Andrea Comas / Reuters

When it comes to reducing pollution, the key isn't the number of trees you plant but where you plant them

Trees clean the air, which is increasingly important in urban areas where high rates of asthma and other health conditions are rampant. They also reduce the urban heat island effect, and of course add to the aesthetic beauty of streets and neighborhoods. For all these reasons, tree planting programs are easy sells for local politicians. New York City, for example, is half way to its goal of planting one million trees. Mayor Michael Bloomberg launched the program in 2007, along with actress Bette Midler and her nonprofit, the New York Restoration Project.

“To walk under the branches of a tree that you have planted connects you to the roots of our past and the aspirations of our future,” Midler noted poetically at the project's launch.

It might seem impossible to argue against trees, but some New Yorkers have found a way. A recent article in The New York Times looks at the high rate of tree-related calls to the city’s customer service line. Some residents, citing concerns about maintenance, have actively opposed the planting of new trees in their neighborhoods.

Even elected officials who profess a love of trees say they fear that the city may not be putting the necessary resources into caring for the trees once they are planted. They cite instances of premature deaths, as well as a pruning backlog, made worse by recent budget cuts. The regular pruning cycle of street trees is now once every 15 years to 20 years, down from once every 7 years.

And while maintenance costs and buckling sidewalks are concerns, officials argue that the aesthetic and public health benefits of trees make them worthwhile.

But a new study suggests that going for that big, round number might not be the best way to add tree cover to a city. Researchers found that paying more attention to where and what types of trees are planted can bring about targeted reductions in the types of air pollutants that contribute to health problems. The study, published in the journal Landscape and Urban Planning, mapped out pollution hotspots in London and modeled the efficacy of planting a variety of trees in these locations. By concentrating trees that have leaves year-round in areas with high pollution rates, the researchers’ model showed that rates of fine particulate matter in the air dropped significantly.

As urban populations all over the world contend with health problems like asthma, it may be wise for city tree-planting efforts aimed at hitting numeric targets to refocus on identifying where those trees might best improve air quality.

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