LICHENOMETRIC DATING OF ROCK SURFACES IN THE NORTHERN CASCADE RANGE, USA

As part of our ongoing effort to refine the accuracy and expand the period over which lichenometric dating can be used in the northern Cascade Range, this study presents a growth curve developed solely from direct and indirect growth-rate data based on lichens of subgenus Rhizocarpon. All of our data are from study sites on Mounts Baker, Rainier, Adams, and Hood in the northern Cascade Range. Our observations of direct growth rates are based on 31 measurements of 11 lichens growing on different surfaces. This direct growth-rate dataset is complemented by indirect growth rates based on measurements of the largest lichen observed on 20 different surfaces over 26 to 34 year periods. Our primary goal is to evaluate northern-Cascade Rhizocarpon sp. growth rates independent of the potential inaccuracies introduced by surface age calibration. In this study we: 1) compare our growth-rate based curve with calibrated curves for the study region; 2) estimate the variability in mean annual growth rates, a key component of error reporting that should be considered in determining the age of lichens; 3) identify changes in the growth rates of lichens over time; and 4) extend the range of lichenometric data for the study domain to the 17th century, 150 years longer than previous growth curves for this region. Our work adds to the relatively few published studies that evaluate Rhizocarpon sp. lichen growth rates, either in terms of direct measurements of the same lichens or indirect measurements of the largest lichen identified on the same surfaces over time. Our numerical results apply to the northern Cascade Range, however, our evaluation of the accuracy and reliability of direct and indirect growth rates versus calibrated curves will apply to lichenometric studies in other regions.