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New York City is planting tens of thousands of trees each year. They provide shade, lower surface temperatures, absorb airborne carbon, and offer wildlife habitat along with just plain beauty. What could go wrong?
Actually, something could, according to a new study. Oaks and sweetgums, which account for a majority of the city’s trees, produce huge amounts of volatile compounds called isoprenes (异戊二 烯). Harmless by themselves, isoprenes interact rapidly with nitrogen oxides emitted by vehicles to form ground-level ozone, a prime factor in many respiratory ailments such as asthma.
The research found that if the city maintains past species patterns, isoprene production in Manhattan will rise by about 140%, and summer ozone levels by as much as 30%.
“We’re all for planting more trees. They bring so many good things.” said study coauthor Roisin Commane. “But if we’re not careful, we could make air quality worse.”
But isoprene from trees alone is not to blame. Ozone cannot form without nitrogen oxides, emitted during the burning of fossil fuels by vehicles and industry. Many city dwellers remain oblivious to this connection, assuming trees can only ever clean the air. “If we lowered emissions significantly, trees would not be a problem,” said lead author Dandan Wei. “It is the cars.”
The Parks Department is aware of the issue. Like the study authors, scientist Novem Auyeung said trees should not be viewed as the enemy. “We could plant any trees we want, if we just rethink our car-centric lifestyle,” she said.
In fact, the department has already reduced the proportion of oaks it plants in favor of a more diverse mix, mainly to protect against pests. Oaks are keystone species, providing food and habitat for native insects and birds. Importantly, northern red oaks can function in high temperatures when other trees shut down. “There are still wonderful reasons to have them around,” said Commane.
28. What does the new study imply regarding oaks and sweetgums?
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