Big trees are very important in ecology. For a start, they provide food for countless other species and shelter for many animals. With their tall branches in the sun, they take in vast amounts of energy. This allows them to produce massive crops of fruit and flowers that much of the animal life in the forest feed on.
Only a small number of tree species have the genetic(基因的) ability to grow really big. The biggest are native to North America, but big trees grow all over the globe, from the tropics(热带地区) to the forests of the high latitudes(纬度). To achieve giant size, a tree needs three things: the right place to establish(建立) its seedling, good growing conditions and lots of time with low adult death rate. Lose any of these, and you will lose your biggest trees.
In some parts of the world, populations of big trees are dwindling because their seedlings cannot survive. In southern India, for instance, an aggressive non-native bush, Lantana camara, is invading the floor of many forests. Lantana grows so thickly that young trees often fail to take root. With no young trees to replace them, it is only a matter of time before most of the big trees disappear.
Without the right growing conditions, trees cannot get really big and there is some evidence to suggest tree growth could slow in a warmer world, particularly in environments that are already warm. Having worked for decades at La Selva Biological Station in Puerto Viejo de Sarapiqui, Costa Rica, David and Deborah Clark and colleagues have shown that tree growth there slows obviously in warmer years. “During the day, their growth shuts down when it gets too warm, and at night they consume more energy because their metabolic(新陈代谢的) rate increases,” explains David Clark. With less energy produced in warmer years and more being consumed just to survive, there is even less energy available for growth.
The Clarks’ theory, if correct, means tropical forests would be smaller over time. The largest, oldest trees would gradually die off and tend not to be replaced. According to the Clarks, this might cause a destabilization of the climate; as older trees die, forests would release some of their stored carbon into the atmosphere, causing a cycle of further warming, forest shrinkage and carbon emissions.
Besides, big trees face threats from elsewhere.
According to the passage, big trees make great contributions to the ecosystem because ________.

A．they can take in large amounts of energy

B．they determine the change of global climate

C．they provide the essentials for many creatures

D．they can improve a new cycle of further warming

All the following factors are a must for making big trees EXCEPT ______.

A．no deadly damage

B．the suitable place for new plants’ growing

C．good environment for growth

D．the highest location

The word “dwindling” (paragraph3) is closest in meaning to “______”.

A．exploding	B．growing
C．changing	D．dropping

What is the best title of the passage?

A．Big trees in trouble.

B．Advantages of big trees.

C．Results of big trees’ disappearing.

D．Importance of big trees to humans.

What will the author most probably discuss after the last paragraph?

A．More threats to the existence of big trees.

B．The effect of human activities on big trees.

C．Benefits of big trees to the whole atmosphere.

D．Comparison between common trees and big ones.

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