Frequency distributions of Rhizocarpon geographicum sl, modeling, and climate variation in Trollaskagi, Northern Iceland

Seven stable moraine surfaces in Trollaskagi, northern Iceland, were sampled to produce frequency distributions of the sizes of up to 1000 Rhizocarpon geographicum s.l. thalli at each site. All frequency distributions showed a similar form, with patterns of disruption at the same points in the curves. In order to examine the possible cause(s) of this disruption, the structure of the observed distributions was compared with randomly generated distributions of lichen sizes. In the absence of snowpack data, temperature observations for the last 115 yr were used to produce duplicated model runs simulating the potential effects of snowkill on the lichen communities, producing results in close agreement with the observed data. It is concluded that in this area of northern Iceland, lichen growth was disrupted at four periods over the last 120 yr, and that at these periods possibly 80 to 100% of lichen thalli may have been lost. This has implications for lichenometric dating of moraines in the region, but as yet it is not possible to determine whether disruptions were solely due to climate, or to a combination of factors, including competition.