Using earthquakes to assess lichen growth rates

Abstract

Botanists make yearly measurements of lichen sizes that describe highly variable radial expansion of young, and old, hizocarpon subgenus hizocarpon that is a function of thallus size and age. Such non‐uniform growth would negate use of lichens to date geomorphic events, such as landslides and moraines, of the past 1000 years. Fortunately, many crustose lichens tend toward circular shapes, which can be achieved only when overall uniform radial growth prevails.

Largest lichen measurements on rockfall blocks that accumulate incrementally as hillslope talus in earthquake‐prone alifornia plot as distinct peaks in frequency distributions. Rockfall surface‐exposure times are known to the day for historical earthquakes and to the year where mass movements damage trees. Lichenometry consistently dates regionally synchronous rockfall events with an accuracy and precision of ±5 years. Only historical records and tree‐ring dating of earthquakes are better.

The four crustose lichens used here have constant long‐term growth rates, ranging from 9.5 to 23.1 mm per century. Growth rates do not vary with altitude or climate in a 900 km long mountainous study region in alifornia, . Linear growth regressions, when projected to the present, constrain estimates of colonization time and possible styles of initial lichen growth.

Key words:

• lichenology
• lichenometry
• landslides
• California
• New Zealand

Acknowledgments

Much of the data used here were collected with the assistance of able students, geology majors who learned exacting lichen selection and measurement procedures. They all had keen questions about landscape evolution and dating of earthquakes. Their significant input improved the efficiency and thoroughness of fieldwork. Nathan Hendler and Patrick Wood worked with me in 2013. Matthew Morriss assisted in 2011 and 2012, and reviewed an early manuscript draft. The completeness and coherence of this paper were much improved by the review process. I especially appreciate the technical insight and candid suggestions made by Stefan Winkler and an unnamed reviewer.

Additional information

Notes on contributors

William B. Bull

William B. Bull, Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, USA

E‐mail: [email protected]