Trees often grow to appear strong, which might make it hard to believe that they're easily harmed on the Inside. But that is the case, according to a worldwide survey of more than 200 types of trees. Researchers studied the inner part of the trees and found that most don't have a built-in safety net that would allow them to survive a serious drought.
"I think this is a really big deal." ecologist David Breshears said. He says it and other studies "keep pointing to: ‘Whoa, our forests are really fragile."'
Studies like this one are particularly worrying about climate change. The average temperature of the planet is rising. Scientists predict that warming will cause changes in rainfall patterns. That could lead to longer, more severe droughts in many regions around the world. As a result, the trees that now grow there could face a serious threat.
When drought strikes, trees begin to fail in the xylem(木质部), special tissue made of many tiny, strawlike tubes. This tissue makes it possible for a tree to draw water from the ground and deliver it to the leaves. During times of drought, tiny air bubbles can develop in the tubes. Too many bubbles block the flow of water——and lead to certain death for the tree.
Brendan Choat, a plant physiologist, worked on the new study along with 23 other researchers. All together the scientists studied 226 tree species in 81 different sites around the world. Choat said that this study is the first to look at all forest types and find so many species living on the edge. Of the 226 different types of trees Choat and his colleagues studied, 70 percent lived close to the point where a serious drought would stop water transport. Trees that flower, such as maples and oaks, were more vulnerable than conifers (针叶树), which, including pines and firs have evergreen, needlelike leaves.
Plant ecologist Bettina Engelbrecht said that in the interest of conservation, scientists can't just concentrate on a few species. "We have to worry about them all."
【小题1】The first paragraph implies that trees .| A．can live through a drought. |
| B．have the ability to protect themselves |
| C．are worth a great deal of money |
| D．are weak as a matter of fact |
| A．The drought. | B．The tube. | C．The water | D．The xylem. |
| A．One-sided. | B．Fruitless. | C．Time-consuming. | D．Meaningful. |
| A．Care about the trees that flower. |
| B．Pay attention to all species of trees. |
| C．Find out why pines and firs are strong. |
| D．Study the trees living on the edge of death. |
| A．Serious drought is a threat to most trees |
| B．Trees face death due to lack of protection |
| C．Global warming reduces the number of trees |
| D．Trees reflect the change of the global climate |
