Lichenometric dating

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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.

The farther from the middle part of the zone, the less precise the dating is.

Another problem, causing dating errors, is how to determine growth curves for the averaged maximum diameter thalli.

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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.

We use size-frequency distributions of lichens growing on well-dated surfaces to fit demographic models for , two species commonly used for lichenometry.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.OSBORN, Gerald D., Geoscience, University of Calgary, Calgary, AB T2N 1N4, Canada, [email protected] MCCARTHY, Daniel, Dept. Catharines, ON L2S3A1, Canada 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.

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