An exotic metallic green beetle is eating its way through North American forests, leaving dead ash trees in its wake. In the 20 years since this Asian pest, the emerald ash borer (Agrilus planipennis), was first discovered in Michigan, it has killed tens of millions of trees and spread to 35 U.S. states and parts of Canada. Now, a new analysis predicts ash populations might not recover, because the beetle also attacks young trees that sprout after older trees are gone, blocking long-term reproduction.
In some places, ash may disappear completely, says forest ecologist Isabelle Aubin of Natural Resources Canada, who was not involved with the work. “Losing key species like ash is weakening forest ecosystems.”
Known as the Venus of the woods, ash trees are valued in both forests and urban landscapes. Tough yet elastic ash wood is quite versatile, and has been used to make skis, baseball bats, guitars, and office furniture. In the forest, the trees provide a home and food for numerous birds, insects, and squirrels.
The staple-size emerald ash borer is believed to have reached North America in wood used to pack freight. As its name implies, its larvae bore into trunks, where they chew through the water—and nutrient-conducting tissues. They can kill a tree in as little as 2 years.
The distribution of ash logs as firewood helped spread the pest, as did the transport of nursery trees from infected areas. The invader is considered the most destructive forest pest in North America, according to the Emerald Ash Borer Information Network, at times killing more than 99% of ash trees in a forest.
To evaluate the extent of the borer’s destruction, Purdue University forest ecologist Songlin Fei and his colleagues turned to the U.S Forest Inventory and Analysis. The federal survey of public and private forest plots in 40,000 locations across the United States periodically records the number, health, and growth of each tree species in a plot.
The data go back to 1930, but Fei’s postdoc Samuel Ward focused on those data collected since the borer was discovered. He divided the data into three time periods: 2002–06, 2007–12 and 2013–18. Fei, Ward, and their colleagues then counted the number of ash trees, saplings, and seedlings and noted which ashes had died between surveys.
Ash makes a quick comeback, the team reports this month in Forest Ecology and Management. Where the borer had taken its toll in the early 2000s, ash seedlings and saplings were thriving, sometimes reaching densities of more than 1200 per hectare within the subsequent decade.
But the data also showed those youngsters were mostly gone by 2018. “Ash recruitment is not keeping up with mortality, and few seedlings appear to reach reproductive age,” says Juliann Aukema, an ecologist the U.S. Agency for International Development who was not involved with the research. With no mature trees to produce new seeds, eventually there will be no seeds left in the soil to replace the dead trees, she says. The ash, Aukema fears, could virtually disappear, just as the chestnut, once the dominant tree in eastern U.S. forests, did during the past century after the introduction of a deadly fungus.
Until now, such long-term analyses of ash populations had been done only in individual forests. They had shown similar outcomes for young ash trees. But, “This paper is providing evidence that what we were observing locally was in fact a global trend,” Aubin says.
Ian Boyd, a systems scientist at the University of St. Andrews, thinks the future of the ash may be less dire than this analysis indicates, however. For one, the ash “doesn’t just roll over and succumb,” Boyd says—also not involved with the work—and lots of seedlings provide a potential way for ash species to bounce back. He also doesn’t think enough time has passed to know the ash’s future. “It will likely take decades for the dynamics of the relationship between the borer and ash to settle down,” he says. This work is “the first chapter of a long story of how a new balance will eventually emerge between it and ash trees.”