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Population pressure shapes urban parks

A study of 10 Northeastern urban forests shows no sign that there is a common urban park plant complex, but does show that population levels affect both native and nonnative species diversity, according to a Penn State study.

"Less than 1 percent of species were common among all 10 parks," says Robert Loeb, associate professor of biology and forestry at Penn State's DuBois Campus. "This is evidence that common urban park flora does not exist and demonstrates that a diverse flora can be maintained in urban parks."

The parks studied were in Boston, New York, Philadelphia, Baltimore and Washington. All of the parks are more than 100 years old except for the three parks in the Gateway National Recreational Area, New York -- Breezy Point, Jamaica Bay, and the Wildlife Refuge which are all less than 60 years old. The other parks are Middlesex Fells, Boston; Pelham Bay and Van Cortland, The Bronx, New York; Pennypack and Wissahickon Creek, Philadelphia; Oregon Ridge, Baltimore and Rock Creek, Washington, D.C.

Loeb looked at populations of native and nonnative vascular plants in existing surveys of these parks and reported on his work in a recent issue of the Journal of the Torrey Botanical Society.

While all the parks have some fresh water habitat, four – Pelham Bay, Breezy Point, Jamaica Bay and Wildlife Refuge – have salt marsh and seashore environments. Loeb suggests that the long history of plantings in the parks appears to have resulted in greater species similarity among the parks in Baltimore, Washington, D.C. and The Bronx than among the parks in the Gateway National Wildlife Refuge, New York and Bronx, New York.

Dividing the 13 species found in all the parks into native and nonnative, 7 were native and 6 nonnative. The native species included red maple, yellow oxalis, and Virginia creeper. The nonnative species, all considered invasive species in the area, included dandelion, Japanese honeysuckle and tree of heaven. Twenty other species existed in 9 of the 10 parks, 13 native and 7 nonnative.

The Penn State researcher believes that at least some of these species do exist in all 10 parks, but were missed during the surveys. The nonnative species in 9 parks include Norway maple, wild garlic, chicory, ground ivy and mulberry. The native species include yarrow, Indian hemp, milkweed, white ash, sedge, Eastern white pine and Eastern poison ivy.

With less than 2.5 percent of plants occurring in 9 of the 10 parks, Loeb concludes that there is no Eastern Park complex of plants, and that the ratio of native to nonnative species is population dependent.

"Increasing human population is significantly related to decreasing native species and increasing nonnative species," says Loeb. "Higher levels of human disturbances such as trampling, air pollution, or arson have more of a destructive effect on native species. Also, large cities have had more money to purchase planting which traditionally have been nonnative."

The Penn State researcher said, "Urban park managers have implemented vegetation restoration programs with mixed success because of the differing abilities of plants to adapt to the harsh growing conditions found in urban parks."

The list of 1391 species reported for the ten parks provides a source to identify native and non-native species, which are growing in urban parks. "The evidence of species survival under the stresses of human use of urban parks is a critical factor for the selection of species for vegetation restoration projects," says Loeb.

"To help further the work of restoring the natural beauty of urban parks, landscape planting stock suppliers can highlight the capacity of horticultural plants, both native and nonnative species, to survive in the harsh growing conditions of the urban environment" he adds.

"The important city park management goals of recreating pre-European settlement plant communities and past landscape design visions of urban parks can have long-term success when species selections are made with consideration of survival in environments with repeated human disturbances," he says.

Vicki Fong | EurekAlert!
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