(CN) — On the Nevada side of Lake Tahoe, situated off the shoulder of Spooner Pass that connects South Lake Tahoe to Carson City in the valley below, an old reservoir doesn’t seem like the type of place tourists would glom on to during the height of tourism season.
In fact, during the summer, the parking lot at the Nevada state park that affords access to Spooner Lake is often half or three-quarters full, as most visitors flock to Tahoe to stake a claim on the shores of North America’s largest alpine lake.
But the arrival of autumn brings a different story: the story of aspen.
Between Spooner Lake and Marlette Lake — located in the middle of the Carson Range that flanks the East Shore — lies a bevy of aspen stands that slough off their green come early October and put on the gold.
The creek-riven gully that connects Spooner and Marlette is called the North Canyon, which is home to some of the most spectacular aspen stands in all the Sierra Nevada.
The sight of innumerable topaz leaves trembling in the declining autumn sun is one reason the tourist season that used to run in the summer and then the winter is now bleeding into fall.
But another sight among the aspen trees is giving natural resource managers in the Lake Tahoe Basin pause.
In June 2018, scientists first noticed that aspen trees around the basin were looking more defoliated than usual — a fancy way of saying the beautiful trees were losing their leaves at an unusual rate.
“It was concerning because, from a landscape diversity perspective, aspens are so priceless in terms of what they contribute up here,” said Will Richardson, executive director of the Tahoe Institute for Natural Science.
While not a large share of Tahoe’s tree species, which is dominated by 11 species of conifer, the aspen is home to a wide array of bird and insect species that rely on the trees for habitat, food and other benefits.
So the signs of major leaf loss at the aspen stand in the North Canyon were disturbing, particularly as land managers in the basin had been undertaking strenuous and expensive efforts to restore aspen stands around the Lake Tahoe Basin as conifer trees began to encroach on their habitat and shade them out.
“I was walking around with some colleagues in the summer of 2018 and it became immediately clear the stand was not looking as good as the last couple of years and so I wondered ‘What are we doing wrong here?’” said Stephanie Coppeto, a forest wildlife biologist with the U.S. Forest Service.
Coppeto runs forest restoration projects intended to improve the health of aspen stands in the basin and recalls being dismayed at the sight of so many sick trees just months after taking steps to ensure their health.
“That’s when we found the caterpillars,” Coppeto said. “I thought, ‘This is a bad sign.’”
The caterpillars were later identified as the larva of the white satin moth, a medium-sized moth with white translucent wings. The moth is native to Europe but has been in North America since at least the 1920s.
“They got established in New England pretty quick,” said Richardson. “They were slow to expand in the American West, but they did work their way down from British Columbia and then east into Idaho and Montana before they circled back through Nevada and arrived in Tahoe.”
Aspen trees are one of the most successful deciduous trees for a reason, including having many adaptations that allow them to live in conifer-dominate forests. But also because they have biological mechanisms for fending off noxious insects.
“In most places, the aspen has grown quite used to the moths,” Richardson said. “Here they are not.”
For reasons not fully understood, the moth caterpillars literally ate several the basin’s oldest and most venerable aspens to death. The sheer number of the caterpillars combined with their voracious appetites meant entire trees were shorn of their leaves by the end of June 2018.
“They cleaned off every tree between Marlette and Spooner Lake,” Richardson said. “It looked like winter in the middle of summer.”
The cause of the infestation is still up in the air.
“There were so many caterpillars and I had to ask myself if our restoration project provided access,” Coppeto said. “But maybe the site restoration just allowed us to see the trees and get a hold of the problem for the first time.”
Richardson said there is some speculation that there are genetic components to the problem, particularly as one evaluates the stands and finds several trees utterly ravaged and others virtually untouched.
There is anecdotal evidence that aspens scorched by past wildfires are more resistant to the depredations of the moth.
“Maybe the scorch kicks out some kind of secondary compound that helps the trees defend themselves,” Richardson said.
For now, Richardson and Coppeto have a cornucopia of maybes but not much in the way of clear answers.
The good news is that the summer of 2018 was by far the worst for aspen and the moth. Last year saw another round of white satin moths in the stands but nowhere near the levels observed the previous year.
Scientists are still aching to get out into the field this year, but Covid-19 means many of the researchers who work for the federal government are prevented from doing research.
A purely unscientific jaunt into the backcountry executed recently did not show legions of caterpillars devouring the leaves off the remaining aspen trees, but it was only one part of the basin and it was still early in the season.
Regardless of whether future outbreaks are as lethal to individual trees and detrimental to the aspen stands that fleck the Tahoe Basin, scientists are eager to study why some individual trees fare better.
The fate of biodiversity in the Tahoe Basin hangs in the balance.
“Aspen trees represent around 2% of the ecosystem in forest communities in the Lake Tahoe Basin, but they support more biodiversity than any other upland type,” Coppeto said.
But it’s more than just ecosystems and food webs — humans love the trees too.
“They are just beautifully aesthetic,” Coppeto said. “They serve a niche in human society and are welcome to those of us who come to the mountains in the fall to see them change colors.”
The aspen stands still pervade in the North Canyon for now, but this fall, that curtain of shimmering gold will have a few holes in it thanks to the white satin moth.