Evidence for multi-decadal fuel buildup in a large California wildfire from smoke radiocarbon measurements

Over the past several decades, the annual burned area in California's
Sierra Nevada mountains has increased considerably, with significant
social, economic, and ecosystem impacts that provide motivation for
understanding how the history of forest management influences the
composition of fuels and emissions in wildfires. Here, we measured the
carbon concentration and radiocarbon abundance (∆14C) of fire-emitted
particulate matter from the KNP Complex Fire, which burned through several
groves of giant sequoia trees in the southern Sierra Nevada mountains
during California’s 2021 wildfire season. Over a 26-hour sampling period,
we measured the concentration of fine airborne particulate matter (PM2.5)
along with carbon monoxide (CO) and methane (CH4) dry air mole fractions
using a ground-based mobile laboratory. Filter samples of PM2.5 were also
collected and later analyzed for carbon concentration and ∆14C.
Covariation of PM2.5, CO, and CH4 time series data confirmed that
our PM2.5 samples were representative of wildfire emissions. Using a
Keeling plot approach, we estimated that the mean ∆14C of PM2.5
was 111.5 ± 2.3‰ (n=12), which is considerably enriched relative to that
of atmospheric carbon dioxide in the northern hemisphere in 2021 (-3.4 ±
1.4‰). By combining these ∆14C data with a steady-state one-box ecosystem
model, we estimated that the mean age of fuels combusted in the KNP
Complex Fire was 40 ± 6 years. This multi-decadal fuel age provides
evidence for emissions from woody biomass, coarse woody debris, and
larger-diameter fine fuels. The combustion of these larger-size fuel
classes is consistent with independent field observations that indicate
high fire intensity contributed to widespread giant sequoia mortality.
With the expanded use of prescribed fires planned over the next decade in
California to mitigate impacts of wildfires, our measurement approach has
the potential to provide regionally-integrated estimates of the
effectiveness of fuel treatment programs.