On a drive from Durango to Cortez, groves of aspens house dead and dying trees, ghostly reminders of the lingering effects of a drought 20 years ago and the region’s changing climate.
Sudden aspen decline – one of the causes of those declining trees – is a widespread, severe and rapid dieback and mortality of aspen groves. The phenomenon is closely associated with the severe droughts in the early 2000s, said Suzanne Marchetti, a technician with the U.S. Forest Service in Gunnison. This rapid aspen mortality was not fully obvious to the Forest Service until 2004 and continued to expand until 2009, Marchetti said.
“It was most prominent at low elevations and on hill slopes already prone to dryness,” she said.
In addition to studies seeking to better understand SAD, the U.S. Forest Service has funded projects to promote the logging of problem aspen areas and ultimately improve the resiliency of aspen groves overall. But experts say similar droughts, fueled by climate change, could spark another high tree-mortality event.
“These droughts associated with high temperatures have had widespread impacts on our forests,” Marchetti said. She added that the research shows “droughts like the one in 2002 will be more common with a warming planet.”
While aspen groves, which share large root systems and have an average lifespan of 100 years, have always been responsive to changes in climate and environmental disturbances, SAD is unique for a couple of factors, said David Casey, a forester in the Dolores Ranger District.
The Forest Service found mature aspens at low elevation in open stands are particularly predisposed to be affected by the hot and dry conditions that hit Colorado from 2000 to 2005. Insects and diseases then exacerbated the problem, killing trees already stressed by the drought. This storm of factors led to a rapid and sudden death of aspen groves, which was determined to be separate from general aspen decline but rooted in similar causes, Casey said.
The continued impact of SAD is exacerbated by climate change and historic fire suppression techniques, he said.
“As it gets hotter and drier, the lower elevation stands are seeing more impact and spread,” Casey said.
A U.S. Forest Service assessment published in 2008 found 139,000 acres in Colorado were affected by SAD in 2006. A year later, that number had more than doubled, topping 358,000 acres of Colorado aspen. In 2013, those SAD stands were remeasured, and the Forest Service found they were continuing to deteriorate, while the stands that were healthy remained stable, Marchetti said.
While there have not been signs of significant expansion into new groves, the aspen groves already exposed to SAD continue to fall apart. Now, about 550,000 acres in the state are affected by the decline, said Marchetti, who helped to collect the original data reported in 2008.
In addition to environmental implications, SAD can also have effects on local businesses.
David Sitton, owner of Aspen Wood Products, said when sudden aspen decline affects a grove, “the quality of the timber is drastically reduced.”
Sitton’s business logs aspens and has a mill located in Mancos. He said there is a prevalence of aspen groves affected by the sudden die-off in that area.
“We just finished harvesting an area this last summer that was a sudden aspen decline area near Mancos,” he said.
If the area is hit particularly hard by SAD, Sitton said the wood will be unusable and it will be waste, used for firewood or left in the forest.
“When it hits an aspen stand, it’s pretty devastating to the aspen,” Sitton said. He added he’s seen “more and more decline in the aspen in our area.”
He said if the grove is bad enough – too dead, too dangerous to cut or trees falling over onto each other – “it doesn’t even make sense to go in and cut it down.”
But Sitton added when his business has gone into log a grove affected by SAD, he saw a healthy regeneration of the aspen in that area afterward.
“The changes are very noticeable,” he said.
Marchetti said the research of the Forest Service partially backs up Sitton’s assessment. In aspen stands with less than 50% die-off from SAD, cutting and logging “was an effective way to promote vigorous regeneration,” she said.
When the stand experienced a higher mortality rate, the regeneration success was spotty. There are also challenges in regenerating areas with high elk populations because “they are just too good at eating the baby trees,” Marchetti said.
While sudden aspen decline is less of a problem than it was in the early 2000s, Marchetti said the recurrence of periods with high aspen decline is “highly likely if this pattern continues of episodic droughts.”