The physical scars burned into Arizona's forests this summer are the most visible proof of wildfires' fury, but the aftermath will spread far beyond the charred landscape.

The loss of so many trees and other vegetation will affect air and water quality for years to come, some Arizona scientists say, reversing the ability of forests to help slow climate change and protect desert water supplies.

Arizona wildfire: Wallow Fire near Alpine

Arizona wildfire: Monument Fire near Sierra Vista

So far, more than 932,000 acres have burned statewide this year, a total closing in on the 975,000 acres in 2005, the worst year on record. Not all the acres were in forests and not all were severely burned, but the toll on the state's wildlands is significant.

"What's really key here is the forest's recovery," said Tom Kolb, a tree physiologist at Northern Arizona University's School of Forestry. His research includes the site of a fire near Flagstaff where a forest that once absorbed carbon dioxide now emits it, the result of decomposing dead trees on the ground and newly grown grasses and shrubs, which absorb far less carbon.

Many scientists believe high levels of carbon dioxide are contributing to higher temperatures around the globe and shifts in climate conditions.

"We are concerned that we are witnessing the conversion of forests that take in carbon and store it to vegetation types that have little ability to do that," Kolb said. "That is a situation where these fires will contribute to global warming. We've reduced the capacity of the land to store carbon dioxide."

On severely burned forest floors, meanwhile, the trees and shrubs that help contain runoff from storms and melted snow are gone. The result, hydrologists say, is floods that will fill streams and creeks with sediment. Aquifers, springs and rivers will suffer losses as less water percolates into the ground.

"It's not a lot of water, but it is important," said Abe Springer, a hydrogeologist at NAU. "It can be the difference in allowing some headwater streams to flow year-round."

Carbon-dioxide role

The role forests play in capturing and storing carbon dioxide is nothing new, part of the basic plant processes schoolchildren study. But the importance of forests in reducing carbon in the atmosphere has grown among scientists and policy makers studying climate change.

"We're looking to forests to take in carbon, thereby lowering the greenhouse gases," said Matthew Hurteau, a forest ecologist at NAU's School of Earth Sciences and Environmental Sustainability.

Big wildfires reverse the process, releasing carbon into the air first as the trees - ponderosa pines and mixed conifers are most common in the high-elevation forests - and other vegetation burn and then as the dead trees decompose, often over decades.

The flames release only a small amount of a forest's carbon during a wildfire. Much more comes as the trees begin to decompose. Grasses and shrubs may sprout within the first year after a fire, but they can't capture as much carbon as a stand of pine trees.

NAU researchers have installed measuring devices to track carbon releases at three sites near Flagstaff for the past five years, including the site of the 1996 Horseshoe Fire, which burned 8,000 acres. Before the fire, the forest could take in 100-200 grams of carbon per square meter each year. Now, it releases 50-100 grams of carbon per square meter each year.

"The site has gone from being a carbon sink, where carbon was being stored, to a carbon source," Kolb said.

One factor in how wide a burned area is converted to a carbon source is the intensity of the fire. If flames kill the trees and the underlying vegetation, recovery will come slowly and more trees will begin to decompose.

If there are patches where the fire burns less intensely, recovery could occur sooner and the long-term effects on carbon releases could diminish. Such patches are evident on the early severity maps of the Wallow Fire in Arizona's White Mountains, now the state's largest recorded wildfire.

Some of those areas could herald the return of aspen or oak trees, which could take over the role of capturing carbon, Kolb said.

Water worries

Fire's effects on a watershed are easier to see and of more immediate concern. A badly burned area can cause flooding in the weeks and months after a fire, sending ash and sediment into streams, creeks and reservoirs.

A large area of the Wallow Fire burned on the Salt River watershed, which helps produce runoff for the Phoenix area through Roosevelt Lake and the canals operated by Salt River Project.

SRP officials will work with the U.S. Geological Survey and the Arizona Department of Environmental Quality to monitor runoff from summer storms and winter snow, said Gregg Elliott, principal environmental scientist for the water utility.

In 2002, the Rodeo-Chediski Fire burned another part of the Salt River watershed, and the result was a flow of ash and debris into Roosevelt Lake. The reservoir is fuller this year, Elliott said, which will help dilute some of the debris.

"We will be watching the area now and then as grass starts to grow," he said. "The long-term effects will be seen for years, up on the watershed as well in as the reservoirs."

Some of the effects can linger for as long as five years, NAU's Springer said, as the root systems of burned trees and shrubs weaken and decompose.

An intense-burning fire can turn the ground hydrophobic, in effect sealing it over in a way that prevents water from rain and melting snow from percolating downward, he said.

With the gloomy predictions comes some hope for the charred areas: Forests that are not severely damaged can recover quickly and resume their natural functions. Studies also suggest that thinning and restoration work can strengthen a forest, even with fewer trees to do the work.

Kolb said the effect is most noticed during a drought. A thinner stand of trees is better able to draw on limited water supplies than a denser stand, which appears to shut down as temperatures rise and the competition for water stiffens.

That's significant because most studies suggest climate change will result in longer, more severe droughts in the Southwest.

The goal in trying to contain carbon levels is stabilizing a forest, Hurteau said. A dense forest can take in more carbon dioxide than one that has been thinned, but that dense forest is more vulnerable to the sort of monster wildfires that leave a forest unable to capture any carbon.

"That's effectively the debt we have to pay to reduce fire risk," Hurteau said. "We're walking that line where we're looking at balance. If we thin the forests, we pay that carbon debt up front. The remaining live trees still pull carbon from the atmosphere. Dead trees release it."