The case for trees

New York City shows that when making the case for urban trees, the numbers are on your side. Gavin McEwan reports.

Nearly a quarter of New York City's area is already under tree canopy but officials have identified the potential to increase this to 42% - image: FlickR/Elena Gailard
Nearly a quarter of New York City's area is already under tree canopy but officials have identified the potential to increase this to 42% - image: FlickR/Elena Gailard

One of the main messages to come out of last month's Urban Trees Research Conference in Birmingham was the need to make a quantified case to policy makers on the benefits of trees. And few make the case like the world's fourth most populous city, New York.

Formerly a tree officer in London, Matthew Wells is now director of tree preservation at the city's Department of Parks & Recreation, which looks after the city's five million-plus trees across its five boroughs. New York is already a surprisingly green city, with nearly a quarter of its area under tree canopy, compared to Chicago's 11 per cent. But Wells says analysis by his department shows the potential to raise cover to 42 per cent - and has already made the case for this in financial terms.

"Trees can be seen as a luxury item," he says. "I lacked the tools to justify them to homeowners or policy managers. We had to show they're not just an expense - they mitigate several effects." Describing the job of monetising trees as "a necessary evil", he explains: "You have to speak the language of the people who make the decisions. Mayor Bloomberg is a numbers guy and he understands that trees represent a good deal for the city."

Key to the task was Stratum, an application based on the US Forest Service's i-Tree package, which incorporates its Urban Forest Effects analysis model. New York City also happens to be home to an urban field station of the Forest Service. "They come here to carry out research, which helps us," says Wells.

The Stratum application not only recorded tree census data but was able to calculate the value of the tree population. Around 1,000 volunteers devoted 30,000 hours to completing a city-wide tree survey, recording details of nearly 600,000 trees that were entered into the system. The numbers were then crunched and extrapolated, revealing that the trees absorb carbon valued at $780,000 (£480,000) each year, bring energy savings of $11.2m and storm water interception worth $36m, among total benefits amounting to $122m.

"That gave us our single most important tool - a way of justifying expenditure," says Wells. "We know how much we spend on them, which at the time was around $22m. From that we worked out that every dollar spent brought $5 in environmental benefits."

This gave rise to Million Trees NYC, an ambitious plan to plant a million more trees on the city's public and private land over a decade as part of the wider strategic plan. Such a plan was never going to gain universal support, particularly as unlike in the UK the city's policy had so far been to plant in response to resident requests. "But that's more likely in areas that already have trees," says Wells. "We wanted to plant in areas such as east Harlem that had a high population and low tree density."

The Stratum survey revealed that more than one in six street trees was damaging sidewalks. "Residents are responsible for trees and might not want them if they are seen as a liability. You have to consider things like possible impact on utilities before you plant a tree in front of houses. But to some extent we are enforcing trees, and we get letters from homeowners to which our response is: 'Well, you might not want it, but you have got it.'"

The survey also revealed the breakdown of the current tree population. "London plane is our most common but we have very few young specimens," says Wells. He adds that using a city-wide geographical information system "lets you up the stocking level, which is very low in some areas. You can have a tree every 65 feet (20m) - that would mean 220,000 more street trees."

Already, 420,000 trees have been planted - "serious numbers", he says. "We have already stocked four areas to full density." Sheer numbers would not be enough to ensure a successful long-term programme. The department found a historical survival rate of 74.3 per cent across the city, though this varied form borough to borough.

"It was worse in where we wanted to plant - the areas that had lower stocking and worse air," says Wells. "Heavy traffic can reduce the rate to 60 per cent." Tree stewardship massively increased survival rates to 97.8 per cent. "It was something we definitely needed to promote," he adds.

"Generally, we plant trees that are most likely to survive. We have to ask, are they as maintenance-free as they possibly could be? We are trying to make our tree planting as bombproof as possible and that includes planning for pest and disease threats. We already have quarantine zones where we won't plant susceptible species. Emerald ash borer is on our radar."

The department is also looking at asthma rates in different areas and has designated six "trees for public health neighbourhoods" with both lower than average stocking rates and higher than average rates of asthma among young people. "We can't yet conclusively link the two, but it's something we're gathering data on," says Wells.

He adds that the valuation of risk is another developing area. "There isn't that much research on it," he says. "We don't prune as much as we used to, yet they are not falling down on us. You have to be careful you don't over-manage trees."

On the question of transferring to his former stomping ground of London, he says: "Having a single parks department across the five boroughs does make it easier for us. We can collect data and plan across the city so much more easily. Unfortunately, for a city like London, fragmentation is a problem." He urges the London Tree Officers Association to "work on a uniform approach to urban forestry".


The i-Tree system is already used in more than 60 countries but only recently found its first application in the UK, in the municipal area of Torbay in Devon, in a pilot carried out by the Hi-line Consultancy.

According to senior consultant Kenton Rogers: "We aimed to measure the ecosystem services provided by Torbay's trees. They have an important role to play - could we put a value on that? We saw i-Tree as a means to do this."

The area has 818,000 trees, or roughly six trees per person, but this represents a perhaps surprisingly low canopy cover of just under 12 per cent. "A 2005 report showed that urban ecosystems were systematically undervalued," says Rogers. "If you don't value your trees, they will be valued at £0."

The project measured three key variables: total carbon store represented by the trees, valued at £15 per tonne; value of air pollutant absorption compared with cost to remove the same quantity mechanically; and effect on property values based on the Chartered Institute of Surveyors method.

The team sent letters to all owners of private trees, found 250 plots, 241 were measured over 12 days, recording eight variables, which were entered into the system, before being analysed by its developers. "It can take up to six months for the data to be returned from the United States Department of Agriculture," says Rogers.

This yielded three headline results. The sequestered carbon was measured at 98,000 tonnes, valued at £1.47m. The structural value of trees was valued at £280m, or around £340 per tree. And the removal of 50 tonnes of pollutants was valued at £281,000, "which seemed not very high", Rogers adds.

He concludes: "These provide baseline figures, which can now form the basis of things such as cost-benefit analyses."


- Twenty-four per cent urban tree canopy

- London plane the most common species

- Estimated $2.3bn value

- Absorb nearly 40,000 tonnes of carbon each year, worth $750,000

- Energy saving of $11.2m

- Storm water interception worth $36m

- Repay planting and maintenance cost five times over

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