Measurement of growth in the lichenRhizocarpon geographicumusing a new photographic technique

Abstract

AbstractAdobe® Photoshop® CS3 Extended software and a photographic time series were used to generate accurate and precise measures of change in the area, perimeter and diameter ofRhizocarponthalli at one, three and seven year intervals. Systematic measurements at a fixed grid of eight diameters per thallus showed a rapid and highly variable diametric growth phase in the smallest thalli (<5 mm2) and slower diametric growth (<0·01 mm2yr–1) in larger thalli (5–500 mm2). When standardized to an annual rate, the areal growth trend was similar, regardless of the number of years studied. This suggests that the areal and diametric growth of small and mid-sizedRhizocarponthalli may be insensitive to annual climatic variation and can be accurately characterized by repeat measurements taken over months rather than decades. Unlike diametric growth rate, change in thallus area and perimeter are statistically robust measures of growth inRhizocarponthalli. Our mean measurement accuracy was 99%. Measurement precision (reproducibility) was >95% (P>0·05) for thallus area and >96% for thallus perimeter. Our technique is tedious, but on flat rocks it can resolve and accurately measure change in thallus morphology at the sub-millimeter scale, and it can be used with recent and/or historical images.