全国100所名校最新高考模拟示范卷 英语卷9 20·MNJ·英语-YNGZGS

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City
trees grow faster and die younger than trees in rural forestry, a new study
finds. Over their lifetimes, then, urban trees will likely absorb less CO2
from the air than forest trees.

As we
all know, the earth would be freezing or burning hot without CO2.
However, CO2 is a greenhouse gas, meaning it traps energy from the
sun as/heat. That makes temperatures near the ground rise. Human activities,
especially the widespread burning-of fossil(化石)fuels, have been sending extra greenhouse
gases into the air. This has led to a rise in average temperatures across the
globe.

Studies
had shown forests readily absorb CO2, but there hadn't been much data on
whether city trees grow, die and absorb CO2 at the
same rate as forest trees do. So some researchers decided to find out.

To
figure out how quickly trees were growing, researchers tracked their diameters
(the width of their trunks) between 2005 and 2014. A tree's diameter increases
as it grows, just as a person's waist size increases as they gain weight. About
half the weight of a tree is carbon, research has shown. Most of the rest is
water. Over the nine years' tracking, the researchers found city trees absorbed
four times as much carbon from the air as forest trees. However, they were
twice as likely to die. So over the lifetime of each type of tree, forest trees
actually absorbed more CO2.

City
trees grew faster because they had less competition for light from their
neighbors. In a forest，trees tend to grow close together，shading their neighbors. Street trees
also benefit from higher levels of nitrogen (氮)in rainwater. Nitrogen helps plants
grow. Waste gases from gas-burning cars also contain nitrogen, thus enriching
city air with nitrogen. Later, rainwater may wash much of it to the ground.
Some street trees may also have better access to water than trees in the
country because the underground water pipes can leak.