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
Weed Communities In an intact plant community, undisturbed by human intervention, the composition of a community is mainly a
Weed Communities In an intact plant community, undisturbed by human intervention, the composition of a community is mainly a
admin
2010-07-19
45
问题
Weed Communities
In an intact plant community, undisturbed by human intervention, the composition of a community is mainly a function of the climate and the type of soil. Today, such original communities are very rare—they are practically limited to national parks and reservations.
Civilization has progressively transformed the conditions determining the composition of plant communities. For several thousand years vast areas of arable land have been hoed, ploughed, harrowed and grassland has been cut or grazed. During the last decades the use of chemical substances, such as fertilizers and most recently of weed killers (herbicides) has greatly influenced the composition of weed communities in farm land.
All selective herbicides have specific ranges of activity. They control the most important weeds but not all the plants of a community. The latter profit from the new free space and from the fertilizer as much as the crop does; hence they often spread rapidly and become problem weeds unless another herbicide for their eradication is found.
The soil contains enormous quantities of seeds of numerous species—up to half a million per m2 according to scientific literature--that retain their ability to germinate for decades. Thus it may occur that weeds that were hardly noticed before emerge in masses after the elimination of their competitors. Hence, the knowledge of the composition of weed communities before selective weed killers are applied is not only of scientific interest since the plant species present in the soil in the form of seeds must be considered as potential weeds. For efficient control the identification of weeds at the seedling stage, i. e. at a time when they can still be controlled, is particularly necessary; for the choice of the appropriate herbicide depends on the composition of the weed community.
The composition of a plant community ______.
选项
A、depends on climate and soil type in a virgin environment undamaged by human beings
B、was greatly affected by human beings before they started using chemical substances on the soil
C、was radically transformed by uncivilized human beings
D、refers to plants, trees, climate, type of soil and the ecological environment
答案
A
解析
转载请注明原文地址:https://kaotiyun.com/show/J8lO777K
0
专业英语八级
相关试题推荐
"littledidtheEnglishpeopleownhimforhisservice"(paragraphone)meansthattheEnglish______.Thepointofthelastsh
ThethirdparagraphexaminesAmerica’sfuture-mindednessfromthe______perspective.Theauthorpredictsinthelastparagrap
ThethirdparagraphexaminesAmerica’sfuture-mindednessfromthe______perspective.Theword"pooh-pooh"inthesixthparagr
______isgenerallydefinedasthestudyofmeaningandusuallypaysattentiontothestudyofmeaningoflinguisticunitsthems
TheWatergateAffair,thepoliticalespionagescandal,finallyledtotheresignationofPresident______in1974.
Theholidaybeganasawaytohonorsoldierskilledduring______.
Onecharacteristicoftherichclassesofadecliningsocietyistheirtendencyto______.WhichofthefollowingisNOTanele
Howdothescientistsmeasurethepitch?
HowImportantIsMoney?Inthefirstpartofyourwritingyoushouldpresentyourthesisstatement,andinthesecondpartyo
DuringtheBritishCivilWar,thesupportersofthekingwascalled______.
随机试题
哪项法洛四联症的表现是错误的
ToGA临床研究证实,曲妥珠单抗(赫赛汀)联合化疗可改善HER2阳性晚期胃癌患者的生存,患者联合用药的中位OS为
甲公司为增值税一般纳税人,甲公司委托乙公司,加工应纳消费税物资(非金银首饰)收回后用于继续加工应税消费品,其由受托方代收代交的消费税,应记入()账户。
下列何种情况不会导致转介的产生:()。
外国在华领事裁判权正式确立于()中,并在其后签订的一系列不平等条约中得以体现。
数据的分析方式说法错误的是()。
战略数据规划方法可略称为J.Martin方法,方法将数据规划过程分为若干步,下述Ⅰ.建立企业模型Ⅱ.确定研究边界Ⅲ.建立业务活动过程Ⅳ.确定实体和活动Ⅴ.审查规划结果哪些属于数据规划的基本步骤?
执行以下程序段a$="VisualBasicProgramming”:b$=”C++”c$=UCase(Left$(a$,7))&b$&RightS(aS,12)变量c$的值为
Eightyearsold.
MasstransportationrevisedthesocialandeconomicfabricoftheAmericancityinthreefundamentalways.Itcatalyzedphysical
最新回复
(
0
)