Blog do AXEL GRAEL: Quanto vale e arborização urbana das regiões metropolitanas?

Pedestrians walk past illuminated trees on a rainy night in Tokyo. Thomas Peter/Reuters

How Much Are Trees Worth to Megacities?

Leafy infrastructure saves bustling metropolises about $505 million each year, according to new research.

Jessica Leigh Hester

In a metropolis teeming with shuffling crowds, cranes and high-rises shouldn’t be the only things reaching skywards. Megacities—those urban centers crammed with more than 10 million people—would be well served to double down on their arboreal assets, according to a new paper in the upcoming issue of the journal Ecological Modeling.

A team of researchers led by Theodore Endreny of SUNY’s College of Environmental Studies and Forestry sought to quantify how leafy infrastructure pays dividends in 10 chock-full cities—and the extent to which the benefits could compound if those areas went greener.

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).

In order to assume a more global perspective, the researchers spent several months crunching the numbers for London to arrive at estimates of trees’ contributions to air pollution removed, stormwater runoff avoided, energy left unguzzled, and carbon sequestered. Researchers then scaled these estimates and units for the various cities.

For each location, the researchers surveyed 500 randomly selected points, and characterized them according to their potential for leafy benefits. Each plot was categorized as existing tree cover, potential canopy—including spots currently serving as parking lots, plazas, or sidewalks—or surfaces that didn’t lend themselves to trees.

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.

Across the 10 megacities in question, the researchers estimated an annual median payoff of $505 million, including:

$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 additional tree 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.

Going greener is sometimes easier said than done. Areas that are strapped for space may wrestle with questions about the best use for each square foot. But branch-and-bud infrastructure doesn’t have to come at the expense of other projects, Endreny says—trees could cluster in pedestrian plazas, or flank sidewalks.

Moreover, past projects have demonstrated that even within a city that has sprouted green initiatives, copses are sparser in some neighborhoods than others, meaning that residents don’t draw equal benefits. To maximize the benefits of a robust urban canopy, “cities will need participation along the lines of public-private partnerships and citizen advocacy, as well as innovations from city planners, environmental engineers, and landscape architects as they redouble their efforts to increase the extent and health of the urban forest,” Endreny says. Still, even if various actors share a common vision, it takes a while for maximum benefits to take root. Endreny adds that trees’ benefits are proportional to their size, leaf area, and length of the growing season, meaning that saplings—while cheaper to plant and crucial to ensuring “future generations of trees”—aren’t quite the powerhouses that more mature trees are.

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.