题目内容：

    Native woodlands can resist the spread of invasive species by blocking daylight reaching the forest floor, researchers have suggested.

The team, consisting of researchers from US universities, decided to focus its attention on common buckthorn(沙棘), which is all invasive species in North America. It out-competes native plants and degrades soils and forests, doing harm to humans and other wildlife. In order to create environments that resist invasion by buckthorn, and thus avoid those costs, it was necessary to understand which forest characteristics offered the greatest influence on the success or failure of buckthorn plants becoming established.

In their experiment, the team grew buckthorn under a variety of different levels of shade and measured the light available to the buckthorn. These included one made up from deciduous species, such as birch, another from evergreen species, such as pine, and another from a mixture of both species.

Dr. Schuster observed: “Results showed that forests that are able to block out 96% of incoming light in the spring or autumn can successfully resist buckthorn invasion.” He added that, in general, evergreen species were much more capable of creating this level of shade compared with deciduous species.

“We anticipated that buckthorn would fail in areas with extremely low light levels, since having some light is necessary for most plants to live,” he said. Yet the team were surprised by how much the buckthorn depended upon light availability in spring or autumn.

He added that if forest managers were seeking to improve the long-term resistance of their forests to buckthorn invasions, then it might be necessary to introduce species that helped reduce the amount of light reaching the forest floor, such as evergreens.

But this approach was not risk-free, warned Dr. Schuster. He added: “If we change forests to get rid of buckthorn, we may push out some desirable native species at the same time. So, the question becomes whether we can select plant communities both to have the species we want and to produce ample shade in spring and autumn to keep buckthorn out.”

1.This experiment was carried out to study ________.

A.why soil conditions matter to plants B.how to help native plants grow better and faster

C.the impact invasive species have on humans D.the factors influencing the growth of buckthorn

2.What kinds of trees were chosen for the experiment?

A.Trees providing different amounts of shade. B.Trees growing and declining at different rates.

C.Trees of different heights. D.Trees of different origins.

3.What does Dr. Schuster imply in the last paragraph?

A.We must get rid of buckthorn whatever the cost.

B.Keeping native species ought to be the top priority.

C.We should be careful about selecting species of plants.

D.Communities need to work together to fight the alien plants.

4.You may likely find this article in a ________.

A.psychological and behavioural paper B.travel guide

C.nature and environment journal D.fashion magazine