字幕列表 影片播放 列印英文字幕 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 simply “meadow” 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 the “High Desert” of 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 these “Tussock” grasslands 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 Earth – the 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 history – the 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 is… just 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 home – to the forests of the temperate latitudes.