A RHIZOCARPON GEOGRAPHICUM GROWTH CURVE FOR THE CASCADE RANGE OF WASHINGTON AND NORTHERN OREGON

Lichen thalli measurements from 22 surfaces of known age on Mounts Baker, Hood, and Rainier are used to construct a regional Rhizocarpon geographicum growth curve that spans 189 years. Growth rates determined using the largest thalli diameters measured on the same surfaces at Mount Rainier in 1976 and 2002 are compared to lichenometric data from Mounts Baker and Hood. Similar lichen thalli diameter vs. age relationships identified in the data from the three mountains suggest the presence of uniform growth rates over the 400 km range. A regional growth curve developed during our study shows three growth phases of successively slower growth from 8 to 20 years, 20 to 145 years, and 145 to 189 years. Uncertainty in lichen growth rates beyond 189 years limits extrapolation past that age; however, the time frame covered by our growth curve is an important period of recent climate variability. When applied in appropriate settings, our growth curve can be used to determine accurate numeric ages (±10 years) for surfaces less than 145 years old in areas where other techniques are not applicable or do not provide unique or well-constrained ages.