This volume 154 is the first comprehensive treatment of the genus Syntrichia in the Holarctic.

Syntrichia (Pottiaceae, Bryophyta) is a genus with a large number of species existing on all continents. Some of the taxa are difficult to distinguish. Many names had to be reclassified taxonomically. The main focus of this work is on the species present in North America as those are often not well understood. Because the Holarctic part of Eurasia is connected to North America, the species of the entire extra-tropical Northern Hemisphere are studied.

About 110 taxa are examined. The studies resulted in 34 species with 59 taxa of Syntrichia and three species which had to be assigned to other genera. Sixteen of the 59 taxa are new to science (five new species, four of them restricted to the Americas, one also existing in Eurasia; four other new taxa are restricted to North America, five to Eurasia, and two are present both in North America and in Eurasia). Three names previously treated as synonyms of other species are recognized as good species (one restricted to North America, two also in Eurasia). Twenty-four other names were designated as new synonyms for other taxa.

In addition to the descriptions of taxa and an identification key, several characteristics (some of them new) are given that may be relevant to a division within the genus Syntrichia.

This work is an important resource for bryologists specifically dealing with species of North America, but also of Europe and Asia. It is a great help for the identification of all taxa of Syntrichia in the Holarctic. It also serves as a basis for molecular-genetic studies on the genus Syntrichia and related genera.

Although Syntrichia Brid. belongs to one of the oldest moss genera, which was distinguished as early as 1801, it did not gain the general acceptance of bryologists for nearly two centuries. Almost until the last two decades of the twentieth century, the species belonging to this genus were placed in the genera Barbula Hedw. or Tor­tula Hedw., within which they were usually classified in separate sections or subgenera. The few bryologists who accepted Syntrichia as a genus in its own right in the nineteenth century were, amongst others, J. W. P. Hübener in his Muscologia germanica of 1833, W. Mitten in Musci Indiae Orientalis from 1859 and J. Juratzka in Die Laubmoosflora von Oesterreich-Ungarn of 1882. It was similar in the last century, when Syntrichia was occasionally recognised as a separate genus, for instance by J. J. Amann in Flore des mousses de la Suisse of 1918, W. Mönkemeyer in Die Laubmoose Europas from 1927 and J. Podpěra in Conspectus muscorum europaeorum from 1954.

The breakthrough in the resurrection of Syntrichia from obsolescence came in the late 1980s and early 1990s with Richard H. Zander (1989, 1993), who redefined this genus and separated it from the broadly conceived genus Tortula. He distinguished it on the basis of a characteristic combination of features, including brick red KOH reaction of the upper laminal cells, the lack of upper marginal border of narrowly elongate cells and the crescent-shaped to semicircular and prominently convex dorsal stereid band of the costa lacking the differentiated dorsal epidermis. So circumscribed, Syntrichia is a cosmopolitan genus, consisting of about 85–90 species. They are known from all continents but are predominantly distributed in temperate and cool to cold regions of both hemispheres, whereas in the tropics they occur mostly on high mountains. The genus has gained much attention in various regional taxonomic treatments and Floras, but no global revision is yet available. Regional studies are certainly very valuable, but they present only fragmentary information about morphological and genetic variability, ecology, physiological reactions and history of the taxa concerned, which are conditioned by local climatic, environmental, geological and historical factors. On the other hand, a global look at some taxonomic features that appear stable in local conditions and may be of high diagnostic value often reveals that they actually represent only a small fraction of their broader variability, thus reducing their value for the circumscription of the species concerned. Hence global taxonomic revisions are of special importance and value, although completing such treatments for large genera is a very time-consuming task.

The present study is not a worldwide taxonomic revision of Syntrichia, but it is a survey of this genus over a vast area of the Holarctic which is covering much of the Northern Hemisphere. Its author has been dealing with this genus for almost half a century, initially treating it as Tortula sect. Rurales De Not. At that time, he published a small paper on species with a bistratose lamina (Kramer 1978) and extensive studies on taxa of this section from the eastern Holarctic, primarily from the Near East and Central Asia (Kramer 1980) and some related taxa from the Southern Hemisphere (Kramer 1988), in which he presented the results of his doctoral thesis. After 35 years, the author of these studies broke his silence by handing over to bryologists the present study on Holarctic taxa with particular emphasis on North America, especially its Pacific part, which he considers to be the centre of the evolution of this genus. As support for his hypothesis, he considers the presence of a number of endemic species and intraspecific taxa, some of which have not been described before. Thus, to some extent this is a complementary work to the author’s former study from 1980, but also containing some information about taxa from South America.

This treatment is a classic taxonomic revision based on extensive research of herbarium materials, although collections from several important North American herbaria, for example ALTA, CAS and UC, appear not to have been studied. The layout and presentation of the data is exactly the same as in the earlier works of this author. In total, he accepts 59 taxa in the study area, including 34 species and 25 subspecies and varieties. Of these, 16 taxa have been described as new to science, including five species (Syntrichia cascadensis, S. coloradensis, S. hermannii, S. novomexicana, and S. ovata), eight subspecies, and three varieties. In addition, the work contains several other taxonomic and nomenclatural novelties, including the reinstatement of three species whose names were previously recognised as synonyms (Syntrichia laeviuscula, S. leptotricha and S. subintermedia) and changes in the taxonomic status of several taxa, as well as the reduction of the names of 24 taxa to synonyms. The arrangement of sp^ecies is alphabetical. For each taxon, the nomenclatural type is quoted, synonyms are given, if they exist, with type citation, morphological description, information on general geographical distribution and the most important iconography. In addition, each taxon is illustrated with line drawings, usually showing the shape of the leaves, transverse sections of the costa and lamina cells, and the shapes of the papillae. These structures for many taxa are also illustrated with colour micrographs from the light microscope, although many are not of good quality and SEM images would certainly be more valuable and useful here. The treatment of each taxon completes the list of specimens examined, although it is nowhere stated whether these are all or only selected specimens that the author studied followed by the taxonomic discussion.

The study ends with a short chapter in which the author discusses the infrageneric classification of Syntri­chia and the problem of the relationships of some species of the genus, as well as their geographical distribution. The summary of the whole is a dichotomous key for the determination of all taxa. It is based mainly on the morphological and anatomical characters of gametophytes, and many couplets are very extensive, which is certainly helpful for users, as it allows them to check the diagnostic value of various traits in the case of critical taxa. As usual, the value and usefulness of the key will be verified by practice.

Kramer’s study is a valuable contribution to the bryological literature concerning the difficult genus Syntrichia which still causes many problems for bryologists. Like any taxonomic revision or monograph, it is a very important tool for identifying species of the genus, which is an important element of the vegetation cover, especially in desert and arid areas. Such studies always contribute to the discovery of more species and better understanding of the biodiversity of poorly explored regions, and many areas of Pacific North America are certainly such. The best example of this is the discovery last year of S. lithophila (Dusén) Ochyra & R. H. Zander in Oregon (Brooks & Jauregui-Lazo 2022), a species previously known only from Antarctica and Patagonia. However, the identity of this species should be thoroughly investigated, also with the use of molecular methods, especially since S. lithophila is perfectly studied genetically (Saługa et al. 2022). Detailed morphological and molecular studies may show that the Oregon specimens represent a separate species, as was the case with Didymodon brachyphyllus (Sull.) R. H. Zander, a Holarctic species reported from Antarctica (Ochyra & Zander 2002), which, however, turned out to be a species different from the Antarctic D. gelidus Cardot (Jiménez & Ochyra 2017; Ronikier et al. 2018). Therefore, Kramer’s work does not require a special recommendation, as it is a summary of all classical taxonomic knowledge about the genus Syntrichia in the Northern Hemisphere. This is very important especially for researchers of the phylogeny of this genus via molecular methods, because they receive a ready classification of taxa in their hands, which they will be able to compare with the classification based on the results of DNA analyses.

Ryszard Ochyra, r.ochyra@botany.pl

Traditionally, mosses have not been considered to undergo adaptive radiations (i.e., high rates of diversification in a particular area). This perception was largely due to their reputation as static evolutionary lines. Their morphology, physiology, and reproduction are widely thought to remain similar to their ancestors, with a limited evolutionary potential compared to the more exciting flowering plants. Laenen et al. (2014) have challenged this traditional view, revealing that bryophytes have experienced several pulses of diversification events since the Cenozoic era. In this context, Syntrichia Bird. in the Holarctic might present cases of explosive diversification, in which lineages diversified into different ecological niches in the Northern Hemisphere, coupled with an unusual increase in morphological disparity. Establishing proper taxonomic foundations, as presented in this book, is the main step to understanding the intricacies of its diversity.

Syntrichia is a clade with an origin in South America (Jauregui-Lazo et al. 2023). Despite its South American origin, this genus has diversified and dispersed widely, with approximately 100 named species found across all continents (Brinda & Atwood 2023). While Syntrichia is common in temperate or cold areas, it also thrives in dryland areas in the Northern Hemisphere. In these areas, it occupies a diverse array of ecological niches, ranging from alpine meadows to biocrusts. Indeed, this genus has emerged as one of the most ecologically dominant groups of mosses across western and northern North America.

Kramer's attention to Syntrichia started in the 1970's and 80's, when the group was treated as Tortula Hedw. sect. Rurales (Kramer 1978, 1980, 1988). His earlier treatments covered most of the species of the Eastern Holarctic and the Southern Hemisphere, in which he included taxonomic and nomenclatural novelties for the southernmost species of Syntrichia. After Zander (1993) clearly segregated the genus from its relative, Tortula,a new reorganization of Syntrichia began. Experts in this group accepted this genus as such and several new regional taxonomic contributions revisited the status of Syntrichia (Gallego 2005; Mishler 2007). More recently, Brinda et al. (2021) presented a new sectional classification. Kramer has now, ~30 years after his last taxonomic treatment, produced a magnificent contribution to Syntrichia taxonomy that complements his earlier works by focusing on the North American taxa.

This book is an extensive taxonomic revision of Syntrichia from the Holarctic with an emphasis on North America. Kramer describes a complex and intricate taxonomy for the group. He introduces 59 taxa further organized into 34 species within the region. Notably, of the 16 taxa new to science, five are recognized at the species level (Syntrichia cascadensis, S. coloradensis, S. hermannii, S. novomexicana and S. ovata Some nomenclatural changes include the transfer of older names from other genera (S. laeviuscula, S. leptotricha, and S. subintermedia) and several new synonyms (24 designated as new synonyms of other taxa). In addition, he proposes transferring three species previously recognized within Syntrichia (S. amphidiacea, S. gemmascens and S. rhizogemmacens) to other genera. For each taxon, the author provides an exhaustive profile that includes nomenclature , taxonomic description, list of specimens examined, and brief distributions. An valuable discussion section is added for the vast majority of the taxa, elucidating historical and contemporary perspectives to distinguish each taxon from its similar counterpart. In addition, the book is supplemented by line drawings, in which the top view of the leaf, costa cross-section, and leaf lamina (i.e., papillae) are depicted. These ilustrations are complemented by light microscope images. Finally, the book concludes with a robust identification key for all the Holarctic taxa, an essential section for those aiming to identify these taxa in the field of laboratory.

The criteria used for the assignment of a group to a particular rank (variety, subspecies, or species) appears somewhat arbitrary. The decision on ranking often depends on the weight a taxonomist gives to the multiple types of evidence (morphological, genetic, ecological, among others). Kramer prioritized grouping by morphological similarity. However, if the author assumes that they are genetically fixed or geographically isolated, why not call them species? It is not clear why some phenetic characters, such as the "tendency of unitrastose lamina" (S. canivernis var. gypsophila) or "the presence of short mucro" (Syntrichia canivervis var. abranchesii) are criteria used for the designation of a variety. It is also not clear why the uncommon presence of vegetative propagules in S. caninervis var. astrakhanica is used for its designation as variety, even when this trait is a rare and definitive character in the context of Syntrichia The fact that many leaf traits in Syntrichia such as the length of the hairpoint, change notably through ontogeny (Mishler 1986), can make traits difficult to interpret from a single snapshot.

The descriptions of new taxa, including the species Syntrichia cascadensis, S. coloradensis, and S. novomexicana, along with three subspecies and two varieties, which are known from limited specimen records, reflect the author's profound expertise and keen taxonomic discernment. The resurrection of some taxa at the species level (i.e., S. laeviuscula), changes to taxonomic rank (i.e., S. ruralis subsp. hirsuta), and proposed synonyms are major contributions for those studying bryology. This rearrangement might capture longstanding gaps of the full diversity of Syntrichia in North America and redefine our identication keys. Nevertheless, significant specimens from major western US herbaria (cas or ucherbaria), which were not covered in this publication, could provide deeper insights into its diversity and distribution, particularly in the western U.S.A. While the proposed taxa appear to be realistic, some components of a comprehensive taxonomic analysis appear to be absent. For instance, illustrations of leaf stance when dry and wet, leaf lamina, leaf border, whole leaf cross-section, as well aspects of the habitat are fundamental when introducing new taxa and are critical for accurate identification of Syntrichia.

The book introduces a few evolutionary concepts, but they appear to be oversimplified. The assertion that serial reduction of cell type represents the sole evolutionary trajectory of Syntrichia (even all Pottiaceae) may not capture the full complexity of evolution. Evolutionary pathways involving the acquisition of particular cell types likely also shaped the diversity of Syntrichia. Inferences about a character evolving in response to environmental pressure (i.e., adaptation) always needs empirical evidence and phylogenetic hypotheses to validate them (see discussion in Mishler 1988). The relationships among taxa stated in the book are largely rooted in morphological resemblance, rather than an explicit phylogenetic framework.

Kramer's comprpehensive revision of Syntrichia which unifies his concepts across the Northern Hemisphere, stands as a remarkable achievement. This book serves as an invaluable asset, not just for bryologists--who have more than once grappled with the challenges of identifying Syntrichia-- but also for those eager to explore the terminology given the array of shapes and structures present. While it is hard to overlook the extensive array of taxa detailed in this book, it is still premature to proclaim a case of accelerated rate of diversification in the Holartic. However, taxonomic descriptions of this quality are the first step towards a full understanding of the genus, good approximation of their diversity, and provide an indispensable framework for those delving into its phylogenetics. I hope to see a South American counterpart in the near future.

Dr. Javier Jauregui Lazo

The genus Syntrichia Brid. was described already in 1801, but for a long time its distinctions from other genera, and first of all from Tortula, were understood in a way different from the current one. For instance, Mönkemeyer (1927) in ‘Die Laubmoose Europas’ accepted Syntrichia including S. subulata Hedw., which was accepted later as a conserved type of the genus Tortula. Currently, this genus includes more than 100 taxa recognized at the species level, distributed throughout the world and playing a significant role in the vegetation of xeric ecosystems, often used as a model object in studies on the physiology of desiccation tolerance.
The modern circumscription of Syntrichia had rapidly spread among the bryologists since the ‘Genera of the Pottiaceae: mosses of harsh environments’ by Zander (1993), however already before that the quite clearly delimited group of species related to S. ruralis was called, either informally as a Tortula ruralis-group, or formally as Tortula sect. Ruralis De Not.
The revision ‘Tortula Hedw. sect. Ruralis De Not. (Pottiaceae, Musci) in der östlischen Holarctis’ has been published by Wolfgang Kramer in 1980, as a volume of Bryophytorum Biblioteca 21, with 165 pages of text and 29 tables of illustrations (20 line drawings, and 9 photograph plates). For many years this publication served as the most comprehensive source of Syntricha taxonomy in many countries of Eurasia. Later on, the treatment for Tortula sect. Ruralis for South America (1988) appeared, and finally in 2023 the taxonomic treatment of Syntrichia for the whole Holarctic was published. It includes 34 species (including 5 species new for science), and many of them include infraspecific taxa, so altogether in the Holarctic region 59 taxa are accepted.
Introducing numerous infraspecific taxa is a rare practice among the present day taxonomic treatments of mosses. Nowadays the bryologists usually try to avoid them, as they provide inconveniences for databasing and checklisting, calculating biodiversity indexes and similar approaches, where all taxa at the same rank are much more easy to use.
The different task is faced by taxonomists, who need to write a key to identification for a large group of very variable plants. The key for Syntrichia in the reviewed publication includes 81 couplets, i.e. it leads to 82 names, including repeats and three names of taxa from the other genera easily confused with Syntrichia. Such a number of repeats seems to be unavoidable in the key, so it works as properly as the key of such length can work. Sorting morphotypes into varieties in this case is helpful for their correct identification.
The reviewed publication is written in a style of classical taxonomic revision, without shortening “specimens examined” for less known species, but,at the same time, not over-flooded with specimens for taxa that are well-known and the least problematic. A special attention is given to the well-known difficulties in differentiation of Syntrichia ruralis from superficially similar species, as well as to the differentiation of its var. ruralis from other varieties of this species, and finally, the differentiation of ‘typical’ var. ruralis from a very troublesome ‘ruraliformis’, with exhaustive explanations why the decision on the rank of each morphotype is made.
Syntrichia ruralis with all accepted varieties is described on 30 out of 155 pages of the treatment, which includes descriptions of accepted species, keys to their identification, discussion on biogeography of the genus and other important parts of the study. In addition to this, the variation of S.ruralis is described in 10 pages of Appendix, where for tens specimens the unusual combination of traits are listed. This part might be useful for refraining from the superfluous description of new species in the S. ruralis complex.
Taxonomic part is followed by the discussion of the infrageneric classification of Syntrichia and geographical distribution of the species.
The focus of the present study was on North American material, and all five new species were described from there. High endemism of mountains in the western part of North America is well known (Carter et al., 2016, https://doi.org/10.3732/ajb.1500484). At the same time, Kramer argued that in Asian Russia and China mostly typical morphotypes of S. ruralis occur, thus he did not expect much novelties in this area and did not request additional material for his study. However, finding of S. lepthotricha in Mongolia, together with its wide occurrence in the western North America northwards to Alaska, suggest that this species can likely be discovered in the East Siberia as well.
The monograph of Syntrichia is illustrated by 63 plates of line drawings, showing leaves, cells, and costa transverse sections, and also by 30 color photographs addressing the laminal cell papillosity pattern and structure of costa, with a special attention to hydroids.
Kramer’s treatment is an invaluable contribution to the study of the genus Syntrichia, a great step forward in knowledge of species which comprise an important component of arid ecosystems, involved in the vegetation recovery.