As part of a long-standing effort to expand green space, New York City plants tens of thousands of trees each year. Trees provide shade, release moisture to cool the ground, absorb incredible amounts of carbon from the air, scrub soot and other airborne pollutants, and provide wildlife habitat as well as beauty. So what’s the problem?
New research suggests that, in fact, something could be wrong: the oaks and sweetgum trees that currently make up the majority of the city’s tree cover produce large amounts of a volatile compound called isoprene. Although harmless in itself, isoprene reacts rapidly with polluting nitrogen oxides emitted by vehicles, buildings and industry to form ground-level ozone, which is a key factor in many respiratory diseases, especially chronic bronchitis and asthma.
The study, conducted by scientists from the Columbia University Climate Department, Lamont-Doherty Earth Observatory Studies by the City of New York and other institutions have found that if Manhattan maintains its historical planting patterns and adds new trees, Manhattan’s isoprene production could increase by about 140% over the next few decades, resulting in a 30% increase in summer ozone levels. Queens, which has the most room to add trees of any borough, could see four times as much isoprene production and a corresponding increase in peak ozone levels. The other boroughs fall somewhere in between. the study The results were published in the academic journal Environmental Science & Technology.
“We are in favor of planting more trees; trees bring us a lot of good things,” said the study’s co-author. Roisin Comane“But if we’re not careful, we could end up worsening our air quality,” said Jonathan Myers, an atmospheric chemist at Lamont-Doherty Laboratory.
“There is no reason to think that trees don’t have an effect on what’s in the air,” said the lead author. Dan Dan Wei“Until now, we just didn’t have the tools to understand this particular aspect,” said Dr. Schneider, who conducted the research as a postdoctoral researcher at Lamont-Doherty.
The leaves of some trees emit isoprene as a by-product of photosynthesis, but the reasons for this aren’t well understood. In the case of oaks, emissions tend to increase exponentially with heat, at least until temperatures reach the high 90s Fahrenheit. Some scientists think this helps prevent leaf tissue from drooping and losing photosynthetic capacity as temperatures rise. The emission of these volatile compounds by trees may also be involved in attracting pollinating insects. Whatever the reason, oaks and maples are particularly prolific, with oaks releasing about 800 times more isoprene than less emitting trees such as maples and sycamores. (Fun fact: the oak-rich Blue Ridge Mountains appear blue-tinted from afar, but this is because large amounts of isoprene and other volatile compounds indirectly react with water to form tiny suspended droplets.)
According to the New York City Parks Department, the city has about 7 million trees covering 22% of the city’s urban area, and is responsible for planting and maintaining public lands. There are about 5 million trees in parks and forests, more than half of which are various species of oaks and sweetgum (37% and 17%, respectively). Of street trees (nearly 700,000 trees at the last count), oaks make up 18% and sweetgum are very few. The most common street tree is the London plane tree, which accounts for one-third of the total. The remainder are about 130 other species of trees.
The new study’s authors analyzed newly available satellite imagery that shows the city’s tree canopy on a 30-meter by 30-meter grid and combined it with Parks Department tree species surveys from 2016 and 2018. They then input data from other scientists, including the study’s co-authors. Andrew Reinman“The researchers are investigating how trees interact with nitrogen oxides emitted by tailpipes and buildings, and they’re able to determine how the trees interact with the nitrogen oxides emitted by tailpipes and buildings,” said Dr. Schneider, an environmental ecologist at the City University of New York Graduate School, who is conducting experiments to measure isoprene production in tree leaves under a variety of conditions.
Researchers found that emissions from trees play a role in regulating ozone production, with levels routinely exceeding the federal safety limit of 70 parts per billion on hot summer days. Currently, levels can reach 100 parts per billion, and the addition of new trees could eventually push them higher, the study said.
But tree-derived isoprene isn’t the only culprit: ozone cannot form without a key precursor called nitrogen oxide, also known as NO.xIt is emitted during the combustion of fossil fuels in vehicles, hot water boilers, power plants and industry.
“If you lower the NOx “Trees aren’t going to be the problem,” Way said. “We’re not trying to convey the idea that trees are polluting the air. Cars are the problem.”
New York has made some progress in reducing nitrogen oxides in recent years, but the pace has been disastrously slow: At the current rate of 2 to 5 percent per year, studies suggest it would take the city 30 to 80 years to cut its emissions by a fifth — at that level, emissions from trees would no longer play a role in building the ozone layer.
There seem to be no quick fixes: The city’s Local Law 97 requires many buildings to be carbon neutral, which would all but eliminate fossil fuels, but that won’t happen until 2050. Electric cars are still just a glimpse into the future: In June, New York Governor Kathy Hochle abruptly halted a decades-old plan to reduce car traffic in Manhattan with a congestion tax; chances of reviving the plan are unclear.
Meanwhile, the city council Passing the 2023 resolution It calls for increasing tree canopy coverage from the current 22 percent to at least 30 percent by 2035, which would require the planting of 250,000 new trees. Research in 2022 A study by The Nature Conservancy found that tree canopy cover could actually be increased by up to 42 percent without impacting existing landscape features such as buildings or roads.
The Park Service is aware of the problem. 2018 Research A study by Parks researchers concluded that the city’s trees emit more than 800 tons of volatile compounds, including isoprene, each year. “We didn’t think it was a big deal,” said Novem Au-Yeung, a senior scientist at the parks department who oversees practical research into how the city should manage its natural resources. Like the new study’s authors, he said trees shouldn’t be viewed as the enemy. “If we rethink our car-centric lifestyle, we can plant as many trees as we want,” he said.
In any case, the department is already planting fewer oaks and more diverse species, but it’s not because of the isoprene issue. The city’s American elm die-offs in the 20th century and ash die-offs in recent years were caused by invasive pests. “We’ve learned our lesson. We’re diversifying, so if one pest shows up, we have others,” Oyen says.
The agency is focusing on trees native to the region, including the native oaks that dominate much of the eastern forests. Of the 55,533 trees planted in the city’s woodlands between 2018 and 2023, oaks made up 20 percent of the total. Of the 57,335 trees planted on streets, 17 percent were oaks, according to agency data. In the fiscal year that ended in June this year, Approximately 18,000 more The species composition is still unknown, but it has been planted.
“We’re not going to cut down big old oak trees,” nor will officials completely stop planting new oak trees, Ouyen said. “We have to think about what we’re going to lose by doing that.” Oaks are keystone species, providing food and habitat for native insects, birds and mammals, she noted. Oaks provide excellent shade, can grow in relatively small spaces, and, unlike other desirable shade species such as tulip trees, are relatively unscathed by urban air pollution, ozone and other disturbances. Importantly, red oaks, especially in the north, can function in high temperatures that shut down other trees. As the climate gets warmer, oaks may thrive even better, to a certain extent, Ouyen said. 2008 Study.
“Oaks are hardy trees, and they may be able to survive a changing climate,” Comane says, “so there are still great reasons to keep them planted.”
The research was funded by the National Oceanic and Atmospheric Administration, with additional support from the New York State Energy Research and Development Authority.
