Trees Grow Faster in Urban Heat

On an Urban Heat Island, Zippy Red Oaks


The New York Times, April 25, 2012


Red oak seedlings in Central Park grow up to eight times faster than their cousins cultivated outside the city, probably because of the urban “heat island” effect, Columbia University researchers report.

The researchers planted seedlings of the native red oak in the spring of 2007 and 2008 in four places: in northeastern Central Park, near 105th Street; in two forest plots in the suburban Hudson Valley; and near the city’s Ashokan Reservoir in the Catskill foothills about 100 miles north of Manhattan. By the end each of summer, the city trees had put on eight times more biomass than those raised outside the city, according to their study, published in the journal Tree Physiology.


“The seedlings grew much larger in the city, with decreasing growth as you get farther from the city,” said the study’s lead author, Stephanie Searle, who was a Columbia University undergraduate when the research began and is now a biofuels policy researcher at the International Council on Clean Transportation in Washington.


The researchers hypothesized that Manhattan’s warmer temperatures — up to eight degrees higher at nighttime than in rural surroundings — could be a primary reason for the Central Park oaks’ faster growth rates.


Yet temperature is obviously only one of the differences between rural and urban sites. To isolate the role played by the thermostat, the researchers also raised oaks in a laboratory setting where all conditions were basically the same, except for the temperature, which was altered to mimic conditions from the different field plots. Sure enough, they observed faster growth rates for oaks raised in hotter conditions, similar to those seen in the field, Dr. Searle said.


The so-called urban heat island effect is often discussed in terms of potentially negative consequences. But the study suggests it could be a boon to certain species. “Some organisms may thrive on urban conditions,” another author, Kevin Griffin, a tree physiologist at Lamont-Doherty Earth Observatory at Columbia, said in a statement.


The results parallel those of a 2003 study in Nature that found greater growth rates among poplar trees raised in the city than among those grown in the surrounding countryside. But the current study went farther by isolating the effect of temperature, Dr. Searle said.


Red oaks and their relatives dominate many forests from Virginia to southern New England. The experience of Central Park’s red oaks could yield clues to what might happen in forests elsewhere as temperatures climb in decades to come with the advance of climate change, the researchers suggested.