Direct growth rates of section rhizocarpon lichens in the Cascade range of Washington and northern Oregon

Abstract

Between 2002 and 2010, diameters of section Rhizocarpon lichens growing on 11 different surfaces on major volcanoes in the northern Cascades range were observed and measured. Growth rates determined from these direct measurements are between 0.3 and 0.7 mm yr-1 and are consistent with a linear rate of growth determined in other regional studies. Data from this study were both added to and compared with a previously constructed growth curve from the same region. The direct lichen growth observations suggest that: 1) a regionally consistent growth rate averages 0.5 mm yr-1 over a broad geographic range, 2) application of the largest lichen technique provides reliable minimum surface ages despite uncertainties in biological or environmental conditions, and 3) assuming the start of a slow growth phase for lichens near 60 mm in diameter yielded grossly overestimated ages for many ice-contact landforms. This latter point suggests reevaluation of the moraine chronologies developed using poorly constructed lichen growth curves.