Megacity trees yield mega benefits

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As of 2016, almost 10% of the global population – 722 million people – lived in megacities, defined as cities with more than 10 million residents each. We usually think of such teeming metropolises as purely human-designed environments: all straight, hard edges of glass, steel, and concrete.

But even in megacities, the urban forest provides substantial benefits to people, according to a recent analysis in the journal Ecological Modelling. What’s more, cities could nearly double the value of ecosystem services provided by urban trees, to roughly $1 billion per megacity per year, simply by planting more of them along the edges of sidewalks, plazas, and parking lots.

The study assessed ecosystem services provided by trees in 10 of the world’s 40 megacities: Beijing, China; Buenos Aires, Argentina; Cairo, Egypt; Istanbul, Turkey; London, Great Britain; Los Angeles, United States; Mexico City, Mexico; Moscow, Russia; Mumbai, India; and Tokyo, Japan. The researchers used a set of freely available computer models known as i-Tree tools.

First, they analyzed aerial images from Google Earth to quantify the area of existing tree cover in megacities. Tree canopy coverage ranges from 8.1% in Cairo to 36% in Moscow, with a median value of 20.9% across all 10 megacities. That amounts to 6.1 square meters of tree canopy per person in Cairo and 98.9 square meters per person in Tokyo, with a median of 39 square meters per person across all cities.

Urban trees reduce air pollution, storm water runoff, and energy (and associated carbon emissions) needed to heat and cool buildings, and they also enhance carbon sequestration. A detailed study of London’s urban forest published in 2015 quantified the value of these ecosystem services for that megacity.

In the new study, the researchers used these London data – taking into account the many differences between cities in weather patterns, burden of air pollution, energy use, length of growing season, canopy cover, human population, and city area – to calculate the value of ecosystem services provided by trees in the other megacities.

Overall, the median value of ecosystem services provided by trees is $505 million per megacity per year. That’s $967,000 per square kilometer of tree cover, or $32 per person, and a median value of 0.12% of megacity GDP.

The bulk of this benefit – a median value of $482 million per megacity per year – comes from reduction of air pollution. But other services are also substantial, and could become even more important as climate change proceeds. “Trees have direct and indirect benefits for cooling buildings and reducing human suffering during heat waves,” says lead author Theodore Endreny of the College of Environmental Science and Forestry in Syracuse, New York. “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.”

Endreny and his colleagues also used the aerial photos of cities to identify additional area that is not currently planted with trees but could be – “sidewalks, parking lots, and plaza areas, where the canopy would extend above the human occupied pedestrian or parking area, and the stem would be positioned to allow for pedestrian passage or parking,” the researchers explain. “Planting trees in this potential tree canopy cover area would increase tree cover by 85%, on average.” It would also roughly double the benefits of the urban forest, to a total of $1 billion per megacity per year.

More urban trees would involve some tradeoffs: In certain circumstances trees can increase the concentration of air pollutants in the immediate area, and a high degree of tree cover can increase winter heating costs in cold climates. And of course, planting and maintaining trees requires a fiscal commitment from cities. But the investment is small compared to the reward, since most of the energy needed to expand the urban forest is provided free by the sun.

Source: Endreny T et al.Implementing and managing urban forests: A much needed conservation strategy to increase ecosystem services and urban wellbeing.” Ecological Modelling. 2017.

Image:passagere-DA

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