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

• Published in: Journal of Quaternary Science
• Main Authors: Bradwell, Tom, Armstrong, Richard A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2006
• Subjects: Iceland
• Online Access: http://dx.doi.org/10.1002/jqs.1058; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fjqs.1058; https://onlinelibrary.wiley.com/doi/pdf/10.1002/jqs.1058

Abstract 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. © British Geological Survey/Natural Environment Research Council copyright 2006. Reproduced with the permission of BGS/NERC. Published by John Wiley & Sons, Ltd.