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In Yosemite Valley, Climate Change Threatens Giant Sequoias, Tiny Animals

by Rachel Crowell
March 27, 2017 | I ❤️ PBS

Have you ever seen a pic of a pika? They're the adorable, hamster-sized relatives of rabbits that make their homes at elevations above 7,500 feet. It’s hard to look at these little guys without gushing over how cute they are, which makes them awesome stars for a new episode of "Nature" on PBS.

But it also makes the harsh reality they face even harder to swallow.

These tiny creatures face a hellish future if the effects of climate change continue to worsen, according to Joseph Stewart, a conservation biologist at the University of California, Santa Cruz.

Stewart doesn’t mince words. If temperatures rise above a certain threshold, these animals, built to withstand some of the harshest conditions on Earth, won’t be able to adapt to their sweltering surroundings.

And then?

"Pikas become toast."

"Nature" episode "Yosemite" focuses on how climate change affects living things in California's Yosemite Valley, from the tiny pika to sky-scraping giant sequoias.

Disappearing water

One theme runs through the episode like a river rushing along its course: the role water has played in shaping the Yosemite Valley, from its early history, to the present, and even looking to the future.

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A scientist climbs a giant sequoia tree in the Yosemite Valley. Photo courtesy of Anthony R. Ambrose.

Take giant sequoias, for instance. These trees are gargantuan–the General Sherman Tree is the largest tree by volume in the world. At 275 feet tall, the Sherman Tree’s height is more than 75 percent of the length of a NFL football field. The tree’s diameter at its base is more than 36 feet, larger than the combined heights of six average U.S. women.

The trees’ thirst for water parallels their towering size. Anthony Ambrose, a canopy biologist at the University of California, Berkeley, studies these giants. During the sweltering summer months, just one of these trees guzzles between 500 and 800 gallons of water every day, he said.

The main water source for these trees? Snow melt from the Sierra Nevada mountain range.

Given the extreme drought conditions California has seen in recent years, including the 2015 drought, described as the state's worst in at least 1,200 years, it’s no surprise that the trees are manifesting symptoms of their parched state.

The effects are grisly.

“Sequoias are losing foliage at an unprecedented rate—in some cases, more than half of the tree,” according to the "Nature" episode.

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Tree biologist Anthony Ambrose hanging out in a giant sequoia. Photo courtesy of Anthony R. Ambrose.

Ambrose has been closely tracking the health of about 50 giant sequoias for the past few years as their water supply has dwindled. He has measured the water status and stress level of each tree by collecting and testing leaf samples from the top of the tree and the base of its crown.

“So the measurements that we’ve been getting so far over the last couple of weeks is indicating that they are definitely at stress levels greater than we’ve ever measured in giant sequoias before," Ambrose said in the film. "This is the first time that I’ve ever been climbing in these trees and actually observed anything that’s noticeable stress.”

While these trees are clearly stressed, “they are doing a lot better than a lot of other species of trees on the landscape.”

Pine, firs and cedars have been hit even harder.

Losing trees by the millions

A U.S. Forest Service aerial survey revealed that more than 100 million California trees are dead, Ambrose said.

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Photo courtesy of Anthony R. Ambrose.

Some of these deaths can be attributed to attacks by bark beetles and other insects. Yet even those deaths can be pinned on an insufficient water supply.

Under less parched conditions, the trees “use water to kind of flood out the bark beetles when they start to attack,” Ambrose said.

The beetles and other insects are opportunistic attackers.

“Once the beetles find a weakened, stressed tree, they kind of swarm it and overwhelm its defenses,” Ambrose noted.

Giant sequoias have inborn chemical defenses that many other types of trees lack. Tannins and other chemicals in the bark and wood help them to bolster their defense against harmful insects, according to Ambrose.


Increasing temperatures are really a driving concern for the future," he said. "Every organism has a (temperature) limit. We don’t know what those limits are in giant sequoias… That’s really a big research question that we need to understand more.”

How cute, furry mammals might survive

Available data does suggest an estimated temperature at which life in the greater Yosemite ecoregion becomes more challenging for pikas, Stewart said.

Stewart and his colleagues surveyed 67 sites that have been home to pikas. The team found that 10 of those sites–or about 15 percent of them–had been abandoned by pikas at the time they were surveyed.

A Yosemite Valley pika. Photo courtesy of Joseph Pontecorvo and THIRTEEN Productions LLC.

The researchers observed a trend at the 10 sites at which pikas were extricated, or locally extinct. The best predictor for whether pikas were present at a given site was the average summertime temperature there. Pikas were more likely to be absent from sites where the mean summer temperature exceeded 57.2 degrees Fahrenheit.

That finding is “completely in keeping with what we would expect” because pikas are “primarily active in the summertime,” Stewart said.

Pikas don’t hibernate and they have high metabolic rates. So, these teeny creatures must collect sizable hay piles during warmer months in order to have enough food to munch on during the winter, Stewart said.

For each day of snowpack in the winter, a pika needs about 20 to 35 grams of food in its hay pile, said Johanna Varner, a biologist at Colorado Mesa University and an organizer of citizen scientist pika efforts across the country.

American pikas weigh between 121 and 176 grams and need to eat the equivalent of between 11.4 and 28.9 percent of their body weight each day. But when daytime temperatures rise, pikas adapt by hiding out in their dens during unbearably warm hours. That’s for a good reason–while they are equipped to deal with low temperatures, their defenses against high temperatures are poor.

“They’ve got a very poor ability to shed heat,” partly because they don’t sweat, Stewart said. The fur that covers the bottoms of their feet and the insides of their ears adds to the problem.

Adapt or die

“Pikas are certainly behaviorally plastic,” Stewart said.

This behavioral plasticity allows pikas to adapt the hours at which they collect food so they aren’t active during the hottest times of the day, Stewart said. But this adaptability only works up to a point.

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Photo courtesy of Anthony R. Ambrose.

Researchers are uncertain about how well pikas can see at night, but they know that these teeny creatures are more vulnerable to attacks from predators in the dark. Stewart called this “a risky combination.” He noted that there seems to be an increased likelihood of pikas becoming locally extinct at a site if there’s a trend towards those pikas becoming active outside of their normal daytime hours.

It appears that pikas are also changing their body sizes and breeding habits in an attempt to adapt to climate change.

“By being a little bit smaller in size, pikas might be able to cope with increasing temperatures,” Stewart said.

However, when pikas become smaller, they are “less able to cope with the cold, which is sort of their niche.”

When it comes to pika reproduction, there are a few different things happening. In some areas, it appears that young pikas are migrating from high elevation sites to lower elevation sites, but “they don’t seem to be able to reproduce.” Many of these pikas die before making it through one winter season. It also seems that in some areas an “exact opposite pattern” might be occurring where pikas are reproducing at increased rates. A similar effect has been observed in many other organisms—especially species of plants—when they are stressed.

Under stressful conditions, “weird, counterintuitive things happen in terms of species survival and reproductive rates,” Stewart said. Sometimes, when it’s unlikely that a living thing will survive, an instinct kicks in for the organism to reproduce, because that promotes survival of the species as a whole.

The future of Yosemite

At the rate climate change is progressing, some researchers think “it’s extremely likely that high elevation communities are going to collapse all at once,” rather than dying out gradually, species-by-species, Stewart said.


It’s scary, definitely scary,” he said.

The outlook for giant sequoias may be guardedly better. Ambrose emphasized the resilience of the trees he has often climbed.

“Giant sequoias are very tough trees… (but) they do need a lot of water.”

Rachel Crowell
Rachel Crowell is a Midwest-based writer exploring science and math. Follow Rachel on Twitter at @writesRCrowell. Reach Rachel at [email protected]
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