地14週

There Are There Major Types of Grasslands



Grasslands occur mostly in the interiors of continents in areas too moist for deserts and too dry for forests.Grasslands persist because of a combination of seasonal drought grazing by large herbivores,and occasional fires-all of which keep large numbers of shrubs and trees from growing.

The three main types of grasslands-tropical,temperate,and cold-result from combinations of low average precipitation and various average temperatures.

One type of tropical grassland,called a savanna,contains widely scattered clumps of trees such as acacia which are covered with thorns that help keep herbivores away.This biome usually has warm temperatures year-round and alternating dry and wet seasons.
humans.

Tropical savannas in East Africa hhumansave herds of grazing and browsing hoofed animals,including wildebeests , gazelles , zebras , giraffes , and antelopes and their predators sush as lions ,hyenas , and humans . Herds of these grazing and borwsing animals migrate to find water and food in response to seasonal and year-to-yea r variations in rainfall and food availability.

In finding their niches , these and other large herbivores have evolved specialized eating habits that minimize competition among species for the vegetation found on the savanna .For example , giraffes eat leaves and shoots from the tops of trees , elephants eat leaves and branches farther down , wildebeests prefer short grass , and zebras graze on longer grass and stems. Savannaplants , like those in deserts , are adapted to survive drought and extreme heat. Many have deep roots that can tap into groundwater.

In a temperate grassland , winters are bitterly cold , summers are hot and dry , and annual precipitation is fairly sparse and falls unevenly through the year.Because the aboveground parts of most of the grasses die and decompose each year , organic matter accumulates to produce a deep , fertile soil. This soil is held in place by a thick network of intertwined roots of drought-tolerant grasses unless the topsoil is plowed up , which exposes it to be blown away by high winds found in these biomes. The natural grasses are also adapted to fires that burn the plant parts above the ground but do not harm the roots from which new grass can grow.

Two types of temperate grasslands are the tall-grass prairies and short-grass prairies of the midwestern and western United States and Canada. Here , winds blow almost continuously , and evaporation is rapid , often leading to fires in the summer and fall. This combination of winds and fire helps maintain such grasslands by hindering tree growth.

Many of the world's natural temperate grasslands have disappeared because their fertile soils are useful for growing crops and grazing cattle.

cold grasslands , or arctic tundra , lie south of the arctic polar ice cap. During most of the year , these treeless plains are bitterly cold , swept by frigid winds , and covered with ice and snow. Winters are long and dark , andscant precipitation falls mostly as snow.

Under the snow , this biome is carpeted with a thick , spongy mat of low-growing plants , primarily grasses , mosses , lichens , and dwarf shrubs.Trees or tall plants cannot survive in the cold and windy tundra because they would lose too much of their heat. Most of the annual growth of the tundra's plants occurs during the 5-to8-week summer , when the sun shines almost around the clock.

One outcome of the extreme cold is the formation of permafrost , underground soil in which captured water stays frozen for more than 2 consecutive years . During the brief summer , the permafrost layer keeps melted snow and ice from soaking into the ground . As a consequence , many shallow lakes , marshes , bogs , ponds , and other seasonal wetlands form when snow and frozen surface soil melt on the waterlogged tundra . Hordes of mosquitoes , black flies , and other insects thrice in these shallow surface pools . They serve as food for large colonies of migratory birds ( especially waterfowl ) that return from the south to nest and breed in the bogs and ponds during the short summer .

Animals in this biome survive the intense winter cold through adaptations such as thick coats of fur ( arctic wolf , arctic fox , and musk oxen ) and feathers ( snowy owl ) and living underground ( arctic lemming ) . In the summer , caribou migrate to the tundra to graze on its vegetation ( Figure 5-11 , bottom ) .

The permafrost in parts of Canada , Alaska , China , Russia , And Mongolia is melting as a result of global warming . This disrupts these ecosystems and releases methane ( CH4 ) and carbon dioxide ( CO2 ) from the soil into the atmosphere . These two greenhouse gases can accelerate global warming and cause more permafrost to melt , which can lead to further warming . The melting permafrost also causes the soil to sink ( subside ) , which can damage buildings , roads , powerlines , and other human structures .

Tundra is a fragile biome . Most tundra soils formed about 17,000 years ago when glaciers began retreating after the last Ice Age ( Figure 4-4 , p . 67 ) . These soils usually are nutrient poor and have little detritus . Because of the nutrient poor and have little detritus . Because of the tundra’s short growing season , its soil and vegetation recover very slowly from damage or disturbance . Human activities in the arctic tundra-mostly the construction of oil drilling sites , pipelines , mines , and military bases-leave scare that persist for centuries .

Another type of tundra , called alpine tundra , occurs above the limit of tree growth but below the permanent snow line on high mountains ( Figure 5-9 , left ) . The vegetation is similar to that found in arctic tundra , but it receives more sunlight than arctic vegetation . During the brief summer , alpine tundra can be covered with an array of beautiful wildflowers .



草原主要是在大陸內部，相對於沙漠太濕，相對於森林乾。草原的保存是由於結合了季節性乾旱，大量草食性動物放牧，意外的大火，以上這些防止灌木和樹的生長。

由於低平均雨量和多變的平均溫度行程熱帶草原、溫帶草原、寒帶草原這三種形式的草原。

熱帶草原被稱為Savanna，包含廣大並散佈的樹叢像金合歡屬植物，那些植物上長滿刺以趨除草食動物，這個生態系全年高溫且乾、濕季交替。

熱帶草原在東非有成群的畜牧動物和有蹄動物包含有牛羚、羚羊，還有肉食動物像獅子、鬣狗、人類，那些動物遷徙去尋找水源和食物去解決季節變化。

在找到牠們合適的地位，牠們和大型草食性動物進化出獨特捕食方式，長頸鹿吃樹頂的葉子；大象吃葉子或樹枝是在下方；牛羚吃短草，斑馬吃長草和莖，此地的植物為了在極端乾旱中生存，許多發展成深根。

在溫帶草原，冬天寒冷，下天乾熱，年雨量少，降雨時間不一定，由於此環境，植物每年死亡跟分解，有機物質聚集在深處，形成肥沃的土壤。這種土壤保存在此由於厚糾纏的根形成細網的覆蓋。除非上層的土被開發。那些暴露在地表被風吹。自然草原會發生火災會燒掉在地表的植物但不會傷害根的發展。

溫帶草原又分高草原和矮草原在美國的中西部和西美洲和中國，在這風不斷的吹，蒸散快速葉子經常被燒掉在夏天和秋天。

世界上許多天然溫帶草原已經消失，因為它們是有用的肥沃土壤種植作物和放牧牲畜。

冷帶草原，或北極苔原，位於南部的北極極地冰冠。在大部分時間，這些樹木的平原是寒冷的，寒冷的風橫掃，並覆蓋著冰雪。冬季漫長而黑暗，降雨量大多是雪。

由於雪，這地生物群落有著厚厚的毛皮，海綿墊低增長的植物，主要是草，苔蘚，地衣，樹或高大植物無法生存在寒冷的天氣凍原，因為他們將失去太多的他們的熱量。大多數年增長率的苔原的植物發生在5 to8週的夏季，當陽光普照幾乎晝夜不停。

成果之一是極度寒冷的形成凍土，地下土壤中，抓獲水停留凍結超過2年。在短暫的夏季，凍土層融化使冰雪從浸種到地面。因此，許多淺水湖泊，沼澤，沼澤，池塘，濕地和其他形式的季節時，大雪和冰凍的表面土壤融化的淹水苔原。成群的蚊子，黑色蒼蠅，和其他昆蟲三次在這些淺層表面游泳池。他們充當糧食大殖民地候鳥（尤其是水禽）返回從南到築巢和繁殖的沼澤和池塘在短期夏天。

動物在這個生物群落的生存，激烈的寒冬通過的修改，如厚外套毛皮（北極狼，北極狐，和麝香牛）和羽毛（雪鴞）和生活地下（北極旅鼠） 。在夏季，北美馴鹿遷移到苔原吃草其植被（圖5月11日，底部。

在多年凍土地區的加拿大，美國阿拉斯加，中國，俄羅斯和蒙古是熔化由於全球氣候變暖。這擾亂了這些生態系統和釋放甲烷（ CH4 ）和二氧化碳（ CO2 ）從土壤到大氣中。這兩個溫室氣體可以加速全球變暖，導致更多的永久凍土融化，這可能會導致進一步變暖。永久凍土的融化還造成了土壤下沉（塌陷） ，它可以破壞建築物，道路，電力線，和其他人的結構。

苔原是一個脆弱的生物群落。最苔原土壤形成約一點七零零萬年開始前，當冰川退卻後，上一個冰期（圖4-4 ，磷。 67 ） 。這些通常是土壤養分窮人和沒有碎屑。由於窮人和養分很少碎屑。由於Tundra的生長季節短，其土壤和植被恢復非常緩慢的損害或擾亂。人類活動在北極苔原，主要建設石油鑽探地點，管道，礦山，和軍事基地，離開恐慌持續存在數百年。

另一種類型的苔原，所謂高山苔原，發生上述限額的樹木的生長，但低於永久雪線高山（圖5-9 ，左） 。植被類似發現在北極苔原，但得到更多的陽光比北極的植被。在短暫的夏季，高山苔原可以涵蓋了一系列美麗的野花。