Reading the Diary of a Tree

Cut a tree trunk across and you will see a pattern most children learn to recognise: a series of concentric rings, alternately pale and dark, spreading outward from the centre. In temperate regions each ring generally represents a single year of growth, so that counting them reveals the tree's age. That much is common knowledge. What is less widely appreciated is how much more these rings can reveal, and how an entire scientific discipline, dendrochronology, has grown from them. The principle is simple. In spring and early summer, when water and sunlight are plentiful, a tree produces large, light-coloured wood; later in the year, growth slows and the new wood becomes denser and darker. A wet, mild year produces a broad ring, whereas a drought leaves a narrow one. The result is a kind of diary in which each year is recorded in the trunk, not in words but in width and density. The breakthrough that turned this curiosity into a method came early in the twentieth century, when the American astronomer A. E. Douglass noticed that trees of the same species in the same region displayed similar sequences of thick and thin rings. By matching, or "cross-dating", overlapping patterns from living trees, from old timber and from ancient beams, researchers can extend a chronology backwards far beyond the lifespan of any single tree. In some regions, such chronologies now stretch back many thousands of years, and teams working with timbers from Anatolia and the Aegean have assembled sequences that allow archaeologists to date ancient buildings with remarkable precision. The applications reach well beyond archaeology. Because a ring reflects the conditions of its year, tree-ring records are used to reconstruct past climates, including droughts and cold spells that predate any written measurements. Scientists also rely on them to check and refine radiocarbon dating, which would otherwise be less reliable. For historians of art, the technique is equally valuable: panels used for old paintings can sometimes be dated by the rings in the wood on which the work was painted. Naturally, the method has its limits. In tropical forests, where there is little difference between the seasons, trees often fail to form clear annual rings. Occasionally a tree produces two rings in a single year, or none at all, and skilled specialists must use cross-dating to detect such irregularities. Nevertheless, as a patient, low-cost source of evidence, the tree remains hard to beat. Every trunk is a record that was never intended for anyone to read, and which therefore, unlike many human documents, has no reason to flatter the past.