Scientists study centuries-long Sierra dry spell

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

The researchers studied tree rings on dead giant sequoias, the largest trees on Earth. They found that during a warm, dry period from the year 800 to 1300, fires were more frequent, suggesting more fires may be ahead for a Sierra facing similar conditions today.

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.

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.

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 in the Giant Forest, the largest concentration of the big trees in California, according to the American Southwest Web site.

Cross sections were cut from the dead sequoias and sanded so they could be studied under microscopes. The tree rings, or growth layers, that form each year the sequoia lives, are wide during wet years and narrow during dry years. 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 in the 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.

Growth rings from these trees reveal the temperature in the past — the rings are wide in warm years and narrow in cool years.

Swetnam said the study shows fire was far more frequent in the sequoia forest during past warmings than it has been over the past several decades.