Trees in megacities are providing services worth an estimated US$505 million (AU$636m) a year per city, according to new research published in journal Ecological Modelling – and there’s plenty more room to scale up green ambitions.

The study found that current services provided by trees made urban environments cleaner, more affordable and more pleasant, with the median value equivalent to US$1.2 million (AU$1.5m) for every square kilometre of tree cover.

Ten megacities (cities with populations of more than 10 million people) were chosen for the project: 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 research team then estimated the benefits of tree cover in terms of reducing air pollution and stormwater runoff, the energy costs associated with heating and cooling buildings, and carbon sequestration.

Pollution control was overwhelmingly the main benefit found, with the present median value from urban trees in all 10 megacities estimated at US$482 million (AU$607m) a year due to reductions in CO, NO2, SO2, PM10 and particularly PM2.5, strongly associated with adverse health effects and mortality.

An additional US$11 million (AU$14m) benefit a year was due to avoided stormwater processing by wastewater facilities. Carbon sequestration was valued at US$8 million (AU$10m), while there was a US$0.5 million (AU$0.6m) benefit due to reduced heating and cooling costs for buildings.

Lead author Dr Theodore Endreny from the College of Environmental Science and Forestry in Syracuse, New York said there were both direct and indirect effects related to cooling the urban environment and reducing human suffering during heatwaves.

“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,” he said.

Co-author Professor Sergio Ulgiati, from the University Parthenope of Naples, Italy, said conducting a large-scale analysis made evident the extent to which nature “supports our individual and community well-being by providing ecosystem services for free”.

“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 wellbeing would also increase,” he said.

Dr Endreny said the value of trees could easily be doubled by planting more trees, with the cities studied having the potential for median tree coverage of 21 per cent to be boosted to 40 per cent, which would increase total median value of trees per city above $US1 billion (AU$1.5b) a year.

“Megacities can increase these benefits on average by 85 per cent,” Dr Endreny 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 wellbeing while providing habitat and resources for other species in the urban area.”

The news supports strategies in place by a number of Australian city councils to increase urban canopy cover.

The City of Melbourne, for example, has an Urban Forest Strategy that aims to increase canopy cover from 22 per cent to 40 per cent by 2040.

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  1. These human health and environmental services provided by city trees are indeed substantial. However, since “no money changes hands”, it is difficult for many people to see the monetary value of these benefits and set aside space for trees in city planning and new developments. As regards air pollution, the deliberate building of multi-storey apartments along busy 4- and 6-lane roadways, without the benefit of a filter belt of trees to adsorb and absorb pollutants, is condemning residents to a lifetime of polluted air. There is a reason that air pollution monitoring equipment is not installed deep within the leafy north shore of Sydney – the trees make sure there is no air pollution to monitor! Take home message – don’t live on a major road, and don’t have your kids in schools on major roads, unless they are well protected by belts of trees.