A Rhizocarpon geographicum growth curve for the Cascade Range of Washington and northern Oregon, USA

Abstract

Lichen thallus measurements from 22 surfaces of known age on Mount Baker, Mount Hood, and Mount Rainier are used to construct a regional Rhizocarpon geographicum growth curve for the Cascade Range of Washington and northern Oregon. Growth rates determined by measuring the largest thallus diameters on the same surfaces at Mount Rainier in 1976 and 2002 are used for comparison with lichenometric data from Mount Baker and Mount Hood. Similar lichen thallus 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: a rapid phase from 8 to 20 yr, a linear phase from 20 to 145 yr, and a slow phase of unknown duration beyond ca. 145 yr. Uncertainty in lichen growth rates beyond 145 yr limits projection of the curve beyond that age; however, the age range of the constrained growth curve covers an important period of recent climate variability. When applied in appropriate settings, our growth curve can be used to determine numeric ages to ±10 yr for surfaces between 20 and 145 years old in areas where other techniques are not applicable or do not provide unique or well-constrained ages.

Rhizocarpon calibration curve for the Aoraki/Mount Cook area of New Zealand

“…Simple comparison shows that most prior reports have nearly similar slopes, but that the growth rate changes. The data reported here fall well in the middle of the reported data from the Cascade Range (Porter, 1981;O'Neal and Shoenenberger, 2003) summary, and above the rates reported in Alaska or Lapland (Denton and Karlen, 1973) (Fig. 9).…”

“…If natural lichen communities obey similar growth patterns as Armstrong (1983) observed, a simple exponential curve may be a better representation than two straight lines. Figure 10 Comparison of largest lichen vs. age from the Aoraki/Mount Cook area with those derived from Southern Alaska and Lapland (Denton and Karlen, 1973) and the Cascades (O'Neal and Shoenenberger, 2003). Note that in the original work, Denton and Karlen (1973) considered the control points as limiting values.…”

Lichen Growth and Lichenometry

“…Hence, colonisation was followed by an exponential phase of growth for approximately the first 20 years succeeded by a phase of slower but more linear growth. The most similar type of growth rate-size curve of R. geographicum, provided by traditional lichenometry, to that obtained by direct measurement has been reported by O'Neal and Schoenenberger (2003). They revisited surfaces in the Cascade Range, USA originally measured by Porter (1981) and found that the growth curve was curvilinear with phases of increasing, constant and then decreasing RaGR, very similar in shape to the curve in Fig.…”

CalibratedRhizocarponspp. Growth Curve for the Mount Waddington Area, British Columbia Coast Mountains, Canada

“…Growth of the Rhizocarpon type (Fig. 1B) begins slowly, accelerates, but slows before it becomes linear (Beschel, 1950;Benedict, 1967;Miller and Andrews, 1972;Denton and Karlén, 1973;Calkin and Ellis, 1980;Gellatly, 1982;Rodbell, 1992;O'Neal and Schoenenberger, 2003;Larocque and Smith, 2004). One possible explanation for the difference in shape is that lichen growth is inherently sigmoidal-as it is for many plants, reptiles, birds, and mammals (Zullinger et al, 1984)-but that the basic S-shaped pattern is interrupted at different stages in its development by a linear growth phase (Benedict, 1985).…”