The i-Tree software project, which helps urban tree managers put a monetary value on their estate based on the range of services it provides, has reached its tenth year. But the project continues to capture a growing range of benefits from trees, extending even to their effect on people's stress levels, its co-developer and US Department of Agriculture Forest Service research leader Dr Dave Nowak told HW.
"We are working on this with Dr William Sullivan of the University of Illinois, looking at it from a city point of view using 3D Lidar data to calculate people's 'green envelope' - how much greenery they see around them," he said. "We then want to tie that to how their bodies react, what the health benefit of that is, whether it's related to people's age, and what the value of that is."
At least with air pollution (HW, 13 May) there is an established link with incidence of asthma or heart attacks, he pointed out. "With stress, is there a physiological response to measure, such as cortisol levels, and if so, what does a rise of x per cent mean? Does it correspond to the incidence of disease? We are building a model of this and waiting for the science to catch up."
Promisingly, a study by Sullivan and colleagues published last month in the journal Environment & Behavior found a "positive, linear association between the density of urban street trees and self-reported stress recovery". The 160 participants in the experiment were first subjected to a standard stress-inducing test, then given an urban scene with varying amounts of tree coverage to view via a 360-degree headset, before having the impact of this on their stress levels assessed by questionnaire.
The next revision of the i-Tree Eco package, version 6, will make it easier for UK users to integrate pollution data into their analyses.
"For the new version we will also look at the age distribution (of people nearby) and calculate the health impact of reduced air pollution, in terms of the number of people not dying, and tying that into the tree population," Nowak said. "Their absorption of PM2.5 particulates (those averaging 2.5(mu)m in diameter) and ozone saves around eight lives a year in Manhattan."
But trees are not a simple panacea he pointed out, as they are less effective at absorbing carbon monoxide. "And at a local scale you can have an inverted effect, as you can change the flow of pollutants, and increase them locally, so design becomes important, and that includes buildings," he said. "A freeway is left open because they want the air to disperse, but in a city you have people and traffic next to each other."
He explained: "i-Tree calculates average effects, so it won't pick up where concentrations are higher or lower, or even local hot or cold spots. Local temperature is another thing we are looking at, but wind is more complicated - do you have to model it in 3D? We are also talking to people in that field."
This offers the prospect of i-Tree helping landscape professionals create smarter designs. "How will you design for 20 or 50 years down the road?" he said. "Can you design better at local level in terms of species or layout, rather than just all trees everywhere? We know the costs better than the benefits, and there are always trade-offs."
On the impact of i-Tree so far, he said: "It's a tool, and how you use it is not something we control." For example the town of Oakville in Ontario, Canada which has been using i-Tree from the start, "has made changes" in response. But he pointed out: "It gives information to make decisions on. Some have been very progressive but with others not much has happened."