长期干旱影响加州的红杉（在线收听）

AS IT IS 2015-10-04 Drought Affecting California’s Sequoias 长期干旱影响加州的红杉

California continues to suffer under a historic drought. The state has received very little rainfallover the past four years.

California is famous for its huge sequoia trees. They are among the largest and oldest living thingson Earth. But scientists say the trees are suffering because of the lack of water.

Now the National Park Service is studying how the drought has affected the sequoias and what canbe done to help them survive.

Anthony Ambrose is a tree biologist with the University of California, Berkeley. One recent day, heslowly climbed up a giant sequoia in California's Sierra Nevada Mountains. The Sierra Nevada rangeis the only place on earth where sequoias live.

"It's an amazing experience to be able to climb up into these things and know that it's beengrowing in this same spot for you know a thousand years or more."

Sequoias can grow to a height of over 90 meters tall. The bottom of the tree can be up to 15meters wide. Some trees are more than 3,000 years old.

The Sierra Nevada range once had plenty of water because of mountain streams. But less water isavailable now because of the drought. Each sequoia needs more than 3,000 liters of water a day. The trees are now getting much less. Some of them are showing signs of thirst, says AnthonyAmbrose.

"We've observed some unusual and abnormal levels of foliage die-back, which haven't beenobserved in the park before."

Researchers are using scientific equipment in tree canopies to help measure air temperature andhumidity levels. They also are studying sequoia seedlings, information from individual trees andimages collected by aircraft to measure the seriousness of the danger.

One possible solution would be to cut down less important tree species because other plantscompete with the sequoias for water. Koren Nydick of the National Park Service is one of thescientific investigators.

"They'll have more water than they would have in a denser forest, more nutrients and light andtherefore be more resistant and resilient to these hot droughts they're faced with in the future."

Scientists say that over thousands of years, sequoias have survived many droughts, forest fires,insect infestations and other disasters. Scientists say the sequoias will probably survive thisdrought, unlike many other California trees.

Words in This Story

drought – n. a long time during which there is very little or no rain

thirst – n. a very great need for something to drink

die-back - n. a condition in which a plat begins, owing to disease or an unfavorable environment

canopy - n. something that hangs or spreads out over an area

humidity – n. wetness in the air