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Some plants and parts of the Earth can withstand heat and dry spells more than others. A Stanford University study has found that these different coping mechanisms are closely related to wildfire burn zones, which poses increasing risks in an age of climate change.

In September 2020, thousands of homes were destroyed by the Almeda Fire in Southern Oregon. (Image credit: Shutterstock)

The ResultsPublished Feb. 7, 2007. Nature Ecology and EvolutionAccording to research, large areas of shrublands and forest in the West are more at risk of fire than previously thought due to the way that local ecosystems use water. Because of the differences in soil and plant traits, more acres tend to burn in these areas under the same dry conditions.

The study’s authors set out to test an often-repeated hypothesis that climate change is increasing wildfire hazard uniformly in the West. “I asked, is that true everywhere, all the time, for all the different kinds of vegetation? Our research shows it is not,” said lead author Krishna Rao, a PhD student for Earth system science.

‘Double-hazard’ zones

The study arrives during the Biden administration Launch preparationsA multibillion-dollar, 10-year effort to expand forest thinning in 11 Western states.

In 18 zones of the U.S. West, plant water sensitivity is high (>1.5), and the vapor pressure deficit is rising faster than average. Both of these factors increase fire hazards and the overlaps are likely will amplify the effects of climate change in burned areas.(Image credit: Adapted by Rao et. al., Nature Ecology and Evolution, 2021)

Climate change is causing a vapor pressure deficit. This refers to how much moisture the air can sucking out of soil and plants. Previous research has shown this to be the case. The vapor pressure deficit has increased in the American West over the past 40 year due to warmer air containing more water. This is the primary mechanism that global warming is increasing wildfire dangers.

The new analysis comes from the LabStanford ecohydrologist Alexandra KoningsAccording to this study, vapor pressure deficits are rising fastest in areas that are more prone to drying out. The combination of highly sensitive, tinder-dry plants and a faster-than-average increase in atmospheric dryness creates what the authors call “double-hazard” zones.

The 18 newly identified double-hazard areas are located in regions that have seen an disproportionately rapid increase in burn area with each uptick in the vapor pressure deficit over two decades. Ranging in size from a few hundred to nearly 50,000 square miles, they’re concentrated in eastern Oregon, Nevada’s Great Basin, central Arizona’s Mogollon Rim and California’s southern Sierra Nevada, where recent wildfires have destroyed thousands of giant sequoia trees that had survived fires for hundreds of years.

According to the authors, the results suggest the distribution of vegetation across the West – that is, the arrangement of scrub, alpine meadows, sagebrush, coniferous forest and other plant communities from the Pacific Coast to the western fringe of the Great Plains – has “amplified the effect of climate change on wildfire hazard” in the region, specifically, the amount of acreage burned.

“California and other Western states are working hard to figure out how to adapt to the changing wildfire risk landscape, including long-term decisions around issues such as land use, vegetation management, disaster planning and insurance,” said study co-author Noah DiffenbaughKara J Foundation Professor, Kimmelman Family Senior Fellow at Stanford, and a senior fellow of Stanford Stanford Woods Institute for the Environment. “There’s a wealth of information in this analysis to support decisions about how to more effectively manage the risks of living in the West in the context of a changing climate.”

Sensitivity to water from plants

The fact that plants rarely act in unison has been a well-known fact for ecologists, plant physiologists, and farmers. “Each plant is different, each species is different and the geography of a place defines how a plant’s moisture level responds to different environmental conditions,” Rao explained.

People have been rapidly growing in areas of high wildfire hazard and plant-water sensitivity in transition zones in the Western U.S.(Image credit: Adapted by Rao et. al., Nature Ecology and Evolution, 2021)

But models for calculating wildfire risk at landscape scale generally don’t account for the diversity of drought responses, “in part because it’s really difficult,” said Konings, who is the study’s senior author and an assistant professor of Earth system science at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). “It’s very labor-intensive to measure how much an ecosystem is drying out, and it’s difficult to predict without those direct measurements because it depends on what kind of soil you have, the topography and what kinds of plants.”

Scientists used satellite data to create a new measure called plant-water sensitive. It incorporates plant and soil hydraulic characteristics that affect the moisture content of plants, such as how much water a soil can hold, how easy water moves through soil when saturated, and root depth.

The authors used artificial Intelligence, statistical analysis, microwave remote sensing and microwave remote sensing to show that this measure in local vulnerability to drying in the face limited rainfall and an adverse atmosphere is closely tied to increases in wildfire burning area and a drying climate within forests and shrublands. In grasslands, they found annual burned area did not increase much with vapor pressure deficit, suggesting that other factors such as fuel availability, ignitions, the plants’ growth stage and strong winds may play a bigger role. The census data was used to track the population growth in vulnerable areas.

Disproportionate growth

The underlying causes for catastrophic wildfires that have scorched so much of the American West in recent years are complex, including not only climate change but also decades of fire suppression and growing populations along the periphery of undeveloped wilderness – a transition zone sometimes called the wildland-urban interface or WUI.

Wildland-urban interface populations grew the fastest in areas where local soil and plant traits increase wildfire danger.(Image credit: Adapted by Rao et. al., Nature Ecology and Evolution, 2021)

In California alone, more than 11 million of the state’s 40 million residents live in the WUI, which encompasses not only densely forested areas like Paradise, a northern California town destroyed in the deadly 2018 Camp Fire – but also parts of the wooded coastal foothills around Silicon Valley, the brush-and-grass covered hills around Santa Barbara and Los Angeles, and neighborhoods in the Oakland hills, just a few miles east of the San Francisco Bay.

The WUI is made up of people from the West, including the Western states, who provide the majority the ignitions for the fires that torch the rich vegetation and threaten human lives. Wildfire dangers are increased simply because more people live in areas with flammable trees, chaparral, and grasses.

The new research shows how climate change is increasing those risks unevenly. It also shows how communities within the WUI are growing in places that are most vulnerable to drought. Between 1990 and 2010, the WUI added 1.5 million people. Populations in parts of the WUI with high plant-water sensitivity have grown 50 percent faster than the West’s wildland-urban interface overall, the study finds.

According to Konings, “This redoubles the need to be thinking about what we can do to reduce wildfire impacts in the WUI in general, including for this subgroup of people who are in the most vulnerable locations.”

Konings is also an assistant professor of geophysics, and a center fellow at Stanford Woods Institute for the Environment. Co-authors are affiliated with the University of California, Los Angeles; Columbia University’s Lamont-Doherty Earth Observatory; and the Australian National University, Acton.