Nature by design

January 14, 2009 by Melissa McMasters

This is an article I originally drafted for Sylvan Communities magazine, but I thought I'd share it here as well.

It's easy to forget when taking a walk through the lush woodlands in our regional parks that these natural wonders are actually designed landscapes. When Frick, Highland, Riverview, and Schenley Parks were being developed from the late 19th to mid-20th centuries, teams of landscape architects worked to shape park users' experience of nature. Working largely in a traditional romantic picturesque style, they laid out parks with open meadows surrounded by woodlands that emphasized a rustic view of nature. While there was certainly some vegetation in place when the parks opened, the selection of trees, shrubs, and herbaceous plants for the park was an important component of these designers' work.

We can still see the efforts of these original craftsmen in places such as the Reynolds Street entrance to Frick Park, which contains open fields and planting beds leading into a series of forested trails. Preserving these historic designs--and balancing them with the needs of today's users--is a guiding principle of the Pittsburgh Parks Conservancy's work with the City of Pittsburgh. But along with history and modern use comes a third, and equally important element--ecology.

Stands of Japanese knotweed like this one in Highland Park grow quickly and are difficult to eradicate.

Today we have much more information about the ecological impact of the plants that are selected for the parks than the original designers had available to them. The parks are collections of native and non-native species, many of which were imported from other areas of the world for their aesthetic beauty or their ability to grow well in this climate. Over the years it became apparent that some of these non-native species were also environmentally invasive, and they began to have a devastating impact on the parks' natural areas. Trees such as Norway maple, Siberian elm, and tree of heaven, shrubs such as jetbead and privet, and herbs such as garlic mustard and Japanese knotweed have overtaken entire sections of parkland, killing or crowding out native species and vastly reducing the amount of biodiversity in the area.

The Parks Conservancy leads numerous volunteer projects aimed at educating the community about invasive species control. Managing invasives is a major task of the Urban EcoStewards, volunteers who provide long-term care to designated areas of parkland. Corporate and community volunteer days provide hands-on training to Pittsburghers of all ages, while putting thousands of hours annually into park maintenance.

In addition to combating invasive species, the Parks Conservancy is working to improve the long-term outlook for native plants. The American chestnut tree made up almost 25% of Pennsylvania's forests before the fungus that causes chestnut blight was imported in the late 1800s. By 1950, the tree had been almost completely eradicated. An extremely valuable source of food, habitat, and timber, the loss of the American chestnut was economically and ecologically devastating. But foresters have hope that through backcross breeding techniques, new cultivars can be created from surviving American chestnut trees and the more blight-resistant Chinese chestnuts. Pittsburgh's Highland Park is home to an experimental orchard that is growing these hybrid trees.

Phil harvests chestnuts at the Highland Park orchard.

The Highland Park orchard was planted in 1996 when the City of Pittsburgh, led by the support of then-City Forester Dale Vezzetti and then-City Councilman Dan Onorato, joined The American Chestnut Foundation's breeding program. Blight-resistant Chinese chestnuts are pollinated with some of Pennsylvania's surviving American chestnuts, with the ultimate goal of creating a tree with the characteristics of the American chestnut but the genes to resist blight. Each year hundreds of seeds are generated, and these hybrid trees have been planted throughout the city parks, including test sites in Frick and Riverview Parks and in the Highland Park orchard. The trees are studied over the years to determine how well the breeding process has worked, with a goal of using a new hybrid chestnut as a restoration planting when invasive trees are removed from a site. The Parks Conservancy has been testing chestnut seeds in areas heavily populated by Norway maples to determine whether chestnuts can grow in soil that has been altered by the Norway maple. The chestnut is an ideal restoration tree because it grows rapidly and can quickly re-establish a canopy, which in turn discourages the growth of sun-loving invasive species on the forest floor.

Another important park tree that fell victim to disease is the American elm, and the Conservancy is also part of efforts to reintroduce a hybrid elm that will be resistant to disease. The Schenley Park Overlook is ringed by six hybrid elm varieties which are being studied by the Conservancy and the Penn State Cooperative Extension to determine their viability as landscape trees. A crucial aspect of the study is whether these trees, which are bred partially from the invasive Siberian, Chinese, and other elm varieties, will become invasive in the surrounding woodlands. The mulch beds around the trees are carefully observed for seedlings, and the forests (which contain no other elms) are watched to determine whether new trees are becoming aggressive and require action. So far, the trees are not exhibiting aggressive tendencies and are functioning as positive replacements to the Norway maples that used to occupy the site.

Despite the many benefits of genetically diverse tree populations, modern horticulture is actually trending toward less genetic diversity in tree propagation. New tree stock is often replicated from tissue culturing and cuttings, which can result in thousands of trees with the exact genetic makeup of a single parent tree. Hybridization and sexual reproduction, by contrast, produce trees with diverse genetic makeup, reducing a tree population's susceptibility to catastrophic loss.

The London Plane trees at Schenley Plaza have defined the space for almost a century.

In 2004, the Conservancy's Director of Parks Management and Maintenance, Phil Gruszka, began working with Dr. Cynthia Morton of the Carnegie Museum of Natural History to study the genetic makeup of 81-year-old London Plane trees at Schenley Plaza. Almost half of the original London Plane population at the Plaza had been lost to disease, and the Conservancy wanted to replant a new, genetically diverse group of London Planes as part of the Plaza's 2006 restoration. The study revealed that the remaining trees were very genetically diverse, providing an excellent alternative to the "Bloodgood" cultivar (which was originally taken from a single-parent cutting) that is most often raised and sold by nurseries. Local nurseries began propagating new trees from the diverse population at Schenley Plaza, reducing the possibility that these new trees could infect each other should some of them contract a disease. Schenley Plaza's London Plane trees now represent a genetically diverse, historically inspired population.

The people of Pittsburgh owe a great debt to the original park designers, whose vision of beautiful natural retreats still enriches our lives today. But the parks are more than attractive places to enjoy a stroll--they are ecological treasure troves, and maintaining them requires extensive knowledge and effort. The Pittsburgh Parks Conservancy is proud to carry on the parks' legacy and continue improving their ecological health for future generations of park users.

Ecological Restoration, Highland Park, Park Threats, Trees and Forestry, Parks Management, Schenley Plaza, Flowers and Gardens, Schenley Park