This study examines water-formed saxicolous lichen trimlines on bedrock outcrops in the Peace-Athabasca Delta, northern Alberta, Canada. We determined the lichen species growing on rock above and below lichen trimlines; described the trimlines with regard to height above water, location, elevation above sea level, and physical factors; and dated the trimlines through lichenometry. Trimlines were found at open, semi-restricted, and perched basins at a mean elevation of 210.85 m asl; 14 of 16 trimline elevations were between 210.26 and 210.92. Mean trimline height above current water was 155 cm (range 40 to 230 cm). The all-lichen species diameter growth rate mean was 0.416 mm yr−1. Physcia caesia grew at 0.331 mm yr−1 and Xanthoria elegans grew at 0.441 mm yr−1. Many of the trimlines may have established after drawdown of high water that existed during the mid- to late-1970's. Based on cemetery data, establishment lag time is about six to 10 years. Most trimlines date from the period 1976 to 1990. The early 1980's appears to be a key time of basin drawdown and trimline establishment. Site and species influences on establishment lag and growth rates, and the complex hydrology, may be reflected in the range of trimline dates. The mean 95% confidence interval range in establishment dates was 6.2 years. In a hydrologically complex ecosystem such as a delta, lichenometric dating may prove a useful tool to reconstruct water level changes.
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Vol. 107 • No. 4