Crown Fires Eliminate Ponderosa Pine Forests

NAU researchers find forests may never grow back as fierce fires pump carbon into the atmosphere


The fierce forest crown fires that have charred hundreds of square miles in Arizona this summer may cause permanent changes in the forest and pump extra carbon dioxide into the atmosphere for decades after the fires flicker out.

Although ponderosa pine forests long ago adapted to frequent, low-intensity fires, the monster fires of recent years threaten to unhinge the system, according to a series of studies by researchers from Northern Arizona University.

“These large fires are devastating our forests,” said Mike Stoddard with the Ecological Restoration Institute at NAU. “We’re concerned that ponderosa pine are not regenerating after these wildfire events.”

Other NAU researchers have studied the after-effects of intense crown fires, like the Wallow Fire, Dude Fire and Rodeo-Chediski Fire. Such fires consume whole trees and jump from treetop to treetop instead of burning through grass and debris on the forest floor.

Those studies show that the fires not only release huge amounts of carbon dioxide as they burn, but continue to release far more carbon than an intact forest in the ensuing decades as the downed logs decompose.

One study suggested that a 10-square-foot patch of forest that absorbed a pound of carbon each year from the air would instead emit half a pound of carbon annually for decades after it burned.

“The grassy vegetation that’s growing is not making up for the amount of carbon that’s being released from the dead trees,” said Tom Kolb, an NAU tree physiologist studying the site of the 8,000-acre Horseshoe Fire, which consumed a ponderosa pine forest in the Coconino National Forest near Flagstaff in 1996.

The studies raise concerns about the long-term effects of the monster fires that have occurred in the region with increasing frequency.

Three of the four largest fires in state history have occurred in the past decade, including this year’s Wallow Fire. The 538,000-acre blaze was apparently started from an abandoned campfire and has so far cost much more to fight than it would have cost to hire crews to thin the same area.

A dangerous rash of fires this year has increased interest in a plan to award timber companies long-term contracts to thin millions of acres in Arizona, in hopes that the use of the thickets of small trees for wood products and power generation will offset the cost of the thinning. The Forest Service is currently seeking bids from private companies for such contracts.

The rash of studies from NAU and elsewhere suggest that the ponderosa pine forests on millions of acres may be doomed without an aggressive thinning plan that allows the return of regular, low-intensity fires.

Rim Country seems particularly vulnerable to dramatic changes in forest composition as a result of high-intensity fires. Areas like Payson sit at the boundary between pinyon-juniper grasslands and ponderosa pine forests.

Ponderosa pines here are living at the edge of the tolerance for heat and drought, which means they will have a hard time returning after a fire — especially if average temperatures rise.

The NAU researchers have discovered that even in the heart of ponderosa pine forests, a high-intensity crown fire can cause seemingly permanent changes.

Stoddard has for years been monitoring the after-effects of the 16,000-acre Hochderffer Fire, which torched an area north of the San Francisco Peaks 15 years ago.

So far, he’s recorded few new ponderosa pine seedlings on his study plots across the area charred by the fire.

NAU silviculturist David Lawrence came to a similar conclusion after studying the 48,000-acre Cerro Grande Fire, which burned another drought-adapted forest near Santa Fe 11 years ago.

He predicted it would take 100 to 200 years for the ponderosa pines to return to that area, if they ever manage to recover. More likely, a brushy grassland will replace the forest permanently, he concluded.

The giant, slow-growing ponderosas have unique adaptations to drought, including specialized bacteria and fungus that grow on their roots and increase their ability to quickly drink up water after rains.

The trees also thrive in poor soils, like the limestone-dominated soils of Rim Country. Such trees remove carbon dioxide from the air and use it to make wood, which means the carbon remains locked up in the tree for centuries.

Low intensity ground fires burned through those old-growth forests every few years, consuming the grass, downed wood and seedlings, but doing little damage to the giant ponderosas — whose lowest branches were 20 or 30 feet above the ground.

The arrival of farmers, ranchers and loggers dramatically changed that ancient old-growth forest, starting in the late 1800s.

Loggers cut down some of the big trees and millions of cattle ate some of the tall grass that used to fill the swales between the wide-spaced giants.

A rare bumper crop of ponderosa pine seedlings in 1919 produced most of the trees still growing today, forming thickets of trees in densities of 500 to 1,500 per acre.

The number and intensity of fires has increased dramatically in the past 20 years.

Those fires have grown to enormous size and intensity, often racing from treetop to treetop instead of ambling along on the ground. The proliferation of small towns and subdivisions built in the midst of these tree thickets has dramatically increased the danger — to both residents and firefighters.

NAU forest ecologist Matthew Hurteau estimates wildfires between 2001 and 2007 contributed about 5 percent as much carbon to the atmosphere as all the nation’s emissions from burning coal, oil and natural gas.

But NAU researchers want to know whether the burned areas will start soaking up carbon as plants return or continue to add carbon to the atmosphere as a result of decomposition of the dead trees.

Kolb used sensors that measured the flow of carbon dioxide on plots consumed by the Horseshoe Fire just off Highway 180 near Flagstaff.

Before the fire, the thickets of trees absorbed about 6 ounces (100 grams) of carbon annually for every square yard of forest. After the fire, the grass and shrubs took in less carbon than the decaying debris produced. As a result,


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