Habitat selection and ecology of Xanthoria elegans (Link) Th. Fr. in glacier forefields: implications for lichenometry

Habitats occupied by the largest Xanthoria elegans (Link) Th. Fr. thalli at seven glacier forefields in the Canadian Rockies were studied to investigate the lichenometric assumption that large thalli occupy ideal sites for growth. The largest thalli were found on steep or overhanging facets at the base of grey limestone clasts that were embedded in moraines. These thalli were unfragmented, had nearly circular outlines, were bordered by barren rock and had SSE to S orientations. This is consistent with the general expectation that south-facing sites offer high solar input and a long snow-free season. Orientations other than south could result by chance or may reflect the importance of microscale factors (e.g. reflected rather than direct solar input). Closure of X. elegans communities and coalescence of thalli was only found at sites that were naturally fertilized with dung. It is concluded that all clasts do not afford homogeneous or ideal environments for lichen growth and do not have an equal chance of being colonized. This raises doubts concerning the validity of statistical normality assumptions in lichenometry and the use of grids to assess closure in lichen communities.