The most endangered tree in the contiguous United States is most likely a battered old oak hidden deep in a Texas mountain range. Its trunk is scarred by a wildfire. Its limbs are weak from a fungal infection. Its habitat is imperiled by climate change. Scientists only realized the species still existed after stumbling upon the ailing specimen during an expedition this spring. And without swift action, researchers warn, Quercus tardifolia could truly disappear.
The species is among some 100 U.S. trees staring down the barrel of extinction, according to a sweeping new assessment published Tuesday in the journal Plants People Planet.
Amid an onslaught of invasive insects, a surge in deadly diseases and the all-encompassing peril of climate change, as many as 1 in 6 trees native to the Lower 48 states are in danger of being wiped out, the scientists say. The threatened list includes soaring coast redwoods, capacious American chestnuts, elegant black ash and gnarled whitebark pine.
Yet only eight tree species are federally recognized as endangered or threatened. And 17 at-risk species aren’t conserved in any botanic gardens or scientific collections — including Quercus tardifolia. If they die off in the wild, these trees will be gone for good.
“It’s easy to feel that gloom and doom because … the scope of the crisis is really, really great right now,” said Murphy Westwood, vice president for science and conservation at the Morton Arboretum in Illinois and a lead author of the study. “We’re losing species before they even get described.”
The new study is the first to list and assess the health of all 881 tree species native to the contiguous United States — an achievement in and of itself, Westwood said, because conservation research rarely focuses on plants.
She pointed to disparities in the International Union for the Conservation of Nature’s “Red List,” the preeminent global inventory for species’ conservation status. The list includes twice as many mammal species as members of the order Lamiales, which includes ash, teak and jacaranda trees — even though the latter group is nearly five times the size of the former.
“Plant blindness” — the human tendency to overlook the plants that surround us — means that fewer resources are devoted to the organisms that supply Earth’s oxygen, feed its animals and store more carbon than humanity will emit in 10 years. Until several years ago, scientists didn’t even know how many tree species existed (the correct number is 58,497).
“It’s this big swath of life that’s totally unstudied or understudied,” Westwood said.
Now a coalition of scientists lead by Botanic Gardens Conservation International is attempting to determine how many of those species are at risk of dying out. Westwood helped lead the U.S. effort.
In the United States, she found, more than two-thirds of species had never been assessed for their extinction risk. Others hadn’t been examined in decades, even as new illnesses and rising global temperatures imperiled their populations.
After five years poring over scientific journals, combing through academic databases and interviewing experts, the researchers uncovered that swaths of America’s forests have silently slipped toward oblivion.
In the rosaceae family — a diverse group that includes hawthorns and apple trees — more than a quarter of species are considered threatened, endangered or critically endangered. Half of all ash species are jeopardized by the invasive emerald ash borer, a jewel-green insect whose larvae feed on the living tissue just beneath a tree’s bark. An emerging disease known as “laurel wilt” is attacking all three native members of the genus persea, imperiling the small, fragrant evergreen trees.
Invasive insects or pathogens are the predominant drivers of extinction risk, the scientists found. Though trees have highly evolved immune systems — a necessity for any creature that survives for centuries — they are easily overwhelmed by disease they’ve never encountered before.
And climate change seems to be making these threats worse, said Stephanie Adams, who oversees plant health care at the Morton Arboretum. Trees stressed by extreme weather become easy pickings for marauding insects and fungi. Prolonged droughts deprive trees of the water they need to produce resin, the sticky substance they use to seal up wounds and trap potential invaders.
“There are trees that have been living in locations for hundreds and hundreds of years and suddenly they’re dying now,” Adams said.
Not all threats are introduced from abroad. In some cases, changing environmental conditions may turn previously benign organisms into killers.
Adams pointed to an outbreak of blight among bur oaks across the Midwest. Though the trees have long coexisted with the fungus that causes the disease, they only started dying in recent years. Researchers think that escalating severe storms and heavy floods — trademarks of rising global temperatures — are promoting the growth of the fungus at the expense of its tree hosts.
Bur oaks have not yet fallen into the IUCN’s “vulnerable” category, Adams said. But it’s not difficult to imagine that the rapid changes in temperature and weather patterns could suddenly send a once-healthy species into precipitous decline.
“Gosh.” Adams took a sharp breath. “That’s a horrible thought.”
The decline of American trees is just one piece of a broader crisis ravaging the planet. A 2019 report from the United Nations Panel on Biodiversity and Ecosystem Services estimated that 1 million species are in danger of dying out. The global rate of extinction is at least tens of hundreds of times higher than normal and still accelerating, threatening to eclipse some of the largest mass die-offs in Earth’s history.
The threats to trees are especially worrying, Westwood said, because of the distinct role they play in nature. Trees are the largest and longest-lived organisms on the planet. They constitute the framework of ecosystems, provide habitat for other creatures and even create their own weather.
And trees have an essential role in humanity’s efforts to avert catastrophic climate change. The United States’ plan to halve emissions by the end of the decade depends on forests to offset about 12 percent of its planet-warming pollution. Disease outbreaks, wildfires, droughts, logging and pollution may jeopardize that plan.
“We have a narrow and rapidly closing window to take action,” Westwood said — but there is still plenty the world can do.
Governments can curb the greenhouse gas pollution — mostly from burning fossil fuels — that threatens to warm the planet by 2.5 degrees Celsius (4.5 degrees Fahrenheit) by the end of the century. Communities can implement stronger policies to protect existing forests and ensure that reforestation projects plant a diverse mix of species that will be more resilient to emerging threats. Researchers can collect endangered species to ensure they are preserved in botanic gardens, and study those garden specimens to develop strategies for protecting their cousins in the wild.
“And then there are things we can all do as individuals,” Westwood said: Plant native species in our gardens. Volunteer in local woodlands. Avoid transporting firewood or other material that might carry dangerous pests.
Human lives depend on the shade that trees cast on scorching city streets, the way their roots and leaves filter the water and air. A healthy forest can slow a wildfire, buffer storm surge from a hurricane and offer solace to a heart in turmoil.
“It’s not altruistic,” Westwood said. “We’re not doing this because we’re tree-hugging nature lovers.”
People need trees as much as trees now depend on us, Westwood continued. “All of these actions are critical to our own survival as a species, and our future on this planet.”
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