MORAINE DEGRADATION PROCESSES AS A PRIMARY CONTROL ON THE SIZE-FREQUENCY DISTRIBUTION OF A LICHEN POPULATION

This study investigates moraine degradation as a primary control on lichenometric dating techniques, with the aim of providing the lichenometrist with a better idea of which portion of a lichen population is likely to provide the most accurate landform construction age. The spatial distribution and maximum diameter of the largest Rhizocarpon sp. lichen growing on each boulder that was resting on the proximal slope of a ca. 135-year-old moraine on Mount Adams, Washington was determined. Using a growth curve developed for the Cascade Range, the age distribution of the lichen population indicates that young boulders dominate the crest and older boulders are located downslope. The distribution of boulder ages, determined using lichenometric techniques, are similar to results obtained from modeling the degradation of a moraine using a diffusion model that integrates boulder movement based on empirical data. The field and model results are significant because they suggest that moraine degradation processes are a primary control on the size-frequency distribution of a lichen population in that they affect the number, orientation, and distribution of boulder surfaces available for lichen colonization, an effect that is enhanced with increasing moraine age. The results of this study indicate that a moraine surface has a continuous range of ages. Therefore, lichenometric dating techniques that are not based on the largest lichen, or a small population of the largest lichens, may result in a landform age that is a fraction of the original construction age. These results are important for any dating technique that assumes a consistant exposure age across a landform that has sustained slope degradation.