The benefits of greening urban areas are well known. Green spaces help reduce the heat island phenomenon, de-waterproof sidewalks and provide residents with refuge during heat waves – in addition to improving their physical and mental health. When we think of New York, the first images that come to mind are mineral: Times Square, the windows of 5th Avenue or the Statue of Liberty. But above the imposing buildings, vegetable gardens bloom and unite in Central Park to make the city breathe. Researchers at Columbia University have taken a close look at New York’s vegetation and concluded that“there is a lot more greenery than we thought”.
ant work
Because it’s not just the trees to consider: the plant cover is also due to lawns, for example. Above all, researchers show that during summer days, “the photosynthesis of trees and grasses absorbs all the carbon emissions produced by cars, trucks and buses, and even more”. The results of their research were recently published in the journal Environmental Research Letters.
These conclusions were the result of painstaking work. The team mapped New York and its surroundings, and analyzed the area in detail in 30-meter squares. The scientists then classified the squares according to their nature: concrete, lawn or leafy forest. All this in order to model the carbon absorption of each plot and study the carbon dioxide content in the air.
Their observation thus recalls the importance of urban forests in the carbon cycle, but also of grasses. “It tells us that the ecosystem matters in New York and if it matters here, it probably matters everywhere else,” points out Dandan Wei, postdoctoral fellow at the Columbia Climate School’s Lamont-Doherty Earth Observatory and lead author of the study.
“It’s not just a gray box”
Cities are responsible for more than 70% of human carbon dioxide emissions. New York alone is the largest human-made CO2 emitter in the United States and the third in the world. But even though its vegetation cover is relatively low, the biogenic absorption of CO2 offsets up to 40% of the increase in human CO2 emissions from the city during summer afternoons. “Most people assumed that New York City was just a gray box, that it was biogenically dead.says Roisin Commane, atmospheric chemist at Lamont-Doherty, co-author of the study. But just because there’s a concrete sidewalk somewhere doesn’t mean there isn’t also a tree shading it.” Enough to motivate large cities to redouble their efforts to expand green spaces.
In order to help determine the most suitable species, this research team intends to characterize the entire vegetation cover of the city. Hardy, fast-growing oak trees are a common choice in this region, but research has shown they give off a fair amount of isoprene, a volatile compound that reacts with vehicle emissions to create pollutant ozone. . Sweetgum, another common tree, produces a similar amount of isoprene but has different growth characteristics. “The more trees, the better. But we could do an assessment to find out which are the best species,” Explique Dandan Wei.
But this good news is not an incentive to rest on one’s laurels. Trees only absorb CO2 during the local growing season, which runs from mid-April to mid-October in New York City. “Vegetation in cities in warmer climates likely plays a larger role in carbon uptake,” Constate Dandan Wei.
