Lichenometry is one of a number of chronological tools used to estimate Holocene timescales accurately. It is particularly useful in arctic-alpine environments above the tree line (Armstrong, 2004). Surprisingly, however, little work has previously been carried out to determine the actual growth rates of lichens, most researchers relying on indirect methods.

In this study the sizes of 2797 Rhizocarpon subgenus lichens have been measured for up to 25 years on 18 glacier forelands (Figure 1) in southern Norway. Section Rhizocarpon and section Alpicola were distinguished where possible. Rhizocarpon sp. was measured primarily because it has been the most successful taxonomic group used in previous lichenometric dating studies, both in the region and worldwide. This study comprises the most definitive set of data of spatial and temporal variation in lichen growth rates. The purpose of this study is to explore the potential of this unique data set to contribute to the understanding of both lichen ecology and direct lichenometric dating.

The measurement programme began in 1981 at 19 sites established on the oldest ‘Little Ice Age’ moraines under investigation (c. AD1750). Additional measurement sites were initiated in 1987 at 28 sites on younger moraines. Subsequently, measurements were undertaken annually at sites on six glacier forelands (three each in Jotunheimen and Jostedalsbreen). These were Styggedalsbreen, Bøverbreen, Storbreen, Austerdalsbreen, Nigardsbreen and Fåbergstølsbreen. The remaining sites were measured every five years. Measurements were made using Mitutoyo dial callipers, with a precision of ±0.05 mm.

In addition to physiological factors the growth rate of lichens is controlled by many environmental variables (Benedict, 1990), but there has been little research into the importance of these factors, despite criticism (Jochimsen, 1973). Climate has a major influence in the growth rate. A prime characteristic being the availability of moisture: the more moisture at their disposal, generally the faster lichens will grow (Beschel, 1973).

Aspects of the environment measured at each site included distance to the glacier; position on the proximal (inside) or distal (outside) slope of the moraine; proximity to moisture sources; an index of snow cover i.e. duration of snow lie derived from the ratio of snow-sensitive black chionophobous lichens to less snow-sensitive Rhizocarpon lichens; vegetation type; lithology, surface roughness of the substrate, crystal size and presence or absence of mineral banding; and geomorphological disturbances e.g. buried by river gravel, slope failure. In addition, characteristics of individual lichens that were recorded included substrate aspect and slope, height above the ground and the influence of competition with other lichens.

Analysis has revealed high levels of variability in lichen growth rates not only between forelands and sites but also between neighbouring lichens. The critical importance of moisture is apparent at all scales. Thus, maritime forelands at low altitude and sites with moist habitats and snow cover of intermediate depth and duration, exhibit the highest growth rates. Preliminary analysis of ten of the forelands has revealed a mean annual diameter growth rate range of 0.43 to 0.72 mm/yr for AD1750 moraines and a range of 0.63 to 0.87 mm/yr for AD1930 moraines. Despite the overlap of these ranges, growth rate on the younger moraine is consistently higher than on the older moraine. The effects of lichen size/age on growth and death rates, and competition between lichens, have also been investigated. These results have implications for the development of lichen growth rate curves, lichenometric dating and the understanding of lichen ecology.