What’s a tree worth?

This year for National Tree Week we have a guest blog post from the Treezilla team. Treezilla is a citizen science platform that has been devised by The Open Science Laboratory. Its aim is to map every tree in Britain which will provide valuable information for researchers, conservationists and other organisations.


In 2017, trees in the UK have been in the headlines for good and bad reasons. The recent launch of the Charter for Trees, Woods and People has led to a renewed focus on the importance of trees to the UK. In contrast, stories of felling of well-established and much-loved street trees from the city of Sheffield serve as a reminder of how easy it is for these natural assets to be lost. Both of these events show how trees are valued parts of our environment, not least in towns and cities.

treezilla1Of course the term ‘value’ has several meanings including both ‘importance’ and, more simply, monetary worth. The developments in assessment and valuation of ecosystem services in recent decades have brought a focus on the monetary value of nature. And although these ideas are increasingly embedded in environmental policy, discussing the natural world in economic terms remains controversial among environmentalists and some academics. Perhaps urban trees, which are in some sense both natural and man-made exist in a less controversial middle ground.

The US Forest Service has led the way in evaluating ecosystem service provision from trees. They have developed a suite of tools under the ‘i-tree’ banner which are designed for a range of scenarios, including from ground-based surveys and aerial imagery and in urban areas. Urban trees (see blog post from last year) provide many ecosystem services: carbon sequestration and storage, removal of pollution from the air, reducing flood risk and mitigating the urban heat island effect, amongst others.

treezilla2They are also important in the design and visual identity of urban areas and have strong cultural resonance for many people. Quite how we go about measuring and valuing these services is opaque to most of us. Many tree ecosystem services derive predominantly through the canopy: air pollution reduction, for example, can be modelled as a function of atmospheric pollution and leaf area. Established allometric relationships allow estimation of leaf area, which is difficult to measure, from tree height or trunk circumference, which is more straightforward.

In the US, a tech company Azavea developed some of the i-tree tools into a mapping application called OpenTreeMap for engagement with citizens. Since 2013 the Open University, Forest Research and a social enterprise, Treeconomics, have worked with Azavea to adapt OpenTreeMap to the UK and built Treezilla: the Monster Map of Trees a citizen science project which is attempting to map trees across the UK.

Treezilla allows users to generate estimates of ecosystem services provision of a tree based on a simple measurement of its trunk circumference. The price paid for this simple window into ecosystem services valuation is that the valuations are themselves somewhat approximate. The system also functions as a more general tree database, allowing users to add photos, record condition and pests and diseases.

Beyond the slightly silly but memorable name, Treezilla has some serious purposes. We know very little about our urban trees: how many there are and which species are planted where, so planning for their future is problematic. Many organisations (mostly local authorities) hold information on urban trees but most have no way of making that information public. Others, including ordinary citizens, may lack the tools to record and monitor their own trees.

This is at a time where, perhaps because the costs of maintaining trees are more apparent than their benefits, our urban trees are under increasing threat. Treezilla aims to raise the profile of urban trees, allow ordinary citizens to engage in ecosystem services assessments and to give people and organisations the tools to monitor and protect their trees.

The latest phase of the project, funded by the OU and a NERC Green Infrastructure Innovation grant has refined the environmental basis for ecosystem services calculations and updated the valuations based on current understanding and the latest UK treasury figures. We have also ramped up the size of the tree database and our ambitions for its future through engaging with partners (The Tree Council, Natural Resources Wales, The Parks Trust and the National Federation of Womens Institutes in Wales), a large number of people, and several organisations which hold large tree databases.

At the time of writing we are approaching 750,000 trees, hoping for 1 million by the end of 2017 and aiming for 5 million by 2019. This is still a fraction of the urban trees in the UK, but would probably be the largest public database of urban trees anywhere.

Anyone can contribute to the Monster Map of Trees, so this National Tree Week, why not grab a tape measure, head out onto your street, park or campus and map the trees around you? You’ll learn something new about them and help build our understanding of the variety, distribution and value of urban trees across the UK.

Phil Wheeler, The Open University, UK


Read all about the UK’s largest tree celebration; National Tree Week

You can also catch up on our National Tree Week blog posts from last year, including a special Journal of Ecology Virtual Issue:

National Tree Week 2016 – Virtual Issue

Right Tree, Right Place: Sustaining Urban Trees Today for the Forest Cities of Tomorrow

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