Zombie Forests in the Sierra Nevada

Katarina Samurović

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California’s Sierra Nevada rates as one of the most biodiverse places in the entire US. Also, about a decade ago, it was identified as one of the ten places in the country where the impacts of climate change are most threatening to endangered species.

Iconic montane forests are one of the most recognizable habitats of Sierra Nevada. Mighty conifers such as sequoias, ponderosa pines, and Douglas firs densely dot the hills and the mountainsides, giving the Sierra their distinct charm.

A gigantic reddish tree with a large burn scar and colorfully dressed people gathered in front of it on a sunny day.
General Sherman is a giant sequoia and the world’s largest tree by volume. Photo: Rebecca Paterson, Sequoia and Kings Canyon National Parks, public domain.

The Sierra Nevada is also home to some notable geographic features. The mountain range contains the highest point in the lowest 48 states, Mount Whitney. The world’s largest tree by volume, a giant sequoia named General Sherman, is also found in the Sierra Nevada.

North America’s largest alpine lake, Lake Tahoe, sits in the Sierra Nevada on the border between California and Nevada. Numerous National Parks, including Yosemite National Park, as well as wilderness areas, national forests, and national monuments can be found in this mountain range.

View of Lake Tahoe from Emerald Bay. Photo: NASA/JPL.
Lake Tahoe in the Sierra Nevada is North America’s largest alpine lake. Photo: NASA/JPL

Conifers are a keystone species

These evergreen forests also provide a home to many of the 3000 vascular plants and almost 600 vertebrate animal species that inhabit the Sierra at some stage of their lives. It’s not just the trees; it’s an ecosystem with conifers as keystone species.

Snow dusted trees against a bright blue sky.
Incense cedar in the Sierra Nevada. Photo: Matthew Brooks, USGS Western Ecological Research Center, public domain.

However, there is new scientific evidence that the habitat is not a hospitable home for these cone-bearing species anymore, putting the faith of the entire ecosystem in question.

Due to increasing and rapid climate change and the consequently dried weather, about one-fifth of trees are now outside of their climatic comfort zone. Thus, researchers estimated that the affected trees are entering a die-out phase and have dubbed them “zombie forests.”

The Vegetation Climate Mismatch Study

The study called “Low-elevation conifers in California’s Sierra Nevada are out of equilibrium with climate”, produced by Avery P. Holl from Stanford University and his team, has found just what the title said. 

The team explored and mapped the phenomenon called vegetation climate mismatch (VCM), a situation when the existing vegetation becomes incompatible with the new climatic conditions. This results in the gradual die-off of the existing trees and the failure of the forest to regenerate via young trees, unadapted to grow and thrive in changed conditions.

Researchers found that almost 20% of conifers in the Sierra Nevada forests are in areas where the climate conditions are no longer favorable to their survival. Of those trees, 95% are found in areas that are below an elevation of 2,356 meters.

A gray shaded relief map of California showing in shades of green where climate mismatched forests are in the Sierra Nevada.
Vegetation Climate Mismatch among conifers in the Sierra Nevada, California. Map: Caitlin Dempsey with data from Hill et. al, 2023.

Ultimately, better-adapted species replace the existing ones. This is called climate-driven vegetation conversion. 

Precise data on Sierra Nevada’s vegetation exists from the 1930s and the Wieslander Survey. It was a perfect database to compare with today because in the time of Wieslander – now almost a century ago – the effects of climate change were still negligible, meaning that all Sierra Nevada conifers were growing within their preferred climatic niche.

Meanwhile, Sierra Nevada has warmed by 1.2 degrees C on average. Expectedly, the lower-elevation trees got pushed out of their climatic equilibrium with their surroundings.

A view across a forest with dead trees from a prolonged drought in California.
View of dead trees from the Colony Mill Trail in Sequoia National Park, taken in 2015 during California’s extreme drought. Lower elevation conifer trees are more affected by factors of climate change compared to higher elevation conifers in California. Photo: Nate Stephenson, USGS Western Ecological Research Center, public domain.

As lower elevations became warmer and drier, conifer populations migrated upwards to cooler higher elevations. Over 90 years, the average elevation of trees in the Sierra Nevada has moved 112 feet up-slope.

However, unlike animal migrations, plant migrations happen gradually over a long period of time. While animals can physically relocate, plants conquer new territories through their offspring over generations – and, unsurprisingly, that can take a while.

Unfortunately, climate change is coming on so rapidly that, in the case of the Sierra conifers, the steady move uphill could not compete with the dramatic increase in temperatures and change in precipitation. 

In response, the tree species slowly disappear from their habitats and the species that are a match to the “new normal” move in.

Researchers say that the percentage of trees suffering from climatic mismatch in Sierra Nevada coniferous forests (now around 20%) could double in the next 77 years.  In time, they will most likely be replaced with chaparral and scrubland.

However, the VCM means much more than just losing species. 

Changes Beyond Forests

Due to the loss of keystone species – those numerous and influential enough to dictate the ecosystem’s main traits – the entire dynamics of the landscape changes, as well as biological diversity.

Take wildfires, for example. The Sierra coniferous forests naturally burn every eight years on average. Climate change is accelerating the frequency and intensity of those forest fires.

Fire and embers glow orange on the floor of a conifer forest at twilight.
A wildfire burns among the conifers in the Sierra Nevada in October of 2018. Photo: Sequoia and Kings Canyon National Parks, NPS/Rebecca Paterson, public domain.

Driven by climate change, there is a significant difference in the wildfire properties.  The forest fires in coniferous forests tend to be more contained. They usually affect and burn away the forest understory – the shrubs and other lower vegetation, and are survived by the tall, mature trees. 

The VCM Maps

One of the highest contributions of this study is the Sierra Nevada Conifer Vegetation Climate Mismatch map

The interactive map, available freely to the public, shows Stable Conifer Distribution, Vegetation climate mismatch (VCM), and Severe VCM, with the option of adjusting VCM layer opacity.

Except for valuable information itself, the maps of Sierra Nevada’s VCM could help long-term land management decisions by distinguishing the areas likely to suffer transition from those expected to remain stable, at least in the near future. 

Last but not least, by being accessible and user-friendly, the map helps raise awareness about the real effects of climate change

“Conservationists know, scientists know, so many people know that ecosystems are changing and expect them to change more, and people are grappling with this,” says Hill.

 “These maps are unique in that you can put your finger on a point and say, ‘This area right here is expected to transition due to climate change in the near future,’ and this forces some really difficult questions about what we want this land managed for and do we try to resist these impending changes.”

There is a debate within the scientific community on how to approach forest conservation under such rapidly changing conditions – a dilemma whether we should stick to the conservative approach or become more proactive regarding adaptation.

However, all experts agree that the action on tackling emissions of greenhouse gasses and slowing down climate change by all means possible is the priority, no matter what road to conservation we take.

Conclusion

While certainly eye-catching, one could argue that the term “zombie forests” is not quite adequate for the situation that the Sierra coniferous forests face.

Besides being undead, zombies, as creatures of our imagination, have many strengths and tricks up their sleeves. They are a force to be reckoned with. 

On the other hand, Sierra Nevada conifers cannot put up a fight. These trees have no choice but to passively face the changes – and they seem to have depleted all their evolutionary survival strategies, such as gradual migration to higher elevations. Unpopular as it is, perhaps it would be more truthful and fairer to name them for what they are – “the dying forests.”

Read next: Climate Change and the Expansion of Ghost Forests

Resources

The Study:

Hill, A.P., et al. (2023). Low-elevation conifers in California’s Sierra Nevada are out of equilibrium with climate. PNAS Nexus, Volume 2, Issue 2, February 2023. https://doi.org/10.1093/pnasnexus/pgad004

The Map:

Sierra Nevada Conifer Vegetation Climate Mismatch (Hill et al., 2023) https://avephill.shinyapps.io/Sierra-Nevada-VCM/

Articles:

Battles, J. J., Robards, T., Das, A., Waring, K., Gilless, J. K., Biging, G., & Schurr, F. (2008). Climate change impacts on forest growth and tree mortality: a data-driven modeling study in the mixed-conifer forest of the Sierra Nevada, California. Climatic change87, 193-213. https://doi.org/10.1007/s10584-007-9358-9

Halofsky, J. E. (Ed.). (2021). Climate change vulnerability and adaptation for infrastructure and recreation in the Sierra Nevada. Chapter 2: Climate Change Effects in the Sierra Nevada. US Department of Agriculture, Forest Service, Pacific Southwest Research Station.

Hernandez, J. (2023, March 13). Climate is changing too quickly for the Sierra Nevada’s ‘zombie forests’. NPR. https://www.npr.org/2023/03/13/1162042220/climate-change-sierra-nevada-zombie-forests

Sierra Nevada wildlife at risk. (2018, September 7). Sierra Forest Legacy. https://www.sierraforestlegacy.org/FC_SierraNevadaWildlifeRisk/SierraNevadaWildlifeRisk.php

Van Deelen, G. (2023, March 1). The forests of the Sierra Nevada are full of zombies. Sierra Club. https://www.sierraclub.org/sierra/forests-sierra-nevada-zombies-climate-change

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About the author
Katarina Samurović
Katarina Samurović is an environmental analyst and a freelance science writer. She has a special interest in biodiversity, ecoclimatology, biogeography, trees, and insects.

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