The Fight to Save Central Park’s Iconic American Elm Trees

When Central Park visitors stroll through its southern reaches, they are greeted by a rare sight: rows of mature elm trees towering over a wide promenade like a vast wooden cathedral. This landscape is known as the Mall & Literary Walk. It is among the Park’s most iconic and recognizable features, home to one of the largest and last remaining collections of American elm trees in the world.

The American elm is an icon. For decades, Ulmus americana was one of the most widely planted shade trees in the United States. Its twisting, vase-like shape and bountiful canopies were a fixture on sidewalks around the country, and their popularity is the reason behind the eponymous Elm Street found in most American cities. Its quick growth and resilience in the face of intense storms also made it an ideal candidate for beautifying cities throughout much of the eastern United States.

But this historic and ubiquitous tree is no longer as prevalent. A century of Dutch elm disease has cratered the continent’s American elm population, turning this beloved native tree into a comparatively rare specimen. Central Park’s current collection of roughly 1,600 American elms is far below its peak. Since the genesis of the Central Park Conservancy, the nonprofit’s team of experts has been caring for these elms, along with the Park’s other 170 species of trees. Without this special maintenance, the Mall would have lost hundreds more of these trees. Protecting the Park’s elms is a full-time job, and our arborists work year-round to prepare Central Park for a healthy, elm-filled future.

Protecting a Vulnerable Species

The fungal pathogen, Ophiostoma ulmi, that causes Dutch elm disease likely snuck into North America via Europe around the 1930s. It was a bad time to be an elm tree. The disease spread up and down the East Coast and decimated the population. Seventy-seven million elm trees in 1930 dwindled to 34 million trees by 1976, and the vast majority lost were American elms.

A mature American elm can grow roughly three feet per year, up to 80 feet. This quick growth makes these majestic trees particularly susceptible to Dutch elm disease. As soon as the ground thaws, American elms exit their winter dormancy, suck up water from the ground, and stretch further skyward. Their vascular system resumes its normal circulation, and their branches become more pliable. Unfortunately, this eagerness leaves the trees more susceptible to infection. These freshly awoken, healthy branches are the perfect snack for Dutch elm disease–carrying beetles.

Dutch elm disease is caused by the fungi-carrying elm bark beetles, or “scolytid,” which like to burrow into branches at the top of elm trees. Eagle-eyed arborists must stay vigilant throughout the year, especially once the trees leave dormancy in the spring. From Memorial Day through August, these diligent tree experts look for the “flagging” that marks an infection. A single wilted, yellowing branch amid an otherwise healthy canopy is a key indicator that the fungal pathogen hitched a ride on a beetle and infected the elm. Arborists will peel back the branch’s bark to check for the disease’s telltale blackish-blue streaks. If they spot these markings, it’s a race against time to prevent a fatal infection and ensure that this pathogen does not spread to the main trunk, explains K Satterthwaite, the Central Park Conservancy’s lead arborist.

“Depending on the size of the tree, it could be within a week that we can no longer save it,” says K, who helps coordinate the Conservancy’s efforts to combat the fungal pathogen. “If we see something that looks suspicious, we will either climb or use our aerial lift truck to examine it immediately.”

Unfortunately, the canopy is not the only way the pathogen spreads. Part of what makes this disease so virulent in a verdant area like Central Park is that it can spread through the trees’ subterranean root grafts. While a dense collection of elms like the Mall can produce spectacular views, their proximity to one another makes it easier for the disease to travel via the root system. The root system of an elm stretches far beyond the trunk and intertwines with the roots of other trees.

Ensuring the Legacy of the Elms

To prevent the spread of Dutch elm disease through the root system, the Conservancy’s arborists inject a fungicide into the root flares of the elm trees. When Tree Care Manager Peter Haupt first started working at the Conservancy in 2010, none of the trees were injected with the fungicide. Peter, who has a degree in urban forestry and arboriculture, oversees the Conservancy’s team of seven arborists who have received specialized training to care for the Park's 18,000 trees in a unique urban setting. Now, there is a rotation of roughly 600 trees that receive the fungicide among the Park’s roughly 2,500 elm trees. These preventative measures began in 2016 after the Mall experienced a significant disease-related tree loss. The fungicide is injected every two to three years, and it is remarkably effective in preventing the spread of Dutch elm disease, says Peter.

“Annually, we probably lose anywhere between 15 to 30 elm trees from Dutch elm disease. That said, as our program has expanded, we've been able to refine the care that we provide to these trees in a very efficient and effective way. It's allowed us to put greater focus on larger areas of the Park,” he says.

The fungicide helps our arborists fight the disease, but they still must stay vigilant and monitor the Park’s elms, says K. Fungicide is not the only tool that arborists use in the fight against Dutch elm disease. Scientists can combine pollen from different varieties of the same plant to create so-called “hybrid trees.” These trees, which take on characteristics of both parents, can sometimes offer more resistance against certain diseases. Some of the first cultivated Dutch elm disease–resistant varieties of hybrid American elms were developed as early as the 1920s, including the Princeton elm.

“If we inject a tree, we can pretty much prevent it from getting an infection in its canopy. However, if there's another tree next to it that is not injected, that tree could get an infection, and it could spread via root graft underneath,” she says. “So, that's one reason the hybrid elms, like all elms, are still susceptible. [There’s a] lower incidence of canopy infection, but [hybrids are] still just as susceptible through root graft.”

While most elm hybrids lack the classic vase-like silhouette that made the American elm famous, they are gorgeous trees and much more resilient to Dutch elm disease. Several of the more disease-resistant elms stand guard on the periphery of the Park’s East Meadow. To help ensure elms’ iconic legacy in Central Park, the Conservancy also planted several baby Princeton elms in a single grove on the Mall last summer.

Dutch elm disease is not the only threat to Central Park’s iconic American elm trees. Trees in heavily trafficked areas rarely survive long. The current collection of American elms on the Mall is actually the third planting of elms in this location, says Peter. The first two collections likely died due to soil compaction caused by foot traffic near their roots. When the current collection of elms was planted in the 1920s, arborists set up rough fencing to ensure that fewer people walk directly under elms and damage their root systems.

“Just keeping people out of the immediate critical root area of these trees—and allowing them to grow undisturbed—works,” says Peter. “It helps them survive.”

These trees may be fragile and susceptible to this fungal pathogen, but with each trip to Central Park, visitors can play a vital role in protecting them.

Timmy Broderick is a freelance science journalist who writes about climate, disability, and LGBTQ+ healthcare. You can find their writings in Scientific American, Slate, and more.