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From pines to prairie? Arizona’s forests susceptible to fire, beetles

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FLAGSTAFF – Drivers headed northwest along U.S. Route 180 from Flagstaff to the Grand Canyon travel through an eerie landscape amidst the otherwise lush and verdant Coconino National Forest.

Charred trunks protrude from a grassy savannah, thin skeletons of the towering ponderosa pines that had dominated these hillsides. Blackened tree limbs are scattered among them. There are barely any patches of green here, and there are no sounds of wildlife.

It resembles nothing so much as a graveyard.

Arizona forestry experts fear such landscapes might be increasingly common over the next century, as warmer temperatures and drier weather create virtually ideal conditions for wildfires. Such fires have already ravaged hundreds of thousands of acres of Arizona’s forests in the past few decades.

“We’re already seeing fires of increased frequency and intensity,” said Tom Kolb, a professor at Northern Arizona University’s School of Forestry in Flagstaff.

“Conditions here are definitely more hot and dry than they were a hundred years ago, and that’s contributing to the fires that have become almost an annual occurrence around here.”

This summer, a blaze swept through over 15,000 acres of the Coconino National Forest, threatening homes and destroying wildlife habitat. The damage is just a fraction of that caused by 2002’s catastrophic Rodeo-Chediski Fire, which consumed over 450,000 acres in the White Mountains, forcing the evacuations of Show Low and other nearby communities.

Recent though the devastation along Route 180 may appear, it’s actually evidence of a wildfire that occurred in 1996. There has been scarcely any regeneration of the vegetation that had thrived here prior to that fire, and experts believe it may take several decades – if ever – before any significant new growth restores natural beauty to these hills.

“Ponderosa pines reproduce only by seeds,” explains Pete Fule, an ecological restoration expert who also teaches at NAU. “These seeds can only carry a certain distance away from parent trees, so when you have a large disturbance event that kills trees over a pretty big area, it’s difficult for them to regenerate back into that site.”

As wildfires continue to eliminate the ponderosa pine and other native trees from their habitat in Arizona, they create an opportunity for invasive species such as bromus tectorum, commonly called “cheatgrass,” to take their place. Cheatgrass has supplanted the native vegetation in many wildfire sites throughout northern Arizona, essentially transforming forests into prairies.

Because cheatgrass is itself very flammable, these resulting savannahs are veritable tinderboxes for the surrounding forests, providing fuel that will only add to the intensity of future fires.

At least one recent study suggests that the proliferation of grassy plains in areas previously occupied by forest vegetation – such as the transformation evident along Route 180 – is to be expected.

Fule explained that a third of large wildfire sites throughout the Southwest in the past half century never returned to their original conditions.

Fire is hardly the only agent of change affecting Arizona’s forests as temperatures climb over the next 90 years.

Pine bark beetles, already responsible for the deaths of millions of pinyon and ponderosa pines in the state, reproduce at a faster rate in warmer weather. U.S. Interior Secretary Ken Salazar has referred to these beetles as “the Katrina of the West,” and their devastation might just be beginning. Healthy pines are able to defend themselves against infestation by pushing the beetles out with sap, but droughts weaken trees, rendering them more vulnerable to attacks.

Such droughts would also threaten Arizona’s spruces, firs and aspens, which rely on moisture more than pines. Such species thrive today in moderately moist habitats such as the temperate hardwood forests around the White Mountains.

Also, as Arizona’s forests become increasingly warm, existing vegetation may relocate, especially at higher elevations, to areas more favorable to their growth. For instance, junipers and pinyon pines – two trees typically found in Arizona’s low-lying woodlands, could conceivably adapt to climate change by moving upwards in elevation to more moderate conditions, supplanting ponderosa pines in the process.

Fule, quipping that “plants aren’t really good at walking,” doubts whether the native vegetation can relocate quickly enough to adapt to the changing climate. If this proves to be the case, the vegetation in low-lying areas might simply die off.

“The key point about modern climate change,” Fule said, “is that it’s happening very quickly. It seems unlikely that modern ecosystems, just acting through natural processes of regeneration, would be able to keep up. In fact, people have done modeling studies to look at how fast plants would have to move, and it’s pretty darn fast. It’s something that exceeds, by orders of magnitude, the fastest movement of plants that we’ve ever seen in the fossil record.”

Kolb’s office looks out over a nearby hill resplendent with healthy ponderosa pines. Asked what he would expect to see were he to look out his window in 100 years, Kolb conjectured that the hillside would have fewer ponderosas and more junipers and pinyons, species that would have crept up in elevation to adapt to the rising temperatures.

Then he turned away from the window and shrugged.

“If it doesn’t look like that,” Kolb said, “then it might just be an ugly, burned landscape out there.”