New York wants more tree cover; a 2024 study found that planting certain species could worsen summer ozone when their emissions mix with traffic pollution

Are trees pollutants? Contrary to popular belief, some trees might unintentionally worsen air quality. New York City wants more trees, and the goal seems to be to expand the city’s tree canopy, bringing more shade, cooler surfaces, cleaner air and wildlife habitat to one of the world’s most densely developed places. But there is a complication hiding among the leaves. A new study found that some of the tree species already common across New York City release large amounts of a chemical called isoprene. On its own, isoprene is harmless. But when it meets nitrogen oxides released by vehicles, buildings and industry, it can help create ground-level ozone, as per a report by Columbia Climate School. That means planting more trees is not necessarily as simple as choosing more of the species the city already has.

Researchers from Columbia Climate School’s Lamont-Doherty Earth Observatory and other institutions examined what could happen if New York continued its existing planting patterns. Their modeling found that isoprene production in Manhattan could rise by about 140% in coming decades, while summer ozone levels could increase by as much as 30%.

In Queens, the potential increase in isoprene production was even larger. The study found it could quadruple, with corresponding increases in peak ozone.


The research, published in Environmental Science & Technology, does not suggest that trees themselves are simply pollutants. Instead, it reveals a more complicated relationship between vegetation and the pollution already present in the city.

Why could more trees affect ozone?

The answer begins with the leaves. Some tree species release isoprene as a byproduct of photosynthesis, although scientists are not certain why plants produce it. Oaks and sweetgums are particularly strong emitters. Oaks can release around 800 times more isoprene than lower-emitting trees such as maples or London planes.

The effect also becomes more pronounced with heat. Oak isoprene emissions increase exponentially as temperatures rise, at least until air temperatures reach the high 90s.

Researchers combined information about the city’s tree canopy with measurements of isoprene production and emissions of nitrogen oxides. The result showed that trees can play an important role in ozone formation on hot summer days, particularly when enough NOx is present.

New York City currently has about seven million trees covering 22% of its surface, according to Parks Department data. More than half of the trees in parks and forests are oaks and sweetgums, while London planes are the most common street trees.

The city is also planning to add many more.

What happens when tree emissions meet traffic pollution?

Isoprene does not create ozone by itself. The crucial ingredient is nitrogen oxides, or NOx, produced through the combustion of fossil fuels by vehicles, boilers, power plants and industry. That distinction is important to the researchers. Lead author Dandan Wei, who conducted the research as a postdoctoral scientist at Lamont-Doherty, said, “If we lowered NOx significantly, trees wouldn’t be a problem. We don’t want to convey the idea that trees pollute the air. It’s the cars.”

New York has reduced nitrogen oxide emissions in recent years, but the study says the reductions have been slow. At the current rate of roughly 2% to 5% annually, it could take 30 to 80 years to reduce emissions by a factor of five, the level at which tree emissions would no longer play a role in ozone formation.

That creates an unusual problem for a city trying to improve its environment. More trees can bring significant benefits, but the choice of species matters when those trees are growing alongside existing sources of air pollution.

Could New York simply stop planting oaks?

Researchers and city officials do not suggest eliminating oaks. Oaks remain valuable urban trees. They offer shade, provide food and habitat for insects, birds and mammals, and can tolerate conditions that affect other desirable shade trees.

Róisín Commane, an atmospheric chemist at Lamont-Doherty and a study coauthor, said: “We’re all for planting more trees. They bring so many good things. But if we’re not careful, we could make air quality worse.”

The Parks Department has already been moving toward a more diverse selection of trees, although that shift was driven by concerns about pests rather than isoprene.

After the loss of American elms during the 20th century and more recent ash tree die-offs caused by introduced exotic pests, officials began placing greater emphasis on diversity.

Novem Auyeung, a Parks Department senior scientist, said: “We’ve learned our lesson. We’re trying to diversify so if one pest comes along, we have other trees.”

The department's planting data show that oaks made up 20% of the 55,533 trees planted in forested areas from 2018 to 2023. On city streets, 17% of 57,335 newly planted trees were oaks.

Still, Auyeung said, “We’re not going to go cutting down any big old oaks,” and the department will not completely stop planting them.

What could change in New York’s tree strategy?

New York City has called for its tree canopy to grow from 22% to at least 30% by 2035, a target that would require about 250,000 additional trees.

A 2022 study by The Nature Conservancy found that canopy coverage could potentially reach 42% without taking space from existing buildings and roads.

The new research does not challenge the value of expanding the canopy. Instead, it suggests that the species selected for future plantings could matter for summer ozone, particularly while nitrogen oxide emissions remain significant.

The city’s efforts to reduce those emissions are therefore part of the same equation. Local Law 97 requires many buildings to become carbon neutral, although that target does not arrive until 2050. The study also notes that electric vehicles remain limited, while a plan to reduce vehicle traffic through Manhattan congestion pricing was canceled in June.

For Commane, the choice is not between trees and clean air.

“Oaks are tough trees. They might be able to survive climate change,” she said. “There are still wonderful reasons to have them around.”

The study was funded by the National Oceanic and Atmospheric Administration, with additional support from the New York State Energy Research and Development Authority.

FAQs

Can trees increase ozone?
Some trees release isoprene, which can react with nitrogen oxides and contribute to ground-level ozone.

Does the study say New York should stop planting trees?
No. The research points to species selection and reducing NOx as important parts of the issue.