Species such as oaks and sweet gums, which make up much of the city’s treescape, emit chemicals that produce a toxic gas when they come into contact with fossil fuel pollution, researchers say.
Caroline Willis/Office of Mayoral Photography
New York City is home to countless cars and buildings that run on fossil fuels and produce more than half of the state’s greenhouse gas emissions, so pollution can be overwhelming.
While planting trees helps curb pollution by absorbing these gases, certain types of trees can actually make the environment worse, according to a recent study published in the journal Environmental Science & Technology.
Species such as oak and sweet gum emit a chemical called isoprene, which is harmless to humans. But when they come into contact with pollutants from fossil fuel combustion, they produce a toxic gas called ground-level ozone, which can irritate the throat, inflame the airways and increase the frequency of asthma attacks.
“What we grow is really only a problem from an air quality perspective because of the amount of fossil fuels we burn in New York City,” says Andrew Reinmann, an ecologist and CUNY professor who worked on the report.
And this is particularly worrying since the Big Apple’s landscape is also populated to a considerable extent by oak and sweetgum trees.
Of the five million trees that make up the city’s parks and forests, more than half are oaks (37 percent) and sweetgums (17 percent), according to the study. That number is much lower for the more than 666,000 street trees that line many sidewalks. Only 18 percent are oaks, and sweetgums make up an insignificant proportion that was not included in the study.
However, if the city maintains its current tree planting patterns, isoprene production in Manhattan will increase by about 140 percent in the coming decades, the report warns. Isoprene emissions tend to rise exponentially in hot weather, so ozone levels are also expected to rise by as much as 30 percent on hot summer days.
To arrive at these numbers, a team of scientists examined the New York City Parks Department’s tree count from 2016 to 2018 and combined it with satellite images of the city’s tree canopy. They also used a model created by study leader and postdoctoral fellow at Stony Brook University Dandan Wei to figure out how trees interact with vehicle exhaust and building emissions.
To curb pollution, city officials need to be very strategic in choosing the tree species they plant, Wei says, “especially when we are committed to planting new trees.”
The parks administration has set a goal of planting one million new trees in the Big Apple, and last year all five borough presidents began gathering support to make this goal a reality by 2030.
“We really need to focus on reducing pollution caused by human activities,” Wei said. “But until then, we need to figure out which tree species are best to plant in terms of air quality.”
Historically, sweetgums and oaks in their many variations have been popular choices for New York City’s treescape, as they are native to many forests in the Northeast. According to the Parks Department’s tree map, there are nearly 20,600 sweetgums in the city’s treescape. In the oak category, the swamp oak is a popular choice, with nearly 75,000 specimens spread throughout the boroughs.
But when it comes to selectively choosing trees to avoid ground-level ozone, the parks department told City Limits that a study alone won’t change its overall planting strategy. The agency said it is committed to creating a “healthy and successful urban forest,” and pointed out that oaks and sweet gums make up only a small portion of the overall tree population: 6 percent and 2.3 percent, respectively.
“Thanks to our consistent collaboration with local nurseries to source unique and diverse trees, we are now able to plant from a palette of over 130 different tree species, up from just 29 species twenty years ago,” said Jennifer Greenfeld, NYC Parks’ deputy commissioner for environment and planning, in an emailed statement.
“Our goal is to continue to plant and maintain a diverse tree population throughout the city that can meet all the challenges of the urban landscape,” she added.
Still, choosing the right tree species can be critical at a time when New York is lagging behind in curbing fossil fuel pollution, experts say.
The rate at which New York has reduced nitrogen oxides, the main air pollution pollutants, has been extremely slow, according to the study. At current rates of 2 to 5 percent per year, it would take 30 to 80 years for the city to reduce these gases to the point where emissions from trees no longer play a role in ozone formation.
The ultimate solution is to invest in electrification and cleaner energy sources, Reinmann says. But New York is still heavily dependent on fossil fuels, with only 28 percent of the state’s electricity generated from renewable sources.
“If we could reach a point where we could largely electrify our vehicle fleet and greatly reduce nitrogen oxide emissions from power plants and other forms of fossil fuel combustion in the city, then it wouldn’t really matter which trees we plant,” Reinmann said.
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