A simple test of lichenometric dating using bidecadal growth of Rhizocarpon geographicum agg. and structure-from-motion photogrammetry

摘要

Digital photogrammetry was used to measure growth rates and track changes at the margins of marked Rhizocarpon geographicum age. thalli on quartzite at the Illecillewaet Glacier in western Canada. The decadal rates were used to validate a lichenometric "growth curve" calibrated using the five largest thalli on historically and tree-ring dated surfaces. Macro-photographs of lichens were oithocorrected by 'Structure from Motion' (SfM) techniques to create a 20 yr image timeline. Model resolution was high (0.01 to 0.03 mm pix), photo alignment was accurate (mean relative error 1.4%) and measurements were validated. Mean areal growth in 59 thalli was highly variable ((x)over bar = 9.43 mm(2) yr(-1) for 1996-2006; and 7.72 mm(2) yr(-1) for 2006-2016). Tiny thalli (5 mm long axis) had jagged outlines and slow irregular growth (0 to 0.2 mm(2) yr(-1)). Long axis growth in the five fastest of 45 thalli (5 mm long axis) with circular outlines was 0.5 to 0.92 mm yr(-1) and mean annual radial growth on four radii per thallus was 0.23 to 031 mm yr(-1). Several [bath kept a nearly circular shape even after fusing with others. Extrapolated mean radial growth of the fastest thallus in either decade dosely predicted the largest and five thallus sizes on historically dated surfaces (-5.43 mm difference at 53 yr, 1.75 mm at 75 to 101 yr). This is an error of about one year at 75 to 101 yr. The findings validate indirectly calibrated rates and surface ages at this site and challenge key assumptions and claims made by proponents of lichenometric dating. (C) 2021 Elsevier B.V. All rights reserved.