Oh, nuts! Wildfire smoke harms a crucial part of California’s economy, study says

It’s no secret that California’s large-scale fires can harm wildlife and the water supply, but researchers say they also can wreak havoc on a major state export.

While most native species have adapted to withstand smaller, naturally occurring fires, they aren’t built for the high-intensity blazes the state has seen in recent years.

But a new study from the University of California, Davis has found that these fire have even longer-reaching effects: Megafires, like the ones the state saw in 2020, also can severely hamper neighboring trees’ ability to produce pistachios, walnuts and almonds.

Researchers looked at Central Valley nut orchards, one of the key components of California’s agriculture, and found that nut trees produced a significantly smaller crop in 2021 following the wildfires. In some cases, farms produced about 15% fewer nuts, but other orchards’ yields were cut in half.

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“It's not just humans that are affected by the presence of smoke, it's trees, too,” said Jessica Orozco, the lead author of the study, which was published this week in Nature Plants. “It could affect ecosystems.”

These effects could also have economic consequences — especially in the Central Valley, which produces a majority of the world’s walnuts and almonds, plus nearly all of the pistachios sold in the United States.

How it works

It might seem like common sense — smoke creates clouds that block light, which trees need to produce energy through photosynthesis. But in practice, it’s a little more complicated.

In short, smoke particles scatter light waves and cause them to be more diffused. You can notice this effect with your naked eye on smoky days, when the skies look like they have a yellowish veneer and sunsets are often a burning, deep red.

This diffusion can be helpful for plants, as the scattered light can help trees photosynthesize and increase their energy storage.

However, in the case of the megafires the Central Valley saw in 2020, the researchers found that the smoke was likely so thick that the diffused light wasn’t able to compensate for the decrease in overall light. Less light means the trees are storing less energy, which in turn led to a smaller crop of almonds, pistachios and walnuts.

The long-term effects

The most surprising result of the study, Orozco said, was that the effects of the wildfire smoke on nut trees persisted for months.

Nut trees typically accumulate energy reserves through photosynthesis during their more productive months during the summer and fall, which allows them to go dormant through the winter and bloom on time in the spring. The critical warmer months coincide with peak wildfire season.

“What we saw was that because of the smoke, they weren't building up as many of these reserves,” Oroco said.

These long-term implications of their study caught Orozco’s team by surprise, especially since this effect hasn’t been studied at length.

“There aren't a lot of studies looking specifically at smoke, but a lot of the time they're mostly instantaneous studies, and none had been so long term,” Orozco said. “So to see that this effect lingered on for so long was really surprising for us.”

Future research

The researchers still aren’t sure exactly what factor led to the lower nut production, though they did determine it was related to the nearby megafires. It could be due to lower light or higher ozone levels, or the particulate matter in smoke could have affected photosynthesis in a way that isn’t yet understood.

Understanding this effect will be key for future research on this topic. Another possible research topic is the economic “collateral damage” of these fires, as Orozco put it.

“Research shows that these types of fires, these intense, long-enduring fires are probably going to increase in the future,” Orozco said. “This research brings up another facet about wildfire research that hadn't really been considered. It could potentially have huge repercussions for growers, for the state, beyond the trees.”