A tree is just a tree... or is it?

2019 was the year that trees became political. In response to increased concern around climate breakdown, and amidst the backdrop of a General Election, all of the main UK political parties pledged to increase tree cover. But what's the connection between trees and climate? Scientist Mat Disney explains.
  Published:  19 Dec 2019    |      2 minute read

A tree is both fantastically simple and yet stunningly complex: an amazing machine that turns sunlight, carbon dioxide (CO2) and water into shelter, food, shade, living space, a thriving community hub. All that and the potential to help address the climate emergency: a large English oak can weigh 30 tonnes or more, have half a million leaves and absorb many 100s of kg of CO2 during its life, as well as sustaining dozens of other species.

3D laser scan of a pair of 200 year old oaks in Hampstead, North London. These trees survived the expansion of the city around them and illustrate how resilient trees can be if given a chance. Animation by Dr Phil Wilkes and Professor Mat Disney, University College London.

One of London’s iconic plane trees may reach the same size in half the time, and help keep cool the increasingly hot summer streets, but needs careful nurturing to survive the pressures of city life. Now think what millions of these trees can do; and then double that.

The UK has some amazing trees, including in our cities, and yet at 11% tree cover we lag behind many comparable countries in Europe, to take just one example. Our lack of tree cover is a product of many things – high population density of course, but also how we manage and value trees more generally. Historically, woodlands have been seen as a source to fuel industrial growth and build military and naval power. The challenges we face now are different, but no less urgent– drastically reducing our carbon emissions, building greener and more liveable urban spaces and maintaining and expanding the green spaces that support biodiversity, revitalise landscapes and improve our wellbeing. This requires making trees central to how we shape our future.

Part of this is understanding the real value of trees. Even working out how much CO2 a tree absorbs is not straightforward – we need to "weigh" the tree somehow, but without cutting it down. New measurements from ground, air and space are helping to do just that, and show that different species do this at very different rates, depending on local conditions.

A laser helps "weigh" the two oak trees in Hampstead, North London.
A laser helps "weigh" the two oak trees in Hampstead, North London.
Credit: Mat Disney

Even a single species will grow very differently depending on where it is planted. Recent research suggested that planting vast numbers of trees could absorb nearly all man-made carbon emissions. The problem is that planting inappropriate species, or in unsuitable areas, can do more harm than good to soil and water cycles. Trees can certainly help, but they are far from a panacea and this also underplays their other more immediate benefits.

So we need to think about what trees we plant where, and why. We also need to consider the value of trees collectively. A large tree in an urban back garden provides vital food and shelter for a multitude of other species. But if that tree is part of a larger patchwork of connected tree canopies, its value becomes far greater.

Doubling tree cover in the UK is absolutely possible and a great way to achieve so many positive outcomes. But this will need new approaches to how we value trees and woodlands, and using this knowledge to re-shape local and national politics to fund and implement this properly.

So a tree isn’t just a tree – the right tree in the right place is an incredible, beautiful and vital part of our lives; more of them benefits us all.