Decades of Tree Data Reveal Forests Under Attack

Smithsonian researchers with ForestGEO found that invasive species are linked to roughly one in four tree deaths in a section of the Blue Ridge Mountains

Smithsonian researchers studied 67 forest plots in a section of the Blue Ridge Mountains. They found that hemlock woolly adelgid had decimated hemlock populations. Alan Tepley, Smithsonian Conservation Biology Institute

The Blue Ridge Mountains make up one of the oldest mountain ranges in the world. They are also home to forests with diverse tree populations. But those trees are under attack from invasive species, and Smithsonian researchers say that the impact over time is becoming ever more concerning.

According to a study published in the journal Ecosystems on April 29, in a section of the Blue Ridge Mountains, about a quarter of the loss of tree biomass—roughly one-quarter of tree death—over the past three decades was linked to invasive species transported by humans. The study is believed to be the first long-term look at the impact of multiple invasive species on aspects of the forest, the researchers reported.

One of the forest sections they studied is part of the Smithsonian’s Forest Global Earth Observatory, or ForestGEO, a network of sites that monitors changes to forests. The initiative dates to 1980, when Smithsonian researchers began mapping, measuring and tagging trees at a site in Panama. That original plot contained a quarter of a million trees. “No one had ever tried anything like this ever before,” says Stuart Davies, director of ForestGEO.

Ash trees
The researchers discovered ash trees infested with emerald ash borer beetles, which get under the bark and disrupt the tissue that brings water and dissolved minerals from the roots to the leaves. Alan Tepley, Smithsonian Conservation Biology Institute

The network now monitors some six million trees inside tropical and temperate forests at 70 sites in 27 countries. The methodology to monitor them is consistent across sites, which makes the program unique. “Unless you measure things in the same way, it’s very hard to compare them,” Davies says. “What we have is a very clearly standardized protocol, where we know if you compare a data set from Gabon with a data set from Brazil, there’s no question.” More than a thousand research articles have come out of the network.

For the new study, the researchers focused on plots inside Shenandoah National Park in Virginia, which contains a section of the Blue Ridge Mountains, and a nearby plot monitored by the Smithsonian Conservation Biology Institute. The team from the Smithsonian and the park looked at 67 plots of forest across 73 acres. The data was from 1987 to 2019 and contained more than 350,000 tree observations.

“We know that pests and pathogens can have a really important impact. But it’s highly variable in time and space,” says Kristina Anderson-Teixeira, a forest ecologist at SCBI and the Smithsonian Tropical Research Institute and lead author of the study. “I wanted to get a broader overall picture of, long-term, how have these been affecting the forests as a whole?”

Shenandoah National Park
“People need to appreciate forests, recognize that our water, our clean air, so many services that we depend on, are the result of fully functioning, healthy forests,” says Stuart Davies, director of ForestGeo. Alan Tepley, Smithsonian Conservation Biology Institute

The researchers found that the invasive species were tied to around 25 percent of tree deaths over the time period studied. “That’s pretty significant for the functioning of the forest,” Anderson-Teixeira says. “We’re losing cool species, species that we value for one reason or the other.” Normally trees have mortality rates of 1 or 2 percent per year, she says. For the trees that invasive species impacted, the figure was as high as 20 percent.

Changes to the forest affect the animals that live in them. This area is known for bobcats, raccoons, eastern striped skunks and opossums. “There are these cascading impacts of the forest composition on. . . the forest animals,” Anderson-Teixeira says. For example, the gypsy moth, an invasive insect, has devastated oak tree populations in the area, and animals such as American black bears, white-tailed deer, Allegheny woodrats, Eastern gray squirrels, and southern flying squirrels rely on acorns from those trees.

“Due to these invasive species,” says William McShea, a wildlife ecologist with the Conservation Ecology Center at SCBI and one of the study’s 20 authors, “you’re getting a lot more young trees, and that’s a much different forest composition. That benefits some species and doesn’t benefit others.” White-tailed deer flourish with young vegetation and woody plants at the forest floor, for example. But other species, including birds, prefer a more mature forest, according to McShea.

Invasive species aren’t just a problem in the Blue Ridge Mountains; they impact forests throughout the United States. People have documented at least 471 exotic insects and pathogens in forests across the country over the past few centuries, and similar situations exist in Europe and Asia.

The problem is species-specific, so particular invasive species impact particular tree hosts. The researchers identified eight combinations of invasive pests or pathogens and their hosts: American chestnuts suffered from chestnut blight, a fungus from Asia; elms had Dutch elm disease, also a fungus from Asia; redbuds had neofusioccum, a fungus of unknown origin; butternut trees had butternut canker, another fungus from Asia; dogwoods had dogwood anthracnose, yet another fungus from Asia; oaks attracted the gypsy moth, an insect from Europe; hemlocks were victim to hemlock woolly adelgid, an insect from Asia; and ash trees attracted the emerald ash borer, another insect from Asia.

Of the eight tree taxa they studied, seven have species that are now threatened or endangered because of the invasive pests or pathogens, according to the study. The researchers found six more combinations, but those trees declined possibly due to other factors.

Invasive species impact the trees through different means. For example, the emerald ash borer, an insect, gets under the bark and disrupts the xylem, a tissue that brings water and dissolved minerals from the roots to the leaves. Gypsy moths cause leaves to fall off trees.

Because of global trade and travel, invasive species will likely remain a problem. Climate change can make the problem worse, because unhealthy forests are less resistant to exotic species, and those pests and pathogens can spread faster under warmer conditions, according to Anderson-Teixeira.

Invasive species are among several ways that forests are under attack. Forest fires, such as those that burned in the Amazon rainforest in 2019 due to human deforestation, climate change and drought are additional ways. Even native species, such as white-tailed deer, which are in high density in certain parts of Shenandoah National Park, can disrupt the balance of the ecosystem if not regulated. As Anderson-Teixeira puts it, “There’s a lot of pressures on forests these days.”

These threats are evolving. Changes to land use are becoming increasingly worrisome to the forest monitors at ForestGEO. “We’re still losing something between 80 and 100,000 square kilometers of tropical rainforest every year,” says Davies, the ForestGEO director. This deforestation and fragmentation of forests allows other threats to increase, according to Davies. For example, hunting often happens when forests are more accessible to humans; fragmented forests are likely more susceptible to climate fluctuations; and when animals do not stay isolated in their forest habitats, they can transmit pathogens to humans, including coronaviruses.

But the researchers found that overall, the forest they studied remained healthy, even as trees impacted by invasive species died. That’s because forests are resilient.

The average above-ground biomass, a measure of trees above the soil, actually increased, as trees that were impacted less by the invasive pests and pathogens grew. Tree biodiversity also did not change much. “In some patches,” Anderson-Teixeira says, “you’d look around and see more species than you would have in the past. It works because it’s a fairly diverse forest, so you’re losing some species, and less affected species are filling in.”

The study suggests that people and governments prevent invasive species through policy regulations, biosecurity and conservation to make forests more resilient.

“People need to appreciate forests, recognize that our water, our clean air, so many services that we depend on, are the result of fully functioning, healthy forests,” Davies says. “Many people who are stuck at home at the moment are probably starting to realize, ‘Heck, I didn’t realize how much I really appreciate being outside and walking in the park.”

Get the latest on what's happening At the Smithsonian in your inbox.