Up to 1 in 6 tree species in the U.S. are at risk of extinction, study finds


The most endangered tree in the contiguous United States is quite possibly an old battered oak tree hidden deep in a Texas mountain range. Its trunk is marked by a forest fire. His limbs are weakened by a fungal infection. Its habitat is threatened by climate change. Scientists only realized the species still existed after coming across the diseased specimen during an expedition this spring. And without prompt action, researchers warn, Quercus tardifolia could really disappear.

The species is one of about 100 U.S. trees threatened with extinction, according to a sweeping new assessment published Tuesday in the journal Plants People Planet.

Amid an onslaught of invasive insects, an upsurge in deadly diseases and the global peril of climate change, as many as 1 in 6 trees native to the lower 48 states are at risk of being wiped out, scientists say. The list of endangered species includes soaring coast redwoods, towering American chestnut trees, elegant black ash trees and gnarled whitebark pines.

Yet only eight tree species are federally recognized as endangered or threatened. And 17 species at risk are not kept in any botanical garden or scientific collection, including Quercus tardifolia. If they die in the wild, these trees will be gone for good.

“It’s easy to feel this sadness and this unhappiness because … the scale of the crisis is really, really big right now,” said Murphy Westwood, vice president for science and conservation at Morton Arboretum in Illinois. Illinois and lead author of the study. “We are losing species even before they are described.”

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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 itself, Westwood said, because conservation research rarely focuses on plants.

She pointed to disparities in the International Union for Conservation of Nature’s “Red List”, the world’s main inventory of the conservation status of species. The list includes twice as many mammal species as members of the order Lamiales, which includes ash, teak and jacaranda – even though the latter group is nearly five times larger than the former.

‘Plant blindness’ – the human tendency to ignore the plants around us – means that fewer resources are devoted to the organisms that supply Earth’s oxygen, feed its animals and store more carbon than humanity does. will issue some in 10 years. Until several years ago, scientists didn’t even know how many tree species there were (the exact number is 58,497).

“It’s that big part of life that’s totally unstudied or understudied,” Westwood said.

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Now, a coalition of scientists led by Botanic Gardens Conservation International is trying to determine how many of these species are at risk of extinction. Westwood helped lead the American effort.

In the United States, she found, more than two-thirds of species had never been assessed for their risk of extinction. Others had not been examined for decades, even as new diseases and rising global temperatures put their populations at risk.

After five years of scouring scientific journals, combing academic databases, and interviewing experts, the researchers found that entire swaths of America’s forests had silently slipped into oblivion.

In the Rosaceae family – a diverse group that includes hawthorns and apple trees – more than a quarter of the species are considered threatened, endangered or critically endangered. Half of all ash tree species are threatened by the invasive emerald ash borer, a green jewel insect whose larvae feed on living tissue just under a tree’s bark. An emerging disease known as “laurel wilt” is attacking all three native members of the genus persea, putting small, fragrant evergreen trees at risk.

Invasive insects or pathogens are the main drivers of extinction risk, scientists have found. Although trees have highly evolved immune systems – a necessity for any creature that survives for centuries – they are easily overwhelmed by diseases they have never encountered before.

And climate change appears to be compounding those threats, said Stephanie Adams, who oversees plant health care at Morton’s Arboretum. Trees stressed by extreme weather conditions become easy prey for marauding insects and fungi. Prolonged droughts deprive trees of the water they need to produce resin, the sticky substance they use to seal wounds and trap potential invaders.

“There are trees that have been living in places for hundreds and hundreds of years and suddenly they are dying now,” Adams said.

Not all threats are introduced from abroad. In some cases, changing environmental conditions can turn previously benign organisms into killers.

Adams pointed to an outbreak of blight among bur oaks across the Midwest. Although the trees have long coexisted with the disease-causing fungus, they have only begun to die off in recent years. The researchers believe that escalating severe storms and heavy flooding – hallmarks of rising global temperatures – promote the growth of the fungus at the expense of its host trees.

Bur oaks have not yet fallen into the IUCN “vulnerable” category, Adams said. But it’s not hard to imagine that rapid changes in temperature and weather could suddenly lead to a precipitous decline of a once healthy species.

“My God.” Adams took a deep breath. “That’s a horrible thought.”

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The decline of American trees is just one part of a larger crisis ravaging the planet. A 2019 report by the UN Panel of Experts on Biodiversity and Ecosystem Services estimated that one million species are at risk of extinction. The global rate of extinction is at least tens of hundreds of times higher than normal and continues to accelerate, threatening to eclipse some of the greatest mass fatalities in Earth’s history.

Threats to trees are of particular concern, Westwood said, because of the distinct role they play in nature. Trees are the largest and most durable organisms on the planet. They form the backbone of ecosystems, provide habitat for other creatures, and even create their own climate.

And trees have a vital role in humanity’s efforts to avert catastrophic climate change. The US plan to halve emissions by the end of the decade depends on forests to offset about 12% of the pollution that warms the planet. Epidemics, forest fires, droughts, logging and pollution can jeopardize this plan.

“We have a narrow and rapidly closing window for action,” Westwood said – but there is still a lot the world can do.

Governments can reduce greenhouse gas pollution – mostly from the burning of 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 botanical gardens and study these garden specimens to develop strategies to protect 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 the local woods. Avoid carrying firewood or other materials that could carry dangerous pests.

Human life depends on the shade trees cast on the scorching city streets, how their roots and leaves filter water and air. A healthy forest can slow a wildfire, dampen a storm surge caused by a hurricane, and offer comfort to a boiling heart.

“It’s not altruistic,” Westwood said. “We don’t do this because we are nature lovers who love trees.”

People need trees as much as trees now depend on us, Westwood continued. “All of these actions are essential to our own survival as a species and our future on this planet.”

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