Quantifying the power of green in cities

By Sheila Moorcroft

Trees, shrubs and green spaces are known to provide significant benefits for city dwellers. That range of benefits is not only growing, but their value in terms of money, lives saved, reduced crime and potentially increased retail expenditure is increasingly quantifiable, and being quantified. The power of green in cities is rising.


City planners will need to engage with citizens, developers, architects to ensure the incorporation of as many different types of greenery in developments. New tools to engage communities in identifying and assessing trees’ impacts will help to reframe a perceived ‘cost’ to a real benefit.

What is changing?

Cities are under pressure. Their populations are growing – not just in mega cities, but also in small and medium sized cities; budgets are under pressure as a result of cuts plus increasing demands; climate change and major weather events are adding to the problems and costs. As a result, budgets for ‘urban forests’ – planting and maintenance of trees – parks and other green places are being cut, and numbers of trees are in decline. A survey across 20 of the largest US cities indicated that 4 million trees were being lost each year; cities in Texas have lost an estimated 5.6 million trees during the recent drought.

The potential significance of that tree loss is reflected in research elsewhere. The arrival of a significant pest which killed an estimated 100 million trees across 15 states in north western America provided an opportunity to establish clear links between human health and tree loss. It identified a clear link between the tree loss and 6.8 additional deaths from respiratory diseases and 16.7 additional deaths from cardiovascular problems per 100,000 adults: a total of 21,000 deaths. Trees are being talked of as part of public health infrastructure; the return on investment clearly measured and quantified.

Air pollution, especially in deep ‘city canyons’ between high rise buildings, can be reduced by as much as 30% by planting trees and shrubs. Trees appear to be particularly effective in removing small – <2.5 micron – particulate matter. Removal rates in one study of 10 US cities varied from 4.7 tonnes in Syracuse to 64.5 tonnes in Atlanta, with annual values also varying significantly from $142,000 per tonne in Atlanta to$1.6 million per tonne in New York. The value of benefits was predominantly related to less illness / fewer deaths. An important tool in quantifying the economic value of trees is iTree, which was originally developed in 2006 and is in its 5th iteration, with new capabilities being added each wave. In Pittsburgh, they established that their street trees – in pavements and on streets only – provided $2.4 million worth of eco-system services – such as shade, reduced temperature, air quality, aesthetics. That represented a return of $3 for every dollar spent on tree planting and maintenance. A later study, including the cities full tree population, valued their services at between $10 and $13 million per year. Then there are the newly emerging benefits: reduced crime and increased shopping. A Philadelphia study has shown that whereas in the past vegetation was presumed to encourage crime, the opposite is in fact true: there were fewer assaults and robberies. The reasons are not yet clear, but it could be that well-kept green spaces increase social interaction, create a sense of ownership, are calming. Research from the University of Washington has highlighted the retail benefits of trees: that interviewees were more likely to travel further, visit more often and be willing to spend more on equivalent goods and services once they arrived in centres which had trees and greenery.

Why is this important?

Never has investing in trees and other forms of greenery been more critical, but potentially under greatest threat. As cities grow, so does their land base – by an estimated 150% by 2030, and what is often prime agricultural land disappears. Or to put in another way, an area three times the size of France is likely to be covered by cities, by 2050.

Communities and city planners, architects and developers are rising to the challenge with innovative solutions. Melbourne has set a target for increasing canopy cover from 22% to 40% by 2040, and within that target improving quality and diversity of plants and species. Green roofs are being incorporated into more and more developments (See Trend Alert). Cities as far apart as Milan, Singapore and Shenzen are experimenting with radical new designs to bring the outside world and nature into high-rise buildings. New York now has the Highline park created along the 1.5 mile raised concrete railroad as was and an entrepreneur in Los Angeles has raised funding to create ‘billboard parks’ by covering billboards with suspended bamboo gardens. A more extreme, and as yet undeveloped, example is going underground. The aim is to create a park underground in an old New York station and that a technology to concentrate and redistribute sunlight may make it possible for plants to grow down there.

New tools and new approaches may yet make our cities more inhabitable as they grow.

– See more at: http://www.shapingtomorrow.com/trends.cfm?output=1&id=22819#sthash.ZJe4s3wL.dpuf


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