Golden quaking aspen leaves won't be making an especially grand appearance in the La Sal Mountains this fall due to a rampant infection of aspen leaf blight, or Marssonina, that has left many stands wilted and defoliated.
The disease, caused by fungal pathogens, is the result of a wetter-than-average May that has allowed the fungi to grow, according to U.S. Forest Service pathologist Elizabeth Hebertson. Hebertson said the condition is widespread throughout the intermountain West from Utah to Montana, and that 80 percent of the stands she has observed are infected.
“Almost in any given year, in any location, you can probably find infected aspen,” Hebertson said. “But this year was unusually high.”
Manti-La Sal National Forest Moab-Monticello Ranger District Ranger Mike Diem told the Moab Sun News that we probably won't see the same spectacular colors that we have in the past.
Stands along the roads to Geyser Pass and Oohwah Lake, and along the east side of the range on the flanks of South Mountain, have been particularly hard hit.
Diem said that under normal conditions, leaf blight isn't a serious concern, and that affected trees should recover the following season. But, he said, prolonged drought conditions and factors associated with climate change have also likely had an effect.
“When you put it in conjunction with other stresses, it's hard to say what the cumulative effects of something like this will be,” he said.
Diem said that aspen forests in the La Sal Mountains, as well as those in other areas of the Southwest, are particularly vulnerable to the effects of climate change and the ravages of drought.
“We're at the edges here,” he said. “Anytime you have a species at the edge of their ecological distribution, they're most at risk of being affected.”
Aspen distribution is widespread throughout the northern latitudes of North America, but in the Southwestern U.S., it is a fringe species with isolated distribution in mountainous areas.
In the Southwest, aspens have become increasingly stressed after more than 15 years of drought, and a rise in temperature associated with climate change. These conditions lessen the trees' ability to ward off disease and insect infestations, and also inhibit their ability to transmit water and nutrients up into their canopies.
In the early 2000s, foresters began to notice a dramatic die-back of aspen in southwestern Colorado, and by 2004, scientists had termed the phenomenon Sudden Aspen Decline (SAD). SAD has resulted in hundreds of thousands of acres of aspen mortality over the past decade.
The current blight, though not directly linked with SAD, can add stress to already-unhealthy tree stands, making their recovery less certain. Hebertson said that ultimately, weather and climate will have implications for the disease.
“Where clones are already more impacted by climate change, the blights can stress the trees out even more and lead to decline or die-back,” she said.
The biology of aspen trees is complex. Thousands of trees in a stand form a single organism of genetically identical clones that are tied to one individual root system. As older trees within the stand begin to decline, the root system sends out suckers, or sprouts, that grow into new trees. But if the clone dies, thousands of trees can be lost.
Hebertson said that the more stressed out a clone is, there will be a resultant decrease in the amount of aspen suckers that sprout each year. And in many cases, she said, the young suckers that are able to sprout are faced with additional threats.
“One of the problems we have is that wildlife and livestock in heavily browsed or grazed areas eat the suckers, and once that clone dies it's gone,” she said.
Greg Montgomery, a former silviculturist for the Moab/ Monticello Ranger District, said that when he first observed aspen forests in the La Sal and Abajo mountains back in 1992, that clones were already exhibiting signs of decline. Montgomery said that the majority of trees were all very old – up to 150 years – and that they showed little sign of rejuvenation.
Montgomery said that since 2002, he has observed extensive aspen mortality on the forest, and that signs of SAD began to appear in 2006.
“Many of our aspen clones are in poor health and are at risk,” he said. “The current phenomena that began to appear last year is just another piece of what is happening.”
Montgomery said that aspens rely on disturbance such as fire to stimulate sucker growth and that suppression over the past 100 years has contributed to the problem.
In 2012, he wrote a paper to help develop aspen management policy for the Moab-Monticello districts of the national forest in an era of climate change. The paper presented several management tools, including controlled burns, mechanical treatments and fencing to protect new suckers from wildlife and livestock.
Diem said that the Moab-Monticello Ranger Districts have been doing treatments to rejuvenate aspen, including the Lackey prescribed burn on the southern end of the La Sals, and the North Elk Ridge forest restoration treatment near Monticello. He likened the projects to “triage,” but said that mortality in clones had decreased over the past couple of years.
Hebertson said that only time and weather conditions would tell how well the trees will recover from the current blight, but said that the Forest Service is being proactive in its measures to try and restore aspen health.
“Hopefully we can increase the aspen components that are out there existing in the landscape,” she said. “Aspen is valuable for all kinds of things such as wildlife habitat, the protection of fisheries, as well as aesthetics. A loss of aspen would be a very bad thing.”