Dear EarthTalk:

What are zombie forests?

B. Morgan, Chico, CA

They may look lush and green, but some forests are already on borrowed time. These so-called “zombie forests” are made up of trees that are still alive but no longer capable of reproducing in the changing climate around them. As Stanford biology graduate student Avery Hill puts it, they persist only because of “ecological inertia”—hanging on even though the conditions that once supported them are disappearing.

In short, they’re living out of place and out of time. Scientists call it a vegetation-climate mismatch: when plants that once thrived in a region suddenly find themselves mismatched with a new reality. Rising temperatures, shifting rainfall and increasing drought are all altering the rules of survival.

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The Sierra Nevada in California offers a striking case. Towering conifers like ponderosa pine, sugar pine and Douglas fir—giants that have defined these mountains for centuries—are struggling to adapt. Since the 1930s, average temperatures there have climbed just over 1°C. “Our maps and models show that the climate has become too warm and too dry to support conifer forests in the long run,” Hill explains.

The numbers are sobering. In less than a century, conifers have crept about 112 feet upslope, while the climate best suited to them has moved nearly 600 feet higher. That gap leaves them exposed, especially after fire. “The speed of change has outpaced the ability of many conifers to adapt,” Hill says. Research shows roughly 20 percent of Sierra Nevada conifers are already mismatched with their surroundings, a figure expected to double within 77 years—even in the most optimistic climate scenarios.

And fire only makes matters worse. A century of fire suppression has packed forests with fuel, setting the stage for megafires that devastate ecosystems. While smaller, mixed-severity fires once helped recycle nutrients and open space for seedlings, today’s intense blazes leave little chance for regrowth. The result? Forests on the brink of transformation. Some parts of the Sierra Nevada, like Eldorado National Forest, are already giving way to chaparral shrubland after back-to-back blows from wildfire and drought.

Still, experts say there’s room for action. Prescribed burns can lower the risk of catastrophic fire, while climate-niche modeling can help pinpoint the forests most at risk. Community science projects like iNaturalist are also making a difference, with local observers helping researchers track changes in real time. Zombie forests may be a warning sign, but they’re not yet a death sentence. With smart management and community involvement, we still have a chance to keep more of these landscapes from turning into “standing dead.”

CONTACTS

• Stanford-led study reveals a fifth of California’s Sierra Nevada conifer forests are stranded in habitats that have grown too warm for them, https://news.stanford.edu/stories/2023/02/zombie-forests.

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