The Bryophyte Flora of Vienna

The bryophyte flora of Vienna is documented only in parts. Old finds often appeared in publications about Lower Austria; only one study addressed the bryophytes of the inner city. Here, we present a bryophyte flora of Vienna, including historical reports and the results of recent investigations. From 1998 to 2023, we recorded 329 bryophyte taxa in Viennese urban territory. Fifty-six of these were liverworts, and 273 were mosses. Sixty-seven taxa are new for Vienna. Forty-nine taxa, given in historical studies, could no longer be found. If we also count these, 378 taxa occurred in Vienna to date. Of the current occurring bryophytes, 67 species have an endangerment classification. Rich in bryophytes were the dry grasslands of the Lobau, the oxbow lakes of the Lobau and the Prater, and large parts of the Wienerwald. But flat roofs and inner-city areas also showed more than 100 species. Compared to other European cities, Vienna is decidedly species-rich and highly responsible for some species in Austria. Reasons for this are the extensive green spaces and the pronounced climatic gradient from the sub-oceanic west to the sub-continental east of Vienna. Awareness raising for bryophytes we recommend in addition to the existing biotope protection.


Introduction
"Vienna is different"-this slogan of the Vienna city government also applies to the size of the green space in Vienna (Figure 1), which accounts for 31% (=128 km 2 ) of the total city area [1]. In addition to numerous large parks, Vienna shares a National Park [2] and a UNESCO biosphere reserve [3], which is unique for a metropolis the size of Vienna. The high proportion of semi-natural vegetation suggests high species numbers in bryophytes.
Historical surveys of bryophytes indicate the high bryophyte diversity of Vienna [4,5]. Many of today's city districts were villages on the outskirts of a capital city until the 19th century. Agriculturally used land lay between the surrounding settlements and the city. Until 1870, the Danube was an untamed river that passed north of the town in many arms, leaving numerous islands and floodplains between the river arms ( Figure 2). Large areas (e.g., the so-called Glacis) lay between the city walls and the suburbs to provide a direct line of fire on any attackers. Many of these areas were investigated by the bryologists of the 18th and 19th centuries. They reported bryophyte finds in Vienna integrated into studies on surrounding Lower Austria: e.g., Jacquin [6], Welwitsch [7], Garovaglio [8], Pokorny [9], Poetsch [10,11], Reichardt [12], Neilreich [13], Juratzka [14], Höfer [15], Höhnel [16], Heeg [17], Matouschek [18], Onno [19], and Baumgartner (W, unpubl.). Although most of these formerly bryophyte-rich habitats have given way to densely built-up areas, near-natural habitats have been preserved, especially on the outskirts of the town. The inner city also shows a high structural diversity and, thus, diverse bryophyte species [20].  Today, bryophytes are arousing interest again in many people, be it from an architectural or artistic point of view. In many places, there are a empts to green urban infra-  Today, bryophytes are arousing interest again in many people, be it from an architectural or artistic point of view. In many places, there are a empts to green urban infrastructure with the help of bryophytes, but mostly with li le success in Central Europe [23]. Architects demand quick greening success, which the bryophytes cannot deliver. There is also li le knowledge about which bryophyte species occur naturally in the urban environment of Vienna. The background of the present work is to expand knowledge about this and to improve the data situation on the biodiversity of Vienna. Today, bryophytes are arousing interest again in many people, be it from an architectural or artistic point of view. In many places, there are attempts to green urban infrastructure with the help of bryophytes, but mostly with little success in Central Europe [23]. Architects demand quick greening success, which the bryophytes cannot deliver. There is also little knowledge about which bryophyte species occur naturally in the urban environ-

Critical Taxa of the Historical Literature
Some historical reports are critical due to changes in taxonomy and nomenclature. However, it was not possible in most cases to consult the corresponding specimens and verify the data. The report of Jungermannia michauxii (today Anastrophyllum michauxii) in 1825 might have been the related species Sphenolobus minutus (Schreb. ex D. Crantz) Berggr., a species described later. Pohlia filum is also doubtful and was probably Pohlia elongata. However, an occurrence of Pohlia filum on the banks of the Danube at that time seems possible. Schistidium confertum is probably S. brunnescens, which we also found during this study. The historical report of Warnstorfia fluitans is most likely a misdetermination since this mire species could hardly occur in Vienna, which was free of Sphagnum species (also in former days), and we thus excluded it from the species list.

Critical Taxa of the Current Surveys
The introduced Nogopterium gracile was found only once in the garden of the Schloss Belvedere. This Mediterranean species might have reached its locality via substrate or plants. It is not assumed established to date, but we included it in the species list under this restriction. Philonotis marchica might have also come to Vienna via plant material. However, we found it several times, mainly in flower boxes and parks in downtown Vienna. An establishment there can be assumed.

Sites Particularly Rich in Bryophyte Species
The areas with the highest species diversity were in the Viennese part of the Donau-Auen National Park, at the Donauinsel, and in the Wienerwald. However, inner-city areas also had considerable species numbers.
The so-called Heißländen of the Lobau ( Figure 3A) deserve special mention: these gravel areas filled up by exceptional floods of the Danube correspond to edaphic dry grasslands. They are all located in the upper Lobau and harbor numerous bryophyte species that have become rare in Austria. In six of those Heißländen investigated, 98 species were found, including the rarities Cephaloziella stellulifera, Entosthodon fascicularis, Mesoptychia turbinata, Microbryum floerkeanum, Microbryum starckeanum, and Aloina ambigua.
Oxbow lakes in the Lobau and Prater ( Figure 3B) are also species-rich sites: although the remnants of the formerly free-flowing Danube no longer have dynamic sections, these almost stagnant waters and their muddy banks, which are often overgrown with reeds, still harbor rare species, including Amblystegium humile, Archidium alternifolium, Campylium polygamum, Fontinalis hypnoides, Riccia warnstorfii, or Ricciocarpos natans.
Also, the loamy soils in the floodplain of the Donauinsel ( Figure 3C) are a habitat for bryophytes. During extreme water levels of the Danube, opening the locks to the New Danube is a flood protection measure. A result of these floodings are deposits of loamy sands on the Donauinsel. These are colonized by pioneers of moist soils (e.g., Physcomitrium patens) soon after the water recedes. Subsequently, these soils dry out until the next flood, which often occurs years later. There, a dry grassland flora develops with sometimes rare elements, such as Pterygoneurum lamellatum, Aloina brevirostris, and A. ambigua. Over a length of 21.1 km, the Donauinsel offers a large number of these sites, which are rare throughout Austria. Also, the loamy soils in the floodplain of the Donauinsel ( Figure 3C) are a habitat for bryophytes. During extreme water levels of the Danube, opening the locks to the New Danube is a flood protection measure. A result of these floodings are deposits of loamy sands on the Donauinsel. These are colonized by pioneers of moist soils (e.g., Physcomitrium patens) soon after the water recedes. Subsequently, these soils dry out until the next flood, which often occurs years later. There, a dry grassland flora develops with some-  The area of the Lainzer Tiergarten also offers important habitats, such as deciduous woodlands, litter meadows, wet meadows, meager dry meadows, spring horizons, and streams. Notable is the virgin forest remnant Johannserkogel ( Figure 3D), hosting considerable deadwood volumes. Also, the Kaltbründlwiese crisscrossed by (coldwater) streams is rich in bryophytes. Overall, we found more than 100 species in the Lainzer Tiergarten-a high number for an area dominated by beech forests.
The variety concerning pH value in the Wienerwald allows the settlement of diverse soil and rock colonizers among bryophytes. Soil reactions in the Flyschwienerwald vary and range from slightly alkaline (e.g., Hermannskogel or Schutzengelberg area) to comparatively strongly acidic (e.g., in the Steinerne Lahn area) [25]. The streams in the Flyschwienerwald enable the occurrence of species, such as Seligeria pusilla and Blindiadelphus recurvatus, which are rare in the other parts of the Wienerwald. The part of the Wienerwald dominated by carbonate is small and lies southwest of Vienna. One of the rarest finds there was the occurrence of Asterella saccata.
Also, cemeteries and parks are habitats for bryophytes in the city. Of Vienna's 55 cemeteries, the Zentralfriedhof ( Figure 3E) is particularly significant concerning bryophyte diversity. Its Jewish part is especially species-rich (70 species) since it is highly diverse concerning structures. Except for the Botanical Garden, generalist species dominate the city's parks, most of which are highly stressed by air pollutants due to traffic. But the parks in the west of Vienna often merge smoothly into the Wienerwald and are correspondingly richer in species.
Even the inner city and rooftops are bryophyte habitats ( Figure 3F). The inner city of Vienna was studied by Hohenwallner [20,26], and the occurrence of bryophytes on the flat roofs of buildings from the turn of the last century was studied by Zechmeister [27]. More recent investigations on the Vienna General Hospital (Allgemeines Krankenhaus der Stadt Wien) brought further finds [28], such as that of Rhynchostegium megapolitanum. In general, the vegetation of the roof surfaces corresponded in part to that of primary dry grasslands. In total, around 100 bryophyte species were found in the densely built-up area of Vienna. The high species number shows that numerous bryophyte species conquer the inhospitable city without human intervention. They only need the necessary time and adequate structures.

Biogeographic Elements of Vienna Compared to Austria
The comparison of the bryophyte flora of Vienna concerning biogeographic elements with that of Austria illustrates its diverging nature. The differences concerning the distribution of biogeographic elements are statistically significant (Chi-square test, p < 0.01): Austria has a high proportion of cool-temperate species (1-3 in Figure 4). In Vienna, boreotemperate species dominate, and no arctic-montane species occur. Vienna's high proportion of southern-temperate species, which have their center of distribution in the Mediterranean region, is also striking ( Figure 4). This distribution of biogeographic elements reflects Vienna's biogeographical position in the transitional area between the Pannonian and Alpine biogeographical regions. Concerning thermal radiation, Vienna has a (sub)Mediterranean influence. The urban heat island ( Figure 5) causes the clustered occurrence of species adapted to a warm climate. . 1-Arctic-montane, 2-Boreo-arctic montane (in tundra and coniferous forest zones); 3-Wide-boreal (from temperate zone to tundra), 4-Boreal-montane (main distribution in coniferous forest zone), 5-Boreo-temperate (in conifer and broadleaf zones), 6-Wide-temperate (from Mediterranean region to coniferous forest zone), 7-Temperate (in broadleaf forest zone), 8-Southern-temperate (in Mediterranean region and broadleaf forest zones), 9-Mediterranean-Atlantic (in Mediterranean region, and extending north in Atlantic zone of temperate Europe); Assessment of species to major biomes following Hill et al. [29]; Y-axis in %.  . 1-Arctic-montane, 2-Boreo-arctic montane (in tundra and coniferous forest zones); 3-Wide-boreal (from temperate zone to tundra), 4-Boreal-montane (main distribution in coniferous forest zone), 5-Boreo-temperate (in conifer and broadleaf zones), 6-Wide-temperate (from Mediterranean region to coniferous forest zone), 7-Temperate (in broadleaf forest zone), 8-Southern-temperate (in Mediterranean region and broadleaf forest zones), 9-Mediterranean-Atlantic (in Mediterranean region, and extending north in Atlantic zone of temperate Europe); Assessment of species to major biomes following Hill et al. [29]; Y-axis in %.

Discussion
The species diversity of bryophytes in Vienna is notable for a metropolis in the Pannonian climate, which is a harsh environment for bryophytes. It has several causes, first and foremost, the high proportion of near-natural landscape elements and protected areas within the urban area. Second, Vienna has an exceptional gradient in precipitation and temperature ( Figure 5), which the bryophyte flora reflects. Oceanic influences from the west and Pannonian-continental influences from the east provide very different climatic conditions depending on the location within the city. With an average of 750 mm/year, the west of Vienna has twice as much precipitation as the east of Vienna (350 mm/year). Average temperatures are also approximately 2 • C lower in the west than in the east. Figure 5 also clearly shows an inner-city heat island, which is classic for cities [30,31]. Bryophyte diversity reflects this climatic diversity: for example, there are more Pannonian flora elements in the east and more sub-Atlantic flora elements in the west.

Comparison of Vienna's Species Diversity with That of Other Cities
Studies on the species diversity of bryophytes in Austrian cities are available for Linz [32] and Salzburg [33]. For Linz, 319 taxa are known. For Salzburg, 323 taxa currently occur, and if we include all taxa mentioned at some time in history, the number of species is 444. In a sense, these species numbers are similar, even though the area size, the geological conditions, and especially the climate are very different in the three cities of Vienna, Linz, and Salzburg. The loss of species is much greater in Salzburg, with a decline of 27% compared to 14% in Vienna. This loss in species is undoubtedly related to the loss of sites rich in bryophytes (e.g., mires) in Salzburg or poorer historical recordings in Vienna. For the city of Graz, an overall survey is lacking, but a study on the areas of Schlossberg and the Botanical Garden [34] mentions 178 taxa. In a more recent study [35] on soil and rock bryophytes of the city, 70 taxa are listed.

Implications for Management and Conservation
Forty-nine species historically known for Vienna were not found in the current surveys. Their loss might have several causes. At the time of the early bryologists, many sites were in villages and communities dominated by agriculture, which were incorporated into the city in the last hundred years (e.g., Roßau, Sievering, Stammersdorf, Floridsdorf). The disappearance of arable bryophytes such as Anthoceros agrestis or Phaeoceros carolinianus is probably due to this. Of course, urban sprawl is also partly to blame for the loss of some species. Wet meadows and fens have disappeared around Vienna. However, a significant impact on species number had the regulation of the Danube and the Wienfluss. On the banks of the unobstructed, markedly branched Danube and its "islands" (Figure 2) were diverse habitats, such as loamy, sandy, or gravelly floodplains. It is thus not surprising that species typical for the banks of large rivers have disappeared with these, such as 4 out of 5 Ephemerum species, Bryum versicolor, Physcomitrium eurystomum, or P. sphaericum.
Also, climate changes over the last 150 years reflect the loss of species that prefer higher humidity or cooler temperatures [44][45][46]. In Vienna, these include Porella arborisvitae, Ptilium crista-castrensis, or the deadwood colonizer Blepharostoma trichophyllum. For other species, such as Orthotrichum scanicum or Neckera pennata, the influence of climate change is probably closely linked to that of air pollution [47][48][49].
The species lost in Vienna have also become rare in the surrounding area. Of the 49 species that no longer occur in Vienna, 37 are classified as endangered in the Red List of Bryophytes of Lower Austria, 7 as extinct, and 12 as threatened with extinction (e.g., Asterella saccata, Figure 6D). Due to the site characteristics and size of the Donauinsel, the dry grasslands of the Lobau, and the habitats along streams in the Flyschwienerwald, some species are more frequent in Vienna than in neighboring Lower Austria concerning the area and marked with a "!" in Table A1 (Appendix A). These are Acaulon triquetrum ( Figure 6A), Aloina rigida, Cephaloziella stellulifera, Dicranella howei, Entosthodon fascicularis ( Figure 6E), Microbryum curvicollum ( Figure 6F), Physcomitrium patens ( Figure 6 C), Pterygoneurum lamellatum, Ricciocarpos natans ( Figure 6B), Seligeria pusilla, Blindiadelphus recurvatus, and Mesoptychia turbinata. Vienna, therefore, is highly responsible for the conservation of these. Asterella saccata, Figure 6D). Due to the site characteristics and size of the Donauinsel, the dry grasslands of the Lobau, and the habitats along streams in the Flyschwienerwald, some species are more frequent in Vienna than in neighboring Lower Austria concerning the area and marked with a "!" in Table A1 (Appendix A). These are Acaulon triquetrum ( Figure 6A), Aloina rigida, Cephaloziella stellulifera, Dicranella howei, Entosthodon fascicularis ( Figure 6E), Microbryum curvicollum ( Figure 6F), Physcomitrium patens ( Figure 6 C), Pterygoneurum lamellatum, Ricciocarpos natans ( Figure 6B), Seligeria pusilla, Blindiadelphus recurvatus, and Mesoptychia turbinata. Vienna, therefore, is highly responsible for the conservation of these.  Campylopus introflexus, as the only neophytic bryophyte species in Vienna, occurred only regionally and in small populations. A contribution to the protection of bryophytes is raising awareness that they are a significant part of the biodiversity of a large city and that the best way to protect them is to let them live where they emerge and grow on their own. A garden wall or roof surface overgrown with bryophytes is beautiful and increases biodiversity-not only of the bryophytes but also of microorganisms living in their protection-because each bryophyte cushion is a small microcosm [50]. Attempts to establish bryophytes outside their natural habitat are doomed to failure in Vienna for climatic reasons [51]. Attempting bryophytes for greening purposes on house walls and indoors should therefore be strictly rejected. Unfortunately, against better knowledge, the use of bryophytes in this area is in vogue. This use is associated with the destruction of bryophyte stands and the creatures hidden in them.
As the present data indicate, urban biodiversity is becoming increasingly important. Vienna's diversity in bryophytes exceeds the species numbers of many cultural landscapes in rural areas [52,53]. Within cities, bryophytes are also indicators of a feel-good climate for people. The high number of boreo-temperate species in Vienna represents a cool-moderate climate that is also digestible for humans [54]. Bryophyte-free areas indicate areas within Vienna dominated by concrete and increasingly inhospitable for humans in the increasing heat and dry summer periods. Bryophyte-rich areas within the city can thus serve as guidelines for future urban planning [55]. In future studies, new bryophyte species will appear in Vienna. We have only recorded bryophytes from publicly accessible areas; further species might occur in old, structurally rich gardens in the suburbs and villa districts, which proved to be rich in species in other studies, e.g., [56].
We collected the current bryophyte data in the period between 1998 and 2023. We conducted no area-wide or quadrant-based search. In most cases, we investigated areas that were expected to have a rich bryophyte species assemblage or where we suspected rare species. Some target regions were determined by projects (e.g., on the dry grasslands of the Lobau or the Biosphärenpark Wienerwald, see funding) or were venues of the "Day of Biodiversity" or excursion destinations within the framework of courses held by the two authors at the University of Vienna. We also collected bryophytes during many walks and hikes in Vienna. In general, we recorded only bryophytes in publicly accessible areas.
Furthermore, we included data from diploma theses supervised by the first author [20,26,[57][58][59] and from a previously published study of the authors [60].
The nomenclature follows Hodgetts et al. [61]. Endangerment classifications are according to the Red List of Bryophytes of Lower Austria [24]. Specimens of all species are in the private herbarium of H. G. Zechmeister.
The climate data originated from 15 climate stations operated by the municipality of Vienna and the Office of the Provincial Government of Lower Austria. The period of the climate data for the Kriging process covers the years 2014 to 2017. For modeling temperature and precipitation for the area of Vienna ( Figure 5), we used the function "Ordinary Kriging" based on the model "spherical" implemented in the ArcGIS (Esri, version 10.8) program. We set the parameters "range", "nugget", "lag size", "number of lags", "search radius", and the "number of neighbors" on default.
For the comparison of the biogeographic elements, we used a Chi-Square Test (p < 0.01) in the program SPSS (version 22.0). Acknowledgments: We thank all those who contributed to the realization of the projects mentioned above, the organizers of the "Tag der Artenvielfalt" for their invitations, the Viennese winegrowers for their extraordinary wines, and all Viennese for their spontaneous condolences at the sight of bryologists kneeling at work. We thank three anonymous reviewers for their valuable comments.

Conflicts of Interest:
The authors declare no conflict of interest.
Appendix A Table A1. List of species found in Vienna-nomenclature according to the Checklist of Bryophytes of Europe [61]; P: Phylum of bryophytes: A-Anthoceratophyta (hornworts), B-Bryophyta (mosses), M-Marchantiophyta (liverworts); O: occurrences: h-historical occurrence, n/h-current and historical occurrences, n-species for which there are no records in the historical literature. We found them during the current surveys, and they are published here for the first time; RL-Red List (according to the Red List of Bryophytes of Lower Austria [24]): R: !: responsibility of the city of Vienna for the respective bryophyte species; F-frequency: vr-very rare (1-2 current occurrences), r-rare (3-5 current occurrences), s-scattered (4-10 current occurrences), w-widespread (more than 10 current occurrences); locality: the locality is amended by information on the habitat, if available; when there are only historical reports of a species, we added the source.