A slow-motion aspen holocaust continues to unfold across Northern Arizona, with 60 to 90 percent of the tremulously beautiful harbingers of fall dying off in many groves in the Kaibab and Coconino national forests, according to a just-released study by Northern Arizona University researchers.

The researchers have dubbed the seemingly inexorable die-off of the white-trunked, tremble-leafed, high-country trees Sudden Aspen Decline (SAD), but still can’t account for it.

The trend could dim the splendor of fall across a wide swath of Rim Country, dealing another bitter blow to the region’s tourist economy — not to mention breaking the hearts of fall-loving locals.

“The fall colors associated with the turning of the aspen leaves have provided a magnet for visitors from throughout Arizona for many generations, said economics professor Ronald Gunderson, with NAU’s W.A. Franke College of Business. “A decline in visitors to the local region is unwelcome news to merchants in the hospitality industry who benefit with the spending by these tourists, particularly when the summer visitation has fallen off.”

The researchers have yet to isolate a single cause for the “extraordinarily rapid” disappearance of whole groves of aspen. Suspects include the effects of a century-long effort to suppress wildfires, the impact of a severe, decade-long drought, the debilitating effects of gradual, global warming and the interaction with various insects and plant diseases.

Individual aspen generally don’t live for more than 150 years, but often a whole grove sprouts from an underground root mass that can eventually cover an entire hillside. So all the visible aspen in a single massive grove can really be clones that have sprouted from a single underground organism — which could make an aspen grove the single largest living thing on earth. Other researchers think they’re also the oldest still-living thing, dating one massive grove in Utah at 80,000 years old.

Moreover, aspen not only draw tourists — they serve a vital role in the forest ecology.

“Aspen have an extremely high ecological value,” said NAU forestry graduate student Tom Zegler, working with the school’s Ecological Restoration Institute. “Per acre, they provide for a greater diversity of wildlife than the sea of ponderosa pine around them and a greater diversity of plants grown beneath them.”

Normally, aspen are among the first trees to recover and spread after a forest fire. The aspen need bright sunlight, so they can’t survive in the shade of the much taller pines. So when a fire creates clearings and a patchy network of open areas, the aspen thrive.

In addition, since they sprout back from the protected root mass, they recover much more quickly from fires than other trees.

Aspen leaves and even bark also provide an important food source for wildlife. The aspen create a quivering mass of small, round, tender leaves. Those leaves have an unusual attachment to the stem, which lets them move in the slightest breeze, creating the distinctive flutter of a quaking aspen grove. Biologists don’t fully understand the advantage to the tree of this distinctive leaf structure, but they suspect that the movement of the leaves enhances photosynthesis and probably enables each leaf to get more sunlight.

The aspen shed these remarkable leaves every fall, but not before turning whole slopes ablaze with a tremble of yellow.

As the hours of daylight dwindle, the tree first withdraws moisture and green-tinted chlorophyll from the leaves and then seals off each leaf by thickening a distinctive layer of cells at the base of the leaf. Once the chlorophyll is gone, other compounds in the leaf show through — giving it a brilliant golden color.

The most recent study by NAU’s Ecological Restoration Institute suggests a dramatic acceleration in a regional trend under way for decades. Previous studies have put most of the blame on the suppression of wildfires, which 100 years ago burned through most of the forest every four to seven years. Those regular fires created openings into which the aspen groves could expand and flourish for a century or two before the returning ponderosas overshadowed them.

However, as a result of the suppression of wildfire and cattle grazing which removed much of the grass that once carried low-intensity ground fires, large areas of the forest haven’t burned in decades.

The more recent declines suggest something else — like the drought or an accelerating warming trend — has redoubled the effect of suppressing fires. As a result, researchers don’t know if they can reverse the trend by returning fire to the system, especially since most of the forest now has so many thickets of trees a fire could scorch even the buried roots of the aspen.

“Since we don’t know why SAD is occurring and what will bring back the aspen,” said Zegler, “land managers don’t want to run fire through or cut down a stand without knowing if the groves will regenerate. If they burn the whole site or cut them all down, they may be able to shut off that hormonal response to the root suckering system and stimulate new growth. But what if nothing grows back? I’m hoping our data will help them.”

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