Forestry researcher Nick Ampersee chops into the pine with his axe and removes a slice of bark. The inside looks as though it has been slathered with white-out.

"Mycelia," he says. "That's a fungus. It usually weakens the tree and then something like a bark beetle will finish the tree off."

Another researcher cuts into a different tree with her knife and shows me what looks like a couple of over-boiled grains of rice.

"We are dealing with some weevil activity here at the base," she says. The tree doesn't have long to live.

Every year over the last decade and a half, the U.S. Geological Survey has descended on Yosemite and Sequoia National Parks in California to give 17,000 trees a physical. But in a growing number of cases, what's starting off as a check-up is turning into an autopsy.

USGS ecologist Nick Ampersee chops into a tree in order to find out what killed it. (Kim Brunhuber)

The cause of death is usually insects or fungus, but researchers suspect it's almost always because of one culprit: lack of water.

Normally, only about two per cent of the trees in their study areas die. But this year, that number has grown to 13 per cent.

"That's a really severe uptick," says U.S. Geological Survey ecologist Nate Stephenson. "We've never seen anything like it before."

Stephenson bends the branch of an incense cedar. Most branches are covered with dry, dead orange needles. The rest are bare.

"I used to call them 'the immortals,' because they just never seemed to die," he says. "In the fourth year of drought, they've started dying by the bucket-loads. So they're no longer the immortals."

Warming temperatures to blame

Stephenson has surveyed some of the oldest, richest forests in the U.S. and British Columbia. Compared to just a few decades ago, he found that the trees' death rate has doubled from one to two percent. It may not sound like a lot, he says, but he says imagine if you were talking about your hometown.

"If you looked back and saw that death rates had doubled, you'd really wonder what was going on," Stephenson says. "The one thing that really stood out is warming temperatures. We think that's what's driving the increase in tree death rates."

For the past four years, California has been going through a record-setting drought. In January, the state's governor, Jerry Brown, declared a state of emergency.

Thirteen per cent of trees in the study areas in Sequoia National Park are dead. (Kim Brunhuber)

In June, Naomi Tague of UC Santa Barbara published a study in the journal New Phytologist on die-off in California forests that found that 12 million trees died due to drought this year alone. Tague, who is Canadian, says the hot, dry weather has been great for the insects and bad for the trees.

While the situation in California is dire now, in the future, Canadian forests may be at greater risk, even if the drought is less severe.

"The trees [in California] are used to drought, and so you have to get this severe drought before you start to see this die-back," Tague says.

"But you can imagine that a spruce forest in the boreal part of Canada, it's not used to seeing drought. So it hasn't developed the same types of defensive mechanisms to insects."

Canada's boreal forest, which stretches across the north of the country, is one of the world's largest intact forests. According to Tague, it may be particularly vulnerable as you move further north.

"The increase in temperature is greater at greater latitudes," Tague says. "A cold drought is not the same as a warm drought."

Their research has found that no tree seems to be immune, including the toughest, most drought-resistant trees in this forest: giant sequoias.

Some of the trees in Sequoia National Park were a thousand years old when Julius Caesar crossed the Rubicon. Last year, Stephenson spent a few days crawling around the forest floor examining sequoia seedlings, convinced they'd be affected by the heat and the drought.

"They all looked really happy," he says. "I sat back, scratched my head and looked up, and there was a huge adult giant sequoia that had a lot of foliage die-back in it. That really got us interested, and we figured the drought was probably the cause of that. And that created a cascade of studies."

They found that a significant number of older trees that had shrugged off the Dust Bowl in the 1930s were losing as much as half of their leaves.

"Ten per cent of the trees had 25 to 50 per cent die-back," says Koren Nydick of the U.S. National Park Service. "This is the first time that this kind of foliage die-back has been observed since this has been a national park."

Aiming for answers

To determine the exact extent of the problem, a small group of scientists is going to extreme -- and dangerous -- lengths.

Diagnosing dead trees (footage courtesy of UC Berkeley researchers Wendy Lloyd Baxter and Anthony Ambrose)

Raw footage of an ascent into an 80-metre giant sequoia, courtesy of UC Berkeley researchers Anthony Ambrose and Wendy Lloyd Baxter 2:05

Anthony Ambrose, a tree biologist currently working in the Department of Integrative Biology at UC Berkeley, aims his crossbow at the uppermost branches of a huge sequoia, fires... and misses.

"That was the perfect shot," Ambrose says ruefully. "It just dropped short."

Ambrose has to shoot an arrow with a line attached over a branch so far away you need a spotter to track it. That's the easy part. Next, he has to hoist himself by hand more than 80 metres up into the canopy to collect samples and take measurements.

"This tree here is in an area that has been exhibiting signs of severe crown die-back, so we wanted to characterize the trees in this part of the grove to see how stressed they are, and compare them with trees in areas that aren't affected by the drought," Ambrose says.

If there's a positive from this tree-killing drought, it's this: for Ambrose and his team, it's literally a dry run. A chance to improve their models in order to better predict what will happen in North America when this hotter, drier climate is the norm.

"This tree here is maybe a thousand, maybe two thousand years old," Ambrose says, looking at the next giant sequoia he's about to climb.

"It's dealt with severe conditions, extreme droughts, fires in the past. They're really resilient trees, but every species, every organism, has a limit, and in the future, there may be a point where drought impact becomes so severe that they shed all their foliage, they stop growing. Maybe at some point, they get susceptible to insects or disease, and start to die back."