The Forests of the Wissahickon


by Burt Froom

West Mt. Airy:  Yesterday and Today

April, 2013 (Article 7)

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The forest of the Wissahickon Park is our wilderness and our park, our inspiration and our concern.

In this article, I would like to introduce two authors and their book entitled, Metropolitan Paradise: The Struggle for Nature in the City. The authors, David Contosta (social and cultural historian, history professor at Chestnut Hill College) and Carol Franklin (landscape architect, professor at the University of Pennsylvania) engage us in a conversation about the roots, changes and future of the Wissahickon Valley. I have drawn upon their four-volume book in this column and I warmly recommend it to my readers! Metropolitan Paradise is available at the Friends of the Wissahickon (

The book opens with the idea of the Wissahickon as Paradise “Sacred to the Lenni-Lenape people and to many early Europeans who settled in this area, the Wissahickon Valley has all the elements of ‘Paradise’ recognized in many cultures – the dramatic gorge with high cliffs, twisted rocks, dark hemlocks, sparkling water and the beautiful rolling terrain directly to the north.  Ironically, this paradise is part of a large, old North American city… and suffers from all the troubles of a modern urban area.”

Our Wissahickon Valley, with its 1,800 acres and 57 miles of hiking trails, is more than an arboretum with ornamental trees. It is more than a park with invented landscapes and lawns and flowers. Our Wissahickon Valley forest is a restored wilderness, a managed wildness that is a recreation of the ancient mature forest where the Lenape once hunted. It is the gift of millions of years of evolution and dynamic geology before we came here. We can treat this magnificent forest as a sacred place. Our task is to reclaim the forest and offer it to the future – so that the forest and surrounding city may both thrive together.

We may wonder what the Wissahickon looked like before Europeans first came here. We live in a temperate latitude deciduous forest. These forests have an impressive architecture of at least five layers of trees and plants.  The tallest trees comprise the canopy, or roof, of the forest (60-150 feet high), whose leaves absorb the energy of the sun and winds and snow of winter.

William Penn, Pennsylvania’s founder, described the forest of his new colony in 1683: ”The trees of most note are the Black walnut, Cedar and Cypress, [tulip] poplar, Gumwood [sweet gum or black gum], Hickory, Sarsefrax [Sassafras], Ash, beech, oak of various sorts…, [and] Chestnut…” In old growth forests, many of these canopy trees, including white pines (up to 230 feet tall) and maples, lived for 200-400 years, and some hemlocks lived to 900 years.


Tulip poplar tree and leaves and flower

The understory layer is comprised of smaller trees like dogwoods, witch hazel, hornbeam, shadbush, Carolina silverbell and sassafras. The shrub layer contains shrubs, woody plants 4 to 12 feet high, like azalea, blueberry, viburnam, mountain laurel and spicebush. The herbaceous layer lies just above the forest floor, and is composed of woody ground covers like trailing arbutus, partridgeberry, sedges, ferns and wild flowers.

Students of the natural landscape speak of “natural succession,” the gradual and continuous replacement of one set of plants and animals for another, as forests are impacted by natural disasters (like disease, hurricanes, fire, flooding), or by human beings (logging, and land clearing for farming and development). The oldest and most complex landscape is the old growth or climax forest, which describes the longest living and least disturbed forests. “Old growth” implies that a forest has never been cut down by European settlers, has experienced little disruption from modern impacts, and that its soil has never been plowed or bulldozed. The Wissahickon Valley of the Lenni-Lenape aboriginal people lived in such a stable forest of very large, diverse, long-lived trees. Yes, it must have been a paradise!

Our picture of the original forest paradise is of large trees and dense undergrowth where the Indian inhabitants left no impact. But recent research tells us that the Indians actually managed the ecosystem like many large gardens. The first Europeans settlers in Maryland remarked at how they could drive a carriage and four horses through the forests because the Indians had burned the underbrush to hunt and travel and plant their crops and build their huts. New England colonists found the same clear forest floor. Instead of virgin forests, our ancestors encountered park-like landscapes. Charles Mann, in his recent book, 1491, points out that it was the settlers who created wilderness by driving out the Indian people and destroying the forests that the Indians had managed.

The European settlers came from an almost completely cultivated world in England and Germany of neat fields and villages and few forests or woods. Our region was covered with an almost continuous forest of tall trees of great girth. The settlers seemed to have had an “unconquerable aversion to trees.” They wanted fields cleared of trees to grow food and flax, to build homes and mills for furniture and transport, and to burn for winter heat.

During the later 1600s to the early 1800s, when there were some 24 mills in the lower Wissahickon Valley and its tributaries,  early manufacturers built roads straight down the slopes of the Wissahickon gorge to operate their mills, because their was no north-south road yet in the narrow chasm. As a result, precious soil was lost to erosion and the creeks were polluted. Photographs of the mills in the later 19th century show bare hillsides devoid of trees. Much of the original forest was lost and changed irretrievably.

The greatest loss was the American chestnut tree. Until a century ago, chestnut trees comprised about one-quarter or more of all eastern forest trees! The chestnut provided food (mast) for many forest animals and humans, from Maine to Georgia. This huge, majestic tree grew wide and tall, to 100 feet, often on hilltops (thus “Chestnut Hill”). Then the chestnut blight appeared. Imported Japanese chestnuts carried the fungus to American chestnuts. The airborne fungus bores through the chestnut’s bark and turns the tree’s pH from 5.5 to 2.8 (extremely acidic), which is toxic to plant cells. The infection girdles the tree trunk, killing the great tree above. Blight-resistant hybrid chestnuts offer some hope for eventual recovery.

The disease was found first in the Bronx Zoo in 1904. Within 40 years an estimated four billion trees were dead and there was a great hole in the natural landscape. The “spreading chestnut tree” of the village smithy in Henry Wadsworth Longfellow’s 1840 poem was gone.



Before the fall: Loggers and giant American chestnut trees standing almost trunk to trunk in the Great Smokey Mountains of  North Carolina in 1910

And another primary tree of the Wissahickon forest is in trouble: the eastern (or Canadian) hemlock, Pennsylvania’s state tree. Formerly there were great stands of hemlocks on the east side of the creek. These hemlocks can grow to 125 feet and have very massive trunks. They are long-lived, the record is a 535 year old tree in our region. They were common throughout northeastern U.S. However, the eastern hemlock has a nasty enemy: the hemlock wooly adelgid (pronounced  a-déll-jid). This is a sucking insect native to Japan, which was accidentally introduced to the U.S. in 1924. It feeds on the sap of young hemlock shoots. Infested hemlocks die of desiccation in four to ten years. The hemlock wooly adelgid was first recognized in Pennsylvania in 1967. It has decimated the hemlocks of the Wissahickon and the great hemlock forests of the Appalachians are threatened with eradication.
The forest of the lower Wissahickon Valley is presently reestablishing itself from these harms, as a thriving environment. The Friends of the Wissahickon (FOW), a 1,600 member organization, was founded in 1924 with a mission “to preserve the natural beauty and wildness of the Wissahickon Valley and stimulate public interest therein.” FOW works to eliminate invasive plant species, monitor watershed management, and restore trails throughout the park.

FOW says that the Wissahickon Valley Park is “home to a healthy American beech, Red maple, oak and hickory forest. Red oaks thrive on the lower slopes and the majestic Chestnut oaks perch on rocky crests. Black, white, pin, and scarlet oaks are found throughout the park.” The tall, straight trunks of tulip poplars, the dominant forest tree, white pines, and eastern hemlocks still tower in the canopy. Other trees in the park include sugar maples, silver maples, mountain maples, black birch, river birch, and gray birch, common alder, serviceberry, hornbeam, bitternut hickory, and pignut hickory, green ash, American holly, whitegum, sweetgum, sassafras, and butternut. We should all bring our tree identification books on visits to the Wissahickon!

What is the health of the Wissahickon Valley forest today? In a recent conversation with Jason Lubar, Associate of Urban Forestry at the Morris Arboretum of the University of Pennsylvania, he said that there are some 300 year old hemlocks scattered throughout the valley There are many trees in the park 100 and perhaps 150 years old. The tallest trees are tulip poplar at 125 feet, seen near Philadelphia University.

However, Lubar pointed out that there are disturbing signs of decline in the park’s health. There is severe erosion, and reduced diversity of native species like the chestnuts and hemlocks and also white pines. Native trees are being crowded out by exotic invasive trees, like the Norway maple. The royal paulownia tree from east Asia (named for a Russian princess), is very tall and has lovely violet flowers, but its huge leaves and rapid growth eliminate native plants in its shade. There are the emerald ash borer and the Dutch elm disease. And there is the Japanese angelica tree, whose thorny trunk, dense roots and thick stands kill many plant species. Jason Lubar also pointed out that the warmer temperatures of global climate change stress many forest plants. Voracious deer herds in the park eat tree seedlings (of mainly native plants). Development, rain run off, pollution and sewage along the banks of the middle and upper Wissahickon Creek poison the forest and creek. There is much to be concerned about, even in paradise.

Sarah West, author of Rediscovering the Wissahickon, points out that, “When more people appreciate the Wissahickon as an environmental jewel, far less effort will be needed to keep the park free of litter…” and for us to stay on established paths. “As earth’s most destructive creature, humans carry a burden of responsibility for preserving our planet. We cannot privately own land in any permanent sense, we only time-share it or borrow it from the future.”

In the May issue of the Yesterday and Today column, we will look at the Wildlife of the Wissahickon.