What do trees mean to you? A home for the birds that pass through your garden? Clean air? A shady spot to sit on a hot day in the park? A helping hand in the fight against climate change?
Trees are all these things and more. But unlike a new road or housing estate, it’s hard to put a proper value on everything they do, so they often lose the battle for space that goes on in many towns and cities.
Calculating the value of trees
The i-Tree Leeds project brings together scientists from the University of Leeds and members of the public to put a value on the city's trees. We aim to increase awareness of the benefits that trees provide, protect existing street trees and encourage more planting.
We're undertaking a tree survey to collect data that will help us to calculate a value for some of the many important things they do. We can quantify the amount of carbon that is taken in by the trees and stored in their branches, trunks and roots as they grow. We can work out how much pollution the trees are likely to remove from the air every year - gases like nitrogen dioxide. And we can estimate the impact of a particular tree on local flood risk.
This year we are focusing on the Middleton & Belle Isle parts of the city. The skills our volunteers learn here can be applied in other parts of the city and across the UK. We would love the project to encompass the whole of Leeds. Want to get involved? Sign up to help with surveying for the i-Tree Leeds project.
What does a survey session involve?
First we need to identify the tree species: does it have broad, flat leaves, or needle-like leaves? Are the leaves arranged in opposite pairs or alternately on the shoot? Do the leaves have smooth or jagged edges? Working out the correct identity is important because this will determine the expected characteristics of the tree, such as the density of the wood, the size of the leaves, and the shape of the crown.
We then need to know how big the tree is. Three main factors are important here: trunk width, overall height and the size of the crown. The width of the trunk and the height of the tree allow us to calculate how much carbon we would expect the tree to store, and how much carbon dioxide it’s removing from the air each year.
Using the data
After our work in the field is done, we take the data back to our lab for analysis. For this we use some innovative computer software called i-Tree that calculates the benefits provided by our urban trees. Once we have calculated these benefits, we will share a report with our project partners, Leeds City Council, which can use the information to defend existing trees and to assign funds and space to the planting of new trees.
Last year, we discovered that trees on the University of Leeds campus store more than 300 tonnes of carbon, with an economic value of over £70,000. Their big leafy canopies are also removing at least 250kg of air pollution per year, equivalent to the vehicle emissions generated by driving 500,000 miles! Imagine what a city would be like without its trees.