How Do Tree Rings Record the Past? A Calendar Written in Wood

Growth happens in the cambium, a thin layer of dividing cells just beneath the bark, which produces new wood inward and new bark outward. In spring, when water is plentiful and growth is rapid, the tree produces earlywood with large thin-walled cells, which is light in colour and low in density. Later in the season growth slows and latewood forms, with smaller thick-walled cells that are darker and denser. The abrupt boundary between the latewood of one year and the earlywood of the next is what makes a ring visible. That requires a seasonal climate, which is why the technique works well in temperate and boreal regions and poorly in the wet tropics, where growth can be continuous and rings may be absent, indistinct or produced more than once a year. Complications exist even in good conditions: a severe drought or defoliation can produce a false ring within one season, and an extremely bad year can produce a missing ring that is absent entirely on part of the trunk.

The technique that makes the whole field possible is crossdating, and it is what separates dendrochronology from simply counting rings. Because climate varies regionally, all trees in an area share a pattern of relatively good and bad years, so the sequence of ring widths is a recognisable signature rather than a random series. Matching that signature between samples allows several things. It identifies false and missing rings, since a sample that will not match without an adjustment reveals its own error. It extends a chronology backwards by overlapping a living tree with older dead wood, then that wood with still older timber, building a continuous sequence far beyond the life of any individual tree. Chronologies built this way now extend continuously for many thousands of years in oak and pine in Europe and in bristlecone pine in North America. And it dates any piece of wood whose rings match the reference chronology, to the exact year of the outermost ring present.

The applications are wider than climate reconstruction and several are unexpected:

• •Archaeological and architectural dating, which can establish the felling year of the timbers in a roof, a ship or a panel painting, and has redated a great many buildings
• •Calibrating radiocarbon dating, since rings of known calendar age supply samples with known dates, which is how the radiocarbon calibration curve was built and why radiocarbon results can be converted to calendar years at all
• •Climate reconstruction, since ring width, density and isotopic composition record temperature, rainfall or both depending on what limits growth at that site
• •Detecting past events, since fire scars, insect outbreaks, floods, landslides and earthquakes leave recognisable marks or growth disturbances that can be dated to the year
• •Identifying volcanic eruptions through frost rings and abrupt growth suppression, which has helped date eruptions with no written record
• •Provenance, since matching timber to a regional chronology indicates where it grew, which has been used in art history and in tracing illegal logging

The method has constraints that its precision can obscure. It dates the year a ring formed and not the year an object was made, so a beam can only be dated to its felling year if the bark edge survives, and reused timber dates the tree rather than the building. Only one variable usually limits growth at a given site, so a chronology records temperature at a treeline and rainfall in a dry lowland, and interpreting the wrong signal produces confident nonsense. Trees respond to age and to competition as well as to climate, so the growth trend must be removed statistically, and the way that is done affects how much long-term variation survives, which was at the centre of a prolonged public argument about a prominent temperature reconstruction. There is also the divergence problem, an observation that in several northern regions ring widths since the mid twentieth century have tracked measured temperature less closely than before, whose cause is disputed and which matters because it questions the assumption that the relationship is stable over time.

A tree in a seasonal climate adds one ring a year, with light earlywood and dark dense latewood making the boundary visible, and the width of each ring records that season's conditions. Crossdating matches the pattern of good and bad years between samples, which catches false and missing rings and chains living trees to ever older timber to build chronologies spanning millennia. The technique dates buildings and ships to the year, calibrates radiocarbon dating and reconstructs past climate, though only one variable usually limits growth at a site.