The biology behind lichenometric dating curves |
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Authors: | Michael G Loso Daniel F Doak |
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Institution: | (1) Department of Earth Sciences, University of California, Santa Cruz, CA 95062, USA;(2) Present address: Department of Environmental Science, Alaska Pacific University, Anchorage, AK 99508, USA;(3) Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95062, USA |
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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. |
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Keywords: | Alaska Lichen biology Lichenometry Pseudophebe pubescens Rhizocarpon geographicum |
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