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In a Central Park plot near East Drive, just north of 102nd Street, a team of biologists is getting a look at what they say could be climate change in action. Oak trees and cocklebur weeds are growing at twice the rate of their counterparts in the cleaner Catskills, scientists have found. It’s not just because city plants, like city kids, are tougher. New York’s polluted air and soil could be the cause.
The botanists think elevated levels of carbon dioxide in the city’s air and increased nitrogen in the soil may account for the difference in growth rate. If the hypothesis holds, it could have major planning and financial implications for management of the city’s parks, as gardeners and ecologists decide which species to grow and how to handle invasive plants that can transform local ecosystems. But for now the scientists from the Central Park Conservancy and Columbia and Fordham universities who are conducting the study caution there isn’t enough data to form hard conclusions.
“Weedy plants in more urban areas are growing more quickly than plants in more rural areas. You might expect the opposite. There’s been this expectation that urban plants grow more slowly. But urbanization is associated with pollutants that some plants respond well to,” said J.D. Lewis, a botanist at Fordham University who has spent 20 years studying the effects of elevated carbon dioxide on plants.
Since plants absorb carbon dioxide, not oxygen, the abundance of carbon dioxide from city traffic gives plants more fuel, Lewis explained. Pollutants in the water stream, including nitrogen, leeches into the soil. Like generations of New Yorkers who have grown stronger from a challenging environment, it appears some plant species have adapted to turn soil pollution into a benefit.
Lewis said the fact that Central Park plants are further south than the Catskill specimens doesn’t completely explain their accelerated growth. The species in the study, oak seedlings and cocklebur – a weedy cousin of the sunflower – are more likely responding to the high levels of carbon dioxide and nitrogen released by car traffic and industrialization, Lewis said. Researchers are focusing on oak and cocklebur because their varied growth patterns (the native oak takes years to mature, the invasive cocklebur only months) provide a broader range of data.
“Organisms evolve because of their interactions with the urban environment,” Lewis said, discussing the changes in the air and soil composition in the nearly 100 years since cars and other vehicles have been belching exhaust in the city. “From the standpoint of plants, this is rapid change.”
Cities have long been considered inhospitable places for anything green, Lewis said. “If the trees are growing faster, that’s potentially a benefit. On the other hand, the trees may be benefiting but the weeds may be growing even faster.” The study began in spring 2005, added the Central Park site last spring, and does not yet have an end date.
Matt Brown, supervisor of the Soil, Water and Ecology lab of the Central Park Conservancy, said the growth rate of weeds really matters because weeds transform the ecosystem that birds and other animals rely on in the park. “Some invasives are so successful they crowd out everything else. That cuts into diversity and affects the wildlife that rely on the ecosystem,” Brown said, adding that Central Park is a crucial stop on the northeastern flyway for migrating birds.
“I think there’s a lot of talk about global warming, and we can possibly see the effects here,” Brown said. “What sort of effect will it have? There’s an enormous amount of complexity to it.”
Brown cautioned against drawing science fiction conclusions about mutant weeds from the study data, but said the Conservancy was excited to be involved in examining the park’s environment. Ecological research to better understand the park has been part of the Conservancy’s mission since it was founded in 1980. “I think it’s really important to value our urban ecosystem,” Brown said, lamenting that cities are often dismissed as not being part of nature. “Sure there’s some domestication happening here because we’re managing the park for 9 million users, but we’re protecting an ecosystem.”