How many leaves on a tree?

To learn more about how trees can mitigate climate change, scientists need to know their leaf area. So how do you work that out?

- AISLING IRWIN

THE EARLY MORNING sun filtered through the trees onto parkland in Milton Keynes, where a group of people were sitting chatting on the grass. But this was no summer picnic - the revellers were volunteers, and they were hard at work.

Their task? To remove every last leaf from every last branch of a freshly felled London plane tree, and to do so as quickly as possible - all in the name of science.

These days we have satellites, drones and laser-scanning technology to unravel the complexities of forests, such as how dense they are and how much carbon they store. But there is one question scientists have not managed to answer, particularly for the trees in our towns and cities: how many leaves do they have? Or, more accurately, what is their total leaf area?

This is what those people in the park were helping to find out, in a research project led by scientist Kate Hand. "Leaf area is a really important metric when it comes to estimating the benefits of urban trees," says Kate. "Trees are an essential part of urban nature - if you live in an area that doesn't have trees, you really notice the difference."

We know only too well the value of the 'urban forest'. Aside from making us feel good and there's plenty of evidence linking greenery with mental health and wellbeing - trees perform a number of vital ecological services.