Only half a dozen giant sequoias have died in the Sierra Nevada’s Sequoia and Kings Canyon National Parks since the drought began, said the parks’ science coordinator Koren Nydick. But the fact that some giant sequoias started to show signs of stress last year caused concern among scientists because the trees are normally long-lived, with some known to be more than 3,000 years old. And all around them in the Sierra, some 6 million trees of other species have died, according to a U.S. Forest Service survey in July.

“We’re seeing firs, pines, incense cedars and oaks are all dying at a rate we’ve never seen before,” said Stephenson. “Even during the 1977 drought in California we didn’t see this many trees dying.”

The severity of the damage on some of the sequoias led him and other biologists to investigate further.

This summer, scientists from the University of California, Berkeley, Stanford University, the USGS and the Carnegie Institution for Science counted the number of giant sequoias with brown leaves, climbed 50 of the trees to see if they were having trouble transporting water to their treetops and flew over giant sequoia groves to make images of their water content using special equipment.

The researchers’ conclusion: some giant sequoias in Sequoia National Park are showing signs of stress, but it’s unclear which trees might be at the highest risk of dying.

Giant sequoias require more water than any other tree, said tree biologist Anthony Ambrose, of the University of California, Berkeley. On a hot summer day, they can suck up 500 to 800 gallons. That’s twice as much water as a California household uses in a day. And it’s not just any water.

“That water comes primarily from snow that slowly melts during the spring and recharges the groundwater,” said Ambrose, “so that during the summer months they have sufficient water to sustain their growth and physiology.”

The problem is that during California’s historic drought, little snow has been available to the trees.

“The last two winters here have been by far the warmest on record,” said Stephenson, “and what that’s meant is there’s been almost no snow on the ground.”

In August and September, Ambrose and Wendy Baxter, another UC biologist, climbed 50 giant sequoias in Sequoia National Park’s Giant Forest. They collected leaves from their treetops –sometimes the equivalent of 30 stories up-- using a simple rig that allows them to hoist themselves up on a rope while hardly touching the bark.

“There’s a beautiful view up here,” said Baxter, as she looked out over the top of the forest from her vantage point 300 feet up. She used small shears to cut a clump of leaves from the treetop and stuffed them in plastic bags.

Even at great heights, giant sequoias are able to draw water up to leaves at their treetops. Inside each of the trees’ cells, water gets pulled up to the top of the tree as if it were being sucked up through a straw.

“When we clip it, the water retracts back into the stem, kind of like a rubber band,” said Baxter.

Back on the ground, she handed the bags to UC Berkeley technician Ken Schwab, who placed a group of leaves inside a round metal device called a pressure chamber.

“When we put our stem into the pressure chamber,” said Baxter, “the amount of pressure that it takes to force the water back out is an indication of how much tension it was under.”

The higher the pressure required to push the water out, the more stressed the tree is.

“Some of the trees are definitely as stressed as we’ve ever measured giant sequoia,” said Ambrose.

This summer, Ambrose and Baxter took measurements in two sites in the forest where some of the giant sequoias had lost half of their leaves in 2014 and in two sites where trees had looked healthier. They found that the most water-stressed trees and the least stressed ones could be found in all sites. Perhaps, Ambrose said, some trees within a particular site have more access to groundwater, their roots reaching deep underground into cracks and crevices that other trees can’t get to.

In comparison with the summer of 2014, fewer giant sequoias lost half of their leaves this summer, said Stephenson. He thinks that perhaps the trees “did all the hard work of adjusting to the drought last year.”

Figuring out what trees will succumb to drought is a difficult business, said Greg Asner, of Stanford University and the Carnegie Institution for Science. Sometimes, Asner said, trees lose lots of leaves and later rebound when they have access to water. Other times, trees look healthy, but are in fact water-stressed.

Asner designed the Carnegie Airborne Observatory, a plane with instruments that measure water, nitrogen and sugars in trees using a technology called laser-guided spectral imaging. This summer he flew over and made images of the giant sequoias in Sequoia National Park’s Giant Forest. On the resulting map, the sequoias on the west side of Giant Forest appear orange and red, a sign that they’re doing worst in terms of their water content. The trees on the east side of the forest appear in blue, a sign that they have more water.

But only repeated flights can accurately pinpoint the most vulnerable giant sequoias, said Asner.

“We cannot tell if a tree is improving or declining in a single mapping,” he said. “By re-flying we can see the total amount of water change in each canopy, and that will be the best possible measure of how each tree is doing.”

So far, scientists say, they can’t draw conclusions about just how stressed the iconic giant sequoia trees are after four years of severe drought, or how many might be at risk of dying.

Scientists say the research, however, is important in establishing a baseline that will allow them to monitor giant sequoias for years to come.

“Our main concern isn’t necessarily the current drought right now; it’s looking to the future,” said Stephenson. “If the climate continues to warm, it will put more stress on the giant sequoias.”

If the drought continues, or if the trees show significant decline as the climate continues to warm, giant sequoias might need some human intervention in the future to survive climate change. That could take the form of prescribed burns to reduce competition for water from surrounding trees. Parks officials could even decide to irrigate some of their most famous giant sequoias, such as the General Sherman, billed as the world’s largest tree, said Nydick.