Many people view urban areas as hostile for plants – concrete stifles root growth, and pollution from vehicles makes it difficult to gain nutrients. A study conducted by The Earth Institute at Columbia University not only discredits those theories, however, but suggests that urban environments have a lot to offer plants to promote growth.

To conduct the research, The Earth Institute researchers planted seedlings in the spring of 2007 and 2008 in three sites around New York. Those locations included an area in northeastern Central Park, near 105th street; in two forest plots in the suburban Hudson Valley; and in more rural surroundings near the Ashokan Reservoir in the Catskill foothills, about 100 miles (161 km) north of Manhattan. Over a five-year period, researchers observed an eight-fold increase in biomass in the urban-grown seedlings.

Researchers attribute the increased growth to high temperatures (particularly at night), carbon dioxide concentrations and atmospheric nitrogen deposition. Saplings planted in Central Park developed a lower root-to-shoot ratio, which reduced below-ground carbon costs to the plant. To compensate for the root development, urban seedlings allocated more growth to leaves than the rural-grown seedlings. This resulted in a ten-fold greater photosynthetic area but the same photosynthetic capacity of foliage as the rural plants.

Barnard College student Acadia Roher counts leaves of experimental red oak seedlings at Lamont-Doherty Earth Observatory (Photo: Alisa Frohman)

The findings mark a step toward understanding how nature and urban environments mix. "With human influence spreading across the globe, nature and urban environments are inseparable. The key is that plants can adapt to these changes in their environments, and in this case, really thrive in a human environment," said Stephanie Searle, lead author on a report of the study. Searle is currently a researcher at the International Council on Clean Transportation in Washington DC.

The report's authors weren't surprised by the results. "Our hypothesis was that the oak seedlings would grow faster in the city due to the higher nighttime temperatures," said Searle. Two previous studies on growth of poplar trees in Biosphere 2 conducted by Principle Investigator Kevin Griffin and co-author Matthew Turnbull, plus a previous study on poplar growth in urban environments by Jill Gregg, laid the groundwork.

The research did uncover new findings, however. "We were surprised at the magnitude of difference in growth between trees at the urban versus rural sites," Searle said. "Also, we had hypothesized that the urban seedlings would grow faster specifically because of their higher photosynthetic capacity boosted by higher temperatures, and this turned out not to be true; these plants got a head start by simply investing more resources in the leaves rather than roots compared to the rural plants."

While the findings reinforce the importance and potential of trees and other plantings in urban environments, many cities including New York City already make great efforts to create green spaces. Central park's 843 acres (341 hectares) are landscaped with trees, though many were knocked down in severe lightening storms in the summer of 2009.

"We believe that the 'urban heat island' effect may help trees in the city grow faster and thrive, but most trees in New York City are planted, so the fact that there are so many trees is really thanks to a great job by the NYC Department of Parks and Recreation," Searle commented.

Now it's up to New York City – as well as other urban areas – to capitalize on the advantages of tree growth, and keep cities green.

The paper on the research was recently published in the journal Tree Physiology.

Source: The Earth Institute at Columbia University