Wissahickon Valley Park’s trees are the lungs of Northwest Philadelphia, providing life to hundreds of wildlife species. But as landscape architect Carol Franklin walks through the park now, she also sees a woodland being attacked on multiple fronts.

Growing up in the 1940s, Ms. Franklin, a Chestnut Hill resident, spent hours after school in the park, picnicking and building dams on the tributaries. She spent her afternoons at the Upper Wissahickon gorges, which were clothed in eastern hemlock — the official state tree of Pennsylvania.

“It was my playground,” said the co-author of a four-volume history of the Wissahickon with historian David Contosta.

Comprising 2,000 acres and more than 50 miles of trails, the park’s terrain has more than 20 tree species. They filter out polluted air, cool down the surrounding neighborhoods, lower the risk of flash flooding and absorb carbon dioxide released from burning fossil fuels. For some of the more than two million visitors a year, it’s a form of therapy.

But the hemlock woolly adelgid, an invasive insect, has devastated the eastern hemlock in the park and throughout Pennsylvania. The emerald ash borer, a beetle first found in the state in 2007, is killing off its ash trees. Beech leaf disease is spreading rapidly, threatening the American beech.

The invasive Japanese angelica and Norway Maple trees have been aggressively spreading and forming monocultures in the area. Porcelain berry, oriental bittersweet, and other invasive vines are strangling and outcompeting natives for spaces and resources. Invasive shrubs like Goutweed, privet, ivy, Japanese knotweed and garlic mustard are also blocking native vegetation growth. Bamboo is spreading from neighborhood yards.

Invasives and tree blights aren’t new phenomena in the Wissahickon. The American chestnut is a rare find in the park following the fungal blight in the early 1900s, with scientists now researching disease-resistant varieties.

Native species still make up most of the Wissahickon Valley Park canopy. The city’s Parks and Recreation Department, Friends of the Wissahickon (FOW), Wissahickon Restoration Volunteers, Wissahickon Trails and other partners are continuously researching and restoring native habitats across the park.

But the number of threats to the forest of Wissahickon Valley Park is making their work more critical. In addition to invasives and blight, deer feast on native vegetation, urban development continues and climate change allows winter survival of some invasive species. American forests like the Wissahickon are experiencing biodiversity loss, and it comes as the National Forest Service is facing layoffs and budget cuts.

Wissahickon Valley Park “is a wonderful place, but it’s also a place where a war is going on. And the trees are constantly being attacked... but they are holding on pretty well,” said Ned Barnard, a Chestnut Hill resident and co-author of “Philadelphia Trees: A Field Guide to the City and the Surrounding Delaware Valley.”

Evolution of the tree canopy

The state of the Wissahickon Valley tree canopy is rooted, literally, in its history. The Lenape, its earliest known residents, used fires to manage the woodland and create meadows to attract wildlife, including deer. This method increased the alkalinity of the soil, fostering the reproduction of mast trees like chestnut, oaks and hickories, according to Franklin’s book, “Metropolitan Paradise, The Struggle for Nature in the City: Philadelphia’s Wissahickon Valley, 1620-2020.”

In the 1600s and 1700s, as European settlers took land from the Lenape, logging and agriculture devastated the native tree population and features of the Wissahickon environment. Towards the end of the industrial era, the Fairmount Park Commission began obtaining park land and planting both native and invasive species in the late 1800s.

The growth of plant nurseries in the 19th and 20th centuries led explorers to import exotic vegetation from Asia and Europe, according to David Contosta.

Today, instead of the once dominant American chestnut and eastern hemlock, much of the canopy consists of native species like the tulip poplar and varieties of maple and oak, Barnard said. These include red, pin, black, chestnut and swamp white oak. The forest is also home to American beech, red maple, umbrella magnolia, American sycamore, common witch hazel, sumac and a few hickories.

Invasives like the Norway maple and Japanese angelica tree have become prominent in the Wissahickon, blocking the growth of surrounding native vegetation. The Norway Maple, brought over by John Bartram, is abundant along Forbidden Drive, and the Japanese angelica is one of the most observed organisms in the park, according to urban forester Andrew Conboy.

Some exotic plants have spread to the park from surrounding neighborhood backyards, Conboy said. Exotic does not mean invasive. This applies to the Tun Tree and Chinese Wingnut, two species from China, Conboy said.

All is not lost

Moving forward, there’s hope that the historic American chestnut could make a comeback one century after the fungal blight decimated the population. In 2023, Conboy discovered a pair of what seemed to be blight-resistant American chestnuts in Carpenters Woods, he wrote in a 2025 Grid Magazine article. If both of them flower, they have the potential to cross-pollinate.

The native American beech is one of the most common species in Wissahickon Valley Park’s mature forest, especially among the northern stretch. Now, it’s dying.

Dhan Parker, an environmental scientist with Philadelphia Parks & Recreation’s Natural Lands Group, is working to save them. He has been tracking the spread of beech leaf disease since it was first reported in Philadelphia in 2022. The disease is caused by foliar nematode, a kind of microscopic roundworm that feeds on the tree’s buds. And it inevitably will infect every single beech tree in all city parks, he said.

“They’re certainly in jeopardy. This is like an extinction-level event,” Parker said.

The disease has spread among the park’s American beech population since it originated around Forbidden Drive and Rex Avenue.

“There are large sections of the Wissahickon in particular, where beeches are 40 percent of the trees in the canopy. So it’s going to have a huge impact,” he said.

This year, he started injecting beech trees with a new treatment. The solution kills the nematodes from inside the tree and suppresses symptoms by 90 percent over two seasons, Parker said.

He’s treated 50 trees in Wissahickon Valley Park. Without treatment, invasive plants could descend on the dying trees, which support wildlife. Beech trees produce heavy amounts of beech nuts every year, feeding many bird species and mammals.

“Hopefully next spring, we’ll come back and we’ll see a nice, clean, beautiful canopy on these trees. But there’s no way to know until then,” he said. “According to other research, it is very effective.”

Restoring the gaps

Research involving Wissahickon Valley Park’s tree canopy has accelerated in the past 30 years, ever since a 1997 study found stark gaps in the canopy. FOW conducted the assessment using funds from the William Penn Foundation.

More than 120,000 trees will need to be planted to fill natural canopy gaps in the Wissahickon, which would cost around $3.6 million (more than $7 million today), the study reads. Canopy gaps are normal in forests and tend to heal on their own. But in the Wissahickon, invasive plants and “deer browse” – the woody vegetation the animals like to eat – made the gaps expand.

“These canopy gaps are like festering wounds that keep growing because the vines keep growing on the outside and cover up trees, [and] invasive exotic plants get in there, and it doesn’t allow the natural regeneration that was happening back in the, say, 1800s,” said Jason Lubar, the former associate director of urban forestry at Morris Arboretum and consultant who helped lead the study.

To fill these gaps, FOW, the Parks and Recreation Department and other agencies have undergone forest restoration in parts of Andorra Run, Cresheim Creek, Cathedral Run and other areas in the park, as outlined in the 2001 Wissahickon Valley Park Master Plan. Fencing to keep out deer and an annual cull were implemented.

Restoring the park has relied heavily on volunteers. Since its founding 25 years ago, Wissahickon Restoration Volunteers has planted hundreds of trees, said Steve Jones, the former president and current board member of WRV. Members remove invasives, plant multiple layers of native vegetation to support wildlife and provide educational programs with Latino Outdoors Philadelphia.

“Our approach is very labor intensive,” he said. “We organize hundreds of people who [dedicate] hundreds of hours of volunteer time in the course of the year to do new plantings, expand our sites or to replace plants that have failed, or to do maintenance. Not every situation allows restoration work to have that much focus of labor connected to it, [but] that’s the model that we use at WRV.”

There are some patches of greater diversity in terms of oaks and hickories, and formerly rarer shrubs like elderberries and chokeberries, Jones said. For example, WRV volunteers planted trees in an area along Carpenter’s Run in 2000 and 2001 and the canopy is now made up of native trees.

“On the one hand, the indigenous canopy is in danger of collapse because of pollution, human-caused climate-change, and so on, and at the same time we can see that it is doing a little better than it would be if we had just sat on our hands for the past 30 years and not tried to use our heads and hands to support it,” Jones said.

“Ideally, much more of the restoration work we do would be carried out by properly compensated Philadelphians who can make a career out of urban ecology. Until we make that possible, we’ll be here filling in the gaps,” he added.

It’s unclear exactly how much these restoration projects have improved the health of the Wissahickon tree canopy. The deer population has decreased, anecdotally, and biodiversity has improved in some areas as a result of the native plantings, according to FOW’s 2020 Ecological Management Plan, which assessed the park’s ecosystem.

Over the next decade, FOW plans to use citizen science to get a real-time picture of the park’s ecological health and features. FOW requested that members of the public record species they see on eBird and iNaturalist and participate in events like the Audubon Christmas Bird Count.

Once they have this data, there’s a lot to navigate. The Parks and Recreation Department needs to come up with an alternative to effectively controlling invasives other than spraying synthetic herbicides, which are a public health risk and can harm native species.

The stress of climate change

Some native Wissahickon trees likely will be stressed in a warming climate. These include the eastern hemlock and black maple, according to Mt. Cuba Center’s Resilient Canopy Project Report. The American sycamore, red maple and chestnut oak likely will adapt well to warming.

Franklin said adjacent landowners should invest in ecological restoration on their own properties, as it would benefit the park. One way is through FOW and the Chestnut Hill Conservancy’s Joint Easement Program for landowners to conserve open spaces.

She said to completely restore the health of the Wissahickon, there needs to be a culture shift, and she hopes more awareness about the state of the park will mobilize more volunteers.

“It relies on people coming to understand what’s at stake and what the consequences are,” she said.

“We just need more people, more money, and more imaginative solutions, and more informed solutions.”