Warming is called a threat to forests

Rising sea levels may be a problem hundreds of miles to the west, but global warming is already hitting much closer to home for Arizonans. There's a good chance it will kill off up to half the West's forests in the next 100 years, according to a University of Arizona scientist.

The forecast is based upon what has already happened in Southwestern U.S. forests, and could happen to an even greater extent just a few decades from now, says Thomas W. Swetnam, a professor of dendrochronology and a fire-ecology expert in the UA's Laboratory of Tree-Ring Research.

Swetnam has attracted national attention. He recently testified before a U.S. Senate committee, saying that global warming's impact on forest-fire frequency and severity is already being felt. He cited Arizona's massive Rodeo-Chediski and Oregon's huge Biscuit fires, both in 2002, as examples.

And this Sunday the UA expert will be a featured guest on a CBS "60 Minutes" segment about the relationship between warming and forest fires.

The recent decade's massive fires in the Western U.S. are widely blamed on many earlier decades of suppressing all wildfires. Swetnam says the policy has left forests thick with fuel — downed trees, underbrush and debris that would have been burned by periodic natural fires.

The fires that ultimately occur in these thick forests and get out of control don't stop with wiping out the understory of downed and small trees, scrub brush and debris, and sparing mature trees, says Swetnam. Instead, they burn so hot that they destroy even the largest trees, killing the forest from ground to the treetop canopy.

The fuel load has not only been built by years of suppressing smaller fires, but by an increase in the amount of drought-weakened trees that were killed by insect infestation.

While reluctant to give a blanket prescription to thin forests that haven't been burned, Swetnam says there is ample evidence that carefully designed thinning of small trees and underbrush and controlled burns can reduce the severity of fires.

Swetnam said some of the burned forests may not rebuild, at least not with the same — and in some cases any — tree species. And that could have dire consequences for the plants and animals that lived in those forests.

The "sky island" view of Arizona's mountainous habitats — and the plants and animals that live there — says many species are isolated by their inability to survive below a certain elevation, or they traverse lower regions to get to another suitable area.

Swetnam said the soil may be too scorched to support new growth of the same tree species, or that they may be beaten out by trees and scrub from lower elevations that are then able to take root and thrive at higher elevations as temperatures rise.

And the soil, with no plants to hold it in place, may wash away, opening the way for invasive species.

In some cases, global warming will push the coolest sky-island zones right off the mountaintops, leaving critical plant and animal species nowhere to go, said Travis Huxman, a UA associate professor of ecology and evolutionary biology.

Non-native grass species, which are able to invade higher and higher areas as temperatures rise, may have an impact on fire for a couple of reasons, Huxman says. Not only may they take over areas where the trees have burned, but they can spread fire among zones up and down the mountains.

"In the Mojave (Desert) we are already seeing these (grasses) fuel fires that race down into the desert," Huxman said.

But solutions aren't easy, even if authorities can agree on forest-undergrowth thinning and prescribed fires.

The Mount Graham red squirrel, a federally listed endangered subspecies that lives in a narrow zone in Arizona's Pinalenos near Safford, is perhaps the best example of habitat sensitivity to climate change and fire. There are thought to be only about 200 of the small squirrels left, says John Koprowski, a UA professor of wildlife conservation and management.

The squirrels pose a particularly tough problem because they thrive and store their food — certain evergreen cones — in the very downed trees and undergrowth that controlled burns and thinning would eliminate, Koprowski says.

"Many of the characteristics that promote fires — downed logs, snags — are the things that squirrels like," he added.

Koprowski says the U.S. Fish and Wildlife Service, the protector of the endangered squirrels, and the U.S. Forest Service, charged with protecting the forest, must balance their needs.

He said the agencies have to come up with a management plan that decreases the risk of catastrophic fires but maintains the squirrels' habitat.

Fortunately, he said, there are some areas on Mount Graham where the squirrels haven't lived for at least a century and that can be managed to decrease the fire risk.

If you watch

University of Arizona scientist Thomas W. Swetnam is a featured guest on CBS' "60 Minutes" at 6 p.m., Sunday on KOLD-TV 13, Comcast Channel 7.