Slowly and painstakingly, University of California tree biologist Anthony Ambrose climbs up a giant sequoia in California’s Sierra Nevada mountain range, the only place on earth where these magnificent trees live.

“It’s an amazing experience to be able to climb up into these things and know that it's been growing in this same spot for a thousand years or more,” he said.

Sequoias grow over 90 meters tall, their trunks are up to 15 meters wide and some of them are more than 3,000 years old.

Once-rich mountain streams have dwindled to trickles and the trees - that each need more than 3,000 liters of water a day - are now getting much less. Some of them are showing signs of thirst.

“We've observed some unusual and abnormal levels of foliage die-back, which haven't been observed in the park before,” Ambrose said.

Sensors installed in tree canopies, examinations of seedlings, data from individual trees and images collected by observational planes will help scientists measure the seriousness of the danger.

One possible solution would be to cut down less important species of trees that compete with sequoias for water.

“They'll have more water than they would have in a denser forest, more nutrients and light and therefore be more resistant and resilient to these hot droughts they're faced with in the future,” said Koren Nydick, who is with the National Park Service.

Scientists say that over thousands of years, sequoias have gone through many droughts, forest fires, insect infestations and other calamities, so they will probably survive this drought, unlike many less-resilient California trees.