A 3,000-year record from 52 of the world's oldest Sequoia trees shows that California's western Sierra Nevada was drought-ridden and often on fire from 800 to 1300, according to a new study published in Fire Ecology.

During those 500 years, known as the Medieval Warm Period, extensive fires burned through parts of the Giant Forest at intervals of about 3 to 10 years. Any individual tree was probably in a fire about every 10 to 15 years.

Knowing how giant sequoia trees responded to a 500-year warm spell in the past is important, the authors say, because climate change will probably subject the trees to such a warm, dry environment again.

Scientists reconstructed the 3,000-year history of fire by dating fire scars on ancient giant sequoia trees (Sequoiadendron giganteum) in the Sequoia National Park in Northern California.

This cross-section of a giant sequoia tree shows some of the tree-rings and fire scars. The numbers indicate the year that a particular ring was laid down by the tree.

(Photo Credit: Tom Swetnam.)

To study tree rings, researchers generally take a pencil-sized core from a tree. The oldest rings are those closest to the center of the tree. However, ancient giant sequoias can have trunks that are 30 feet in diameter – far too big to be sampled using even the longest coring tools, which are only three feet long. To gather samples from the Giant Forest trees, the researchers were allowed to collect cross-sections of downed logs and standing dead trees.

"It's the longest tree-ring fire history in the world, and it's from this amazing place with these amazing trees." said lead author Thomas W. Swetnam of the University of Arizona in Tucson. "This is an epic collection of tree rings."

The team also compared charcoal deposits in boggy meadows within the groves to the tree-ring fire history. The chronology of charcoal deposits closely matches the tree-ring chronology of fire scars.

The health of the giant sequoia forests seems to require those frequent, low-intensity fires. As the climate warms, carefully reintroducing low-intensity fires at frequencies similar to those of the Medieval Warm Period may be crucial for the survival of those magnificent forests, such as those in Sequoia and Kings Canyon National Parks.

Citation: Swetnam et al., 'Multi-Millennial Fire History of the Giant Forest, Sequoia National Park, California, USA', Fire Ecology, 2009, 5(3), 120-150; doi:10.4996/fireecology.0503120