Everybody, who once hiked over carpets of lichens in northern boreal latitudes has experienced the enormous biomass produced by the genus Cladonia, and in particular the species representing the reindeer lichens. In these latitudes, Cladonia also displays its full phenotypic potential, which is sometimes difficult to capture at the species level. This may now be easier using the fifth volume of the Nordic Lichen Flora which covers Cladoniaceae, including the genera Cladonia, Pilophorus and Pycnothelia. Few people have studied this group as intensively as Cladonia experts Teuvo Ahti and Soili Stenroos, who edited this monograph together with Roland Moberg. Exactly 100 species are recognized in this volume: Cladonia with 95 species, Pilophorus with 4 species and the monotypic genus Pycnothelia. After a brief introduction and description of the family, the text continues with a description of Cladonia and a key to the species. The authors follow a narrow and phenotypic concept of species in the notoriously difficult genus Cladonia. For example, for slight phenotypic variation found in the C. chlorophaea complex, several species are segregated, but even so, variation of chemical patterns can still be recognized within the segregates, for example, C. novochlorophaea. So far it is not clear whether future molecular data will help resolving species in this case or many others, which remain as a challenge for systematics. Wherever molecular data are available, these are discussed in the light of phenotypic variation, but in many cases, molecular data offer limited resolution or show incongruency across molecular markers (which may result from incomplete lineage sorting and/or hybridization). This monograph is certainly one of the finest examples of phenotypic classification in lichens these days, which is a consequence of the life-long experience of the authors. However, phenotypic plasticity and homoplasy (‘convergent evolution’) may blur the delimitation of Cladonia species as entities, and both hybridization and the presence of multiple lineages has been suspected in various species complexes of this genus. Therefore the ‘user’ of this book is often tempted and may wish to consider a species group as a first unit of recognition (and evolution?), and then try to improve taxonomic accuracy, using detailed description of the phenotypic and ecological correlations. The wealth of such information makes the book a valuable help both for beginners and experts in lichenology. Together with the morphological descriptions there is comprehensive survey of secondary chemistry (its patterns and its variations) and habitat preferences. In the notes to each species, the authors provide additional data. For example, they explain their concept of C. coniocraea, which is otherwise difficult to distinguish from C. ochrochlora, or the difficulties in the delimitation of C. arbuscula. In other instances, I wished the notes to the taxa would be more extensive. It would be interesting, for example, to know more about the intriguing segregation in substratum choice of C. parasitica, which correlates with geography. The authors also emphasize that many nomenclatural issues still need to be solved in Cladonia (including the nomenclatural conservation of C. chlorophaea). All species are represented by colour photographs which are also available on the attached CD. The images vary in their photographic quality, partly due to the different original sources (including some pressed herbarium specimens on white background). Nevertheless, the pictures represent the characters of the species, as recognized by the authors. Black and white distribution maps are given for all accepted taxa to show their presence in provinces of Northern European countries.
Vernacular names as well as information about red list status are listed for the respective countries. Notably, many species are red-listed for Denmark in the present work, which may result from loss of habitats or interpretation of threat categories. After the shorter accounts on Pilophorus (citable as an individual contribution by T. Ahti and S. Stenroos) and Pycnothelia, four taxonomic novelties are included in the appendix. An index to the names concludes the monograph. As a comprehensive and updated source of information on Cladoniaceae in Northern Europe, this book is of prime value for lichenogists and conservation biologists. It should not be missed on anyone's bookshelf who wants to recognize and know Cladoniaceae better.
