Up to one-sixth of the tree species found in the continental United States face possible extinction, yet only a handful enjoy federal protection under the U.S. Endangered Species Act, a new study finds.
The study, which focused on 881 tree species native to the continental United States, drew on field data indicating where trees occur and scientific literature detailing threats they face. (Hawaii has a vastly different flora that’s being assessed separately.) Researchers evaluated how endangered each tree is according to criteria developed by the organizations NatureServe and the International Union for Conservation of Nature (IUCN). As a result of invasive insects, pathogens, climate change, development, and other threats, the team found, 11% to 16% of those trees—as many as 135 species—face possible extinction.
“That’s a lot of species,” says Murphy Westwood, vice president of science and conservation at the Morton Arboretum and lead author of the study, which was published this week in the journal Plants People Planet.
The number is consistent with extinction estimates for other groups of organisms, says Noah Greenwald, endangered species director for the Tucson, Arizona–based Center for Biological Diversity, who was not involved in the research. Earlier this year, for example, researchers reported that one in five of the world’s reptiles is threatened.
Still, Greenwald says, the level of threat is “quite concerning.” Trees play especially foundational roles in ecosystems: When trees die out, whole swaths of biodiversity can perish along with them, as can ecosystem services that humans depend on. “Trees and forests are really the bench that we all rest on,” he says.
Invasive insects and pathogens are the top killers of U.S. trees, the authors found. Nearly half of ash species, for example, are threatened by emerald ash borer, a beetle that arrived from Asia some 2 decades ago and has spread across half the continent. Chestnut, hemlock, pine, and laurel species also face deadly pests.
Human-caused climate change registered as the second most pervasive threat. A “poster child” for the risks posed by global warming is a medium-size, thick-leafed oak, Wesley Knapp, chief botanist at NatureServe, says. Quercus tardifolia. Just one known individual is left in the wild, located in Texas’s Big Bend National Park, and climate change is rapidly making its habitat unsuitable. “The tree is not long for this world,” Knapp says. (Overall, the oak and hawthorn genera contain the largest numbers of threatened species.)
Although Q. tardifolia may be doomed, other threatened species can be saved, the authors emphasize. Organizations restoring forests, for example, could include threatened trees in their plantings, Westwood says.
Preventing new tree killers from reaching the United States is also critical, says Leigh Greenwood, a forest specialist at the Nature Conservancy, which was not involved in the research. “This paper is very much a call to action to bolster the prevention strategies that we have against the entry of new forest pests and pathogens.”
Strengthening efforts to collect seeds and tissues from threatened trees and place them in long-term storage or grow them in protected places could help provide a crucial insurance policy, researchers say. Seventeen species flagged in the study don’t appear in any seed bank or collection, the authors found. “If those threatened species disappear from the wild,” Westwood says, “we have no backup.”
The U.S. Fish and Wildlife Service (FWS) currently lists just eight U.S. tree species as threatened or endangered. But not all species flagged by the new study would necessarily qualify for federal protection, Westwood says, because the government uses criteria that differ from those used by IUCN and NatureServe. And even if a tree species does qualify, it could take years for officials to add it to the list. In a 2016 study, Greenwald found the service took an average of 12.1 years to list a species.
A spokesperson for FWS declined interview requests.
Despite the grim news, Westwood says the United States has the wealth and expertise to save at least some of its threatened trees. “We have the technology and resources to shift the needle,” she says. “We can make a difference. We have to try.”