Big Tree Measure 2025: Milestones and Magic

A milestone.

A significant achievement, an important event.

Originating from stones placed alongside ancient Roman roads to mark the distance travelled, the word comes from the Latin mille passus, meaning 'a thousand paces'.

For those able to join us for Big Tree Measure 2025, your feet may feel like you walked much further than a thousand paces. Measuring 6,400 trees in nine days is an epic achievement. An achievement that marks a significant milestone in the journey.

Five years of citizen science, measuring 6,400 trees every year. That’s 32,000 trees with multiple data points. 6,400 bar codes installed and now checked every year by volunteers. Over the nine days of the tree measure event, there are approximately 150 volunteer days contributed each year. The statistics are impressive, but 2025 is a significant milestone that is about much more than measuring the trees. It is about collaboration and community.

That is the magic.

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Rain or shine, the day begins, the kettle is boiled, and the flapjacks are ready. People gather over conversations about trees, science, climate, enhanced rock weathering, soil, drones and more before sitting down to learn about the Glandwr Forest Carbon Study and this year’s tree measurement protocol.

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And then we put on wellies and head out to measure the trees. The Glandwr Forest Carbon Study is extensive, with 25,600 trees spanning 11.5 hectares or 18 football pitches. The study examines two types of forestry, conifer and broadleaf, and two nature-based treatments, enhanced rock weathering and soil microbiome inoculation, studied separately and in combination. Each of the eight cells, plus a grassland control, is replicated across eight blocks in the study for statistical robustness. If you are heading out to measure trees in Block H, you will be nearing a thousand paces.

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Out in the forest, everything is quieter as teams of three find their rhythm.There are pauses to marvel at toads, spiders and the redpoll overhead. So many people with so much expertise. Over nine days, rain or shine, citizen science volunteers measure the trees.This year’s measure started with epic rain and ended with glorious sunshine. Whether you joined the Big Tree Measure for the first time this year or whether you join us every year, thank you.

None of this would be possible without a dedicated team of supervisors to answer questions, offer coaching, and help teams spot the tree’s height. With many trees now over 5 metres, there were plenty of call-outs of ‘a little bit higher’! Thank you, Paul, Ric, Phil, and Sonia.

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Over 150 volunteer days were contributed by individuals and organisations this year. We are incredibly grateful to SAP UK, Office of the Future Generations Commissioner for Wales, Heidelberg Materials, Waring Ecology Lab from ImperialCollege London, Tubex Tree Shelters, and Natural Resources Wales for giving your team members volunteer time to spend at Glandwr Forest. And a big thank you to the Department of Geography at Swansea University and Black Mountains College for bringing your students to gain valuable fieldwork experience.

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When the Glandwr Forest Carbon Study started in 2021, we had a broken sun umbrella, a picnic table, and a crazy idea for a citizen science program to measure the trees and take care of the project on the ground. The trees are now taller, and the facilities have improved, but the community is just as special. Thank you to those who live nearby, those who joined us while on their holiday (again), and those who travel from far away. A special shout-out to four students from Imperial College London, who heard about the project and, a few days later, took the train to Wales for a weekend of measuring trees and making a direct contribution to environmental science.

And that brings us back to milestones and collaboration.

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“The Glandwr Forest Carbon Study has been a genuine collaboration between the scientists working on the project, The Carbon Community, and the citizen science volunteers. 2025 marks the fifth year of the project, and a significant scientific milestone. At last, enough time has elapsed for us to rigorously quantify changes in carbon held belowground - in the roots and in the soil. When we combine these data with the tree measurements you have collected, we will obtain the first holistic picture of how our treatments are impacting ecosystem carbon capture.”

Bonnie Waring, Associate Professor of Ecosystems Ecology, Imperial College London

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2025 is a milestone for the Big Tree Measure.

2025 is a milestone for the Glandwr Forest Carbon Study.

From all of us at The Carbon Community, thank you to everyone for your hard work and encouragement. You are the magic.

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