In 1920, Washington planted 507 Norway maples along 16th Street to honor its war dead; the invasive species later spread into Rock Creek Park

In 1920, Washington, DC, created a unique living memorial to residents who had died during World War I: 507 Norway maple trees planted along 16th Street NW. This was dedicated on Memorial Day, May 30, 1920, along the stretch between Alaska Avenue and Allison Street. The memorial's legacy goes beyond the commemoration, as the Norway maple species (Acer platanoides) has the capacity to reproduce and establish itself beyond cultivated lands and was widely planted during that time as an urban shade tree. Over the following decades, it became one of the non-native trees found in Washington’s Rock Creek Park, with the National Park Service now classifying it as an aggressively invasive species.

A memorial consisting of living trees

The original idea of the memorial was conceived by the District’s American Legion as a way to commemorate the dead through a landscape instead of a conventional monument. A 1986 Washington Post article reported that one tree and one plaque were dedicated for each of the 507 District servicemen who had died in World War I. The memorial created a long tree-lined memorial corridor along 16th Street NW. The species chosen for the memorial was Norway maple, which had become popular in American cities because of its hardiness, rapid establishment, and ability to tolerate urban conditions. According to the University of Connecticut Extension, Norway maple was introduced into the United States in the mid-1700s and became widely planted as a street tree during the 19th century.


The memorial did not remain at exactly 507 trees: historical reports say another 23 trees and markers were added in early 1922 for people omitted from the original memorial, bringing the total to 530. For years, the trees and markers were decorated during Memorial Day, but over time the maintenance gradually declined. According to the Washington Post, by the beginning of World War II, most of the trees had died, and many plaques had been damaged or stolen.

When an urban tree moved into the forest

The ecological story of the species, however, continued long after the original 16th Street plantings died. Planted directly along the eastern boundary of Rock Creek Park, the 16th Street corridor shared an immediate physical border with the forest edge. Because Norway maples can begin producing seeds in just ten years, the mature memorial trees had a nine-year active seeding window to cast their winged seeds, known as samaras, directly into the park via autumn winds before the roadside population collapsed. This initial wave of wind-dispersed seeds established a self-sustaining beachhead within the park's interior. Crucially, while urban stress and neglect wiped out the parent roadside trees by 1939, the wild offspring thriving inside the protected park escaped this collapse. Having reached their own reproductive maturity, this wild generation continued to multiply independently from the original planting site. Additionally, the species’ ability to tolerate relatively low light allows these seedlings to establish beneath forest canopies. The NPS identifies Norway maple as a non-native, aggressively invasive species in Rock Creek Park and notes that it can outcompete native plants for resources.

Research helps explain why this particular species can become difficult for native forest communities. In a 1996 study published in the Bulletin of the Torrey Botanical Club, the authors compared vegetation beneath invasive Norway maple with the vegetation beneath native sugar maple and American beech. They observed significantly lower understory species richness beneath Norway maple, with many of the stems present being seedlings and saplings of the species. Therefore, the findings provide supporting evidence for concerns regarding the tree’s ability to alter forest understories.

A later 1999 study published in Biological Invasions examined a natural forest stand in New York and similarly found substantially lower sapling density and species richness beneath the Norway maple vegetation than beneath sugar maple. The research, therefore, indicated that the invasive species could establish within a natural forest instead of being restricted to disturbed edges. Another 2006 study published in the Journal of Ecology further tested the tree’s ability to establish itself in intact eastern forests, finding that the maple could germinate and survive under dense forest conditions. While these studies do not specifically measure the entire Rock Creek Park forest, they help explain why the NPS treats Norway maple as an ecological threat there.

It is worth noting that today, Rock Creek Park contains a mixture of native forest communities and non-native plants. With the park spanning roughly 1,754 acres of dense urban canopy, the structural impact of this species is massive. The NPS lists Norway maple as one of the most dominant invasive species found in the park, estimating that the species has successfully infiltrated and compromised nearly 30% of the park's interior native forest land by aggressively outcompeting native canopy saplings.