Tree rings reveal a fiery past — and future

Note: This article is a sidebar to this issue's feature story, "Written in the Rings."

Tom Swetnam, the director of the Arizona tree-ring lab, grew up with wildfire. His father was a forest ranger in northern New Mexico, and after Swetnam graduated from college in the late 1970s, he spent two years as a seasonal firefighter in the Gila National Forest of southern New Mexico (HCN, 11/08/04: Keepers of the flame). Inspired by a pack trip with Forest Service scientist Jack Dieterich, who was participating in some of the first tree-ring studies of fire history, Swetnam eventually made his way to Tucson for graduate work.

At the tree-ring lab, he studied the black fire scars nestled in the rings of ponderosa pines, giant sequoias, and other species. He used existing tree-ring chronologies to precisely date each fire, and helped prove that many forests in the Southwest and California were long familiar with wildfire. In the 1980s, these weren’t popular findings — many foresters still viewed wildfire as a malevolent aberration — but the evidence of recurrent fires was tough to dispute.

"When you have skeptics, you can bring in fire-scarred trees and say ‘Look, see all these scars,’ " says Swetnam. "If the fire-scar record shows that the tree has lived through 30 fires, it’s clearly well-adapted to fire." The record, he says, "was essential to turning the tide of opinion among fire managers and the public about the necessity for prescribed fire" — forest fires intentionally ignited to improve ecological health.

Swetnam also noticed that some years, such as 1748, saw numerous fires in widely separated sites. He reasoned that such massive fire years were caused by broad changes in climate. When he compared the fire record with the drought histories constructed by his colleagues at the tree-ring lab, he found his hunch was correct: Drought was strongly correlated with wildfire.

Swetnam and his longtime friend and collaborator, U.S. Geological Survey researcher Julio Betancourt, eventually uncovered a connection between swings in ocean temperatures and Southwestern fire. In a 1990 paper in the journal Science, they argued that extremely dry La Niña years, caused by a cooling of the sea’s surface in the tropical Pacific, often lead to big fire seasons. Since then, more subtleties have emerged. It appears that wet El Niño years, caused by a warming of the tropical Pacific, encourage fuels to build up in the forests, while ensuing La Niña-driven dry periods make it easy to light the bonfire.

In the Science paper, says Swetnam, "we explicitly say, ‘Hey, this has potential for forecasting, for anticipating what fire systems might be like.’ But people didn’t see it at the time." Now, however, the National Interagency Fire Center in Boise, Idaho, has a Predictive Services Group, which uses the insights of Swetnam and other climate scientists to help plan for the future.

Researchers are currently using tree rings to further untangle the influences of distant ocean temperatures, in the Atlantic as well as the Pacific, on fire behavior in the West. Such work may eventually guide not only firefighters, but also forest managers. Research by Betancourt and others indicates that some variations in sea-surface temperatures encourage periodic, extensive droughts, leading to widespread forest die-offs. Subsequent wet periods encourage the simultaneous growth of new trees, creating uniform stands highly vulnerable to drought, fire, insect outbreaks, and other disturbances.

The task of forest managers, Betancourt says, may be to "throw these systems out of synch" to cushion the effects of large-scale catastrophes. That might mean removing seedlings of a particular age from one part of a forest, he says, while allowing them to grow undisturbed in another area. More diverse forests, with a mix of trees ranging from seedlings to old growth, would likely be better protected from wholesale damage, and would recover more quickly from disturbances.

In a drought-prone — and warming — world, preservation may lie in patchiness.

• Climate Change
• Forests