1 . City trees grow faster and die younger than trees in rural forests, a new study finds.

Studies had shown forests take in greenhouse gas. But there hadn’t been much data on whether city trees grow, die and take in CO₂ at the same rate as forest trees do. So Lucy Hytyra and her workmates at Boston University in Massachusetts decided to find out.

To figure out how quickly trees were growing, the researchers tracked their diameters (直径) between 2005 and 2014. The team focused on red oaks and red maples growing on Boston streets. These trees grew four times faster than the same species did in a nearby forests. Faster-growing trees take in more CO₂. Over the nine years, the researchers tracked these trees, and they found city ones took in four times as much CO₂ from the air as the forest trees did. The city trees also, however, were twice as likely to die. So over the lifetime of each type of tree, forest trees actually take in more CO₂.

“City trees grow faster mainly because they have less competition for light from their neighbors,” Hutyra says. In a forests, trees tend to grow close together, shading their neighbors. Street trees also benefit from higher levels of nitrogen (氮) in rainwater. Nitrogen is an element that helps plants grow. Still, she says that her team’s findings may not apply to arid cities. “In locations short of water like Phoenix or Los Angeles, city trees might respond differently because it’s very hot in the city and they have so little water.” At such sites, she says, “Trees might grow slower.”

So why were Boston’s streets trees twice as likely as their country cousins to die young? City roads can leave big-tree roots less room to grow. Plus, people often decide to cut down trees that are sick, or in the way of some desired building or view.

1. What can we learn from the research by Lucy’s team?

A．City trees live twice longer than forest trees.

B．Over their lifetime, forest trees actually take in more CO2.

C．Over the nine years, faster-growing trees breathe in less CO2.

D．On Boston streets red oaks grow much faster and taller than red maples.

2. City trees grow more quickly mainly because ________.

A．they live in hot weather	B．they take in more water
C．they tend to grow close together	D．they benefit from more light and nitrogen

3. What does the underlined word “arid” in paragraph 4 probably mean?

A．Dry.

B．Busy.

C．Crowded.

D．Large.

4. Where is the passage most likely from?

A．A travel journal.

B．A story book.

C．A science magazine.

D．A medical report