Within‐site variation in lichen growth rates and its implications for direct lichenometry

Abstract

Variation in lichen growth rates poses a significant challenge for the application of direct lichenometry, i.e. the construction of lichen dating curves from direct measurement of growth rates. To examine the magnitude and possible causes of within‐site growth variation, radial growth rates (RaGRs) of thalli of the fast‐growing foliose lichen Melanelia fuliginosa ssp. fuliginosa (Fr. ex Duby) Essl. and the slow‐growing crustose lichen Rhizocarpon geographicum (L.) DC. were studied on two S‐facing slate rock surfaces in north Wales, UK using digital photography and an image analysis system (Image‐J). RaGRs of M. fuliginosa ssp. fuliginosa varied from 0.44 to 2.63 mm yr–1 and R. geographicum from 0.10 to 1.50 mm yr–1.5. Analysis of variance suggested no significant variation in RaGRs with vertical or horizontal location on the rock, thallus diameter, aspect, slope, light intensity, rock porosity, rock surface texture, distance to nearest lichen neighbour or distance to vegetation on the rock surface. The frequency distribution of RaGR did not deviate from a normal distribution. It was concluded that despite considerable growth rate variation in both species studied, growth curves could be constructed with sufficient precision to be useful for direct lichenometry.

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

“…mm yr –1 ). A limitation of RaGR, however, is that it is strongly correlated with thallus size and exhibits a high degree of variability both within and between thalli (Armstrong ). As a consequence, measurement of relative growth rate has been used to compensate for these variations in size, growth being scaled to an existing thallus measure such as increase in area per unit of area in unit time (mm 2 mm –2 , time –1 ) (Armstrong , ).…”

“…), R. geographicum thalli occur exclusively at the top of the face, whereas on surface B, there is a peak of abundance close to the top but then a declining trend of abundance down the face. Furthermore, there may be a gradient of thallus size from top to bottom on some faces, largest thalli occurring at the top, but with little significant difference in measurable RaGR down the face (Armstrong , ). These results support initial colonization of the upper part of the face, possibly from diaspores carried by birds, and subsequent colonization down the rock from propagules carried in surface runoff (Armstrong , ).…”

“…There are four potential methods of estimating the age of the largest thallus: (1) calibrating lichen size against surfaces of known age (‘indirect lichenometry’) (Locke et al. ; Innes , ; Matthews ; Benedict ); (2) establishing a lichen growth rate/size curve from direct measurement of lichen growth (‘direct lichenometry’) (Armstrong , ; Trenbirth and Matthews ); (3) using radiocarbon (C‐14) dating (Maguas and Brugnoli ; Clark et al. ; Bench et al.…”

Glacier change from the early Little Ice Age to 2005 in the Torngat Mountains, northern Labrador, Canada

“…Typically, lichenometric dating of historical moraines uses the indirect approach where lichenometric field data is used in conjunction with known dates on local moraine surfaces to create a lichen size-age calibration curve used for dating (Armstrong, 2015). In contrast, the direct approach to lichenometric dating has been rarely employed owing to the relative lack of long-term measurements of lichen growth rates (Armstrong, 2014). Given the paucity of surfaces with known ages in the Torngat Mountains, reliable calibration curves could not be constructed using the standard indirect technique.…”

“…However, owing to the availability of historical photography of lichens at several growth stations established by McCoy (1983) and Rogerson et al (1986a,b), it was possible to create lichen growth curves using the direct method described in Armstrong (2014Armstrong ( , 2015. The direct method of lichen growth curve construction results in larger uncertainties relative to the indirect method because of the highly variable nature of shortterm growth rates displayed by crustose lichens (Armstrong, 2014;Roof and Werner, 2011). As a result, age ranges that explicitly incorporate lichen growth rate variability are provided rather than individual dates for the initiation of ice recession in the Torngat Mountains.…”