Lichenometric dating (lichenometry) and the biology of the lichen genus rhizocarpon: challenges and future directions

Abstract

Lichenometric dating (lichenometry) involves the use of lichen measurements to estimate the age of exposure of various substrata. Because of low radial growth rates and considerable longevity, species of the crustose lichen genus Rhizocarpon have been the most useful in lichenometry. The primary assumption of lichenometry is that colonization, growth and mortality of Rhizocarpon are similar on surfaces of known and unknown age so that the largest thalli present on the respective faces are of comparable age. This review describes the current state of knowledge regarding the biology of Rhizocarpon and considers two main questions: (1) to what extent does existing knowledge support this assumption; and (2) what further biological observations would be useful both to test its validity and to improve the accuracy of lichenometric dates? A review of the Rhizocarpon literature identified gaps in knowledge regarding early development, the growth rate/size curve, mortality, regeneration, competitive effects, colonization, and succession on rock surfaces. The data suggest that these processes may not be comparable on different rock surfaces, especially in regions where growth rates and thallus turnover are high. In addition, several variables could differ between rock surfaces and influence maximum thallus size, including rate and timing of colonization, radial growth rates, environmental differences, thallus fusion, allelopathy, thallus mortality, colonization and competition. Comparative measurements of these variables on surfaces of known and unknown age may help to determine whether the basic assumptions of lichenometry are valid. Ultimately, it may be possible to take these differences into account when interpreting estimated dates.

Growth of Rhizocarpon geographicum in the summit region of Volcan Barú, Panama

“…Other studies with Rhizocarpon species that have included a larger size range of up to 80 mm initial radius (e.g. Iceland (Bradwell & Armstrong 2007); Wales (Armstrong 2016)) consistently report the highest radial increments in thalli of similar size to those of the present study and decreasing Δradius in smaller and larger lichens. Although the present study included only one larger thallus (radius c .…”

“…50 mm), the much lower Δradius of this particular specimen is suggestive of a similar size-related trend. In absolute terms, the maximum growth rates observed in Rhizocarpon thalli at this tropical montane site are considerably higher than those reported for most temperate sites which are typically below 1 mm y −1 (Armstrong 2016). Much higher growth rates have been reported from the tropical lowlands in central Panama for another crustose lichen, Cryptothecia sp.…”

Lichenometry and Schmidt hammer tests in the Kaunertal glacier foreland (Ötztal Alps) during the AMADEE-15 Mars Mission Simulation

“…The logarithmic line was chosen (Fig. 6) because it mirrors the time-dependant decrease in lichen growth rate (Winkler & Shakesby 1995;Sass 2010;Dąbski 2014;Armstrong 2016). For a comparison, we present the lichen growth rates developed by Winkler and Shakesby (1995) who worked in Rofental, a valley immediately east of Kaunertal (Fig.…”

Establishing a multi-proxy approach to alpine blockfield evolution in south-central Norway

“…mortality due to snow, the radial growth of lichens, their growth behavior in general, and the influence of competition (e.g. Osborn et al 2015;Armstrong 2016). We used the successful applied growth Tab.…”