Central Park trees in autumn. (Photo by Sean Pavone on Shutterstock)
NEW YORK — As New York City races to combat climate change and improve quality of life for its residents, an ambitious plan to dramatically increase the city’s tree cover is taking root. But new research suggests this well-intentioned greening effort could have an unintended consequence: worsening air pollution on the city’s hottest summer days.
A study led by researchers at Columbia University, Stony Brook University, and the City University of New York concludes that increasing NYC’s tree canopy could lead to higher levels of ground-level ozone, a major air pollutant that can trigger asthma attacks and cause other respiratory issues. This finding presents a complex challenge for urban planners and policymakers as they attempt to balance the many benefits of urban forests with potential risks to public health.
At the heart of this paradox are two types of trees that dominate New York’s urban forest: oaks and sweetgums. These arboreal powerhouses, which account for more than half of the trees in the city’s parks and forests, are prolific producers of chemicals called isoprenes. Trees naturally release isoprene into the air, especially on hot days. While harmless on its own, isoprene can react with other pollutants in urban air to form ozone. In most rural areas, this isn’t a significant problem. But in cities like New York, where the air is already laden with pollutants from vehicles and industry, additional isoprene can tip the scales toward dangerous ozone levels.
The study, published in Environmental Science & Technology, used advanced computer modeling to estimate current isoprene emissions from NYC’s existing trees and project future emissions under various tree-planting scenarios. The results were eye-opening: if the city achieves its goal of increasing tree cover from the current 22% to 30% by 2035, isoprene emissions could increase by 1.4 to 2.2 times in Manhattan alone. On the hottest summer days, this could lead to ozone levels spiking by 8 to 19 parts per billion – as much as 30% – a significant increase that could push the city over federal air quality limits more frequently.
The situation is even more dramatic in Queens, where isoprene production could quadruple due to the borough’s greater capacity for new trees.
This doesn’t mean NYC should abandon its tree-planting ambitions. Trees provide numerous benefits to urban areas, from reducing the “heat island” effect and absorbing carbon dioxide to improving mental health and increasing property values. The key, according to the researchers, lies in being strategic about which trees are planted and where.
“We’re all for planting more trees. They bring so many good things,” said study coauthor RóisÃn Commane, an atmospheric chemist at the Columbia Climate School’s Lamont-Doherty Earth Observatory. “But if we’re not careful, we could make air quality worse.”
Not all trees are created equal when it comes to isoprene emissions. Oak trees, for example, are prolific isoprene producers, while other species like ginkgo or linden trees emit very little. The study suggests that by favoring low-emitting species, especially in areas with high air pollution, cities can reap the benefits of increased tree cover while minimizing the risk of exacerbating ozone problems.
“There is no reason to think that trees don’t play a role in what’s in the air. We just didn’t have the tools before this to understand this particular aspect,” says the study’s lead author Dr. Dandan Wei.
The findings highlight the complex interactions between urban greenery and air quality, and underscore the need for a holistic approach to urban planning. As cities worldwide grapple with the dual challenges of climate change and air pollution, the lessons from this NYC study could prove valuable far beyond the five boroughs.
But before we vilify oaks and sweetgums, it’s crucial to understand that these trees aren’t the root of the problem. The real culprit is the high level of nitrogen oxides (NOx) in New York’s air, primarily from vehicle emissions and other fossil fuel combustion. Without these pollutants, the isoprenes from trees wouldn’t pose a significant threat to air quality. “If we lowered NOx significantly, trees wouldn’t be a problem,” Wei explains. “We don’t want to convey the idea that trees pollute the air. It’s the cars.“
Novem Auyeung, a senior scientist with the New York City Parks Department, acknowledges the complexity of the situation. While the department has already begun diversifying its tree plantings for other reasons, such as disease resistance, she emphasizes that oaks won’t be eliminated from the urban forest. “We’re not going to go cutting down any big old oaks,” Auyeung states, highlighting the multiple benefits these trees provide.
The study underscores the urgent need for a multi-pronged approach to improving urban air quality. While diversifying tree species can help mitigate the isoprene issue, the researchers stress that dramatically reducing NOx emissions is the key to solving the ozone problem. Unfortunately, progress on this front has been slow, with current reduction rates suggesting it could take 30 to 80 years to bring NOx levels down to a point where tree emissions would no longer contribute significantly to ozone formation.
As NYC and other cities push forward with ambitious plans to green their landscapes, this research serves as a crucial reminder that even the most well-intentioned environmental efforts can have complex and sometimes counterintuitive effects. By approaching urban forestry with a nuanced, science-based strategy, cities can work towards creating greener, healthier, and more livable urban environments for all their residents.
Paper Summary
Methodology
The researchers used a high-resolution computer model called NYC-MEGAN (Model of Emissions of Gases and Aerosols from Nature) to estimate isoprene emissions from trees in New York City. This model incorporated detailed data on land cover, tree species, and weather conditions at a 30-meter resolution – much finer than previous studies. They then used a “box model” to simulate how these isoprene emissions would interact with other pollutants to form ozone under various conditions. The researchers also compared their results to actual air quality measurements taken in the city.
Results
The study found that on hot summer days, isoprene emissions from trees in NYC could be up to twice as high as previously estimated. If the city increases its tree cover to 30% using high isoprene-emitting species like oak, isoprene emissions could increase by 1.4 to 2.2 times in Manhattan. This could lead to ozone increases of 8 to 19 parts per billion on the worst air quality days. The model also showed that biogenic volatile organic compounds (BVOCs) like isoprene are already major contributors to ozone formation in NYC, even with current tree cover.
Limitations
The study has several limitations. It focuses only on isoprene and doesn’t account for other BVOCs that trees emit. The model also doesn’t fully capture the complex urban environment, such as the effects of buildings on airflow and temperature. Additionally, the projections are based on specific tree-planting scenarios that may not exactly match NYC’s actual plans. The researchers also note that their model may represent an upper limit of BVOC contributions to ozone formation due to some assumptions made in the calculations.
Discussion and Takeaways
The study highlights the need for careful planning in urban greening efforts. While increasing tree cover has many benefits, the type of trees planted and their location matter for air quality. The researchers suggest favoring low isoprene-emitting tree species, especially in areas with high air pollution. They also emphasize the importance of continuing to reduce other sources of air pollution, particularly nitrogen oxides from vehicles and industry. The study underscores the complex relationship between urban vegetation and air quality, and the need for interdisciplinary approaches to urban planning that consider multiple environmental factors.
Funding and Disclosures
The study was supported by the National Oceanic and Atmospheric Administration and the New York State Energy Research and Development Authority. The authors declared no competing financial interests.
