After more than a week, firefighters in Southern California are finally getting a handle on the massive Thomas Fire. The blaze, which tore across 242,500 acres and two counties over the last 10 days to become one of the largest fires in California history, was 30 percent contained as of Thursday.
Researchers on not so clear on what happens now to the vast swaths of forest and chaparral that the flames have consumed. While scientists have long known that single fires can be rejuvenating for certain fire-adapted ecosystems—low-intensity fires promote biodiversity, renew soil nutrients, and may even be necessary for some plants to produce seeds—a new study shows that repeated fire events can reduce soil fertility and carbon storage, negatively affecting even those ecosystems whose regular re-growth depends in part on wildfires.
The weather conditions that fueled the Thomas Fire are not unusual for the region: little to no rain, dry vegetation, hot Santa Ana winds blowing with hurricane force over the mountains. While Southern California is usually a tinderbox by the end of the summer season, in the past, wildfires would occur only every few decades, giving native plants time to recover between firestorms. Now, climate change is making Southern California hotter and drier for longer periods of time, and wildfires have become both more frequent and more severe.
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That realization—that climate change is already leading to more fires in California and around the world—motivated Stanford University's Adam Pellegrini and his colleagues to look into what effect those more frequent fires were having on soil in particular. In a study published this week in Nature, the researchers looked at data from grasslands, broadleaf forests, and needleleaf forests around the world, collected over a period of up to 65 years. Some of the sites were protected from wildfires, while others faced a higher number of fires than normal.
In savannas and broadleaf forests that were frequently hit with fires, the difference was dramatic: There was a 36 percent drop in carbon storage and a 38 percent loss of soil nitrogen—a nutrient critical for plant growth. Needleleaf or conifer forests however—much like the area where the Thomas Fire is burning—did not have the same loss of soil fertility and carbon capture. "In needleleaf forests, fire frequency actually enriched carbon and nitrogen," says Pellegrini, a post-doc and lead author on the new study.
The difference, Pellegrini speculates, could be one of intensity. The needleleaf forests the team analyzed experienced only low-intensity, prescribed burns, rather than raging wildfires. "When you get a really severe wildfire, you see almost complete consumption of that litter layer," Pellegrini says. "You see so much consumption of the organic matter that it's unlikely that enrichment would take place." In prescribed fires, on the other hand, the dense, squishy forest floor smolders, releasing some carbon and nitrogen atoms from their bonds without burning them up, giving the nutrients a chance to trickle down into the soil.
That's good news for land managers, according to Pellegrini. "It suggests that, when we do these prescribed burns, you can put these nutrients in the soil."
"The other big implication," Pellegrini says, "is that the contribution of fire to global carbon emissions may be much higher than we thought." Plant growth is what draws down carbon out of the atmosphere, but the loss of carbon storage and soil fertility after repeated wildfires inhibits the ability of plants to regrow, which means frequent fires both emit carbon as organic matter burns, and inhibit its sequestration.
That can lead to a dangerous feedback loop, Pellegrini says. As climate change creates warmer and drier conditions in fire-prone areas, it will lead to more fires, which leads to more carbon emissions, which leads to more warming, and ultimately more fires.