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.
