The conifer forests of California’s Sierra Nevada cover some of America’s most beautiful landscapes. From Lake Tahoe to Yosemite Valley, ponderosa pines, sugar pines, and Douglas fir signal the transition from the foothills to the mountains.

But that landscape is threatened due to climate change. About one-fifth of conifer forests in the Sierra Nevada are living in areas too warm for them to continue regenerating at their normal rate, according to a study published yesterday in the journal PNAS Nexus. 

It's hard to kill a large, mature conifer unless there is a disturbance, like a wildfire. So these “zombie forests” are still living but with little hope of regenerating—that is, producing future generations of trees—as the climate warms. “They're missing some key qualities that a healthy living forest has,” said Avery Hill, postdoctoral researcher at the California Academy of Sciences and the study’s lead author.

Hill and the other authors on the study reached this conclusion after comparing Forest Service Existing Vegetation maps, which were generated using satellites, with extremely detailed plot surveys conducted in those same forests in the 1930s. They found that since the '30s, the climate in the Sierra Nevada has increased about 1.2 degrees Celsius, shifting an average microclimate up the mountain slope by about 182 meters. The trees, though, haven’t kept up—the average elevation of conifer species only moved upward about 34 meters. As a result, nearly one-fifth of Sierra Nevada conifers are experiencing what the study describes as “vegetation climate mismatch.”

This mismatch increases the likelihood that any disturbance to these forests—like drought, wildfire, or logging—will lead to their being replaced with more shrublike species, such as chaparral. The changes will most likely happen at the seedling stage—since young plants are more vulnerable than adult trees, conifer seedlings will be less likely to grow back in the drier, warmer climate than seedlings of chaparral species.

The rate of these changes is uncertain, since each forest transition relies on a disturbance event as a catalyst. The paper is the first to show, on a large scale, how the coniferous forests of the Sierra are mismatched with their climate, so integrating Hill’s findings with other research on the frequency and likelihood of forest disturbances could give forest managers a more powerful understanding of how the forests they are responsible for are changing, said Winslow Hansen, a forest ecologist at the nonprofit Cary Institute of Ecosystem Studies who was not involved in the new paper. “This paper is demonstrating, across broad scales, the outcomes of many really fine-scale environmental processes operating on individual tree seedlings,” he said, describing it as “a frontier in terms of this type of work.”

In 2017, the Nuns Fire near Napa Valley burned through the property where Hill grew up. While Hill had always intended to study the interaction between climate change and plant life during his PhD, the disaster pushed him to study fire ecology as well. 

The transition from coniferous forest to chaparral will also mean a change in wildfires. In the Sierra, a coniferous forest will burn every eight years on average if left to its own devices. Chaparral forests don’t burn as frequently—about every 30 to 90 years. When wildfires do break out in chaparral, they tend to burn at very high temperatures and completely clear out a landscape. In contrast, a coniferous forest fire just burns away the brush and understory, while leaving the tallest, most mature trees. 

These changes have implications for people living in the margin between urban and wild landscapes, who already are dealing with increased risk of wildfires. Still, there’s a lot that scientists have yet to understand about what those changes will look like, said Hill. For example, scientists still aren’t sure exactly how vegetation climate mismatch impacts the severity of a wildfire. 

As these forests transition, the types of wildlife living there will change too. While there is some overlap, mature conifer forests are ideal habitats for species that thrive in cooler, shadier spots, such as spotted owls, fishers, and pine martens. Chaparral ecosystems tend to support species adapted to drier landscapes, such as jackrabbits and coyotes.

Hill emphasized that a changing forest isn’t necessarily good or bad. Instead, the findings are a tool to understand how to manage forests in the future. Climate change is raising questions for forest managers about what, exactly, their responsibility is to a forest. Is it better to preserve existing conifers, despite the warming climate? Or to allow the landscape to change instead? Which choice is best for the humans and wildlife that live in or near these zombie forests? “If these changes are inevitable, does it make sense to prioritize resources to these places? Or does it make sense to prioritize resources to conserving conifers in places where the climate still closely aligns with their niche?” said Hansen. 

Information about where vegetation climate mismatch is happening “helps us then decide what we can do about it,” said Hansen. “We have a number of really difficult questions to address in terms of what we want our forest ecosystems to look like in the Western United States.”

What they decide will depend on what forest managers value about a particular landscape, and what can realistically be accomplished, he said. A forest manager charged with protecting a specific species or the historic character of an area might decide to lower fire risk in the area by thinning out trees and removing dead wood and dry brush. Or they might transplant coniferous seedlings to a recently burned area and tend to them until they are mature enough to survive on their own. 

Another forest manager might choose to not interfere in the transition to chaparral, or even support species that thrive in chaparral ecosystems, by allowing wildfires to move through an area without interference or by setting prescribed burns instead. Chaparral species are threatened by climate change as well and could benefit by moving into new areas.

“It's the thinking about it that's necessary,” said Hill. “The only bad thing is not being aware and not thinking through what's best for a particular area.”