The disease of drought
Research links sudden aspen decline and hantavirus

A healthy stand of aspen trees waits out the winter north of Durango. A new study by Fort Lewis College has linked sudden aspen decline, a mysterious infestation that hit many of Southwest Colorado’s lower elevation stands starting in 2006, with an increase in the prevalance of hantavirus-carrying deer mice. The theory is that the dead and dying trees led to a decline in biodiversity, thusgiving rise to the deer mouse, which is not as picky about its habitat./Photo by Stephen Eginoire

by Missy Votel

The recent die-off of aspen trees could have consequences not juss for the health of the forest, but humans as well, according to a study by Fort Lewis College. The tree-killing syndrome known as sudden aspen decline (SAD), which has wiped out swaths of trees throughout the West, has been linked to a rise in deer mouse populations and, more importantly, the sin nombre virus, which causes the deadly hantavirus in humans.

According to the study, which was overseen by Fort Lewis College Associate Professor of Biology Erin Lehmer, deer mice found at the sites hardest hit by SAD were nearly three times as likely to carry sin nombre. The findings were delivered last week at the annual meeting for the Society for Integrative and Comparative Biology in Seattle and reported on in the latest issue of ScienceNews.

The 2009 study was conducted jointly with FLC Associate Professor of Biology and Agriculture Julie Korb and the help of Fort Lewis College students as well as the San Juan National Forest. Lehmer has conducted similar studies on how small animal communities respond to habitat disturbance, including the Missionary Ridge Fire, ORVs and human encroachment. However, up until now, the effects of SAD, a relatively new phenomenon in the last few years, had never been closely documented.

“There wasn’t a lot of research on SAD, that’s why we wanted to study it,” said Korb, who researched SAD with seven Fort Lewis College biology majors for their capstone independent research projects.

Another reason for getting involved is that Southwest Colorado has been ground zero for the disease, which was first noticed on the San Juan National Forest by by land managers in 2006. Although die-off of aspens is part of any forest’s natural ebb and flow, what was so alarming about the discovery of SAD was its rapid and extensive spread. Since it first struck, SAD has grown exponentially in size and severity. In 2008, nearly 553,000 of the state’s 3.2 million total forested acres, or 17 percent, were found to be dead or dying aspens. Of that, 40,000 acres of the San Juan National Forest’s 300,000 acres of aspen were found to be afflicted, mostly on south- and west-facing and low-elevation slopes. Although the epidemic seems to have abated for the time being, Lehmer said she suspected there were more implications to the outbreak than meets the eye.

“The reason SAD caught my eye is because infectious diseases have been on the rise throughout the world for the last 25 years,” she said. “With the change in climate, there can be a change in range of animals or a change in community dynamics, and new diseases can emerge.”

She pointed to the recent outbreak of avian flu, previously only confined to wild birds, which jumped to domestic birds, such as chickens, and then humans. Lyme disease followed a similar progression. Hantavirus has a similar history. Despite the fact that sin nombre has been around for thousands of years, it only recently made the jump from mice to people, with the first documented case of hantavirus in the Southwest in 1994.

Turns out, Lehmer’s hunch about SAD proved true. With the decrease in the forest canopy brought on by SAD, there was a decrease in understory habitat and vegetation, leading to a shift in species inhabiting the diseased stands. Using live-animal traps, Lehmer and her students sampled multiple sites, ranging in intensity of infection from low (less than 30 percent of trees affected) to moderate (30.1 to 70 percent) to high (more than 70.1 percent affected.) “We found that the prevalence of hantavirus at the ‘high’ sites was three times that of the ‘low’ and ‘moderate’ sites,” she said.

The likely reason is that with less coverage, other aspen-dwelling animals, such as voles – which are not a good host for sin nombre – went off in search of more suitable terrain. Meanwhile, less picky inhabitants, like deer mice – who do make a good host for the virus – stayed. With less available habitat, more mice were crowded into a smaller space, giving rise to the spread of the virus. “An increase in SAD leads to a decrease in small animal diversity,” said Lehmer. “Higher biodiversity lowers the odds of spreading disease and disease prevalence.”

A similar scenario has taken place with the lyme disease epidemic in the East. As wildlife habitat became more fragmented, species that made poor hosts for the Lyme pathogen dwindled. This, in turn, gave rise to the white-footed mice that carry Lyme disease, allowing them to readily pass around infections.

Despite the findings, Lehmer stopped short of drawing any conclusions, noting that the findings of a second, current study on SAD should be out this spring. “SAD is really a rapidly changing phenomenon. Different patterns are emerging as the SAD progresses,” she said, adding that more long-term study is needed. “Anytime you study disease in wild animals, you have to look at long-term data to see any trends.”

Despite uncertainty over the outcome of SAD, its origins are better understood. The general consensus is that the drought of the early 2000s stressed the trees, opening them up to a host of invaders including borers, beetles, fungi and other pathogens. Further complicating matters is the fact that trees that succumb to SAD are not able to regenerate through their roots, as healthy aspen do. “With SAD, the chemical link that signals trees to regenerate is now gone and roots don’t sprout,” said Korb, whose research centered on the trees and surrounding vegetation. She said the end result of SAD goes beyond aesthetics and tourism, to affecting the entire ecosystem. “Aspen provide large amounts of diversity in these conifer-dominant forests in the West,” said Korb.

She said the Forest Service has been experimenting with SAD treatments, whereby aspen stands are clear cut or burned in an effort to remove dying trees and trigger new, healthy growth, with some promising response. “Aspen need some sort of disturbance, such as fire, to regenerate, like in the case of the Missionary Ridge Fire,” she said.

Korb said she is hopeful the study may result in better strategies for managing SAD, such as clues to which stands will better respond to treatment. “The question is, ‘How far is too far gone?” For example, the presence of two insects, the bronze poplar borer and aspen bark beetle, is usually indicative of higher mortality. “If we see those two agents in high frequency, we know that stand is going to be really susceptible, and management might not want to treat it vs. another stand that might respond better,”

She also said managing grazing in affected stands could also play a role in offsetting the effects of SAD on other species. “We saw an increase in yarrow in severe SAD groves because it’s not palatable to cows, thus lowering plant diversity because everything else but the yarrow is being eaten,” she said. “But with this, we also see a shift in species composition.” She said one potential answer is to alter grazing allotments so the understory has a chance to recover in severely affected areas.

However, these are only ways of managing SAD and its impacts once on the ground. Tackling the root cause itself – climate change – is a much more complicated and thorny issue that’s not likely to go away quite as easily as the disease. “The stands that are affected are going to keep deteriorating, but no new ones are going to be hit,” said Korb, “… until the next drought.” •