No surprise, trees are good for urban spaces. US forestry scientist Dr Greg McPherson says the benefits can now be quantified using tools to show the triple bottom line benefits of green spaces, helping draw bigger budgeting. New York, for example, quadrupled its tree management budget when it found every US$1 ($1.50) put in yielded U$5 in returns.

Dr McPherson told Environmental Management News that “hundreds” of cities in the US were using the tools developed by the US Department of Agriculture’s forestry arm in collaboration with organisations such as the Arbor Day Foundation and American Forests. They are being employed to measure the value of their green spaces, with results showing “dramatic effects and benefits” in energy savings, air quality improvement and carbon sequestration.

The most dramatic effect of the quantification tool, he said, was in “changing the perception of local officials and residents” about the role and value of trees, leading to more investment into the management of “urban forests”.

Dr McPherson and researchers have developed three tools backed by 20 years of studies: the i-Tree software suite, which can be used by communities, government agencies and others for urban forestry benefits assessment; CITYgreen software, which analyses “ecosystem services”, tabulating dollar benefits for the services provided by trees and other green spaces such as water and air quality improvements; and a tree carbon calculator that estimates the carbon benefits of an individual tree.

The tools can be adapted for use in Australia. He estimates a two-year research period to gather information on the predominant tree species in the various local climate zones, twinned with sampling and measurements “to better understand how they grow and how big they get and their height and size and width area”, and developing models to see “how the trees perform”. Researchers would also need to collect data on rainfall, temperature, air quality and air pollutant concentrations, all of which help program the tools for local use.

He expects the US team that developed the original software could work with local research communities, the green industry and governments – where funding could come from – who are interested in how to address problems related to climate change and water management, air quality and quality of life.

“[We need to] get it on their radar that while trees are not the only answer, and urban green spaces aren’t a silver bullet, they can be part of the solution where investment will payback many times over,” he said.

“Our urban green space has been considered a frill, it’s been taken for granted,” Dr McPherson said. “We’re trying to equip ourselves to make a better case that it is a necessity, that it is essential to our health and wellbeing.”

He cites the example of New York, where the i-Tree system assessment found “that for every $1 spent in managing street trees, there was $5 benefits from those trees”.

“That, along with some other information … resulted in an increase in their budget from $12 million a year to $50 million a year.”

Scheduled to speak at the Urban GreenScapes Symposium 2009 in Canberra this week, Dr McPherson plans to draw on the example of San Francisco’s use of the tools and the findings regarding carbon sequestration, water and the “urban heat island” effect.

“Carbon sequestration, for example, was 65kg per year for a 20-year-old ash tree in [the city which has] a climate similar to Melbourne,” he said. “But it can vary from 50-100kg per year of CO2 that’s removed from the air, or emissions that are reduced from the power plant because the tree is located in such a way it shades an air-conditioned building and it reduces the need for air-conditioning, it reduces the need for electricity, which reduces the demand for production of electricity and the emission of CO2 at the power plant.”

Trees also lower air temperature – “10% increase in canopy is associated with a one degree centigrade reduction” in temperature through “evapotranspirational cooling [because] like people, trees sweat, and then moisture evaporates and it cools the air”.


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