Where species will find refuge

Nooks and crannies offer safe harbor from a changing climate.

 

Connie Millar picks her way across a boulder field in California’s Lee Vining Canyon. The ramparts of the High Sierra, which mark the east entrance of Yosemite National Park, claw at the western horizon. Across the valley, RVs grumble down the curves of Tioga Pass Road. But Millar is listening for something else on this autumn day: pika. 

American pikas are endearing, pocket-sized mammals with prodigious ears and a distinctive high-pitched call. But they can’t tolerate heat — a trait that has made the pika into something of a climate change icon. Even President Obama has noted that warming has pushed the creatures uphill in search of cooler conditions.

But Millar, an athletic, strawberry-blonde ecologist with the Forest Service’s Pacific Southwest Research Station, says there’s more to the story. While some studies have reported problems for pikas in the Great Basin, Millar and others have found that many populations in the Sierra Nevada and Rocky Mountains have held their ground, a feat she attributes to the unique qualities of the talus fields they call home. 

For the past 10 years, Millar and her colleagues have monitored temperatures in boulder piles like those in Lee Vining Canyon. At the hottest times of day, there can be a 12 degree Fahrenheit difference between the rocks’ surface and the shaded chambers between them. Those chambers often retain ice and stay cooler than the average air temperature, the metric researchers usually use to predict future conditions.  

These differences matter here, just as they do for people living in places like Phoenix. “If we were out on the tarmac, we would probably all die,” Millar says. “But we have air-conditioned offices and buildings, so we just go inside.” The chambers in talus offer pikas a similar option, and could help the critters survive climate change — at least for a while.

Places like talus fields, where local climate conditions defy regional trends, are called refugia, and they can be found in the nooks and crannies of almost every landscape. But only in recent years have scientists and managers begun to recognize their importance for climate adaptation. For as long as these pockets remain sheltered, they could offer much-needed protection for threatened plants and animals — and a rare glimmer of hope for the people trying to save them. 

Connie Millar, an ecologist with the Forest Service’s Pacific Southwest Research Station, hangs thermometers in trees in the Sweetwater Mountains of California to monitor air temperature. The measure will be used to compare temperatures in pika taluses, and other micro-habitats, to the standard air temperature and determine if climate refugia exist in the area.
Scotty Strachan

Like many ecologists, Millar first came across the concept of refugia while trying to make sense of the sometimes-perplexing places species exist today. Why, for instance, do limber pines grow in the mountains of Nevada, far from the tree’s Northern Rocky Mountain stronghold? And why does the Wyoming toad live only in the Laramie Basin, 500 miles south of its close relative, the Canadian toad?

The answer, researchers discovered, is that these populations are most likely leftovers from the last ice age, which ended roughly 12,000 years ago. As the planet warmed, many cold-adapted species retreated to higher latitudes and elevations — except in refugia, where local conditions allowed them to persist. Because these places remained colder or wetter or foggier than surrounding areas, they shielded species from what had become an inhospitable climate.

Scientists hope that refugia will play a similar role in the future, buying time while managers come up with long-term climate adaptation plans, or until greenhouse gas emissions drop. But first, researchers must figure out how refugia form and where they occur.

One of the most common mechanisms for producing refugia in mountainous terrain is cold-air pooling, which happens when cool, dense air flows downhill on calm nights and collects on valley floors. In 2008, Jessica Lundquist, a hydrologist at the University of Washington, developed a way to pick out which drainages might host cold-air pools. She and her students have applied the method to entire mountain ranges, including the Sierra Nevada, providing ecologists with a critical resource: maps of likely refugia. 

In other cases, the ability to hold onto moisture is all it takes to create refugia. Researchers working in the Klamath-Siskiyou Mountains, an area of exceptional biodiversity on the California-Oregon border, have identified north-facing slopes, old-growth forests and river valleys as places that retain more water. In the Mojave Desert, climate models suggest that entire mountain ranges could serve as refugia because they squeeze more precipitation out of the dry air.

Complex landscapes produce a wide range of microclimates, but they exist everywhere — even in talus fields and frost pockets that occur in farmers’ fields, Lundquist says. “You can look at almost any scale and see these setting up.”

An American pika runs toward one of its winter food caches with a mouthful of plants. Pika don’t tolerate heat very well, but are able to find cool places of refuge in the nooks and crannies of talus fields.
Tom & Pat Leeson

As scientists’ understanding of refugia grows, land managers must decide how to use them. The National Park Service and Bureau of Land Management now recognize refugia as important climate adaptation tools, but few management plans include concrete ways to use them, says Pat Comer, chief ecologist at the conservation nonprofit NatureServe.

One approach simply involves remembering they exist when planning for the future. For instance, managers have long flirted with the idea of reintroducing wolverines to the Sierras. But scientists worried that the scrappy carnivores, which were eradicated in the early 20th century, would not survive in a warmer future. Wolverines den in snowdrifts, and declining snowpacks could make it hard for them to breed.

So in 2011, Lundquist’s team partnered with researchers at the U.S. Geological Survey to see how local climate processes, such as cold-air pooling, alter future snow forecasts, and thus, the outlook for wolverines. The results suggested that wolverines might be able to find more spring snow in previously unrecognized mid-elevation refugia. “They didn’t need the whole mountain range to be that cold,” Lundquist says. “They just needed a spot for their den.”

Managers could also focus existing conservation efforts on refugia, says Toni Lyn Morelli, a research ecologist at the USGS’s Northeast Climate Science Center in Amherst, Massachusetts, and lead author of a new study on managing refugia. Investing in places that naturally resist warming would allow managers to target some of the other stresses that organisms experience, Morelli says.

For example, managers could help cold-adapted fish like bull trout by protecting slow-warming mountain streams on public lands that  face threats from logging. Scientists working in the Klamath-Siskiyou region have made similar arguments for shielding vulnerable refugia from grazing and mining.

Those who manage protected areas could also make the most of their safe-harbor qualities. Devils Postpile National Monument, 20 miles south of Lee Vining Canyon, boasts a deep valley with subalpine meadows where frequent cold-air pooling and spring-fed creeks keep temperatures low. “We are kind of like a poster child,” says Deanna Dulen, the park’s superintendent. Dulen sees an opportunity to protect valuable meadow habitat, which is under threat from climate change, by limiting visitor impacts and removing encroaching conifers.

Vulnerable species could even be relocated to healthy refugia like Devils Postpile. Artificial refugia could be developed, as well: Lundquist’s group’s recent research shows that small forest clearings, created as part of routine tree thinning, can boost snow cover. A hole in the forest canopy about 100 feet wide allows more snow to pile up, and the shade of the surrounding trees prevents it from melting. “You’ve just created a nice big snow trap,” she says.

Her team has found that in the Cascade Mountains, the snow in such clearings lasts between two weeks and two months longer, supplying soil moisture and stream flow through the spring and summer. It might seem like a small way to address the daunting problems of climate change, Lundquist says, but “this is something you can do at a scale you can control.”

The Trinity Alps in northern California are part of the Klamath-Siskiyou mountain range where north-facing slopes and places that retain more water have been identified as climate refugia.
Miguel Vieira/CC Flickr

After a few minutes of scrambling over the rubble, Millar hears a pika — a long squeak like the sound made by a squished dog toy. The call comes from the bottom of the talus field, where air circulation inside the rock pile creates the coolest conditions. A lush meadow that provides food for the pika sprawls below, fed by moisture trapped under the talus. The idyllic scene illustrates why Millar think the animals should be emblems of resilience — not vulnerability.

But refugia alone can’t save imperiled species. Habitat fragmentation makes it hard for many plants and animals to reach places that could meet their needs. And species will need so-called climate corridors, which allow migration to higher latitudes and elevations as temperatures rise, Comer says.

Refugia also won’t last forever. If warming continues unabated, these places will eventually grow too hot, or too dry. Michael Dettinger, a hydrologist with the USGS in Carson City, Nevada, sees three possibilities: Refugia will resist warming, they will reflect warming, or they will disappear altogether.

Dettinger has studied long-term temperature records from two spots that experience cold-air pooling but have followed different trajectories in recent decades: the Lake Tahoe Basin and the nearby Truckee River drainage in California. Stubbornly resistant, Tahoe hasn’t warmed at all, while the Truckee has heated up at about the same rate as its surroundings, reflecting the regional trend.

For now, both places remain unusually cool for their elevation, but that could change. New weather patterns might disrupt the area’s cold-air pooling, for example, by bringing more wind. “The nightmare scenario is that the microclimate that makes the refugia work could dissipate in a way that actually made it the fastest-warming thing in the area,” Dettinger says.

Still, Millar thinks focusing on refugia is better than doing nothing. Balanced on a jumble of jagged rocks, she surveys the valley — a place she has visited every summer for more than a decade and which she hopes will host pikas for at least a few decades more.

“We have to work with interim strategies,” she says, “because what else can we do?”

Note: A previous version of this article misspelled Toni Lyn Morelli's name.