LICHENOMETRIC DATING: MORE OF AN ART THAN A SCIENCE

The popular technique of estimating ages of deposits from sizes of crustose lichens continues despite past valid criticism, probably due in large measure to the apparent ease of application and general lack of expense, along with the ubitquity of lichens. But there is a startling lack of agreement on range of utility, treatment of error, and methods of measurement, sampling, and data handling. The measurement of any size property of a lichen is problematic, and every search strategy ever proposed has been criticized by others for logical reasons. This mutual criticism suggests there may be no good way to collect and process data. Meanwhile, the basic assumption employed in lichenometry, that the largest lichen on a deposit colonized soon after deposition and survived indefinitely, is untenable. In independent long-term studies of Rhizocarpon populations most thalli died within a few decades, and mean annual death rates (0.3-5%) are only weakly correlated with thallus size. Extrapolation of death rates in these populations suggests life spans of 150-160 years for most thalli. Original colonists may not be long-lived, and the largest lichens on a deposit may not be original colonists. Furthermore, R. geographicum thalli do not exhibit radial growth from a single origin, and thalli often fuse with neighbors at young ages.

Experiments conducted on independently dated deposits generally indicate that numeric lichenometric ages are not reliable. There are a few studies suggesting validity, and indeed there may be cases where lichens and growth curves actually provide realistic numerical ages. But it cannot be foretold which lichen assemblages will provide good ages and which bad ages. The logical conclusion is that no assumption of good ages can be made, and that it is folly to assign numerical ages to a deposit on the basis of lichen sizes.