California’s ‘Zombie Forests’ Are Cheating Death—but Maybe Not for Long

A fifth of conifer forests in the state’s Sierra Nevada mountains are stranded in unsuitably warm conditions

Ponderosa pines
Ponderosa pines at Yosemite National Park Jodie Wilson under CC BY 2.0

In California’s Sierra Nevada mountain range, ponderosa pines, sugar pines and Douglas firs stand tall. But despite their imposing nature, many of these cone-producing trees, some of the tallest living things on Earth, have a secret: They’re living on borrowed time.

According to a study published last week in PNAS Nexus, about 20 percent of these conifers are living in “zombie forests,” stranded in climate conditions that are too warm to suit them. That number will likely double by the end of the century, researchers say, even if greenhouse gas emissions decrease to the lower end of scientific projections.

This “vegetation climate mismatch” effectively puts the trees just one wildfire, extreme drought or logging event away from disappearing.

“They’re cheating death, in a way,” says Avery Hill, an ecologist and plant biologist at Stanford University and lead author of the study, in a video. “As soon as some kind of disturbance event—in California this is probably a wildfire—shakes things up, they won’t rise again. They won’t keep standing.”

Using 90 years of vegetation data, researchers created a computer model that shows how the conifers’ range has climbed nearly 112 feet upslope since the 1930s. However, as the planet warms, the range with the best temperatures for the trees has also climbed—almost 600 feet upslope on average during the same period, outpacing the forests.

While mature trees are able to continue growing in the mismatched conditions, young trees can’t become established as easily. Should a destructive event occur to the fully grown plants, they would likely die off and be replaced by shorter, shrublike vegetation that can handle the warmer and drier habitat.

Zombie forests: Stanford researchers map where climate change has left forests vulnerable

The “zombie forests” are found in lower elevations of the mountain range, largely below 7,730 feet. At the same time, humans have been moving into the foothills of the Sierra Nevada, which are vulnerable to fires.

“Given the large number of people who live in these ecosystems and the wide range of ecosystem services they confer, we should be looking seriously at options for protecting and enhancing the features that are most important,” says Chris Field, a biologist at Stanford focusing on climate change and a co-author of the study, in a statement.

The researchers point to climate factors, like changes in annual precipitation and winter temperatures, as the main reasons for the shift in suitable conditions. Several decades of aggressive fire suppression strategies have also left the forests more vulnerable to high-intensity wildfires.

Mapping these past and potential forest transitions can help forest managers and policymakers prioritize where to employ limited resources, whether by working to prolong the lives of zombie forests or by supporting areas where vegetation and climate are still matched, Winslow Hansen, an ecologist with the nonprofit Cary Institute of Ecosystem Studies who did not contribute to the study, tells the New York Times’ Elena Shao.

“Our maps force some critical—and difficult—conversations about how to manage impending ecological transitions,” Hill says in the statement. “These conversations can lead to better outcomes for ecosystems and people.”

The findings of the new study “are what I would expect to see,” says Tara K. Miller, an ecologist and evolutionary biologist at the University of Virginia to New Scientist’s Lois Parshley. Like Hansen, Miller hopes that understanding where these vegetation climate mismatches are can guide conservation efforts.

“We may be losing some of the distinctive species that we’ve long appreciated on our landscapes,” Miller says to New Scientist.

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