In August 2013, a forest fire blazed through California’s Sierra Nevada. It burned approximately 257,314 acres, or 402 square miles, and suppression costs alone numbered around $127 million.
According the Sierra Nevada Conservancy’s estimates, the Rim Fire released 11,352,608 metric tons of greenhouse gas emissions, which is akin to annual greenhouse emissions form 2.3 million cars, or annual carbon dioxide emissions from 3.2 coal fired power plants.
Ecologists, according to the Sierra Nevada Conservancy, estimate forest areas subject to high intensity burns, around 40% of the total burn area, may take 30 to 50 years to reestablish themselves.
Now, a team of researchers led by the Jon Muir Institute of the Environment at the Univ. of California, Davis, report high-elevation fires are increasing in the Sierra Nevada region. Their study was published in the journal Ecosphere.
The study’s lead investigator Prof. Mark Schwartz, of the John Muir Institute of the Environment, called the observed increase “another harbinger of climate change.”
“And with California currently in the midst of a four-year drought, low snowpack in the mountains and related forest stress are further increasing the chances of large, destructive fires that move high into the Sierra,” he said.
According to the Indicators of Climate Change in California report, snow accumulation between October and March is responsible for most of the year’s water supply. Over the 20th century, the runoff to the Sacramento River has decreased by 9%.
The study used fire data, spanning 1908 to 2012, available from the California Fire Perimeter Geodatabase. The fires’ elevation ranged from 10 to 4,421 m, the peak of the mountain range. The study classified large fire areas as 500 acres.
“The probability of fires at elevations above 3,000 m has increased through time with 30 of 1,534 fires occurring since 1989 above 3,000 m, while just 7 of 1,531 recorded fires prior to 1950 were above this elevation,” the researchers write.
Purely an observational study, the researchers posit a few hypotheses regarding the uptick in fires at higher elevations. Due to forest fire suppression in the 20th century, the Sierra Nevada has an increased stem density, consisting of both live and dead fuels, according to the study. “Combined with rising temperatures, this has contributed to increasing fire severity in low- and mid-elevation forests,” the researchers write.
“Historically, high-elevation forests of the Sierra Nevada, with their thin soils, sparse structure and cool climate, burned infrequently,” the researchers continue. “However, while the significant increases in overall density experienced in these forests over the last few decades may be driven more by climate than fire suppression, the increased fuels at high elevation may connect previously discontinuous forest stands and allow fires to carry upslope.”
The researchers suspect the increasing fires could lead to shifts in vegetation in the Sierra Nevada’s subalpine forests. Further, the build-up of dead fuel biomass correlates to higher tree stand density, which can potentially fuel larger fires and lead to changes in forest structure.
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