONGO

LIBERIA

IVORY COAST KENYA

Mediterranean Sea

GHANA

ATLANTIC OCEAN

INDIAN OCEAN

savanna


is grown in some areas of savanna for this purpose. In drierareas, where the rainfall is eight to 24 inches (200600 mm),several species of wire grass (Aristida) occur, especially A.stipoides, as does Indian sandburr (Cenchrus biflorus), anannual grassone that lives for only one yeargrowing tosix to 24 inches (1560 cm) tall.

Trees grow among the grasses. Thorn trees (Acacia species)are the most characteristic woody plants, and A. tortilis is themost widespread. It grows to a height of 1350 feet (415 m)and has the umbrella-shaped top that is typical of acacias.According to tradition, wood from this tree was used to makethe biblical Ark of the Covenant. A. senegal is the principalsource of gum arabic, a material used to make glues andpastes and a component of some medicines. A. laeta, anotherwidespread savanna thorn tree, is also an important commer-cial source of gum arabic.

Possibly the most extraordinary savanna trees are thebaobab (Adansonia digitata) and the candelabra tree(Euphorbia candelabrum). The baobab grows up to 40 feet (12m) tall, but its lower trunk is as much as 30 feet (9 m) indiameter. Its peculiar appearance gave rise to a legend thatthe devil pulled the baobab tree from the ground and pushedit back upside down, leaving its branches below ground andits roots sticking up in the air. The baobab is very long lived.Specimens have been reliably dated at 2,000 years old, andless reliable methods suggest that some baobabs are verymuch older. The tree does not grow each year; sometimes itshrinks, probably because of the loss of fluid in times ofdrought. Every part of the tree is useful: Cloth is made fromits outer bark and rope from its inner bark; its timber is used;its seeds are edible and rich in vitamin C; and its leaves areeaten as a vegetable. The candelabra tree, found on the EastAfrican savanna, is closely related to the spurges. Its many

(opposite page) African savanna. Africa has the worlds largest areaof tropical grassland. It occupies most of the continent between theSahara in the north and the Kalahari and Namib Deserts in the south,except for the tropical forest on either side of the equator centered onthe Congo Basin.

branches grow from the top of the trunk, all pointingupward.

Australian grasslandsThere was a time long ago when Australia was a land ofrivers, lakes, and forests. Its climate turned drier about 10million years ago. The lakes disappeared and as they shrank,so did the forests. Trees need a plentiful supply of water, butgrasses can manage with less. Consequently, the retreat ofthe forests was accompanied by the expansion of grassland.Today much of the interior of Australia is desert. There issome tropical forest in the north and east, and eucalyptusforest is widespread in the east, but the natural vegetationover a large part of the continent is a mixture of grasslandand scrub. Mallee, in the south of the country, composesdense thickets of dwarf eucalyptus. The Australian grasslandforms a savanna landscape similar to the African savanna butcomposed of different species. The map shows the tropicalgrassland areas of Australia.

Mitchell grasses (Astrebla species) cover the northern partof the region, known as the Mitchell Grass Downs. Theseextend for about 930 miles (1,500 km) from the center of theNorthern Territory to the middle of southern Queensland, asa belt of almost treeless grassland. Mitchell grasses grow1236 inches (3090 cm) tall and occasionally to as much as48 inches (1.2 m). They resist drought and provide nutritiouspasture. Sheep and cattle graze the Downs. The annual rain-fall averages 1430 inches (350750 mm). Temperaturesoften exceed 100F (38C) in summer, but sometimes frostsoccur in winter.

20 GRASSLANDS

(opposite page) Australian grasslands. Grasslands of several typessurround the central deserts. The Mitchell grass (Astrebla pectinata)

resists drought and provides good pasture. It grows on rolling hills(downs) across the north of Australia. There is savanna grassland to

the south. Between them are desert sand dunes that are stabilizedwith Triodia grass. The remaining areas of Australia are dominated

by scrubland, where different Acacia species grow among the grasses.


Mitchell grasses give way in the north to bluestem grasses(Dichanthium species). These grow only about six inches (15cm) tall in the drier areas, but where there is more moisturethey sometimes stand 6.5 feet (2 m) high. The Mitchell grasspastures separate areas of woodland dominated by thorntrees (Acacia species). Australia is home to more species ofAcacia than any other continent, and the most common vari-eties found beside Mitchell grassland are gidgee (A. cambagei)and brigalow (A. harpophylla).

Tasmania

SOUTHERN OCEAN

INDIAN OCEANCoral Sea

Mitchell grass downs and brigalow scrub

mulga scrub

mulga and saltbush

dunes fixed with Triodia

savanna with saltbush

mallee

Savanna with brigalow extends down most of the easternside of Australia. It covers 132,200 square miles (342,400km2), approximately from Townsville, Queensland, at lati-tude 19.25S, to the QueenslandNew South Wales border, at28.58S. Here the rainfall ranges from more than 30 inches(750 mm) a year in the east to less than 20 inches (500 mm)in the west. Gidgee replaces brigalow in the drier areas, andin some places there are open grasslands, dominated bybluestem grasses (Dichanthium).

Brigalow scrub also includes some other trees. These arepredominantly species of Eucalyptus, another characteristi-cally Australian genus. There are ironbarks (E. melanophloiaand E. crebra), poplar box (E. populnea), and Browns box (E. brownii). Farther south there are also spotted gum (Corym-ba maculata) and red bloodwood (C. gummifera); Corymbaspecies are types of eucalyptus. In places there are openwoodlands of brigalow and belah (Casuarina cristata).

Mulga scrub, composed of the dwarf thornbush mulga(Acacia aneura) and Spinifex species of grasses, extends downthe western side of Australia, and in the center of the countrythere is an area of mulga with saltbushes (Atriplex species,especially A. vesicaria). Savanna grassland with saltbushesextends across much of the continent on the southern side ofthe desert. As the name suggests, saltbushes can grow inplaces where the soil is salty. Mulga grows up to 20 feet (6 m)tall. Livestock feed on its leaves, and its wood has many uses.Boomerangs, for instance, are traditionally made from mulgawood. Spinifex grasses are 2040 inches (50100 cm) tall, andthey grow from underground stems rather than in tussocks.This makes them very useful for stabilizing loose sand dunes.Porcupine grass (Triodia species), which is used for this pur-pose in the desert, grows in the same way, usually to a heightof eight to 48 inches (20120 cm) but sometimes to eight feet(2.4 m) tall.

Upland grasslandsFew trees are able to grow anywhere the average summertemperature is lower than 50F (10C). Grasses, however, tol-erate lower temperatures. Consequently, if the average sum-

22 GRASSLANDS


mer temperature decreases over a distance, the line where itfalls below 50F (10C) marks the limit for tree growth. Treesgrow on one side of this line and grasses on the other side.The boundary is called the tree line, timberline, or forest limit.

The boundary between trees and grasses is not always quiteso abrupt as this description makes it sound. Between thedense part of the forest and the forest limit, the plant compo-sition of the forest changes gradually. If at first there arebroad-leaved evergreen trees, these may give way to decidu-ous treestrees that shed their leaves in winterand then toconiferous trees, such as firs, pines, and spruces. The conifer-ous forest then becomes more open. Instead of being closelypacked so that their shade makes the forest floor very dark,the trees are more widely spaced, and there are gaps andclearings where the sunshine reaches the forest floor, allow-ing grasses and herbs to grow. As the trees become even morewidely scattered, they also become smaller and increasinglystunted. Finally there are no trees at all; this is the tree line.Rather than a clearly marked line, however, it is more like abelt in which the concentration of trees gradually decreases.

One type of tree line is latitudinal. Average temperaturesdecrease with increasing distance from the equator, andforests give way to temperate grasslands when the averagesummer temperature dips below 50F (10C). The term treeline is more usually applied to mountains, however. Becausetemperature decreases with altitude, there is a height beyondwhich trees cannot survive. The forests that blanket thelower slopes of a mountain gradually become sparser andfinally disappear, and above this altitudinal tree line there aremeadows forming alpine savanna in the Tropics and alpinegrassland elsewhere.

The rate at which temperature decreases with height isknown as the environmental lapse rate (ELR), and it varies fromplace to place and day to day. It is the average ELR that mat-ters, however, because it is the average summer temperaturethat determines whether or not trees will survive. The aver-age ELR is 3.6F per 1,000 feet (6.5C/km). If you know boththe average summer temperature in a particular place and theheight of that place above sea level, you can calculate theapproximate altitude of the tree line. At El Paso, Texas, for

example, the average temperature (counting both day andnight) in the warmest month is 80F (27C). The temperaturemust decrease by 30F (17C) to reach the transition point of50F (10C), and it will do so at a height of about 8,300 feet(2,500 m). El Paso is already 3,920 feet (1,196 m) above sealevel, so the average summer temperature will be 50F (10C)at an elevation of 12,220 feet (3,700 m). Consequently, thatwill be the average height of the tree line in the mountainsabove the city. At Banff, Alberta, the average July temperatureis 57.9F (14.4C), and Banff is about 4,583 feet (1,397 m)above sea level in the Canadian Rocky Mountains. Hence thetree line there will be at about 6,780 feet (2,070 m). The treeline descends with increasing distance from the equator,because with increasing distance the summers become pro-gressively cooler. In northern Canada and Eurasia, where thesummer temperature never reaches 50F (10C), the tree lineis at sea level.

Calculating the height of the tree line on a real mountainis rather more complicated, however. Some parts of themountain face the Sun and others face away from the Sun,and some areas will be in full sunshine, while other areas areshaded for much of the time. Consequently the average tem-perature will vary markedly from place to place on the moun-tainside. Cold air from higher on the mountain, perhapsfrom the region that is permanently covered with snow, willfrequently subside down the mountainside, lowering thetemperatureand therefore the tree line.

Above the tree line, whatever its height, there are alpinegrasslands or savannas. The composition of the grasslandsvaries, but they are always rich in flowering herbs and, often,heathers that grow among the grasses. Fescue grasses (Festucaspecies) are common in most mountains. They grow inclumps and are known as bunchgrasses or tussock grasses.

Alpine meadows are valuable to the farmers living in thevalleys. Traditionally they are cut to make hay to feed ani-mals through the winter, and in many parts of the worldsheep and, to a lesser extent, cattle graze them through thesummer. In late spring herders drive the animals from thevalleys and stay with them throughout the summer, living intents or cabins high in the mountains. This type of farming is

24 GRASSLANDS


called transhumance. It is less common now than it used tobe, but it has not died out, although the methods havechanged in some places. In the European Alps, for example,the sheep now travel to and from the mountains in hugearticulated trucks, rather than being driven on foot.

Movement of continentsGrasslands occupy the centers of continents, far from theocean, where the climate is too dry for forests but not dryenough to produce deserts. They do not move, althoughclearing forest to provide pasture will produce new grassland,and abandoning the pasture may allow the forest to return,even centuries after it was originally cleared. Our planet hasexisted for a very long time, however, and these are short-term changes.

Although grasslands continue to occupy continental inte-riors, the continents themselves are moving. About 350 mil-lion years ago, for example, North America was pivotedabout a quarter turn from its present position and the equa-tor passed through it, from the eastern end of Hudson Bay toapproximately where San Francisco sits today. At that time,however, the whole of what is now the Great Plains and all ofthe land to the west of it lay beneath a shallow sea, and theRocky Mountains had not formed. The prairie could not haveexisted then, not only because sea covered the entire area,but also because grasses had not yet evolved. The earliest fos-sils of grassesand grasslandsare found in North America,and they are about 45 million years old. The earliest clearrecord of African grasslands is about 14 million years old,although fossilized grass pollen that is older has been found.

Grasses probably appeared first in the Tropics, close to theedge of tropical forests. The first grasslands were of the savan-na type, but as climates became cooler the grasslandschanged. They continued to be grassland, but savanna wasgradually transformed into prairie, pampa, and steppe. Thetropical grasslands became temperate grasslands.

Climates change for several reasons; the changes that con-vert savanna to prairie result from the movement into higher

GEOLOGYOF GRASSLANDS

CHAPTER 2

26

GEOLOGY OF GRASSLANDS 27

latitudes of the continents carrying them. The process iscalled continental drift.

Conclusive evidence for continental drift was not discov-ered until the middle of the 20th century, but the idea was farfrom new. Some scholars were proposing something ratherlike it as early as the 16th century. With access to the first rea-sonably accurate maps of the world, they could see that thecontinents fit together rather more neatly than was likely tobe due to chance. Evidence continued to accumulate over thecenturies: The rocks forming the mountains of Scotland arevery similar to those of the Appalachian Mountains; NorthAmerican coal, made from plant remains, is very similar tothe coal found in Europe; certain plants and animals arefound only in particular places, and those places are separat-ed by thousands of miles of ocean. Early in the 20th centurythe German meteorologist Alfred Wegener (see the sidebar)drew together these and other strands of evidence to proposethat at one time all the continents had been joined to form asingle supercontinent, which he called Pangaea, surround-ed by a vast ocean, Panthalassa.

One of Wegeners supporters was the South African geolo-gist Alexander Logie Du Toit (18781948), who had foundsimilarities in the rock formations of South Africa and SouthAmerica. He renamed Wegeners continental displacement,calling it continental drift, which is the name that survived.Then the American oceanographer Robert Sinclair Dietz(191495) proposed the theory of seafloor spreading todescribe the way oceans grow wider from a central ridge.Finally in 1967 Dan McKenzie (born 1942) of CambridgeUniversity synthesized all that was known at the time andproposed a new theory, plate tectonics. Tectonics refers to thedeformation of the Earths crust and structures resulting fromit, and the theory proposes that the solid crust consists of dis-crete sections, called plates. A scientific theory is an explana-tion of a natural process, with solid evidence to support it.

There are two types of crustal plate: continental plate andoceanic plate. They differ in the rocks from which they aremade and in their thickness. Oceanic plates are made fromvery dense rock and are three to nine miles (515 km) thick.Continental plates are less dense and 1950 miles (3080 km)

thick. Both types of crust rest upon the material of the man-tle, but because it is less dense, continental crust projectshigher than oceanic crust and oceans fill the basins between

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Alfred Lothar Wegener and continental drift

Since the first realistic maps of the world were published in the 16th century, many geog-raphers had puzzled over the fact that the continents on each side of the Atlantic Oceanlooked as though they might fit together. Some thought it mere coincidence, but otherssuggested ways a single continent might have split into two parts that then moved apart.

The German meteorologist Alfred Lothar Wegener (18801930) went much further.Wegener compiled a mass of evidence to support what he called continental displace-ment. This phenomenon came to be called continental drift. He studied the scientific lit-erature for descriptions of rocks that were similar on each side of the Atlantic. He foundplants with limited distribution that are separated by vast oceans and fossil organisms thatare also distributed in this way.

Finally he proposed that about 280 million years ago, during the Upper Paleozoicsubera, all the continents were joined, forming a single supercontinent, which he calledPangaea (from the Greek pan, meaning all, and ge, meaning Earth), surrounded by anocean called Panthalassa (thalassa means ocean). He theorized that Pangaea broke apartand the separate pieces drifted to their present locations; the continents are still drifting.

In 1912 Wegener published a short book outlining his theory, Die Entstehung derKontinente und Ozeane (The Origin of the continents and oceans). He was drafted into theGerman army in 1914 at the start of World War I but was wounded almost at once. Hedeveloped his ideas further while recovering in a hospital, and in 1915 he published amuch longer edition of his book (it was not translated into English until 1924).

His idea found little support. Geologists at the time believed the mantlethe materialbeneath the Earths crustto be solid, and they could not imagine any way that conti-nents could move. They also found that some of Wegeners calculations of the rate of con-tinental displacement were incorrect.

But support for Wegeners idea began to grow in the 1940s, when for the first time sci-entists were able to study the rocks on the ocean floor. These studies indicated that theoceans had grown wider by spreading outward from central ridges, where underwatervolcanoes were erupting, laying down new rock. Wegeners theory was generally accept-ed by the late 1960s, but by then Alfred Wegener was dead. He had died in 1930 duringhis third expedition to study the climate over the Greenland ice sheet.


continents. There are seven major plates: the African, Eu-rasian, Pacific, North American, South American, Antarctic,and Australian plates. There are also several lesser plates: theCocos, Caribbean, Nazca, Arabian, Indian, Philippine, andScotia plates. In addition, there are minor plates such as theJuan de Fuca plate, microplates, and fragments of formerplates that have broken apart. The map shows the presentlocation of the major and lesser plates.

Plates fit tightly together and move in relation to oneanother in three basic ways: apart, causing oceans to widen;toward each other, causing oceans to grow narrower and con-tinents to collide; or past one another, traveling in oppositedirections. Some plates are no longer moving, and some havebeen permanently incorporated into other plates, but manyare still on the move. The North American and Eurasian

Eurasian Plate Eurasian PlateNorth American Plate

Philippine Plate

South American PlateAustralian Plate

Juan de Fuca Plate

Pacific Plate

Cocos Plate

Antarctic Plate

Nazca Plate

Scotia Plate

Caribbean PlateArabian Plate

African Plate

Indian Plate

Tectonic plates. The map shows the majorplates and larger minorones into which theEarths crust is broken.Continents move acrossthe Earths surfacecarried on the plates.

plates are moving apart, for example, at about 0.8 inch (2cm) a year, and the Pacific and Nazca plates are moving awayfrom each other at about six inches (15 cm) a year. Theboundaries between moving plates are called active margins,and they are sites of earthquakes and volcanic eruptions.

Beneath the Earths crust the rocks of the mantle areextremely hot and are compressed under tremendous pres-sure. They are very much denser than the crustal rocks, butthe high temperature and pressure cause them to deformthey can be squeezed, bent, and stretched by the movementof material around them. They can also transmit heat by con-vection.

Convection is the process by which liquid heats up in apan sitting on a stove. The pan is heated from below and theliquid at the bottom is first to warm. As it warms, the liquidexpands. Expansion makes the liquid less dense because itoccupies a bigger volume without gaining any more mole-cules. Cooler and therefore denser liquid sinks beneath thewarmer liquid, pushing it upward. The warm liquid cools asit rises away from the source of heat, and the cool liquid, nowat the bottom of the pan, heats up and rises in its turn. A ver-tical circulation develops, with warm liquid rising, cooling,moving to the sides at the surface as warmer liquid pushesupward through it, and sinking to be warmed and rise again.The liquid moves in convection currents, and a set of rising andsinking currents is known as a convection cell. There are usual-ly several convection cells in a pan of liquid on the stove.

Rocks in the Earths mantle are not liquid, but they areheated from below because the Earths core is its hottest part,and they deform sufficiently for convection cells to developin the material on which the crust rests. As the mantle mate-rial moves horizontally before sinking, very, very slowly itdrags the continental and oceanic plates with it.

Scientists have been able to determine the movements ofplates over hundreds of millions of years, and this capacityhas allowed them to reconstruct maps of the world as itappeared at different times in the past. The illustration showshow the continents and oceans were arranged 135 millionyears ago and 65 million years ago compared with their pres-ent arrangement. Look further into the past and the map ofthe world becomes even more unrecognizable. Today plates

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are still moving, seafloors are spreading, and continents aredrifting. Eventually North America will collide with Asia, andperhaps the prairie and steppe will join into a continuousbelt of grassland. As Australia continues its journey north-ward it will one day enter the humid Tropics, and perhapsforests will blanket what are now its dry grasslands.

How mountains rise and wear awayWhen drifting plates collide, mountain ranges are born.There are two ways in which this can happen. Oceanic crustis denser than continental crust. If an oceanic plate and acontinental plate collide, the denser oceanic crust will slide

Southern Ocean

PacificOcean

PacificOceanIndian

Ocean

AtlanticOcean

Arctic Ocean

Continental drift. The continents areconstantly in motion.The map shows the way they weredistributed 135 millionand 65 million yearsago, compared withtheir familiararrangement today.

usually rather jerkilybeneath the continental crust. Thisprocess is called subduction. As it sinks deep into the mantle,the ocean crust melts and some of the material rises to thesurface again some distance away in the direction of the con-tinent, producing volcanoes.

The dense rock of the crust is covered by sediment. Finerock particles are washed from the continents, are carried to

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Geologic time scaleEon/ Era/ Period Epoch/ BeganEonothem Erathem Subera System Series Ma*

Quaternary Pleistogene Holocene 0.11Pleistocene 1.81

Phanerozic Cenozoic Tertiary Neogene Pliocene 5.3Miocene 23.03

Paleogene Oligocene 33.9Eocene 55.8Paleocene 65.5

Mesozoic Cretaceous Upper 99.6Lower 145.5

Jurassic Upper 161.2Middle 175.6Lower 199.6

Triassic Upper 228Middle 245Lower 251

Paleozoic Upper Permian Lopingian 260.4Guadalupian 270.6Cisuralian 299

Carboniferous Pennsylvanian 318.1Mississippian 359.2

Devonian Upper 385.3Middle 397.5Lower 416

Lower Silurian Pridoli 422.9Ludlow 443.7Wenlock 428.2Llandovery 443.7

Ordovician Upper 460.9Middle 471.8


the ocean by rivers, and then settle onto the ocean floor.Over the many millions of years of the oceans existence,these sediments slide down the submerged sloping edges ofthe continents until they cover every part of the ocean floor.Gradually the weight of the upper layers of sediments com-presses the lower layers until they form rockcalled sedimen-tary rock. Sedimentary rock is less dense than the rock of theoceanic crust itself (called igneous rock, from the Latin ignis,meaning fire), which is formed directly from the hot materi-al in the mantle. Where the plates collide, the continental

Eon/ Era/ Period Epoch/ BeganEonothem Erathem Subera System Series Ma*

Lower 488.3Cambrian Furongian 501

Middle 513Lower 542

Proterozoic Neoproterozoic Ediacaran 600Cryogenian 850Tonian 1000

Mesoproterozoic Stenian 1200Ectasian 1400Calymmian 1600

Paleoproterozoic Statherian 1800Orosirian 2050Rhyacian 2300Siderian 2500

Archean Neoarchean 2800Mesoarchean 3200Paleoarchean 3600Eoarchean 3800

Hadean Swazian 3900Basin Groups 4000Cryptic 4567.17

Source: International Union of Geological Sciences, 2004.

Note: Hadean is an informal name. The Hadean, Archean, and Proterozoic eons cover the time formerly known as thePrecambrian. Quaternary is now an informal name and Tertiary is likely to become informal in the future, althoughboth continue to be widely used.

*Ma means millions of years ago.

plate scrapes sedimentary rock from the surface of the ocean-ic plate and crumples it upward to form a mountain rangemade from a mixture of continental rock and sedimentaryrock from the ocean. A mountain range formed in this way iscalled a cordillera.

Where two continents collide, the rocks of both plates areequally dense, so one cannot sink beneath the other. Instead,the two crumple upward, in the way a tablecloth crumplesupward if you place your hands some distance apart on it andpush them together.

More than 100 million years ago, during the Cretaceousperiod (see the sidebar showing the geologic timescale), theAtlantic Ocean began to open and North and South Americabegan to move westward against the Nazca and Pacific plates.These oceanic plates began to be subducted beneath theNorth American and South American plates, crumpling therocks, and by about 50 million years ago the Andes andWestern Cordillera had started to form. The WesternCordillera is the mountain chain down the western side ofNorth and Central America, including the Rocky Mountains.The Nazca and Pacific plates are still disappearing beneathNorth and South America, at a rate of one to three inches(28 cm) a year. The Pacific plate is also being subducted onthe western side of the ocean, and this motion produces thevolcanoes and earthquakes that surround the Pacific Basinwith what is often called a ring of fire.

During the Silurian period the closure of two oceans pro-duced a chain of mountains that extends from the southernAppalachians, across Ireland and Scotland, to Svalbard, thegroup of islands to the north of Norway. This was anotherexample of a collision between continents and oceans, withoceanic crust sinking beneath continental crust.

Between 54 million and 49 million years ago, the IndianPlate, traveling northward, collided with the southern mar-gin of the Eurasian plate. This was a collision of a differentkind: Two continental plates had collided. This collisionraised the Himalayas and the Tibetan plateau, causing amajor change in the global climate (see Evolution of grass-lands on pages 7173). India is still moving northward, at1.52 inches (45 cm) a year.

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Since the collisions that have produced the Andes, RockyMountains, and Himalayas are still continuing, the processesthat raise mountains also continue. This means that themountains should continue to grow taller. In fact, though,two other processes check the growth of mountains: isostaticadjustment and erosion.

Together the rocks of the Earths crust and the uppermostlayer of the underlying mantle compose the lithosphere andthe plates that form continents and ocean basins are some-times called lithospheric plates. Immediately below the litho-sphere there is a layer of the mantle called the asthenosphere.The lithosphere rests on top of the asthenosphere; individuallithospheric plates float on the asthenosphere as ice shelvesand icebergs float on the ocean surface.

The thickness of the lithosphere varies from place to place.Ocean floors are about three miles (5 km) thick, continentsare thicker, and lithosphere carrying mountain ranges can benearly 40 miles (64 km) thick. Ocean crust is made fromdenser rock than continental crust, but the difference is notsufficient to compensate for so great a difference in thick-ness, and where the crust is very thick it is also heavier thanadjacent crust and the heavy section of lithosphere sinks intothe asthenosphere. The lithospheric plates are free to moveup and down until their weight, pushing downward, bal-ances their buoyancy, pushing upward. Ice floating on waterbehaves in the same way, with the result that most of an ice-berg lies below the ocean surface. This situation is calledisostasy, from the Greek words isos, equal, and stasis, sta-tion. When rocks are crumpled to build mountains, themass of material increases, making that section of plate sinklower into the asthenosphere, as shown in the diagram. Inthis process, isostatic adjustment, as material is added to makethe mountain higher, the mountain sinks down, making itlower. Isostatic adjustment consequently offsets part of theincrease in height.

The moment rocks are thrust into the air they are exposedto the weather. Rain, made slightly acid by the carbon diox-ide dissolved in it, reacts with some of the chemical com-pounds in the rock to produce soluble substances that arewashed away by the rain, leaving tiny cracks, holes, and

weaknesses. Water seeps into cracks and freezes there,expanding as it does so, widening the cracks along lines ofweakness and breaking off fragments of rock. When the icemelts, these fragments fall away. Rivers transport small rocks,rolling them against one another and knocking away thesharp corners and projections. Eventually the rock fragmentsare reduced to the size of sand grains, and in this form someof them are carried out to sea, where they settle to the bot-tom as sediment that one day may be raised above the sur-face once more in a new range of mountains.

Over millions of years this continual process of erosionwears away even the mightiest mountain chain. We maythink of mountains as being eternal, but they are not. Theyappear and disappear. The processes seem slow to us, butcompared with the age of the Earthapproximately 4.6 bil-lion yearsthe few hundred million years required to raise and lower a mountain chain is not really a very longtime at all.

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mountain

asthenosphere

sea level

ocean floor

Isostasy. The Earthscrust is thicker in

mountainous regionsthan it is elsewhere, and the mass of the

crust depresses the topof the asthenosphere.

As wind and rain wearaway the mountains

and they grow smaller,the reduction in theirmass causes them to

float higher in theasthenosphere, so the

mountains rise.


Grassland soilsGrasses and the herbs associated with them grow in soil, noton bare rock. Soil is a mixture of mineral particles derivedfrom rock and the remains of plants and animals. Bare rock isexposed to the weather (see How mountains rise and wearaway on pages 3136), and erosion by wind and waterbreaks it down into small particles. This process is calledphysical weathering. Water moving between the particlesreacts with some of their chemical ingredients to producesoluble compounds that enter the water, producing a soilsolution. This is chemical weathering, and the compounds itreleases include some that nourish plants.

Once plants become established, they begin to addorganic material in the form of fallen leaves and deadplants. Animals that feed on the plants and on each otheralso add material. When plants and animals die, other ani-mals, followed by fungi and bacteria, break down theorganic material into simpler chemical substances, some ofwhich are absorbed by plant roots. At this stage the physi-cal, chemical, and biological processes have together pro-duced a soil.

As time passes, the soil changes. Organic matter forms alayer on the surface. At the base of that layer, the organicmaterial is being decomposed. Water, draining downward,carries some of the soluble substances resulting from decom-position into a lower layer, where they accumulate. Beneaththat layer there is a layer consisting mainly of mineral mate-rial derived from the bedrock at the base. A trench cutthrough a soil reveals these layers as series of horizons thattogether form a soil profile. The illustration shows a completesoil profile, but many soils contain fewer horizons than this,because the soil is not fully developed.

Soil is alive with organisms, some big enough to be visibleand countless billions more that are microscopically small,but the amount of life in the soil depends on the climate andthe amount and type of plants that it supports. Climates varygreatly (see the sidebar How climates are classified on page49), and plants have particular climatic requirements.Consequently there are many different types of soil. The soilbeneath the Arctic tundra is very different from the soil of

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O2

A1

A2

A3

B1

B2

B3

C

R

O1 organic debris

partly decomposed organic debris

mineral material with fine particlesof organic matter

mineral material into which nutrients have moved from above

transitional layer

transitional layer

layer in which most nutrients accumulate

transitional layer

parent material

bedrock

Soil profile. An ideal soil forms a series

of layers, or horizons.Few soils possess all of

these layers.


the Mojave Desert, and both are different from the soils ofthe tropical rain forests. Scientists have devised systems forclassifying soils (see the sidebar).

How soils are classified

Farmers have always known that soils vary. There are good soils and poor soils, heavy soilscontaining a large proportion of clay, sandy soils that dry out rapidly, and light, loamy soilsthat retain moisture and nutrients. Loam is a mixture of sand, silt, and claymineral parti-cles of different sizes. In the latter part of the 19th century Russian scientists were the first toattempt to classify soils. They thought that the differences between soils were due to thenature of the parent materialthe underlying rockand the climate. They divided soilsinto three broad classes. Zonal soils were typical of the climate in which they occur, intra-zonal soils were less dependent on climate for their characteristics, and azonal soils werenot the result of climate. Azonal soils include windblown soils and those made from siltdeposited by rivers on their floodplains. Individual soil types were placed in one or other ofthese broad groups. This system remained in use until the 1950s, and some of the Russiannames for soils are still widely used, such as Chernozem, Rendzina, Solonchak, and Podzol.

American soil scientists were also working on the problem, and by the 1940s their workwas more advanced than that of their Russian colleagues. By 1975 scientists at the UnitedStates Department of Agriculture had devised a classification they called Soil Taxonomy.It divides soils into 10 main groups, called orders. The orders are divided into 47 subor-ders, and the suborders are divided into groups, subgroups, families, and soil series, withsix phases in each series. The classification is based on the physical and chemical prop-erties of the various levels, or horizons, that make up a vertical cross section, or profile,through a soil. These were called diagnostic horizons.

National classifications are often very effective in describing the soils within their bound-aries, but there was a need for an international classification. In 1961 representatives fromthe Food and Agriculture Organization (FAO) of the United Nations, the United NationsEducational, Scientific and Cultural Organization (UNESCO), and the International Society ofSoil Science (ISS) met to discuss preparing one. The project was completed in 1974 and isknown as the FAO-UNESCO Classification. Like the Soil Taxonomy, it was based on diagnos-tic horizons. It divided soils into 26 major groups, subdivided into 106 soil units. The classifi-cation was updated in 1988 and has been amended several times since. It now comprises 30reference soil groups and 170 possible subunits. The FAO has also produced a WorldReference Base (WRB), which allows scientists to interpret the national classification schemes.

The soils of the South American llanos and cerrado grass-lands are predominantly old soils from which most of theplant nutrients have been lost. In many places there are lay-ers of laterite (see the sidebar), which give them a red or yel-low color. African savanna soils are much younger and morefertile. These shade into arid soils in the north and into wet-ter soils on high ground. On the southern side there arepoor, exhausted, lateritic soils typical of tropical rain forest.The soils of temperate grasslandsthe prairie, steppe,pampa, and veldare deep and fertile, making them idealagricultural soils in places where the climate is suitable forfarming.

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Laterite

Tropical soils are often red or yellow, as a result of the presence of oxides and hydroxides,chiefly of iron and aluminum. These compounds sometimes form hard lumps or continu-ous layers of a rock called laterite. The name is from later, the Latin word for brick.

Most laterite is porous and claylike in texture. The surface is dark brown or red, but ifthe laterite is broken, the interior is a lighter red, yellow, or brown. Laterite is fairly softwhile it remains in the soil, but it hardens when it is exposed to air. It has been mined as asource of iron and nickel. Bauxite, the most important aluminum ore, is very similar to lat-erite. In some lateritic soils aluminum combines with silica to form the mineral kaolinite,also known as China clay, which is used

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