When lightning ignited fires in California’s Big Basin Redwoods State Park north of Santa Cruz in August 2020, the fire spread quickly. Redwoods naturally resist burning, but these fires reached the canopies of trees over 300 feet tall. “It was shocking,” says Drew Peltier, a tree ecophysiologist at Northern Arizona University. “It really seemed like most of the trees were going to die.”
Yet many of them lived, according to a report in Science magazine, and in a paper published in Nature Plants, Peltier and his colleagues explain why: The burned trees, despite losing their needles, mobilized their long-held energy reserves, the sugars that were produced from sunlight decades ago. The trees routed this energy into dormant buds under the bark.
“This is one of those papers that challenge our previous knowledge on tree growth,” says Adrian Rocha, an ecosystem ecologist at the University of Notre Dame. “It is amazing to learn that carbon taken up decades ago can be used to sustain its growth into the future.”
When the wildfires in 2020 burned through Big Basin Redwoods State Park, reported the San Francisco Chronicle, they left some of the oldest trees on the planet badly burned; researchers now have estimates of just how old the energy reserves of those redwoods are. Researchers studying a stand of severely burned old-growth redwoods found the buds were more than 1,000 years old.
Mild fires burn through coastal redwood forests about every decade, and the giant trees resist flames in part because the bark is up to a foot thick on the lower trunks, and it contains tannic acids that are fire-resistant. But in 2020 even the uppermost branches of many trees burned and their ability to photosynthesize went up in smoke along with their needles. Giant sequoias — which are different from the redwoods — can live for up to 3000 years, but in 2020 about 10 to 14 percent of the giant sequoias in the Sierra Nevada that were at least four feet in diameter were killed in the Castle Fire on the Sequoia National Forest.
A sprout emerges from thick redwood bark in Big Basin Redwoods State Park — 2021 photo by Drew Peltier, Northern Arizona University
Fire managers weren’t sure the trees on the Sequoia and in Big Basin would make it, but visiting the state park a few months after the fires, Peltier and his colleagues found fresh growth emerging from the trunks of blackened redwoods. They knew that shorter-lived trees can store sugars for several years. Because redwoods can live for more than 2000 years, the researchers wondered whether the trees were drawing on much older energy reserves to grow these new sprouts.
Melissa Enright with the USFS covered parts of 60 blackened tree trunks with black plastic to block out sunlight, ensuring that any new sprouts would grow with only stored energy, not new sugars produced from current photosynthesis. After 6 months, the team brought some sprouts back to the lab, and they radiocarbon-dated them to calculate the age of those sugars. At 21 years, they are the oldest energy reserves shown to be used by trees. But the mix of carbohydrates contained some carbon that was much older, and Peltier calculated that the redwoods’ carbohydrates were photosynthesized nearly 6 decades ago.
“They allow these trees to be really fire-resilient because they have this big pool of old reserves to draw on,” Peltier says. These redwoods have formed new sprouts, but Peltier and other forest researchers wonder how the trees will cope with far less energy from photosynthesis, considering that it will be many years before the trees can grow as many needles as they had before.
“It is likely that other long-lived trees also harbor carbon reserves that are much older than previously recognized,” said Peltier. The carbon stores observed in the trees, he told a Forbes reporter, date back as far as 1500 years, and they may provide hope for other ancient trees “destroyed” by fire.
