Smoke from megafires puts orchard trees at risk

Nature Plants published the study today (Oct. 2).

"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," said lead author Jessica Orozco, a postdoctoral researcher with the UC Davis Department of Plant Sciences. "Our study suggests that trees are just as vulnerable as humans."

Dark skies, less energy for trees

Scientists studied almond, pistachio and walnut trees at 467 orchard sites in California's Central Valley from 2018 to 2022. In 2020, megafires scorched more than 4.2 million acres in California, filling the skies with smoke and ash. At the time, researchers were studying how trees store energy, in the form of carbohydrates, to cope with heat and drought. But Orozco said the team saw an opportunity to study how smoke affects carbohydrate levels.

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

Photosynthesis changes under smoke-filled skies. Smoke particles block some sunlight but also reflect light, creating more diffused light. The diffused light can help trees make more carbohydrates. However, Orozco said the study found that while diffused light increased, the smoke was so thick that it likely didn't compensate for the loss of direct light.

Lingering effects, less yield

The team found that megafire smoke not only reduced the amount of carbohydrates in trees but also caused losses that continued even after the fires were out. This led to nut yield decreases of 15% to as much as 50% in some orchards. The most active time for wildfires also coincides with the time trees start storing carbohydrates to sustain them through winter dormancy and spring growth.

"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 said.

Orozco said researchers still don't know what components in megafire smoke caused the decrease in tree carbohydrates. During the 2020 megafires, the smoke reduced light and increased both ozone and particulate matter levels, all of which affect photosynthesis. One or a combination of these factors could have led to the drop in tree carbohydrates.

Additional authors on the study are Professor Maciej A. Zwieniecki and postdoctoral researcher Paula Guzmán-Delgado of the UC Davis Department of Plant Sciences.

The Almond Board of California, the California Pistachio Research Board, the California Walnut Board and the California Department of Food and Agriculture supported the research.