The biology behind lichenometric dating curves

Abstract

Lichenometry is used to date late-Holocene terminal moraines that record glacier fluctuations. Traditionally, it relies upon dating curves that relate diameters of the largest lichens in a population to surface ages. Although widely used, the technique remains controversial, in part because lichen biology is poorly understood. We use size-frequency distributions of lichens growing on well-dated surfaces to fit demographic models for Rhizocarpon geographicum and Pseudophebe pubescens, two species commonly used for lichenometry. We show that both species suffer from substantial mortality of 2-3% per year, and grow slowest when young-trends that explain a long-standing contradiction between the literatures of lichenometry and lichen biology. Lichenometrists interpret the shape of typical dating curves to indicate a period of rapid juvenile "great growth," contrary to the growth patterns expected by biologists. With a simulation, we show how the "great growth" pattern can be explained by mortality alone, which ensures that early colonists are rarely found on the oldest surfaces. The consistency of our model predictions with biological theory and observations, and with dozens of lichenometric calibration curves from around the world, suggests opportunities to assess quantitatively the accuracy and utility of this common dating technique.

Lichen Growth and Lichenometry

“…In addition, small thalli may merge early in life to establish larger thalli than would be expected by their initial growth rates. There may also be substantial mortality of young thalli (Loso and Doak 2006) resulting in relatively few survivors of the first colonists on a mature rock surface. Therefore, studies of very early growth processes and mortality may enable a more accurate estimate of the time lag to be made and improve the accuracy of dating curves.…”

The biology of the crustose lichen Rhizocarpon geographicum

“…All frequency distributions were similar, with a pattern of disruption at the same point in the curve, growth being disrupted at four points over the first 120 years with 80-100% of thalli being lost. Using size frequency data, Loso and Doak (2006) found rates of mortality in R. geographicum of between 2 and 3% of thalli per year, and postulated that mortality may explain the apparent contradictions between the results of lichenometry and lichen biology. Hence, the 'great period' of growth could be explained, in part, by mortality which ensures that early colonists are rarely found on the oldest surfaces.…”

“…In addition, very small thalli may merge early in life to establish larger colonies than would be suggested by their initial growth rates. There may also be substantial mortality of young thalli (Loso and Doak 2006) such that there are few survivors of the early colonists on a mature rock surface. Therefore, studies of early growth processes and mortality may enable a more accurate estimate of the time-lag and improve the accuracy of dating curves.…”

Lichenometric constraints on the age of the Huashan Grottoes, East China

“…Therefore, the largest (presumably the oldest) lichen could provide a minimum estimate of the exposure age of the sediment or rock surface on which they are colonizing, once its growth rate is known adequately (Loso and Doak, 2006). Owing to the slow (normally millimeters per decade) and nearly constant growth rate of lichen thalli, lichenometry has become a reliable technique for dating the exposure age of geomorphic landforms during the very recent geological time (Beschel, 1973;Innes, 1985).…”