On the corner of Constitution Avenue and Ninth Street, nestled in the pollinator garden of the Smithsonian National Museum of Natural History, there’s a tree. It’s surrounded by a soft carpet of grass and ringed with “summer snowflakes,” whose flowers point downward like clusters of little bells. When I visited the tree on a morning in April, it seemed quiet and restful, its soaring branches bare for the moment.
It’s surprising that this particular tree could feel so unassuming — after all, it’s been around far longer than anyone alive today. Estimated to be more than a century and a half old, this American elm preceded the museum it stands beside by some 60 years. It’s seen 33 different presidents, multiple global pandemics and the Great Depression. It’s no wonder that the tree is known as the “Smithsonian Witness Elm.”
I stood with Sylvia Schmeichel, lead horticulturist with the Smithsonian, as a group of children filed along the path beneath the tree’s boughs. “It’s one of the oldest and largest trees that we have in our collection,” she said. “In a highly-visited urban setting, it’s unusual to have trees this old and this size.”
Caring for the gentle giant requires planning and precision. And the experts who look after it say there are plenty of reasons to do so. While the Witness Elm possesses immense significance in itself, it also reaches across time and space — as both an installment in the long saga of elm trees in the United States and a member of Washington, D.C.’s beloved urban treescape.
That this tree still stands at all is the result of both good management and sheer luck. That’s because American elms have had a rough go of it over the last century, after the emergence of a deadly disease caused their numbers to plummet.
Before the disease took hold in the U.S. in the 1930s, the American elm (Ulmus americana) was a staple among the country’s parks and streetscapes. The U.S. Forest Service has estimated that, at that time, there were 77 million of these trees in incorporated areas alone. Thanks to its elegant branches and shady green canopy, the American elm had stolen the hearts of city-dwellers across the eastern U.S.
Washington, D.C., was no exception. By the early twentieth century, the city was dense with American elms, including one planted at the White House by John Quincy Adams in 1826 and a pair planted at the Lincoln Memorial in honor of the World War I Armistice. The tree would become even more of a fixture of the D.C. landscape in the years that followed, as the National Mall, flanked with American elms, took shape.
“Elms were chosen because of their beautiful form and their urban tolerance,” said Jake Hendee, an arborist with Smithsonian Gardens. “That tree was relied upon to create this really compelling ‘Grand Avenue’ between the Washington Monument and the Capitol Building.”
But depending entirely on one tree species has its downsides. “The shortcoming of that approach became clear when Dutch elm disease entered the continent and worked its way relatively quickly to Washington, D.C.,” he said.
Out on a limb
First identified in the Netherlands in 1919 and detected in the U.S. in 1930, Dutch elm disease is caused by a fungus. It typically spreads from tree to tree with the help of the European elm bark beetle, which carries the pathogen as it flits among decaying branches. When the fungus arrived in D.C., the elm-lined National Mall supplied a leafy paradise.
“Once the pathogen is inside the tree, it clogs up the vascular tissue,” Hendee said. “Starting at the point where the infection began, you see die-back.” Leaves turn yellow, wilt and fall, and branches begin to die as the infection spreads. Unable to transport crucial water and nutrients from roots to leaves, an untreated tree eventually dies. By the 1970s, the U.S. Forest Service estimates the pathogen had killed tens of millions of American elms.
“It’s a very difficult disease to treat once it’s inside the tree, so most of our efforts are aimed at preventing it from getting inside,” Hendee said. That means keeping a close eye on the elms and removing dying or diseased wood as quickly as possible to prevent the fungus from hopping to healthy trees. And all tree pruning is performed between October and March, when the bark beetles are dormant.
That intensive management, Hendee said, has made Dutch elm disease less devastating in recent decades. But there’s another piece of the puzzle, too.
“In the population of American elms grown from seeds, there’s a great deal of genetic diversity,” Hendee said. “A few of those trees are resistant to Dutch elm disease, and so a lot of research has focused on identifying those individuals and propagating them throughout our landscape” using techniques like grafting, which produces genetically identical trees.
One of the individuals that showed a remarkable ability to fend off the disease is the Witness Elm’s neighbor, an American elm near the Smithsonian Institution Building that’s managed, along with the other elms on the Mall, by the National Park Service. Known as the “Jefferson” cultivar of Ulmus americana, clones of this hardy tree have been planted around the country to bolster Dutch elm disease resistance.
Unlike its relative down the road, however, the Smithsonian Witness Elm is not thought to have defenses against the disease. “Most really large, very old American elms that remain on our landscapes are not Dutch elm disease-resistant,” Hendee said. “They have just survived the game of chance, and we’d like to think we helped them along with good management.”
The Witness Elm requires its fair share of attention. Some of that care has less to do with disease and more to do with the tree’s imposing size — 85 feet tall and more than five feet in diameter.
“A lot of our trees have large metal cables that help support them, and sometimes we have to do structural pruning to help offload some of the weight,” Schmeichel said. These trees are also regularly visited by specialists who perform sonic tomography scans, which allow them to look inside each tree similarly to the way a doctor sees inside the body using a CT scan.
“They can determine if there are any weak spots we need to be aware of, without actually drilling into the tree and causing extra harm,” Schmeichel said.
On top of that, experts do everything they can to fortify the Witness Elm against disease. For instance, Hendee said, the tree is injected every three years with fungicide, which should help stop any Dutch elm disease infection in its tracks.
Hendee also said it’s important not to — quite literally — miss the forest for the trees. After all, the Witness Elm is part of the much broader city treescape. Disease-resistant cultivars are important, he said, but an urban forest with a wide variety of species and cultivars has the best chance against new diseases.
“They’re not just nice to have around,” Hendee said. “Our trees are really directly tied to our human wellbeing.” Among other things, they provide cleaner air, reduce surface temperatures and have been linked to improved mental health.
I plan to visit the Witness Elm again soon, when I can see the jubilant canopy in all its glory. But even leafless, the tree occupied a scene full of life. A robin bathed in a puddle near its base. Insects darted between its trunk and the bell-like flowers. Two passersby leaned their heads back to look at its branches. Life goes on, and the tree watches.
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