More trees are dying and drying out in California's forests due to a hotter temperatures and prolonged droughts, which is causing more severe wildfires and a mass altercation in how forests store carbon.
That's according to Cal Fire-backed researcher Marissa Goodwin at a research webinar Wednesday. Goodwin, a graduate student at the University of New Mexico, serves as project manager for the Teakettle Ecosystem Experiment, a long-standing study that tests how repeat burning and thinning affects an experimental, old-growth forest in the Sierra Nevada.
Goodwin said the recent increase in size and severity of wildfires in California partially has to do with a legacy of fire suppression in forests, which has caused both an increase in fuel density and an altercation in how much carbon a forest would naturally contain with regular burning.
California forests that have not had regular, healthy burns to clear out dead trees and debris have become heavy carbon pools, which makes big wildfires that much more devastating to the environment, said Goodwin. She referred to this excess carbon stored in forests as carbon "owed to the atmosphere."
"We've altered the natural disturbance regime through fire suppression," Goodwin said. "As a result, over the past century, there hasn't been that regular fluctuation in the live tree carbon stock from wildfires, and instead there's been a steady increase in the amount of carbon that's stored in these ecosystems, and overshoot of the carbon-carrying capacity results in what we call a 'carbon debt.'"
Prolonged, warmer temperatures are also lengthening the fire season, and unpredictable precipitation patterns are drying out forests, which in turn is causing the majority of carbon in forests to be in dry, dead brush and trees, rather than living vegetation with a higher moisture content. Goodwin said that in mixed conifer forests, dead fuel usually has moisture content of 10 to 50 percent, while live confer trees range from 110 to 150 percent.
"Dead fuels are inherently drier than live fuels, and they now are becoming even drier with climate change. As these dead fuels dry out, they become more readily available to burn, and will release more energy as heat when wildfire occurs," Goodwin said. "Because the energy released plays an important role in wildfires spread, fires that occur where there's a lot of dead and dry fuels can produce mega fires that are characterized by high intensity and high rates of spread."
There's also another large contributor to drier fuels in forests, she said - a vapor pressure deficit in the atmosphere, which is the strength in which the atmosphere pulls moisture from soils and plants. Years of below-average winter snowpacks and increased temperatures have caused deficits in the atmosphere to increase, which causes fuel loads to become drier.
"As a result, we're witnessing hotter and drier droughts that occur more often and last longer," said Goodwin. "We've seen this dynamic play out over the past decade in California, starting with the occurrence of the 2012 to 2016 drought, which was the worst drought period to occur in the state over the past 1,200 years."
There's also unusual way that changes in climate lends itself to higher tree mortalities; for example, higher temperatures led to an increase in bark beetle outbreaks, and with trees already stressed due to unusually hot and dry weather conditions, this can cause more killer infestations.
She said bark beetle outbreaks from the 2012 to 2016 drought were responsible for killing 58 percent of trees in the Sierra National Forest.
Goodwin and other Teakettle researchers examined two wildfire events from the 2020 wildfire season to investigate how tree mortality and climate work together to affect fuel dryness and the energy released during fires.
Upon examining the Cameron Peak fire in the Colorado Rocky Mountains and the Creek Fire in the Sierra Nevada, Goodwin and her team concluded that a decrease in fuel moisture content over the past two decades resulted in higher energy values from wildfires, up to two-fold, and a greater proportion of easily burnable biomass.
"Our results suggest that the abundant dead and dry fuel loads that are produced by climate driven tree mortality and climate driven fuel aridity may result in wildfires that are perpetuated not only by available fuels, but also by the substantial amounts of energy these fires releases heat," Goodwin said.
Goodwin also highlighted a study that Teakettle researchers conducted between the 2012 to 2016 drought that examined how climate change is impacting how much carbon is released in forest management techniques, like thinning and burning.
She concluded that fire officials may have to rethink conducting prescribed burns in forests heavy with dead trees, as both managed and unmanaged forests are expected to have more of their carbon makeup in dead fuel, as drought events continue to worsen.
"Prescribed fire can reduce the risk of wildfire associated with drought related fuel inputs by consuming dead trees and surface fuels," Goodwin said. "But the expectation that carbon emissions will decrease with subsequent management may no longer hold with ongoing climate change, and we should expect to see increases in [carbon emissions] of both initial and repeat management in Sierra Nevada forests."
Goodwin said areas with a lot of dead fuel may have to begin regular burning at smaller scales, based on the rates at which dead trees are becoming surface fuels, and at intervals that can still get rid of these fuels in a timely fashion.
"When it comes to the carbon costs of forest management or the emissions associated with prescribed burning, we need to recognize that the carbon cost of prescribed burning is likely to be a lot higher than it was 10 to 20 years ago, as a result of drought, and bark beetles, tree mortalities, and our policies surrounding the use of prescribed fire," said Goodwin.
The presentation was part of a series of meetings put on by the Forest Health Research Program, a grant project that has distributed over $20 million to 60 projects dedicated to finding a deeper understanding of California forest health. The funds come from revenue made in the state's cap-and-trade program, which makes companies pay allowances to emit a certain amount pollutants.
"It's our intent to fund sound scientific studies that support forest landowners and agencies and fire management organizations within the state," said Tadashi Moody of CALFIRE. "And we importantly want to ensure that the scientific information that we generate from those funded projects gets into the hands of decision makers and landowners and those making policy, so they have good science supporting good policy."