Growth rates of Rhizocarpon geographicum: a review with new data from Iceland

• Published in: Journal of Quaternary Science
• Main Authors: Bradwell, Tom, Armstrong, R A
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2007
• Subjects: Ecology and Environment; Earth Sciences; Iceland
• Online Access: http://nora.nerc.ac.uk/id/eprint/2361/; https://nora.nerc.ac.uk/id/eprint/2361/1/BRADWELL_ARMSTRONG_2007_nora.pdf; http://www3.interscience.wiley.com/cgi-bin/fulltext/113510816/PDFSTART

This paper reviews evidence from previous growth-rate studies on lichens of the yellow-green species of Subgenus Rhizocarpon - the family most commonly used in lichenometric dating. New data are presented from Rhizocarpon section Rhizocarpon thalli growing on a moraine in southern Iceland over a period of 4.33 yr. Measurements of 38 lichen thalli, between 2001 and 2005, show that diametral growth rate (DGR, mm yr-1) is a function of thallus size. Growth rates increase rapidly in small thalli (<10 mm diameter), remain high (ca. 0.8 mm yr-1) and then decrease gradually in larger thalli (>50 mm diameter). Mean DGR in southern Iceland, between 2001 and 2005, was 0.64 mm yr-1 (SD = 0.24). The resultant growth-rate curve is parabolic and is best described by a third-order polynomial function. The striking similarity between these findings in Iceland and those of Armstrong ([1983]) in Wales implies that the shape of the growth-rate curve may be characteristic of Rhizocarpon geographicum lichens. The difference between the absolute growth rate in southern Iceland and Wales (ca. 66% faster) is probably a function of climate and micro-environment between the two sites. These findings have implications for previous lichenometric-dating studies, namely, that those studies which assume constant lichen growth rates over many decades are probably unreliable.