Wildfire smoke may be reducing California nut tree yields: Study

Long-term exposure to wildfire smoke may be depleting the energy reserves of California's orchard trees — reducing their nut production by as much as half, a new study has found.

Smoke can impact such trees for months following a megafire, with far-reaching effects on seasonal blooms and subsequent harvests, according to the study, published on Wednesday in Nature Plants.

“A lot of research focuses on the impact of smoke on humans but there is less study on the effects of smoke on plant health,” lead author Jessica Orozco, a postdoctoral researcher in plant sciences at the University of California Davis, said in a statement.

“Our study suggests that trees are just as vulnerable as humans," she added.

Orozco and her colleagues studied almond, pistachio and walnut trees at 467 orchards from 2018 to 2022 in California’s Central Valley, the state’s agricultural core.

At first, the scientists focused on how trees store energy as carbohydrates amid heat and drought. But after a slew of wildfires scorched 4.2 million acres across the state in 2020, they expanded their efforts to explore how smoke impacts carbohydrate levels.

Smoke particles can influence the photosynthesis process not only by blocking some of the sun's rays, but also by reflecting light, the authors noted.

Although that new source of light can actually help trees create more carbohydrates, the thick smoke prevents that light from compensating for the substantial losses, according to the study.

Instead, the authors found that smoke can hamper carbohydrates storage in trees both during a blaze and in its aftermath. Making matters worse is the fact that wildfires are usually most active during the period in which trees start storing carbohydrates for winter sustenance.

“Photosynthesis produces carbohydrates, which are critical elements for tree survival,” Orozco said. “Trees need carbohydrates not just to grow but to store energy for when they’re under stress or when photosynthesis isn’t happening.”

The nut yields of orchard trees affected by megafires plunged by 15 to 50 percent, depending on the location, the researchers observed.

The authors characterized the impact of these massive blazes as "unequivocal evidence that persistent exposure to high levels of smoke leads to widespread decline in energy reserves."

While they remain uncertain as to which components of smoke cause that depletion, the researchers noted that ozone and particulate matter are known to affect photosynthesis.

“We were expecting to see some impact especially in the months when the smoke was really dense, but we weren’t expecting the smoke to have such a lingering effect and result in a significant drop in yield,” Orozco added.