To estimate the existing tree cover in Beijing, Buenos Aires, Cairo, Istanbul, London, L.A., Mexico City, Moscow, Mumbai, and Tokyo, the researchers adapted the i-Tree model, which was developed by the U.S. Forest service in 2006. i-Tree employs aerial photography to gauge the dollar value and environmental payoff of the urban canopy. To date, it’s only been used to snap a birds-eye-view of the canopy across U.S. cities (past analyses have zoomed in on L.A. and Austin).
It likely comes as no surprise that overall tree cover in these bustling areas was just a sliver of the global average—39 square meters per capita, compared to 7,800. The density of the urban canopy varies widely among megacities, too. In Cairo, only 8.1 percent of the land is flecked with trees; in Moscow, 36 percent of the land is leafy. (Among the megacities, the median was 20.9 percent.) All told, Tokyo has the highest tree canopy cover per person.
Many of trees’ benevolent effects are general and constant: They help mitigate the heat-island effect, for example, and curb pollution. But the more specific benefits dispersed by trees vary from place to place. According to the researchers’ model, Cairo—which has little precipitation to begin with—didn’t reap a particular benefit in terms of stormwater remediation, for instance. Likewise, Mumbai, which has lower energy expenditures relative to the other megacities, didn’t recoup much in that realm. Meanwhile, owing to its extended growing season, L.A. accrued the most benefit from CO2 sequestration.
- $482 million per year in decreased air pollution (predominantly from smaller particulate matter, a byproduct of combustion and diesel engines)
- A benefit of $11 million annually through improved stormwater remediation
- A half-million saved in heating and cooling costs
- $8 million in C02 sequestration
Of course, these are simply estimates—but the researchers argue that they’re an appealing alternative to the costly, time-consuming prospect of collecting boots-on-the-ground data. While the scalable model slices through bureaucratic slog, the researchers also acknowledge drawbacks. For instance: The data doesn’t account for differences across ecosystems, such as varied seasons or site-specific pollutants and species.
The researchers conclude that the payoffs of green infrastructure track closely with density—which is to say that megacities have a lot to gain. Endreny and his collaborators call for city stakeholders to build out their tree canopies. The researchers found that the 10 cities had a median tree cover of 611 square kilometers, and a potential additionaltree cover of 455 square kilometers. Endreny and company argue that rededicating parking lots and other available surfaces to trees could nearly double the benefits that the existing leafy residents confer.
Hanging a dollar sign from a tree branch won’t immediately resolve debates about the best ways to marry density and green initiatives—but quantifying the ecological and financial savings that the urban canopy promises could be a crucial step toward megacities that are crowded with both buildings and tree trunks.