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  3. 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

admin2010-07-19  66
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.
Why are there problem weeds?
选项 A、Because they are the weeds that cannot be eradicated by herbicides.
B、Because all selective herbicides can encourage the growth of previously unimportant weeds by eliminating their competitors.
C、Because they were hardly considered before so that their seeds were not prevented from germinating.
D、Because they benefit greatly from the fertilizer applied to the farm land.
答案B
解析
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