During the heat of summer, Moabites often find sanctuary in the nearby La Sal Mountains, where temperatures are cooler and shade is plentiful. The mountains are home to vast groves of Quaking Aspens. These iconic Western trees are known for the characteristic way their leaves shake and shimmer in the wind, and for their vibrant foliage in autumn. Visitors to the La Sals may notice a decline in the health of the aspen groves this season. Stands of aspen with little to no understory regrowth in the form of saplings or young trees, or stands with prematurely dying leaves, are likely struggling. The trend of dying aspens is expected to continue.
A combination of long-term and temporary issues are plaguing these trees. According to experts, the decline of the aspens in the Manti-La Sal National forest can be traced to an interconnected set of causes, most of which are linked to drought conditions exacerbated by a changing climate.
In 2002, scientists identified what they called “Sudden Aspen Decline,” or SAD. According to Bill Anderegg, a professor of biology at the University of Utah whose research centers around the intersection of ecosystems and climate change, “SAD was first really applied to the rapid wave of aspen death that happened after the 2000 to 2003 drought. What’s kind of wild is that not as many trees seem to die during the drought itself—instead, there’s a lagged pulse in mortality. For the SAD, this seemed to peak in 2008 or so. We’re fairly confident that drought is the primary factor in this rise in mortality.”
Because aspens tend to suffer several years after a severe drought, Anderegg speculates that the increased aspen death evident in the La Sals this summer might be related to a drought in 2012. “There’s a decently severe drought now,” said Anderegg, “but my sense is most of the death that’s happened last year and the beginning of this year is beginning to ramp up from the 2012 drought. Of course, it’s difficult to disentangle the causes.”
This summer, Moab is facing a particularly dire drought, which may continue to diminish aspen groves in the La Sals for several years. According to Heather McLean, a fire prevention technician for the Forest Service in the Moab-Monticello Ranger District, the La Sals received very low snowfall this past winter. The resulting lack of snowmelt left the mountain soil extraordinarily dry this season. “The snow-pack was a little higher percentage of normal in the spring months,” she explained, “but in the winter we only received 40% or 50% of the normal snowpack.”
As a fire prevention technician, McLean keeps an eye on the moisture levels in the Manti-La Sal National Forest. “It’s been very dry,” she said. “The fuels data from the first of May showed that the large fuels—logs laying on the ground three inches in diameter or bigger—were at 4% humidity. They were drier than kiln-dried lumber at that time.” “Fuels” here refers to any material in the forest available to burn, and dry fuels indicate that wildfires can start easily. Dry fuels also correspond to dry soil.
Drought conditions negatively impact aspens in a variety of ways. Like all trees, aspens need to draw moisture from the soil in order to live, and require a slightly wetter climate than many other trees in Utah. According to John Guyon, a forest pathologist for the Forest Service in the Intermountain Region, “Aspen is a tree that likes ‘mesic’ sites, meaning sites with a bit more moisture. For that reason, it’s not surprising that aspens would be a canary in the coal mine for the expression of drought stress.”
In very dry conditions, aspen trees can even suffer from a kind of embolism. “Embolism is the equivalent of a tree heart attack,” explained Professor Anderegg. “Trees have to pull water out of the soil and get it up to their leaves. The water is under tension: it’s like a spring. During a drought, tiny air bubbles shoot in and fill the water transport pipes. The little air bubbles block the water passages, and eventually cause a breakdown of the vascular system.”
Anderegg has actually listened to these “tree heart attacks” in real time. “With the right equipment,” he said, “you can hold a microphone up to a tree on a dry summer day and actually hear those embolisms form. It sounds like pops and pings—it’s a little like popcorn popping inside of a tree.”
Drought also impacts aspens indirectly. Low soil moisture in the mountains means less food for large ungulates, cattle, and other herbivores. Under these conditions, grazing animals will forage more on young aspen trees.
“This year, we’ll probably see more browsing because of the drought, because there isn’t as much herbaceous forage. Lots of browsing by deer, domestic livestock, and other animals keeps many saplings and young aspen from surviving,” said Barbara Smith, a wildlife biologist for the Manti-La Sal National Forest. “They’ll eat anything green, including young aspens.”
Smith explained that in some places, the Forest Service is taking measures to erect tall fences around aspen stands to prevent excessive grazing by herbivores, but that fences haven’t been built in the La Sals.
While overgrazing due to drought diminishes the new growth in the understory, drought-weakened trees are also more susceptible to bugs and diseases that can destroy the overstory. According to Guyon, aspen trees are vulnerable to insects even under the best conditions, but trees weakened by drought fail to recover when insects damage the canopy.
“When trees in an area suddenly look like they’re losing their leaves often and losing color before autumn, that’s usually due to one of a few different defoliating insects,” said Guyon. “One common one in Utah is the Large Aspen Portrix. We also have a few other caterpillar-like insects called Aspen Leaf Tiers. Those are pretty common as well.”
While defoliation due to insects is normal, healthy aspens recover from defoliation events within a year. “The next year, they usually foliate as normal,” said Guyon. “They gradually recover. But when they’re under physiological stress, they have a lower capability of responding. That’s one of the agents involved in SAD. It’s also tied back to drought stress. If the stands are under stress, a complex of insects and diseases will hit the stressed trees and trigger a decline.”
While certain insects destroy the overstories of aspen trees, another set of insects, fungi and diseases attack the trunks of weak trees. According to Guyon, “canker diseases affect the stems of aspens. Besides the canker diseases, there’s also a group of three insects that attack the stems of the trees, and those are called borers. Often it’s a chain: you have a triggering event with defoliation, and then you get the cankers and bores coming in.” These cankers and bores, explained Guyon, are often the agents responsible for the trees finally dying. All of these agents are involved in Sudden Aspen Decline.
Paradoxically, drought conditions increase the chances of wildfires that could help aspen stands regenerate—but extremely hot and dry conditions make fires too dangerous to allow anywhere near developed areas. Aspen stands are clonal colonies, sending shoots up from one connected root system. Disturbances like wildfires can trigger the colonies to send up more shoots.
“Aspens are a seral species, meaning that they come into ecosystems after disturbances,” said Smith. “After fires or landslides, for instance, they’re the first to come back.”
According to these experts, human prevention of forest fires has disrupted the pattern of disruption and regeneration that makes aspen stands thrive. Guyon explained that two thirds of aspen forests come in as “pioneer species” in the usual ecological succession, when fires and other disturbances create clearings. Often, conifers take over and aspens wane as part of a natural succession. “What’s abnormal about this situation is that because of firefighting efforts in western forests, stigma against fires, as well as some changes in the structure of our forests, there are much longer intervals between fire related disturbances than there would be in a normal ecological sequence.”
When possible, the Forest Service does controlled burns to help aspen stands regenerate. McLean said, “Two years ago, we had a controlled burn down on South Mountain to reduce the encroachment of the fir and spruce coming up on the aspen stands. The aspens really responded well.”
The Forest Service is planning more controlled burns, but fire restrictions make controlled burns impossible this season. “The narrow window we had for safe burns closed too quickly this year,” said McLean.
Fire doesn’t always help aspens. If there isn’t enough moisture in an ecosystem, fire is simply destructive. Smith emphasized the importance of following fire restriction protocol when visiting the La Sals this summer. “Do not start any fires,” she said. “We keep emphasizing that every chance we get.” Though under some conditions controlled fires can help aspen groves regenerate, other fires—especially in drought conditions—will simply kill trees. “We need there to be some moisture, or the fire will kill everything,” said Smith.
Visitors to the La Sals should also avoid carving into the bark of aspen trees. According to Anderegg, “carving on Aspens is bad—they’re easily infected by things through wounds, so carving on the bark can be pretty hard on them.”
Scientists are not optimistic about the future of aspen stands in the Western United States. According to Guyon, “If you look at any of the climate change prediction scenarios, the general indication is that we can expect warmer, drier summers, which can directly affect groups of trees. The trees are going to be experiencing some drought stress for a long time.”
Some of the diminishment of aspen groves might be due to the organisms attempting to move upslope towards cooler, wetter ecosystems in response to a warming climate. According to Guyon, the sprouts that shoot up from the clonal root system are dying back from the edges and attempting to gain higher ground, though very slowly. “Kind of like an amoeba, they contract, and they’ll gradually move upslope,” he said. “That’s happening on ecological time scales over the course of centuries. If climate change occurs at a fast enough rate, aspens might not be able to make that upslope journey fast enough.”
Anderegg is concerned about continual decline of aspens. “In many areas of the west, we’re anticipating a steady decline,” he said. “The studies done so far suggest that the suitable climate for Aspen could shrink between 40 and 90 percent before the end of the century. A huge amount depends on what we do about climate change. If we slow climate change, that number is a lot lower, but if we do nothing…that’s the 40-90 percent scenario.”
While the fate of aspens in the West is dire, Guyon sees a ray of hope. “Aspen has been in western landscapes for tens of thousands of years,” he said. “They’re unlikely to completely go away.”