首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
(1) Located in tropical area at low altitudes, savannas (热带草原) are stable ecosystems, some wet and some dry consisting of vast g
(1) Located in tropical area at low altitudes, savannas (热带草原) are stable ecosystems, some wet and some dry consisting of vast g
admin
2022-03-02
26
问题
(1) Located in tropical area at low altitudes, savannas (热带草原) are stable ecosystems, some wet and some dry consisting of vast grasslands with scattered trees or shrubs. They occur on a wide range of soil types and in extremes of climate. There is no simple or single factor that determines if a given site will be a savanna, but some factors seem to play important roles in their formation.
(2) Savannas typically experience a rather prolonged dry season. One theory behind savanna formation is that wet forest species are unable to withstand the dry season, and thus savanna, rather than rain forest, is favored on the site. Savannas experience an annual rainfall of between 1000 and 2000 millimeters, most of it falling in a five-to-eight month wet season. Though plenty of rain may fall on a savanna during the year, for at least part of the year little does, creating the drought stress ultimately favoring grasses. Such conditions prevail throughout much of northern South America and Cuba, but many Central American savannas as well as coastal areas of Brazil and the island of Trinidad do not fit this pattern. In these areas, rainfall per month exceeds that in the above definition, so other factors must contribute to savanna formation.
(3) In many characteristics, savanna soils are similar to those of some rain forests, though more extreme. For example, savanna soils, like many rain forest soils, are typically oxisols (氧化土) (dominated by certain oxide minerals) and ultisols (老成土) (soils containing no calcium carbonate), with a high acidity notably low concentrations of such minerals as phosphorus, calcium, magnesium, and potassium, while aluminum levels are high. Some savannas occur on wet, waterlogged (溃水) soils; other dry, sandy, well-drained soils. This may seem contradictory, but it only means that extreme soil conditions, either too wet or too dry for forests, are satisfactory for savannas. More moderate conditions support moist forests.
(4) Waterlogged soils (溃水土壤) occur in areas that are flat or have poor drainage. These soils usually contain large amounts of clay and easily become water-saturated (饱水状态) . Air cannot penetrate between the soil particles, making the soil oxygen-poor. By contrast, dry soils are sandy and porous, their coarse textures permitting water to drain rapidly. Sandy soils are prone to the leaching of nutrients and minerals and so tend to be nutritionally poor. Though most savannas are found on sites with poor soils (because of either moisture conditions or nutrient levels of both), poor soils can and do support lush rain forests.
(5) Most savannas probably experience mild fires frequently and major burns every two years or so. Many savanna and dry-forest plant species are called pyrophytes (耐火植物) , meaning they are adapted in various ways to withstand occasional burning. Frequent fire is a factor to which rain forest species seem unable to adapt, although ancient charcoal remains from Amazon forest soils dating prior to the arrival of humans suggest that moist forests also occasionally burn. Experiments suggest that if fire did not occur in savannas in the Americas, species composition would change significantly. When burning occurs, it prevents competition among plant species from progressing to the point where some species exclude others, reducing the overall diversity of the ecosystem. But in experimental areas protected from fire, a few perennial grass species eventually come to dominate, outcompeting all others. Evidence from other studies suggests that exclusion of fire results in markedly decreased plant-species richness, often with an increase in tree density. There is generally little doubt that fire is a significant factor in maintaining savannas, certainly in most regions.
(6) In addition, humans have contributed to the conditions favoring the formation of savannas. On certain sites, particularly in South America, savanna formation seems related to frequent cutting and burning of moist forests for pastureland. Increase in pastureland and subsequent overgrazing have resulted in an expansion of savanna. The thin upper layer of humus (腐殖质) (decayed organic matter) is destroyed by cutting and burning. Humus is necessary for rapid decomposition of leaves by bacteria and fungi and for recycling by surface roots. Once the humus layer disappears, nutrients cannot be recycled and leach from the soil, converting soil from fertile to infertile and making it suitable only for savanna vegetation. Forests on white, sandy soil are most susceptible to permanent alteration.
According to Paragraph 6, human activity affects soils in all of the following ways EXCEPT________.
选项
A、decomposition of leaves occurs too fast for surface roots to obtain nutrients
B、nutrients are not recycled
C、humus is destroyed
D、certain soils become unable to support vegetation other than savanna vegetation
答案
A
解析
推断题。在某些地区,热带草原的形成可能和为了建造牧场而对湿润林地进行频繁的砍伐和焚烧有关。过度放牧导致了热带草原的扩张。稀薄的腐殖质(腐烂的有机物)上层被砍伐和焚烧行为破坏。腐殖质是树叶被细菌和真菌迅速分解和根部表面循环的必需品。一旦腐面层消失了,营养物质就不能循环并渗入土壤中,土壤由肥沃变得贫瘠,故选A。
转载请注明原文地址:https://jikaoti.com/ti/quzMFFFM
0
专业英语四级
相关试题推荐
Itisnaturalforyoungpeopletobecriticaloftheirparentsattimesandtoblamethemformostofthemisunderstandingsbetw
Researchershaveestablishedthatwhenpeoplearementallyengaged,biochemicalchangesoccurinthebrainthatallowittoact
Intheearly1800’s,over80percentoftheUnitedStateslaborforcewasengagedinagriculture.Machineryand【C1】______technol
WemetRihanaandherhusbandatapartytwomonthsago.______we’vehadnofurthercommunication.
A、Manyintervieweeshavesufferedfromcrimesinrecentyears.B、HalfofthepeopleinAmericahavegunstokeepthemsafe.C、Th
RiteofPassageisagoodnovelbyanystandards;______,itshouldrankhighonanylistofsciencefiction.
Thegovernmentshoulddo______liesinitspowertoimprovethelivingstandardofthispoorregion.
Intoday’sjobmarket,applicantswhohavestudiedabroadarewidelywelcomedbycompaniesandschools,andareofferedwithmor
Huangjianxiang’spassionate_______madethefootballmatchbetweenItalyandAustraliaveryinteresting.
RainforestsTropicalrainforestscanbefoundinplacesneartheEquator./Despiteoccupyingarelativelysmallarea,/
随机试题
根据公式,房地产评估值=房地产年收益额/资本化率,此公式假设前提包括【】
乙型肝炎病毒复制活跃乙型肝炎感染恢复,已产生免疫力
患者,男,48岁。口苦,心烦,闷胸不舒,入睡困难,舌质红,脉数。用药宜选()
外企白领朱某与王某于2006年4月结婚,二人进行了婚前财产公证,并订立了婚后财产的归属协议,协议约定:婚前个人财产以及结婚后双方各自取得的财产,归各自所有。2006年7月,朱某为出国进修借朋友李某30万元,后李某急需用钱,遂要求王某还款。以下关于本案的说法
黄强因遗嘱继承纠纷将其妹黄月诉至法院,并在质证时提出黄月举示父亲的自书遗嘱是伪造的,申请笔迹鉴定,法院准许。关于本案的鉴定,下列哪些说法是正确的?()
官府菜在规格上一般不低于宫廷菜,而又与庶民菜有极大的差别。()
有一阅览室,读者进入时必须先在一张登记表上登记,该表为每一座位列出一个表目,包括座号、姓名,读者离开时要注销登记信息;假如阅览室共有100个座位。试分别用信号量和P、V操作以及管程来实现用户进程的同步算法。
A、Theoldpackageiscolorless.B、Theoldpackageisuninteresting.C、Peopleshouldprotecttheenvironment.D、Theoldpackagei
A、Theexamisveryeasy.B、Berelaxandcalmandthenshewillmakeit.C、It’snecessarytobringadictionarywithher.D、It’s
Thefirststeptodefeatinganenemyistopindownhisexactidentityandhowheoperates.Hypertension—chronic,abnormallyrai
最新回复
(
0
)