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Visitors hiking in a park.
Giant Sequoias Yield Longest Fire History From Tree Rings
Firefighters ignite prescribed fires in sequoia groves to encourage giant sequoia germination.
Prescribed fire simulates the natural role of fire in the parks.

NPS photo

California's western Sierra Nevada had more frequent fires between 800 and 1300 than at any time in the past 3,000 years, according to a new study based upon tree-ring research.

The report, Multi-Millennial Fire History of the Giant Forest, Sequoia National Park, California, USA, was published in the electronic journal Fire Ecology in February.

A 3,000-year record from 52 sequoia trees show that California's western Sierra Nevada was droughty and often fiery from 800 to 1300, according to the study led Thomas Swetnam, Director of the University of Arizona Laboratory of Tree Ring Research.

Anthony Caprio, Fire Ecologist for Sequoia & Kings Canyon National Parks, is a co-author.

"Because of their long lives giant sequoias have an exceptionally long chronology of past fires recorded in their rings. The study is important because it looks at how fire regimes responded to past climate changes over the last 3,000 years," Caprio said.

Scientists reconstructed the history of fire by dating the years in which fire scars were found in ancient giant sequoia trees, Sequoiadendron giganteum, in the Giant Forest of Sequoia National Park.

Scientists can crossdate multiple samples (no live trees were sampled) by matching characteristic patterns of signature rings, usually years in which the trees grew little. For example, 1580 was an exceptionally dry year that is reflected as a very small ring in most giant sequoias. Once dated the actual calendar years in which fire scars were formed can be determined and a history of fire reconstructed.

The scientists found the years from 800 to 1300, known as the Medieval Warm Period, had the most frequent fires in the 3,000 years studied.

"While its intuitive that climate patterns influence fire regimes, this study's data provides some of the first looks at how past fire regimes responded in a sequoia grove. Periods of droughts saw more frequent, though smaller fires, as a rule. Wetter periods saw less frequent fire. However, this also gave fuels a chance to accumulate and this led to larger fires."

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

During 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.

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

Such charcoal-based fire histories can extend much further into the past than most tree-ring-based fire histories. The charcoal history of fire in the giant sequoia groves extends back more than 8,000 years.

Click here to read the study.

Researchers count the tree rings of a giant sequoia.
The history of fire in the Sierra Nevada is gleaned from analysis of a giant sequoia's tree rings.

NPS photo by Tony Caprio

Scientists mark the years when this tree witnessed wildland fire.
Natural fires occurred approximately every 5-15 years in sequoia groves. Sequoias thrive in a forest with frequent fire which opens the canopy for this sun-loving tree and clears the ground so there tiny seeds can germinate and grow.

NPS photo by Tony Caprio.