Dear Readers, I might have mentioned that my Open University course includes a number of projects on ecosystem services this year – an estimation of the amount of money that having trees saves us in terms of carbon captured, flooding ameliorated and air quality improved. So my first task has been to measure some trees. By looking at the circumference of the trunk plus the species, it’s possible to estimate the size of the canopy and the overall bulk of the tree.

So, off we go with a notebook and a tape measure into the wild streets of East Finchley, First up we measure ten trees in the County Roads – these are rows of Victorian terraces, narrow residential streets with a lot of character. The trees are a fine  mixture of species – crab apples and cherries predominate, but there are also hawthorns and rowans, and more recent trees such as crape myrtle, shadberry and hibiscus.  However, these are also small trees, with a maximum circumference of 101 cms. In total, they store about 1500 kgs of carbon dioxide, and remove about 190 grams of particulates from the air every year.

A Rowan tree on the County Roads

But then today my husband and I measured some of the street trees along the High Road, and enormous bruisers they are. Mostly these are London Plane trees, with some large lime trees further north, and a lone ginkgo tree.

Well, some of these trees were so enormous that I couldn’t get my tape measure round them, with a maximum circumference of 332 centimetres. When I plug in the calculations for these ten trees, it turns out that they’ve sequestered a whopping 12,000 kilograms of carbon dioxide so far.

Converted to monetary value, the trees on the County Roads have saved us £94 so far, while the ones on the High Road have saved us £757.

Now, if I had to choose between the trees on the High Road and the trees on the County Roads with my accountant’s hat on, it’s clear that the High Road trees would win – they sequester more carbon, and also alleviate more flooding and improve air quality to a greater degree. However, the calculations don’t include the costs incurred by such large trees – they need to be pollarded regularly, the London Planes are a major cause of hay fever, and their roots may sometimes impinge on buildings and sewers.

What also isn’t taken into account is the biodiversity value of the trees involved. London Plane trees provide nesting and roosting sites for birds, but that is about it. Crab apples, cherries and rowans provide food for a wide range of insects, their fruit and blossom is eaten by birds, and all in all they provide a much wider range of habitats and feeding opportunities than the trees on the High Road.

In the hypothetical scenario that we were presented with, it wasn’t possible to save some High Road trees, and some County Roads trees – it was all or nothing. What a conundrum! I love the way the temperature drops when you walk under the London Plane trees on your way home from the station on a hot summer day. I love the way that the starlings have chosen one of the Plane trees to roost in, and the way that they swirl around it before they settle down. But I also love the audible hum of bees in the cherry blossom, and the way that the parakeets munch on the crab apples. In terms of saving the planet from a climate change point of view, I should probably save the High Road trees, but that can feel as if it’s about just saving ourselves, rather than saving the whole range of living things that we share the planet with.

So, I shall crunch some more numbers and knock up a few graphs and see if there’s any way of coming to an answer that I’m comfortable with. If nothing else, it gives some insight into the difficult decisions that will have to be made going forward, though hopefully not too many of them will actually involve cutting down trees. If this exercise has shown anything, it’s that we need all the trees we can get.