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  • Grass.

  • If there is one plant that has come to dominate our world, it's this.

  • Occupying every biome on earth except the icesheets, grasses have colonised every patch

  • of soil, from tropical to temperate forests, savannah to steppe, scrubland to desert.

  • Humans have taken these grasses and shaped them over generations into crops that feed

  • the world today.

  • Their homelands are the seas of grass we call Meadow, Prairie, Pampas, Veldt and Steppe

  • - the world's Grasslands.

  • Grasslands are one of only two biomes on our planet to be completely dominated by a single

  • class of plant.

  • But what is grass?

  • A simple definition is a non-woody plant containing a hollow stem for strength, bladed leaves

  • for photosynthesis and a flowering bundled seed head for reproduction.

  • This simple but highly successful formula has evolved since the end of the Cretaceous

  • period, about 66 million years ago, to form over 12,000 species today, and grass's ability

  • to colonise poor soils, survive fire, and turf out competing plants has allowed them

  • to spread to almost every corner of earth.

  • The prairies and steppes are the biomes where this class of plant is fully dominant.

  • This is in comparison to the Savannahs of the tropics, so often confused with grasslands,

  • where we find a mix of grasses, shrubs and trees.

  • Grassland areas occur in relatively temperate drier climates, either in the mid-latitudes,

  • or in high-altitude regions such as the Himalayas and Andes, where rainfall is insufficient

  • for tree growth, or where historical conditions, such as pervasive fire or large animal grazing,

  • have prevented trees from gaining a foothold.

  • There is no one Koppen climate type that matches the global grassland distribution, with continental

  • and cool semi-arid zones, along with humid and highland subtropical zones and even oceanic

  • areas having grasslands.

  • This indicates that the coupling between grassland and climate is not as strong as in the other

  • biomes so far looked at, with major Koppen groups B, C and D being represented here.

  • If we consult our Holdridge Lifezones chart, we can find the grasslands in the centre as

  • steppe”, where conditions are neither particularly cold or hot, but where overall

  • rainfall is on the drier side, indicating that grasslands can dominate in these more

  • arid areas where trees might struggle.

  • So you've probably heard the terms Prairie and Steppe, and possibly Pampas and Veldt too.

  • These terms are more regional in usage than technical.

  • Prairie is almost exclusively used to refer to the grasslands of North America, and is

  • a word borrowed from the early French settlers to this region, meaning simplymeadow

  • in that language.

  • Steppe refers mostly to the grasslands of Eurasia, and is taken directly from the Russian

  • word for this biome.

  • Pampas is the most common word for the extensive grasslands of mid-southern South America and

  • is from the Quechua word for plain.

  • Veldt, the Afrikaans word for field, is a loose term to describe the open upland country

  • of eastern South Africa that is sometimes grassland but might also be scrub.

  • Beyond the vernacular terms, grasslands vary more technically in the height of grass, and

  • in the absence of grazing animals, this relationship is determined by the amount of rainfall and

  • soil drainage.

  • Where the soil is wetter, the grass will grow taller, with some species growing up to 4m

  • in height, while more arid conditions lead to shorter grass.

  • Anyone who has tried to dig up the turf or sod of their lawn will know how well grasses

  • bind to the soil, and the tallgrass species also follow this pattern, with roots burrowing

  • into the earth as deep as their structures are above ground.

  • As grasses are flowering plants, meadows burst into vivid colour during the season of plant

  • reproduction which occurs in spring in the mid-latitudes, and at the beginning of the

  • wet season in subtropical highland areas.

  • So where in the world do we find the grasslands?

  • The two northern continents of North America and Eurasia form most of the land area of

  • this biome with the great plains of their heartlands being made up of prairie and steppe.

  • The southern hemisphere by comparison has more isolated areas.

  • The North American prairies extend unbroken in vast plains from Alberta, Saskatchewan

  • and Manitoba in Canada south and east into the central part of the USA, including the

  • states of Montana, the Dakotas, Minnesota, Wyoming, Nebraska, Iowa, Kansas, Missouri,

  • Oklahoma, New Mexico and Texas.

  • In these regions, the eastern parts receive more rain and are naturally tallgrass prairies

  • which blend into the temperate forests of the eastern part of that continent.

  • As we travel west toward the Rockies, less rainfall leads to shorter grass until semi-arid

  • and scrub conditions occur.

  • Parts of theHigh Desertof Washington, Oregon, Idaho and Nevada have grassland, as

  • does the low-lying central valley of California.

  • In practically all these areas, however, the excellent soils and suitability for wheat

  • and corn farming have led to decimation of natural prairie, with very few areas remaining.

  • South America has three areas of grassland.

  • The first, known as Puna, can be found in upland subtropical areas of the Andes, where

  • much of this remains preserved in its natural state.

  • It's a different story for the Pampas of North-Eastern Argentina, Uruguay and Southern

  • Brazil, however, where the extensive Pampas has been converted almost entirely to farming,

  • with ranching dominating.

  • Lastly, the eastern tip of this continent, from Tierra del Fuego to Southern Patagonia,

  • as well as the Falkland Islands, are dominated by grasslands in the most polar latitudes

  • of any in the world.

  • In Africa, that continent dominated by the tropical Savannah, only a small portion of

  • it in eastern South Africa is considered a part of the true grassland biome, found in

  • uplands, where it is known as the Veldt.

  • We find a surprising area of natural grassland on both islands of New Zealand, a country

  • otherwise dominated by temperate forests owing to its Oceanic climate.

  • The causes of theseTussockgrasslands are poor soils in the centre of the North

  • Island, and a rain shadow in the South Island east of the Southern Alps that reduces rainfall

  • significantly.

  • In the Middle East, we have relatively isolated pockets of grassland in upland areas of Turkey

  • and the southern Caucasus, while many subtropical areas surrounding the Tibetan plateau have

  • this biome in a similar way to those of the Andes.

  • However, these areas pale in comparison to the largest area of grassland on Earththe

  • vast Steppe plains of Eurasia.

  • Running in an unbroken line at around 45°N in latitude, the steppe separates the temperate

  • and boreal forests of the north from the deserts of the south.

  • Starting in the west of the continent, in Eastern Europe, they encompass the whole of

  • the Ukraine and the southern half of European Russia, where most of the natural steppe has

  • been transformed into the Wheat Belt that feeds these countries in a similar way to

  • that of the North American prairies.

  • Further east, the steppe runs along the boundary of Russia and Kazakhstan, and then east again,

  • ending with the border of Mongolia, where the northern half that country is dominated

  • by steppe still mostly in its natural state.

  • Species diversity in natural grasslands is high.

  • Region by region, we find in the North American Prairie, among others, Big bluestem, Indiangrass

  • and Switchgrass species.

  • In the Eurasian Steppe, common species families include Stipa, Festuca and Agropyron.

  • The eponymous Pampas grass is found in that South American grassland, while the equally

  • eponymous Veldtgrass (Ehrharta calycina) is from South Africa, and in the New Zealand

  • tussock grasslands, Chionochloa is common.

  • Like the savannahs of Africa, the presence of large areas of edible grass in the prairies

  • and steppes has resulted in the support of vast herds of grazing animals.

  • In North America there were tens of millions of Bison roaming the prairie before the mass

  • slaughter of the 1800s.

  • And let's not forget that the grasslands of North America and Eurasia led to the evolution

  • of perhaps the most important animal in human historythe horse.

  • As mentioned, most of these areas of natural grassland have vanished, being replaced by

  • agriculture, such is the quality of their soils and climatic suitability for cereal crops.

  • But unlike other biomes that have been diminished by human activity, the grasslands have, really,

  • just been adjusted, because what has replaced these grasslands isjust more grass.

  • The domestication of various species of the grass family into cereal crops is regarded

  • as one of the key developments of human civilisation.

  • Wheat, corn, rice, barley, oats, millet, sugar and many more crops that directly, or through

  • fodder for livestock, feed us, are all grasses.

  • And so this humble family of plants that colonised the world and later went onto feed ours can

  • be thought of in many ways as the most important in our world.

  • And that is the grasslands.

  • I hope you enjoyed this foray into these green seas.

  • If you did, please like and share this video, and let me know your thoughts in the comments.

  • Don't forget to subscribe, so you don't miss future episodes.

  • Thanks again for watching.

  • I'll see you in the next part of this series when, for many of you, and myself, we'll

  • be coming hometo the forests of the temperate latitudes.

Grass.

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B2 中高級 美國腔

草原(The Grassland Biome - Biomes #5)

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    Chia Teh Lin 發佈於 2021 年 06 月 02 日
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