Climate change is creating “zombie forests” in California. One-fifth of the conifer forests in the Sierra Nevada mountains are in areas that are no longer suitable for them – meaning they may not be able to regrow if wiped out by wildfire or disease – and that number is expected to double by the end of the century.
at the California Academy of Sciences first began investigating the relationship between climate change and forest fires several years ago, exploring how the combination might determine which plants could continue to grow where they had historically thrived.
His inspiration was quite personal: the devastating of 2017 came so close to his family home in Napa County that the windows cracked. “I had to evacuate my parents and grandparents, and we didn’t know for weeks if their houses were still standing,” he says.
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In the aftermath of one of the worst wildfires in the state’s history, provided some of the first empirical evidence of how fire is intensifying changes across the West. Using US Forest Service data from nine states, he found that wildfires can actually accelerate the process by which certain tree species migrate across the landscape in response to climate change – fleeing warmer temperatures.
“The next question is where are these important vegetation transitions already occurring in California?” Hill says. To find out, he and his colleagues at Stanford University in California went back through 90 years of vegetation data. They found that the mean elevation of conifers has risen 34 metres in that time, but the temperatures at which the trees thrive has shifted upslope 182 metres. The Sierra Nevada has warmed an average of 1.2°C during the time studied.
The group found that currently, one-fifth of the Sierra Nevada’s conifer forests are in areas that are no longer suitable for regrowth; in other words, they’re “zombie forests”. If a fire or disease sweeps through, it is unlikely they will be able to regrow, says Hill. He and his colleagues projected that, by the turn of the century, that number will double.
at the University of Virginia says the findings “are what I would expect to see”. Last year, Miller showing that across North America, warming temperatures are causing a mismatch in the timing of plant leaf-out and flowering. Understanding where this misalignment is most severe “can also help us prioritise conservation efforts in places where species are most affected,” says Miller.
In California, the most dramatic mismatches are occurring on the drier, warmer edge of the conifer forest, downslope from Yosemite National Park. Many of these areas are likely to transition into shrubland or mixed broadleaf forests, which contain a mix of deciduous and coniferous trees, says Hill.
That could also have a significant impact on future wildfire modelling. “We hope this study brings awareness to the idea that underlying vegetation affects how fire moves through the landscape, and how it’s changing,” he says. Shrubland known as chaparral, for example, historically burns in 40-year cycles, but when it does, it is usually a severe fire. In contrast, conifer forests are adapted to burn more frequently, but less severely. “These fires behave very differently, with very different implications for the human communities around them,” says Hill.
Though it is outside of the study he conducted in the Sierras, back home in Napa, Hill says the Douglas firs he grew up with are already failing to regrow. It is a warning sign of further changes to come. “We may be losing some of the distinctive species that we’ve long appreciated on our landscapes,” says Miller.
PNAS Nexus
Lois Parshley is an independent investigative journalist. Follow more of her climate reporting @loisparshley on , , or .
