Lichenometric Studies in the Cascade Range of Washington: Establishment of Rhizocarpon Geographicum Growth Curves at Mount Rainier

Abstract

Measurements of Rhizocarpon geographicum thalli on rock surfaces of known age at Mount Rainier National Park permit growth curves to be constructed for andesitic and granitic substrates spanning the last 125 yr. Control points for the younger part of each curve come mainly from man-made structures, whereas older control points are from rock outcrops in the foreland of Nisqually Glacier, an area which as experienced progressive deglaciation since the early 19th century. The growth curves indicate an initial period of very rapid growth until thalli reach about 10 mm diameter, after which growth proceeds at an exponentially decreasing rate for the next century. Growth rates are slightly faster for andesitic substrates than for granite. If extrapolated, the curves should be useful for dating substrates as much as 3 to 4 centuries old with an accuracy of ± 20 yr or better. However, the curves are applicable primarily between altitudes of about 1000 and 1700 m; below this zone growth rates are higher, whereas above it lichenometry may be of limited value because of widespread lichen kill during the early 19th century along the crest of the Cascade Range.

Comparative data from other Northern Hemisphere sites suggest that (1) R. geographicum growth curves are broadly similar in shape and show an exponential decrease in growth rate during the first century or two after an initial “great period” that typically lasts from about 20 to 80 yr, and (2) the lichen factor (diameter in millimeters of century-old thalli) is related in a general way to climatic environment.