New York City is committed to expanding its green spaces by planting tens of thousands of trees each year. These trees provide numerous benefits, such as shade, lowering surface temperatures, absorbing carbon, filtering pollutants, and providing wildlife habitats. However, a recent study has found that certain tree species, particularly oaks and sweetgums, emit volatile compounds called isoprenes, which can react with nitrogen oxides to form ground-level ozone, a known respiratory irritant.
Researchers from the Columbia Climate School’s Lamont-Doherty Earth Observatory discovered that if the city continues planting these high isoprene-emitting trees, isoprene production in Manhattan could increase by 140% in the upcoming decades, leading to a substantial rise in summer ozone levels. In Queens, where there is ample room for additional trees, isoprene production could quadruple, resulting in higher peak ozone levels. The study highlights the potential negative impact of tree species selection on air quality.
While trees play a significant role in urban environments, it is essential to consider their emissions in the context of air pollution. Trees emit isoprenes as a byproduct of photosynthesis, with oaks and sweetgums being among the highest emitters. Despite this, trees alone cannot generate ozone without the presence of nitrogen oxides from sources like vehicles and industrial activities.
Efforts to reduce nitrogen oxide emissions in New York City have been slow, with current reductions unlikely to mitigate the influence of tree emissions on ozone formation for several decades. The city recently set ambitious goals to increase tree canopy coverage to 30% by 2035, necessitating the planting of 250,000 new trees. The Parks Department is actively diversifying tree species to mitigate potential risks from pests and diseases, while still maintaining the environmental benefits of trees like oaks.
Ultimately, trees are a valuable asset to the urban landscape, providing numerous ecological and social benefits. By carefully selecting tree species and continued efforts to reduce air pollution, New York City can enhance its green spaces without compromising air quality.
