Trees have a proven positive impact on cities: They lower wind speed and thus energy consumption. They reduce pollution and heat. They generally improve how neighborhoods are perceived. They make cities more livable. But the best way to prove why cities should plant more trees might be the fact that they have a significant economic impact.
A new study published in the online journal Ecological Modelling puts a number on just how much money trees save cities. After studying 10 megacities around the world and taking into account air pollution, storm water, building energy, and carbon emissions, the researchers found that trees have an economic benefit of about $505 million every year. From Beijing and Cairo to London and Los Angeles, researchers from the SUNY College of Environmental Science and Forestry and Parthenope University of Naples found that trees are worth $1.2 million per square kilometer, or $35 per capita.
But in the future, those numbers could be much greater. In megacities, which are defined as cities with more than 10 million people, tree cover today ranges from 8.1% of the metropolitan area to 36%. But the potential tree cover ranges from 15.6% to 24%, meaning there’s significantly more space to plant trees. “Planting more trees in potential tree cover areas could nearly double the benefits provided by the urban forest,” the researchers write.
To determine the economic impact of trees in the megacities, the researchers used a tree cover estimator called i-Tree, which requires analysis of 200 or more plots of trees within a city and then extrapolates economic benefit from there. But only a few megacities have been surveyed and the tool doesn’t take into account the larger metropolitan area of these urban centers. Instead of relying on people on the ground to survey each city, the researchers randomly selected points from aerial imagery and categorized each one as tree canopy, potential tree canopy (like a plaza or parking lot), or another category like agricultural or water where there’s no opportunity to plant trees. Using established models of tree benefits for London, they were able to extrapolate the urban forest’s advantages to other cities.
They found that the median reductions in air pollution save $482 million per year, the reduction in the amount of stormwater processed by wastewater plants saves $11 million, the reduction in carbon emissions saves $8 million per year, and the reduction in energy heating and cooling saves $500,000 per year.
Combined with the strong scientific evidence that trees are an ideal way to make life in cities better, the study shows that there’s a serious economic reason to invest in them. And indeed, some cities are already taking this body of research to heart through tree-planting initiatives. It’s an urban design no-brainer.