Trees’ value can now be calculated
Nobody plans to chop down trees in parks and reserves for their timber, but their “ecological services” have a value that’s been hard to determine until now.
The monetary value of Christchurch’s public trees is being calculated for the first time. Nobody plans to chop down trees in parks and reserves for their timber, but their “ecological services” have a value that’s been hard to calculate until now.
Ecological services include such things as carbon sequestration, storm water abatement and pollution reduction.
So the US Forest Service developed a software package – called i-Tree Eco – that puts a dollar figure on what trees do for humans and the environment.
Developers know exactly how much a square metre of land is worth, but what’s the value of the trees on the land, asks Justin Morgenroth, an associate professor in the School of Forestry at the University of Canterbury.
Nobody really knows and the aesthetic appeal of trees hasn’t been enough to save them, in many cases. Assigning them a dollar value might tip the scales towards keeping them rather than chainsawing them, he said.
The city council also spends public money planting and maintaining the urban forest. For the first time, i-Tree will tell us what return comes from that investment.
i-Tree Eco was customised for New Zealand by the US Forest Service with data provided by Morgenroth over a couple of years. He furnished New Zealand tree species, as well as meteorological and pollution data and other variables.
All this cost about $100,000, with Christchurch, Wellington and Auckland councils contributing equally. Calculating the value takes, for the moment, a pair of graduate students. They’ve been measuring tree height, girth, crown diameter, proximity to each other and buildings and other factors.
The data is entered into i-Tree and it models tree functions. Under pollution, for example, the software calculates the amount of ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide absorbed by trees.
i-Tree also calculates how much runoff is avoided when trees intercept and absorb rainwater, and the effects of trees on nearby building energy use caused by shading, and other functions.
And then it assigns a dollar value to these services. Within a few weeks, the students will have collected data from 100 plots in public parks and reserves across Christchurch. Next research season, they’ll collect data on trees in places like Cathedral Square and alongside roads.
Another research project is testing whether this kind of data can be collected for trees on private land using remote sensing called LiDAR. Researchers can’t enter private land to measure trees, so maybe it can be done from above.
i-Tree’s outputs were only estimates built by modelling, and “every model including this one is wrong”, Morgenroth said.
But i-Tree provided the “best guess that we could possibly make” at this time, he said.
The Christchurch City Council was looking to integrate its existing tree data into the i-Tree system, acting head of parks Rupert Bool said.
Now that i-Tree has been customised for New Zealand, it’s available for use by those with the gear to measure trees well.
Later this year, an app version of software – called MyTree – will become available, allowing New Zealand civilians to calculate the functions and value of individual trees on their own property, for example. It will also be free.