Last September, US Geological Survey ecologist Nate Stephenson hiked into Sequoia National Park’s Giant Forest to look for dying seedlings. California was suffering through its third year of severe drought, and trees were dying in the park in greater numbers than usual. The roadside leading up to Giant Forest was pincushioned with trees faded brown – dead oaks, sugar pine, fir, incense cedar. But the forest’s namesake trees, which are among the world’s oldest and largest, were faring better. They’re tough – they have to be to live for thousands of years – and tend to grow in the wettest parts of the landscape.

Still, Stephenson thought the effects of the drought might have started to become visible on sequoia seedlings, which are typically more vulnerable to environmental fluctuations than mature trees. He searched the forest floor, but found nothing out of the ordinary. It was only when he looked up that he was startled: he saw a towering old sequoia loaded with tufts of evergreen foliage turned brown.

The tree wasn’t dead, but such foliage die-back is an uncommon sign of stress. “I’ve been studying sequoias for 35 years or so and had never seen anything like this,” Stephenson says. He deployed a field crew to hike through Sequoia and its sister park, Kings Canyon, to document the die-back. About half of the more than 4,300 trees they surveyed had lost 10% to 50% of their foliage, while 1 in 100 had lost more than 50%.

I’ve been studying sequoias for 35 years or so and had never seen anything like this Nate Stephenson, US Geological Survey ecologist

How likely giant sequoias are to survive as the climate changes, making grueling droughts increasingly common, is uncertain. “One of the big questions is, just how much drought can giant sequoias survive?” explains Koren Nydick, an ecologist and science coordinator for Sequoia-Kings Canyon.

The stressed trees may hold clues to their species’ future that can help managers figure out how to protect them, and this summer, Stephenson and others have been racing to document them.

“We’re treating the drought as if it’s a possible preview of the future,” Stephenson explains. “Our goal is to map completely for all sequoia groves in the Sierra Nevada where they’re most vulnerable to drought.”

Increasingly sophisticated technology has made such an ambitious proposition possible. Greg Asner, a professor at Stanford University and lead scientist for the Carnegie Airborne Observatory, has spent the summer flying over California’s forests to assess their condition. His plane is equipped with instruments that capture the chemistry of individual trees across entire landscapes, generating colorful 3D maps that allow land managers to identify hotspots of stress or resilience. “It’s like going in and getting a blood test, and the doctor saying you’re OK or you’re not,” Asner explains.

The only time I’ve seen it this bad is in the Amazon in 2010, when I mapped millions and millions of dead trees Greg Asner, Stanford University professor

For the state as a whole, the results so far are sobering. “The only time I’ve seen it this bad is in the Amazon in 2010, when I mapped millions and millions of dead trees,” Asner says.

Worst off are the forests south of the Sierra Nevada, almost all of which are “in huge trouble,” according to Asner.

He’s mapped significant mortality in the low and mid-elevation forests in the Sierras, and found evidence of widespread stress in northern California as well, though it’s less extreme than in the south. The least stressed forests are near the Oregon border and at high altitudes in the Sierras.

As for the sequoias, Asner has flown the parks a few times since May, collecting data on trees’ biomass and their water, sugar and nitrogen levels, which together give researchers a good sense of how they’re holding up.

At the same time, Stephenson’s field crews are again surveying giant sequoia die-back from the ground, and Anthony Ambrose, a University of California, Berkeley, researcher with expertise in giant sequoia physiology, is climbing trees to sample their foliage.

Ambrose’s study trees span a spectrum, from severe foliage die-back to none at all. When he climbs, he clips off foliage and immediately measures its water content. The tension in trees’ water columns grows during drought, and can become so great that the columns collapse, which can kill the tree. Ambrose’s field measurements will help gauge how close they are to that point, and he’ll do more detailed chemical analyses later in the lab.

His findings, as well as the results from Stephenson’s ground surveys, will be used to calibrate Asner’s data. Collectively, they’ll piece together a picture of the sequoias’ health that will be unprecedented for land managers here.

Dropping leaves is a water conservation strategy, and it might help a sequoia maintain a good water supply to the foliage that remains. “It’s not a desirable thing for a tree to have to lose up to half of its foliage”, Ambrose says, “but it is an indication that they do have mechanisms to deal with severe drought”.

The team will also look for spatial patterns in the die-back and try to explain why certain areas are more sensitive than others. That would help park managers prioritize efforts to protect sequoias when the drought lifts. They could use prescribed fire to thin certain parts of the forest and reduce competition for water, for instance.

Early results from Asner’s flights show that the giant sequoias are doing better than many California trees. “There are some that are showing some drought stress, but many are not,” Asner says.

A map of the Giant Forest with blue indicating high water content in the trees, or low drought stress; green indicating moderate water content, or mild drought stress; and red indicating low water content, or high drought stress. Photograph: Courtesy of Greg Asner, Carnegie Airborne Observatory

The grove appears to have a stress gradient, Asner explains, with trees doing well on the east side, where it’s relatively flat, and stress building to the west, where the land slopes more. Stressed trees are also more common near meadows. Asner suspects that soils hold water better on the flatter terrain, while the trees near meadows might have shallower root systems, or suffer from hotter temperatures because there’s less shade.



Sequoias are resilient, and this isn’t the first drought the ancient trees have endured. But it is a different kind of drought because temperatures are warmer. Heat intensifies drought by robbing plants and soils of additional moisture. And already, Stephenson says, “it seems to have put extra stress on sequoias”.

After noticing the die-back last fall, Stephenson searched park records for mention of similar foliage loss during the droughts of the 1920s and 30s, which impacted California as well as the Great Plains.

“During the Dust Bowl period, they did see a lot of pines dying,” Stephenson says. “But not a whisper of anything going on with the sequoias. We couldn’t find any reference to anything like this being seen in the past.”