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

Growth of crustose lichens : a review

Crustose species are the slowest growing of all lichens. Their slow growth and longevity, especially of the yellow-green Rhizocarpon group, has made them important for surface-exposure dating (lichenometry). This review considers various aspects of the growth of crustose lichens revealed by direct measurement including: 1) early growth and development; 2) radial growth rates (RGR, mm yr−1); 3) the growth rate–size curve; and 4) the influence of environmental factors. Many crustose species comprise discrete areolae that contain the algal partner growing on the surface of a non-lichenized fungal hypothallus. Recent data suggest that 'primary' areolae may develop from free-living algal cells on the substratum while 'secondary' areolae develop from zoospores produced within the thallus. In more extreme environments, the RGR of crustose species may be exceptionally slow but considerably faster rates of growth have been recorded under more favourable conditions. The growth curves of crustose lichens with a marginal hypothallus may differ from the 'asymptotic' type of curve recorded in foliose and placodioid species; the latter are characterized by a phase of increasing RGR to a maximum and may be followed by a phase of decreasing growth. The decline in RGR in larger thalli may be attributable to a reduction in the efficiency of translocation of carbohydrate to the thallus margin or to an increased allocation of carbon to support mature 'reproductive' areolae. Crustose species have a low RGR accompanied by a low demand for nutrients and an increased allocation of carbon for stress resistance; therefore enabling colonization of more extreme environments

Dismantling the treasured flagship lichen Sticta fuliginosa (Peltigerales) into four species in Western Europe

In the framework of a worldwide project on the phylogeny of the lichen genus Sticta, dedicated sampling was performed in four regions of Western Europe, roughly along an East-West line between N 48°02’ E 07°01’ and N 52°01’ W 09°30’, ranging from France/Vosges to Ireland/Kerry. Five clearly distinct ITS haplotypes were detected for isidia-producing species where only two were expected. Subtle anatomical and morphological characters, together with a strongly supported 4-loci molecular phylogeny, permit to distinguish, besides the easily recognized S. canariensis and S. limbata: • the two « well-known » S. fuliginosa and S. sylvatica whose type collections have been carefully reassessed; the former is widespread in both hemispheres, while the latter is correctly identified only from continental Europe and the Andes in Colombia; the barcode ITS of S. fuliginosa differs by a single substitution from S. limbata (with a single exception), and the 4-loci phylogenetic tree does not resolve them as distinct lineages, most probably highlighting a very recent divergence and incomplete lineage sorting; • three species that were formely included in S. fuliginosa: the resurrected S. ciliata Taylor, belonging to a complex group yet to be disentangled and occurring in the Neotropics, Africa, Macaronesia and Western Europe, and two species described as new for science, S. fuliginoides, found in continental Europe, the Canary Islands, eastern North America and Colombia, and S. atlantica only known from Ireland and the Azores archipelago. Molecular inferences demonstrate active divergence and dispersion within S. ciliata that may require recognition of further species. Fresh material can be identified with a morphological and anatomical preliminary key provided here. We propose that the taxonomy of all lichen species be urgently reviewed in the light of molecular data in an evolutionary context, particularly those used as bioindicators of environmental change and woodland management