Estimation of lichen growth rates using archived Google Street View images

Abstract

This paper proposes a practical method for estimating lichen growth rates by comparing field measurements with archived Google Street View (GSV) images, offering a cost-effective alternative to traditional decades-long monitoring. Lichen colony sizes in past GSV images were estimated relative to a stable reference length, such as the distance between a lichen colony and a nearby fixed structure (e.g., a tree knot or a branch fork), ensuring consistent measurements over time. To minimize errors related to tree growth and orientation of trunk face, growth rates were determined based on vertical colony expansion, as this direction is less affected by trunk deformation. The radial growth rate (mm/year ± standard error) was calculated as half of the increase in the total vertical colony size. The estimated growth rates for lichens in Tsukuba City, Japan, were as follows: among foliose lichens, Dirinaria applanata grew at 1.8-2.5 ± 0.2-0.3 mm/year, Parmotrema austrosinense at 3.8 ± 0.1, P. tinctorum at 3.6 ± 0.2, Phaeophyscia spinellosa at 2.1-2.6 ± 0.1-0.4; among crustose lichens, Graphis aperiens at 0.4 ± 0.1, Lecanora cf. argentata at 0.5 ± 0.1, Pertusaria pustulata at 0.3 ± 0.1, and Trypethelium eluteriae at 1.3 ± 0.2. These values are comparable to previously reported growth rates for the same or related species, suggesting that the estimation method provides reasonably accurate results. The method introduced in this study reduces the burden of long-term field surveys and enables large-scale monitoring, making it a valuable approach for studying lichen growth.