Just as they do in forests and other natural ecosystems, trees deliver a variety of ecosystem services in cities. They sequester carbon and reduce air pollution and stormwater runoff, for instance.
Researchers looked at 10 megacities on five continents that lie in five different biome types: Beijing, China; Buenos Aires, Argentina; Cairo, Egypt; Istanbul, Turkey; London, UK; Los Angeles, United States; Mexico City, Mexico; Moscow, Russia; Mumbai, India; and Tokyo, Japan.
They determined that trees provide an average of $505 million in benefits to each megacity every year, or about $1.2 million per square kilometer of trees.
Most people probably don’t think of megacities — urban areas with 10 million or more residents — as being part of nature and hence in need of conservation efforts. New research shows that this perception is not just erroneous but counter-productive, however, as trees actually provide ecosystem services worth millions of dollars to the megacities of Earth every year and have the potential to be doing even more.
Just as they do in forests and other natural ecosystems, trees deliver a variety of ecosystem services in cities. They sequester carbon and reduce air pollution and stormwater runoff, for instance. They cut winter winds and help cool their surroundings in summer via shade and transpiration, as well, leading to avoided greenhouse gas emissions and lower energy bills for owners of homes and buildings.
“Trees have direct and indirect benefits for cooling buildings and reducing human suffering during heat waves,” Theodore Endreny of the College of Environmental Science and Forestry in Syracuse, New York, the lead author of a study published this month in the journal Ecological Modelling, said in a statement. “The direct benefit is shade which keeps the urban area cooler, the indirect benefit is transpiration of stormwater which turns hot air into cooler air.”
In order to determine how much these services are worth to megacities, Endreny and team looked at 10 megacities on five continents that lie in five different biome types: Beijing, China; Buenos Aires, Argentina; Cairo, Egypt; Istanbul, Turkey; London, UK; Los Angeles, United States; Mexico City, Mexico; Moscow, Russia; Mumbai, India; and Tokyo, Japan.
The researchers used a tool called i-Tree Canopy to survey each megacity and estimate their levels of tree cover, which ranged from 8.1 percent in Cairo to 36 percent in Moscow. On average, about 20 percent of the 10 megacities examined for the study were covered in tree-based ecosystems.
The team also looked at the potential for each city to increase tree cover, and found that an addition 19 percent of the land area of each city would provide suitable terrain, on average.
Using detailed estimates of the magnitude and value of tree-based ecosystem services, Endreny and his colleagues determined that all of those trees provide an average of $505 million in benefits to each megacity every year, or about $1.2 million per square kilometer of trees. That’s $35 in free services provided by trees to each megacity resident.
The bulk of that $505 million annual average value of urban trees, some $482 million, is due to reduction of pollutants like carbon monoxide, nitrogen dioxide, sulfur dioxide, and particulate matter released by the burning of fossil fuels. The stormwater processing that is avoided by wastewater facilities thanks to trees is worth another $11 million per year, while carbon sequestration is worth $8 million and energy savings are worth $0.5 million, the researchers found
Endreny suggests that these benefits could be nearly doubled if those cities were to make use of additional land areas that are suitable for planting more trees.
“Megacities can increase these benefits on average by 85 percent,” he said. “If trees were to be established throughout their potential cover area, they would serve to filter air and water pollutants and reduce building energy use, and improve human well-being while providing habitat and resources for other species in the urban area.”
Endreny and his colleagues note that, as of 2016, there were 40 megacities with a combined 722 million residents, which is nearly 10 percent of the total human population, all of whom “would benefit from nature conservation plans where they work and live.”
The researchers write in the study that a shift in how we think about trees’ place in urban landscapes may be necessary to unlock the full potential benefits of urban trees, however: “The most common mind set separates cities from the rest of nature, as if they were not special kinds of natural habitats. Instead, awareness that urban systems are also nature and do host biodiversity and ecosystem services opportunities, should push urban people towards increased urban forest conservation and implementation strategies.”
Sergio Ulgiati, a professor at the University Parthenope (UP) of Naples, Italy and a co-author of the study, said in a statement that, in order to follow up on the findings of the present research, UP had created an Urban Wellbeing Laboratory to be jointly run by researchers and local stakeholders.
“A deeper awareness of the economic value of free services provided by nature may increase our willingness to invest efforts and resources into natural capital conservation and correct exploitation, so that societal wealth, economic stability and well-being would also increase,” Ulgiati said.
- Endreny, T., Santagata, R., Perna, A., De Stefano, C., Rallo, R. F., & Ulgiati, S. (2017). Implementing and managing urban forests: A much needed conservation strategy to increase ecosystem services and urban wellbeing. Ecological Modelling, 360, 328-335. doi:10.1016/j.ecolmodel.2017.07.016
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