Tree rings reveal a fiery past — and future

  • Thomas Swetnam, director of the Laboratory of Tree-ring Research, stands among archived samples

    Edward McCain
 

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.

 

High Country News Classifieds
  • GRAND CANYON DIRECTOR
    The Grand Canyon director, with the Grand Canyon manager, conservation director, and other staff, envisions, prioritizes, and implements strategies for the Grand Canyon Trust's work...
  • ADMINISTRATIVE ASSISTANT
    Great Old Broads for Wilderness seeks a part-time Administrative Assistant to support the organization's general operations. This includes phone and email communications, office correspondence and...
  • HISTORIC LODGE AND RESTAURANT - FULLY EQUIPPED
    Built in 1901, The Crazy Mountain Inn has 11 guest rooms in a town-center building on 7 city lots (.58 acres). The inn and restaurant...
  • ONE WILL: THREE WIVES
    by Edith Tarbescu. "One Will: Three Wives" is packed with a large array of interesting suspects, all of whom could be a murderer ... a...
  • PROGRAM DIRECTOR, SALAZAR CENTER FOR NORTH AMERICAN CONSERVATION
    The Program Director will oversee the programmatic initiatives of The Salazar Center, working closely with the Center's Director and staff to engage the world's leading...
  • WILDEARTH GUARDIANS - WILD PLACES PROGRAM DIRECTOR
    Salary Range: $70,000-$80,000. Location: Denver, CO, Portland, OR, Seattle, WA, Missoula, MT or potentially elsewhere for the right person. Application Review: on a rolling basis....
  • RIVER EDUCATOR/GUIDE + TRIP LEADER
    Position Description: Full-time seasonal positions (mid-March through October) Organizational Background: Colorado Canyons Association (CCA) is a 10 year old nonprofit organization fostering community stewardship of...
  • BOOKKEEPER/ADMINISTRATIVE ASSISTANT
    Position Description: Part-time, year-round bookkeeping and administration position (12 - 16 hours/week) $16 - $18/hour DOE Organizational Background: Colorado Canyons Association (CCA) is a 10...
  • LAND STEWARD
    San Isabel Land Protection Trust seeks a full-time Land Steward to manage and oversee its conservation easement monitoring and stewardship program for 42,437 acres in...
  • EXECUTIVE DIRECTOR
    Ventana Wilderness Alliance is seeking an experienced forward-facing public land conservation leader to serve as its Executive Director. The mission of the Ventana Wilderness Alliance...
  • COMMUNICATIONS AND DEVELOPMENT DIRECTOR
    The Quivira Coalition (www.quiviracoaltion.org) is a Santa Fe-based nonprofit that builds resilience on arid working lands. We foster ecological, economic, and social health through education,...
  • GRANT WRITER
    "We all love this place we call Montana. We believe that land and water and air are not ours to despoil, but ours to steward...
  • DEVELOPMENT DIRECTOR
    The Development Director is responsible for organizing and launching a coherent set of development activities to build support for the Natural History Institute's programs and...
  • WILDLIFE PROJECT COORDINATOR
    Founded in 1936, the National Wildlife Federation (NWF or Federation) is America's largest and most trusted grassroots conservation organization with 53 state/territorial affiliates and more...
  • EXECUTIVE DIRECTOR
    The Cinnabar Foundation helps protect and conserve water, wildlife and wild lands in Montana and the Greater Yellowstone Ecosystem by supporting organizations and people who...
  • TRUSTEE AND PHILANTHROPY RELATIONS MANGER,
    Come experience Work You Can Believe In! The Nature Conservancy in Alaska is seeking a Trustee and Philanthropy Relations Manager. This position is critical to...
  • EXECUTIVE DIRECTOR AT FRIENDS OF CEDAR MESA
    -The Land, History, and People of the Bears Ears Region- The Bears Ears and Cedar Mesa region is one of the most beautiful, complex, diverse,...
  • CONSERVATION SPECIALIST
    Position will remain open until January 31, 2021 Join Our Team! The New Mexico Land Conservancy (NMLC) is a non-profit land trust organization dedicated to...
  • OLIVERBRANCH CONSULTING
    Non-Profit Management Professional specializing in Transitional Leadership, Strategic Collaborations, Communications and Grant Management/Writing.
  • GREAT VIEWS, SMALL FOOTPRINT
    Close to town but with a secluded feel, this eco-friendly home includes solar panels, a graywater reuse system, tankless hot water, solar tubes, and rainwater...