Lichenometric dating (lichenometry) and the biology of the lichen genus rhizocarpon:challenges and future directions

Lichenometric dating (lichenometry) involves the use of lichen measurements to estimate the age of exposure of various substrata. Because of low radial growth rates and considerable longevity, species of the crustose lichen genus Rhizocarpon have been the most useful in lichenometry. The primary assumption of lichenometry is that colonization, growth and mortality of Rhizocarpon are similar on surfaces of known and unknown age so that the largest thalli present on the respective faces are of comparable age. This review describes the current state of knowledge regarding the biology of Rhizocarpon and considers two main questions: (1) to what extent does existing knowledge support this assumption; and (2) what further biological observations would be useful both to test its validity and to improve the accuracy of lichenometric dates? A review of the Rhizocarpon literature identified gaps in knowledge regarding early development, the growth rate/size curve, mortality, regeneration, competitive effects, colonization, and succession on rock surfaces. The data suggest that these processes may not be comparable on different rock surfaces, especially in regions where growth rates and thallus turnover are high. In addition, several variables could differ between rock surfaces and influence maximum thallus size, including rate and timing of colonization, radial growth rates, environmental differences, thallus fusion, allelopathy, thallus mortality, colonization and competition. Comparative measurements of these variables on surfaces of known and unknown age may help to determine whether the basic assumptions of lichenometry are valid. Ultimately, it may be possible to take these differences into account when interpreting estimated dates

The rocky intertidal biotopes of Helgoland: present and past

Abstract: Nineteen of the 57 littoral rocky shore biotopes and 4 of the 26 variants as well as 1 sublittoral fringe biotope classified for Britain and Ireland have been recorded on Helgoland in this baseline study. Most of the wave-cut platform is at sublittoral and lower littoral levels and most high eulittoral biotopes are confined to narrow zones on seawalls. Large areas of gently sloping lower eulittoral and also boulder areas adjacent to seawalls are characterised by a mixture of two or more biotopes. Only 4 of the 23 littoral biotopes and variants are characterised by faunal species. Comparison with past descriptions of intertidal communities suggests continuing presence of most of the autochthonous biotopes over the past 80 years but also change due to the invasion of the macroalgae Mastocarpus stellatus and Sargassum muticum over the past 20 years. One previously recorded cave biotope and a sublittoral seagrass site have become extinct due to habitat loss while other biotopes probably have extended their range due to habitat increase. The presence of 4 intertidal biotopes (20%) considered rare or scarce in Britain supports the recognition of Helgoland as a site of special conservation in a regional and international context