Reassessing the taxonomic diversity of Plagiothecium section Orthophyllum in the North American bryoflora

At the beginning of the twentieth century, Plagiothecium nemorale s.l. and P. succulentum, both belonging to P. sect. Orthophyllum, were considered to be distributed across almost the entire Northern Hemisphere. However, in the mid-twentieth century these taxa were recircumscribed resulting in their exclusion from the North American bryoflora and restricting their distributions to Asia and Europe, and in the case of Plagiothecium nemorale s.l., also North Africa. More recently, it was found that P. nemorale s.l. is a taxonomic complex comprising three distinct species: P. nemorale s.s., P. longisetum, and P. angusticellum. I revised the North American material of P. section Orthophyllum deposited in three herbaria (NY, FH, F) and found that five examined taxa of the section are present in the North America. Two of the species P. angusticellum and P. longisetum, have not been recorded from North America previously, while two others, P. nemorale and P. succulentum, had not been listed in the North American bryoflora for 50 years. One taxon is new for the U.S.A. – P. succulentum f. propaguliferum. Here I provide detailed descriptions of the anatomical and morphological features of the recorded taxa along with photographic documentation of their most important characteristics. I also summarize their known distributions in North America and ecological preferences and provide a key for their identification.

In the nineteenth century, Plagiothecium sylvaticum (Brid.) Bruch & Schimp. was included in the North American bryoflora, it being the only species recorded from North America at the time that is now considered to be a synonym of P. nemorale (Sullivant & Lesquerux, 1865;Macoun & Kindberg, 1892;Renauld & Cardot, 1892;Macoun, 1889). Although studies of the North American bryoflora increased in the twentieth century (e.g., Andrews, 1921;Thomas, 1952;Ireland et al., 1987;Anderson et al., 1997), records of Plagiothecium were relatively rare and few additional taxa of the genus were recorded from the area. Species which are now considered to be synonyms of P. nemorale and P. succulentum were recorded only extremely rarely and mainly in studies from the first half of the twentieth century (Grout, 1939;Sharp, 1939;Gier, 1949;Clebsch, 1954;Githens, 1957;Norris, 1967).
Trends in the t axonomic history of Plagiothecium in North America are well illustrated by analysis of the bryophyte lists for the continent. Grout (1932Grout ( , 1940 recognized 31-32 taxa of Plagiothecium in North America, including P. sylvaticum and P. sylvaticum var. succulentum (Wilson) Husnot. Over time, many of these taxa were transferred to other genera, leaving only 13 taxa of Plagiothecium (including P. sylvaticum and P. succulentum) in North America in the list of Crum et al. (1965) and subsequently supported by the research of Worley and Iwatsuki (1970).
Undoubtedly the most influential works on North American Plagiothecium were Ireland's (1969Ireland's ( , 1985Ireland's ( , 1986 taxonomic revisions, in which he recognized only six to eight species of Plagiothecium in the North American bryoflora. In those works, he synonymized several taxa, mainly within P. cavifolium, and excluded from North America others, including P. sylvaticum, P. neglectum, P. nemorale, and P. succulentum. The distributions of these last two taxa, previously considered to range across the whole Northern Hemisphere, were thus restricted to Asia and Europe, and in the case of P. nemorale s.l. also northern Africa (Hill et al., 2006;Ignatov et al., 2006;Ros et al., 2013;Suzuki, 2016).
Ireland's taxonomic concepts were largely followed in subsequent bryological lists for this part of the world. For example, Crum et al. (1973) listed only seven taxa of Plagiothecium in their next list for North America, which was then followed by Anderson et al. (1990). In a worldwide revision of the Plagiothecium, Wynns (2015) added a number of taxa of the genus to the North American bryoflora, but continued to exclude P. nemorale and P. succulentum.
My revision of North American material of Plagiothecium sect. Orthophyllum suggests that recent treatments underestimated the taxonomic diversity of the group in North America and brings into question the widespread adoption of Ireland's taxonomic concepts. This article presents the results of this research.

Materials and methods
Nine-hundred specimens of Plagiothecium sect. Orthophyllum from three North American herbaria (NY, FH, F) were reviewed. All of them from these collections were studied more closely and qualitative and quantitative characteristics of their gametophytes were measured. Characters examined includedqualitative features related to the stem leaves: such as leaf shrinkage, symmetry, shape and concavity, the curvature and serrations of the leaf apex, the shape of leaf cells, the shape of decurrent cells, and the formation, or not, of distinct auricles by these cells, and quantitative features (length and width) of the abovementioned structures. The resulting data were used to make species descriptions and to direct photographic imaging. Taxon distributions and ecological preferences were summarized from specimen label data. The taxon descriptions are based exclusively on specimens originating in North America. All examined specimens are listed in Appendix 1. The names of individual taxa were adopted from Wynns (2015), with the exception of the P. nemorale complex (Wolski, Nowicka-Krawczyk, 2020).

Results
The revision indicated that in North America there are two species that have been excluded since the widespread adoption of Ireland's (1969) classification and have not subsequently been recorded for the continent: P. nemorale and P. succulentum. In addition, a form of the second species has been found -P. succulentum f. propaguliferum E. Bauer, and this taxon is listed for the first time in the U.S.A. area. Also, the revision indicates one species that until now was considered a synonym but now is treated as separate -P. longisetum, and one new for North America -P. angusticellum. The analysis of herbarium materials indicates that those species in the past years were most often wrongly determined as P. cavifolium and P. denticulatum (Hedw.) Schimp.
D i s t r i b u t i o n a n d h a b i t a t . -D u r i n g this research, specimens of Plagiothecium succulentum f. propaguliferum were noted only from one locality in eastern North America (U.S.A., Vermont) (Fig. 6), where it occurs in epilithic habitats (in damp crevices in the shade of cliffs; see Appendix 1).

Discussion
Species of Plagiothecium sect. Orthophyllum are frequently misidentified in herbaria and the literature, and representatives of the section are often considered to be taxonomically problematic (Nyholm, 1965;Iwatsuki, 1970;Lewinsky, 1974;Smith, 2001;Wolski, 2017Wolski, , 2018. However, many of these challenges are the result of a lack of basic taxonomic research to document intraspecific variability in problematic taxa (Wynns et al., 2017;Ignatova et al., 2019;Wolski, Nowicka-Krawczyk, 2020). Moreover, many of the taxa of the section can be distinguished in both Euasia and North America on the basis of easily recognizable features (Grout, 1932;Sharp, 1939;Greene, 1957;Nyholm, 1965).
Plagiothecium sylvaticum (at present a synonym of P. nemorale) among representatives of the genus Plagiothecium was described as a species with leaves more or less shrunken in dry condition (Grout, 1932;Sharp, 1939). This is a very important taxonomic feature that helps distinguish, among others, P. nemorale from other species of the genus (Green, 1957;Nyholm, 1965;Iwatsuki, 1970;Smith, 2001, Wolski & Nowicaka-Krawczyk, 2020. Within North America, all five of the documented taxa of section Orthophyllum occur in eastern North America, with a distinct dominance of P. succulentum, but in the Pacific Northwest only P. longisetum occurs. In comparison with the Eurasian material of P . n e m o r a l e , P . l o n g i s e t u m a n d P . angusticellum (Wolski, Nowicka-Krawczyk, 2020), the North American material of these species tends to have lower values for the analyzed quantitative features. Despite this, values of the most taxonomically significant featuresthe length and width of leaf cellsare relatively similar to the average values in the Eurasian populations.
The specimens cited in this study were previously identified as either P. denticulatum or P. cavifolium. The easiest way to distinguish the species treated here from P. denticulatum is by the symmetry of the leaves and form of decurrent cells. Plagiothecium denticulatum is characterized by very clearly asymmetric leaves (most leaves rounded asymmetric) and decurrent cells that are rounded, inflated and form distinct auricles (best viewed in situ on the stem). Plagiothecium nemorale and P. succulentum (including P. succulentum f. propaguliferum) have symmetric leaves, while P. longisetum and P. angusticellum have asymmetric or slightly asymmetric leaves. However, in all four of these species, the shape of decurrent cells is rectangular or quadrate, not inflated, and they do not form distinct auricles (Green, 1957;Nyholm, 1965;Iwatsuki, 1970;Lewinsky, 1974;Smith, 2001;Wolski, 2017Wolski, , 2018Wolski, Nowicka-Krawczyk, 2020).
Other species that could be confused with those treated here are P. japonicum and P. fallax Cardot & Thér. The first is known from North America only from Alaska and was treated by Iwatsuki (1970) as a form of P. nemorale -P. nemorale f. japonicum (Sak.) Iwats., but is now considered a separate species (Wynns 2015). Plagiothecium japonicum is quite similar to P. nemorale, but has larger, broadly ovate, concave leaves, with an acuminate apex, larger costae, and rigid, dilated cells; it is quite easy to distinguish from other species. Genetic analysis indicates that P. japonicum may be a hybrid or an allopolyploid of P. cavifolium and P. nemorale (Wynns 2015), although this requires further research. Plagiothecium fallax is also known in North America from Alaska, but has a broad distribution in Japan. It is similar to some of the treated species due to