Daniel P. McCarthy

Estimating Ecesis for Tree-Ring Dating of Moraines: A Comparative Study from the Canadian Cordillera

Studies from glacier forefields in the Canadian Cordillera report that conifers ecize in ca. 5 to 60 yr. Comparative studies of four methods of ecesis determination in two glacier forefields in British Columbia indicate that the traditional approach of sampling trees on small dated surfaces may produce ecesis estimates that are too large. Results from sampling on transects running orthogonal to moraines, interpolating surface age between fixed points based on historical photographs, provided improved estimates of ecesis by enlarging the area and range of environments sampled. Although both the traditional and transect techniques may give comparable results in well-vegetated forefields, the transect technique is recommended as a better estimate of ecesis in sparsely vegetated terrain.

SAMPLING HEIGHT-AGE ERROR CORRECTION FOR SPRUCE SEEDLINGS IN GLACIAL FOREFIELDS, CANADIAN CORDILLERA

Sampling height-age errors in tree-ring dating are estimated by extrapolation of mean apical growth rates for 188 Engelmann spruce growing in four glacier forefields in the southern Canadian Cordillera. Average correction factors obtained are 1 to 4 yr for sampling heights of 10 cm and 6 to 19 yr for sampling heights of 50 cm. These data suggest sampling height is a relatively minor source of error in the determination of tree age in glacier forefields if the trees are sampled within 10 cm of the root crown.

Snow-avalanche impact pools in the Canadian Rocky Mountains

Snow-avalanche impact pits (Corner, 1980) are large circular to semicircular erosional hollows located at the foot of long, steep avalanche slopes (e.g., Fitzharris and Owens, 1984; Nyberg, 1985). They are especially conspicuous where they intercept the local water table and are transformed into water-filled pools (e.g., Davis, 1962; Liest0l, 1974). Most avalanche pit/pool sites are bordered by arcuate debris accumulations up to several meters in height on the downpath side (e.g., Schytt, 1965; Corner, 1980). These deposits consist of rock, soil, and organic debris that is almost certainly the product of avalanche-induced transportation (Peev, 1966; Ballantyne, 1989; Luckman et al., 1994) although some authors have suggested a protalus (Fitzharris and Owens, 1984) or even meteoritic origin (Corer, 1975, 1980). Avalanche impact landforms have received scant attention in the North American cordillera (Luckman, 1977), and no detailed studies have been reported from Canada. This paper describes the morphology and recent history of three avalanche impact pools in the Canadian Rocky Mountains. Observations of the geomorphic effects of avalanche impact are presented for each site and the origin of these features discussed with respect to the magnitude/frequency of the events responsible for their formation.

Estimating Lichenometric Ages by Direct and Indirect Measurement of Radial Growth: A Case Study of Rhizocarpon agg. at the Illecillewaet Glacier, British Columbia

Abstract Data for Rhizocarpon agg. thalli at the Illecillewaet Glacier in British Columbia were used to see whether a single growth curve could provide accurate age estimates on either side of the Continental Divide, to determine whether modern and historical growth rates are similar, and to test a published model that estimates lichen age from short-term radial growth rates. A lichen growth curve was developed for Rhizocarpon agg. using thallus-size data from 14 tree-ring and historically dated substrates. Comparison of this curve with one developed on similar materials 250 km north of this location found similar growth rates for the first 150 yr but slightly faster growth over the next 150 yr at the Illecillewaet site. Radial growth was also measured annually at an average of five points at 105 Rhizocarpon agg. thalli to see if direct-measurement data could be used to reliably estimate lichenometric ages. Radial growth from 1996–2000 ranged between 0.262 and 0.412 mm yr−1and showed large variation within and between thalli. Mathematical analysis found that radial growth was not a positive function of the radius, and linear regression incorrectly predicted that growth rates increase with thallus size. Ages estimated by linear extrapolation of the 4-yr mean growth rate were ca. 10 yr less than those estimated by the indirectly calibrated growth curve on surfaces <200 yr old. Progressively less accurate minimum estimates were obtained using linear extrapolation of radial growth rates for old surfaces and thalli >60-mm diameter. These findings lend support to the assumption that modern growth rates of Rhizocarpon lichens averaged over several years can potentially provide close estimates of lichenometric age.

Digital Analysis of Lichen Cover: A Technique for Use in Lichenometry and Lichenology

Image analysis software was used to obtain various measures of thalli in crustose lichen communities. Lichens growing on flat-faced quartzite boulders in two glacier forefields were photographed and the images were transferred to a computer. Barren rock was digitally removed using Adobe Photoshop and Idrisi geographic information system (GIS) software was used to measure thallus size, perimeter, compactness, and total lichenized area. Replicate analyses show that the technique produces reproducible results that allow a user to distinguish very small differences in lichen coverage and obtain other quantitative measures of a lichen population. The technique is suitable for use with smooth, evenly illuminated surfaces where there are clear color differences between the various lichens and the substrate is a uniform color. Excellent results can be obtained using a 35 mm film camera, inexpensive flash units, a film or print scanner and DOS based versions of Idrisi that permit batch processing of images. The technique can be used to examine lichen colonization patterns and explore new approaches in lichenometric dating.

Snow-Avalanche Impact Landforms: A Brief Discussion of Terminology

It is suggested that features produced by the erosional impact of snow avalanches on water bodies be termed snow-avalanche impact landforms and deposits. The specific examples described by Matthews and McCarroll (1994) and Smith et al. (1994) are seen as distinctive end members of the range of depositional landforms produced by these processes.

Measurement of growth in the lichen Rhizocarpon geographicum using a new photographic technique

Adobe® Photoshop® CS3 Extended software and a photographic time series were used to generate accurate and precise measures of change in the area, perimeter and diameter of Rhizocarpon thalli at one, three and seven year intervals. Systematic measurements at a fixed grid of eight diameters per thallus showed a rapid and highly variable diametric growth phase in the smallest thalli ( 2 ) and slower diametric growth ( 2 yr –1 ) in larger thalli (5–500 mm 2 ). When standardized to an annual rate, the areal growth trend was similar, regardless of the number of years studied. This suggests that the areal and diametric growth of small and mid-sized Rhizocarpon thalli may be insensitive to annual climatic variation and can be accurately characterized by repeat measurements taken over months rather than decades. Unlike diametric growth rate, change in thallus area and perimeter are statistically robust measures of growth in Rhizocarpon thalli. Our mean measurement accuracy was 99%. Measurement precision (reproducibility) was >95% ( P >0·05) for thallus area and >96% for thallus perimeter. Our technique is tedious, but on flat rocks it can resolve and accurately measure change in thallus morphology at the sub-millimeter scale, and it can be used with recent and/or historical images.