Where Are Britain's Oldest Trees? Notable, Veteran and Churchyard Trees in OpenStreetMap

Britain is unusually rich in ancient and veteran trees, but the fullest inventory of them, the Woodland Trust's Ancient Tree Inventory, is not open for reuse. This note asks what open data can say about where Britain's oldest trees stand. From 70,394 OpenStreetMap trees in the United Kingdom with a name, girth, age or similar tag, I identify 1,626 notable trees and 276 with a sign of great age (a credible girth of 3 m or more, or an ancient, veteran or natural monument tag), and set them beside Wikidata and English Wikipedia. Coverage is driven by mappers: 42% of notable trees lie in 10 of 428 occupied 10 km squares, 40% are unnamed trees of unknown species tagged by a few local mappers, and only 48 of 90 Wikidata remarkable trees are on the map. Where species is known, oaks make up 47% of notable trees and 51% of those with a sign of age. Yews are strongly tied to churches: 18% of 176 mapped yews stand within 50 m of a Christian place of worship, against 1.3% of other trees, and almost all of these are by Anglican, Church in Wales or Catholic churches rather than nonconformist chapels. Trees with a sign of age are over-represented near former royal forests (4.7% of them within 5 km of a Wikidata royal forest point, against 1.3% of the land), but this rests on 13 trees. Mitchell's rule of thumb of 2.5 cm of girth a year gives plausible ages for trees of a few centuries but falls short of the ages claimed for famous ancient yews by a factor of roughly 3 to 12. Open data can locate some of Britain's great trees; they cannot yet map them. The Woodland Trust Ancient Tree Inventory was not used because it is not open for reuse. Tree data contain OpenStreetMap data (© OpenStreetMap contributors, ODbL 1.0). Interactive version: https://tristan-hotham.github.io/ancient-trees/. Data and code: https://github.com/Tristan-Hotham/ancient-trees. Blog post: https://blog.tristanhotham.com/2026/10/28/ancient-trees/.