Urban heat island effect has upside for oaks in NYC

Red oaks in New York City grew eight times faster than rural oaks. Scientists think the the urban heat island effect was the primary reason.

Native red oak seedlings grew as much as eight times faster in New York’s Central Park than in more rural, cooler settings in the Hudson Valley and Catskill Mountains, in an experiment conducted by Columbia University scientists. The study, published in the journal Tree Physiology in April, 2012. These scientists say the urban heat island – a well-documented phenomenon that makes large cities hotter than surrounding countryside – is the primary reason. A fallout of airborne nitrogen — a fertilizer — from urban pollution might have helped the trees as well.

Red oaks in New York City were found to grow eight times faster than nearby rural oaks. The difference is thought to be due to the urban heat island effect. Image via Inhabit NYC

Tree physiologist Kevin Griffin of Columbia University’s Lamont-Doherty Earth Observatory oversaw the study, which was led by Stephanie Y. Searle, a Washington, D.C., environmental researcher who was a Columbia undergraduate when she started the research.

The urban heat island effect is what sends city-dwellers on extended vacations to the cooler beaches or the mountains in the summertime. The effect makes nighttime temperatures, in particular, significantly hotter than they would be otherwise. According to a press release from Columbia:

Griffin said that the city’s hot summer nights, while a misery for humans, are a boon to trees, allowing them to perform more of the chemical reactions needed for photosynthesis when the sun comes back up.

Central Park in NYC. In 2007 and 2008, researchers planted red oak seedlings here, as well as in two rural locations, and watched how quickly the trees grew. Image via Columbia University Department of Earth and Environmental Sciences.

In spring 2007 and 2008, these scientists planted seedlings in northeastern Central Park, in two forest plots in the suburban Hudson Valley, and near NYC’s Ashokan Reservoir, in the Catskill foothills some 100 miles north of Manhattan. They cared for all the trees with fertilizer and weekly watering. Maximum daily temperatures around the city seedlings averaged more than 4 degrees F higher. Mininum temperature averages – that is, nighttime temps – were more than 8 degrees higher in contrast to the more rural locations. By August, the city seedlings had developed eight times more biomass than the country ones. Most of the increase was in the form of leaves, the researchers said.

The researchers largely ruled out other factors that might drive tree growth, in part by growing similar seedlings in the lab under identically varying temperatures, and showing much the same result. Due to air pollution, the city also has higher fallout of airborne nitrogen — a fertilizer — which could have helped the trees as well. But the scientists said they believe higher temperature from the urban heat island effect were the main factor.

Red oaks and their close relatives dominate areas ranging from northern Virginia to southern New England, so the study may have implications for changing climate and forest composition over a wide region. The researchers said in their press release:

With half the human population now living in cities, understanding how nature will interact with urban trees is important … Some things about the city are bad for trees. This shows there are at least certain attributes that are beneficial.

Bottom line: Red oak seedings planted in New York’s Central Park grew eight times faster than the same trees planted in more rural locations, in an experiment conducted in 2007 and 2008. Tree physiologist Kevin Griffin of Columbia University’s Lamont-Doherty Earth Observatory oversaw the study, which was led by Stephanie Y. Searle, a Washington, D.C., environmental researcher who was a Columbia undergraduate when she started the research. The results were published in April, 2012 in the journal Tree Physiology.

Read more at the Earth Institute of Columbia University

Deborah Byrd

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