Lichenometric dating — the use of lichen thallus diameter to estimate the exposure age of rock surfaces — has long relied on the assumption that Rhizocarpon geographicum grows at a constant radial rate after an initial colonization lag. A 2024 study in northern Iceland challenged this by combining high-resolution photogrammetric measurements of 340 thalli on moraines of known age with growth data from fixed photographic stations monitored since 1964. The results showed that growth rates are not constant but follow a logistic curve, decelerating significantly after thalli reach approximately 60 millimeters in diameter. For surfaces older than roughly 250 years, the standard linear model overestimates age by 15 to 40%.
This matters because lichenometry has been the primary dating tool for Little Ice Age moraines and rockfall deposits in regions where radiocarbon material is absent and cosmogenic nuclide dating is prohibitively expensive. Hundreds of published glacial chronologies in Scandinavia, Patagonia, and New Zealand rest on the linear assumption. If growth is logistic rather than linear, then the apparent synchrony of glacier advances across these regions — a cornerstone of Little Ice Age paleoclimatology — may be partly an artifact of systematic age overestimation at the older end of the calibration range.
The deeper lesson is about the danger of extrapolating biological processes beyond their calibrated range. A lichen growing on a surface exposed fifty years ago provides excellent calibration data. Extending that calibration to surfaces exposed five hundred years ago assumes that the organism's physiology, microenvironment, and competitive landscape remained unchanged across an order of magnitude in time. Every dating method carries hidden assumptions, but biological clocks carry an especially treacherous one — the assumption that life is linear when, in truth, nearly every biological rate eventually bends.