Scientists study sequoia tree rings for fire clues

Scientists have found evidence in Sequoia National Park of a centuries-long dry spell — and clues about how the Sierra Nevada could be changing.

The researchers studied tree rings on dead giant sequoias, the largest trees on Earth. They found that during a warm, dry period between A.D. 800 and 1300, fires were more frequent, suggesting more of them might be ahead as the Sierras face similar conditions.

Their findings have been published in the most recent edition of the journal “Fire Ecology,” said Thomas W. Swetnam, lead researcher and director of the Tree Ring Laboratory at the University of Arizona in Tucson.

Information about how sequoias responded to the 500-year warm spell is important because scientists predict climate change may subject the forest to a similar environment again, Swetnam said.

One lesson: The Sierra will see more fires.

The study also answers doubts about whether there really was a 500-year warm period in the western United States, he said.

“It is thrilling to see 3,000 years of history recovered from these amazing trees,” Swetnam said. “This is the longest tree-ring history that’s been established in science.”

Swetnam and his collaborators, including fire ecologist Anthony Caprio in Sequoia National Park, focused on samples from 52 dead and downed sequoias.

Cross sections were cut and sanded so they could be studied under microscopes. The tree rings, or growth layers, which form each year the sequoia lives, are wide during wet years and narrow during dry ones. Within the rings, distinct scars are left whenever the tree was injured by fire.

By counting the rings, scientists can precisely tally and date the fire scars — even pinpointing whether they occurred early or late in the year.

Scientists checked the dates of their findings with other researchers who drilled and removed sediment cores from the ground below Giant Forest. In the cores, charcoal sediments from past fires were analyzed for their age. They matched the age of the fire scars in the tree rings.

The researchers also studied temperatures by looking at high-elevation trees, called foxtail pines. Some foxtails are more than 2,000 years old.

Growth rings from these trees reveal the temperature in the past, adding wide layers in warmer years and thin ones in cooler years.

Swetnam said the study shows fire was far more frequent in the sequoias during past warmups than it has been over the last several decades. If the warm, dry conditions continue, it could lead to more catastrophic fires in the dangerously overgrown forests.

The forests are thicker now because authorities snuffed most fires in the past century. Thick forests tend to create large, destructive wildfires.

But nobody knows for sure what might occur if the climate warms over the next century, Swetnam said.

“The past is not a perfect guide,” Swetnam said. “We are in a much different situation now compared to the past. Our point is that it’s likely we’re heading into something like that 500-year warm period.”