‘They’re critically important’: Virginia’s ash trees are under siege
The emerald ash borer. (David Cappaert, University of Georgia Center for Invasive Species and Ecosystem Health)
The emerald ash borer looks like nothing so much as an Elvis impersonator in insect form.
When hit with light, its green and gold body sparkles as if the bug is wearing a sequined jumpsuit; its eyes, glistening protuberances that consume most of its head, command attention. Both the insect and the impersonator are objects of fascination, things that are strange and extravagant, out of place in the sober light of day.
But where the latter is a curiosity, the former is a threat. No mere interloper, the emerald ash borer, a native of Asia, has for more than a decade been penetrating the U.S., often by hitchhiking on bundles of firewood carried by campers. Wherever it has spread, it has invaded ash trees, tunneling beneath their bark, disrupting the internal systems that carry nutrients and water and eventually killing them. Once infested, few trees survive: Mortality is nearly 100 percent, and nationwide losses are already estimated to number in the tens of millions.
“There are places that it is very obvious that you have a lot of dead standing trees,” said Elizabeth Matthews, a botanist who oversees efforts to inventory and monitor forests in the National Park Service’s National Capital Region, an area that includes Virginia, Maryland, Washington, D.C. and West Virginia.
In Virginia, where the emerald ash borer has been found in so many counties that the U.S. Department of Agriculture placed the entire state under a quarantine in August 2012, the death tolls are already grim. Northern Virginia has been perhaps the hardest hit: According to the most recent NPS data, national parks in Virginia’s section of the National Capital Region lost 27 percent of their ash trees — about 30,000 in total — between 2013 and 2017. Over the same period the George Washington Memorial Parkway alone saw a 40 percent drop in its ash population.
Impacts stretch far beyond national parks, however. Throughout the state, ash trees are ubiquitous not only in forests, where they thrive in both uplands and environmentally significant wetlands, but also in cities, where they were planted by the hundreds decades ago because of their durability, fast growth and beauty. In many of the commonwealth’s urban areas, ash represent anywhere from 1 to 5 percent of the tree canopy, according to Virginia Tech’s Street Tree Assessment Project; in Abingdon, they make up 10 percent.
Those percentages aren’t as high as those found in other states, where ash trees can constitute up to 50 percent of a city’s tree canopy. But they aren’t insignificant either: in Richmond, for example, where ash are about 3 percent of the city’s trees and 4 percent of its canopy, the loss of these species could mean the disappearance of about 1,400 trees — enough holes in the arboreal landscape to be noticed by almost anyone.
At the same time, with climate change driving temperatures steadily upward, trees are becoming more important in urban areas than ever. Cities tend to be hotter than the surrounding countryside due to a phenomenon known as the heat island effect, which occurs when densely developed and populated areas see rises in temperatures due to large expanses of pavement, fewer shaded areas and a lack of natural features that can absorb heat. Increasing the tree canopy is one of the primary ways that neighborhoods can bring down the heat.
“There’s many clear human effects of losing tree canopy,” said Leigh Greenwood, a program director with the Nature Conservancy and manager of the Don’t Move Firewood campaign, which aims to stem the emerald ash borer tide sweeping across the U.S. “These things have huge impacts on neighborhoods if all the trees have to be removed.”
Those impacts are more than increased discomfort and worse aesthetics. Tree losses have been correlated with negative health effects — one study by the U.S. Forest Service’s Pacific Northwest Research Station that specifically focused on the emerald ash borer found an association between ash losses and increases in deaths from cardiovascular and lower respiratory diseases. And large-scale die-offs like those caused by the borers pose unique public works challenges related to removing dead trees that pose risks to life and property in crowded cities.
“When a single tree dies in a forest, it’s often not that catastrophic,” said Greenwood. “When a single tree dies in a city, it can fall on a car or a house or a powerline.”
Less obvious but perhaps even more consequential is the ripple effect that the disappearance of ash trees from forests may have across the commonwealth. Ash trees, which occur in four distinct varieties in Virginia — white, black, green and pumpkin, the latter named for the shape the base of its trunk often takes — are what Meredith Bean, the emerald ash borer coordinator for the Virginia Department of Forestry, calls “foundation species.”
Because these trees often prefer to grow along stream banks or in wetlands, they play a key role in soil stabilization, a concern particularly important to policymakers in the Chesapeake Bay watershed seeking to cut down on the amount of sediment flowing into rivers and creeks in order to meet water quality goals by 2025.
“When you lose those really important trees, the whole geomorphology of the area is being changed as well,” said Bean.
Greenwood agreed: “They’re critically important for holding the whole system together,” she said. “Losing ash trees will have echoing effects throughout the entire ecosystem.”
With the downing of ash trees en masse will also come changes in flora and fauna. In the NPS’ National Capital Region, at least 27 types of insect “are restricted to ash species,” a brief from the agency reported. As they die or find new habitats, the bird species that rely on them will also shift. At the same time, decreased tree canopy will let more light reach wetland waters, potentially driving up their temperatures and affecting fish and other aquatic species, as well as the forest floor, altering the mix of plants that can survive and thrive there.
“You go from an open wetland with big trees to something really dense and shrubby,” said Matthews.
Many of those shifts can’t be easily classified as good or bad, she pointed out — just different from the former status quo. But one development that researchers are already bracing themselves for is an uptick in non-native plants that are likely to fill the vacuum left by the vanishing ash and tend to outcompete natives accustomed to slower, successional growth.
“As soon as ash is gone from an ecosystem, it’s common for people to see more invasives take over and kind of wipe out the native plants,” said Bean.
And just as the disappearance of ash from city streets will occur as climate change is making urban heat island issues more acute, so too will its exit from forests unfold as greater climate variability exerts additional stress on such ecosystems. In the wooded wetlands of Northern Virginia, which are often home to only a handful of tree species, Matthews worries that the loss of ash will further undercut ecological diversity, a characteristic that can help an ecosystem survive climate shocks.
“When you lose two of only three to five tree species, it’s really obvious you’ve lost that component,” she said.
The losses now being logged aren’t permanent, although Matthews said that in the DC area, “We’re definitely at the point now where there are more dead than there are living trees.” Ash trees will regrow, and those that emerge may have a better chance of surviving the onslaught of emerald ash borers, although on that point there’s no certainty. Seed banks are also more sophisticated today than they used to be, as are biological controls that may be able to reduce the borer population to a more manageable size that would allow some trees to develop defenses against them.
Still, said Greenwood, “These are big-horizon projects. … Sometimes they don’t gain a big foothold effect for 50, 75 years.”
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