Liverwort flora of Ayan – a gained link between subarctic and hemiboreal floras in West Okhotiya (Pacific Russia)

Abstract The liverwort flora of Ayan was first investigated one hundred and fifty years after the first exploration of vascular plants. A number of factors has determined the relatively high taxonomic diversity of liverworts in this hemiarctic flora of small-sized area: 118 species and one subspecies were revealed. These data are new not only for the studied area, but also for the huge land adjacent to the western coast of the Sea of Okhotsk. The liverwort flora possesses the domination of taxa common in the hemiarctic, although with a lot of taxa more common in boreal as well as arctic-alpine environments. The presence of Mega-Beringian and calciphilous taxa is the peculiar trait of the studied liverwort flora. Based on detrended correspondence analysis (DCA), Ayan liverwort flora shows relationships to the continental mainland floras situated both in North-East Asian hemiarctic and hemiboreal East Asia and is, therefore, the link between both. The flora of Ayan surroundings is one of the newly-filled ‘blank spots’ in the possible floral exchange way between Arctic Northeast Asia and mountainous floras of temperate East Asia.


Introduction
The Russian-American Company under the High Patronage of His Imperial Majesty was founded in 1799 and urgently required new surface tracks to access the North Pacific from Europe via Siberia. The way from Yakutsk Town to the newly-founded Ayan Settlement at the coast of the Sea of Okhotsk was constructed and opened in 1844 and this was the only uninterrupted way in the middle of 19th century to access the Pacific Ocean by land across Eurasia (Lightfoot 2003). Fortunately for biologists, this way crossed a lot of geologically unique places with various climates and even the first indications by Regel and Tiling (1859) showed its richness and resulted in the description of several taxa bearing epithet ' ajanensis'. Though some of such epithets were published even before that time, they appeared from the land adjacent south to the Ayan: i.e. Picea ajanensis Fisch. ex Carrière was described, based on materials collected by Middendorf in 1844 from the huge area "jugo Stanowoi … prope Udskoi … insula Schantar…" (=Stanovoy Range, Udskaya Bay, Shantar Islands) (von Trautvetter and Meyer 1856: 87) and does not belong to Ayan surroundings in the strict sense.
Thirty years after its establishment, the Ayan track was abandoned and was never established again as a summer-operated road, although some remnants of that monumental construction are still observable here and there along the Aldoma River. The closing of the track operation immediately stopped the investigations in the area for a long time. Then, the exploration of Ayan flora was continued in the second half of 20th century, after one hundred years (Khokhrjakov and Berkutenko 1979, Kharkevich et al. 1983, Schlothgauer 1990, Schlothgauer and Kryukova 2005. The exploration process was made possible to re-start and it was fruitful due to the wide use of helicopter transport as the only possibility to visit some of the more inaccessible places. The excellent materials collected at that time and reviewed later are confined to vascular plants, but did not include bryophytes. Although, in 1980-th, the bryologist Valentina Cherdantseva (see the description of her life and career in Bakalin 2013a) visited the Ayan Settlement and collected mosses, but those, unfortunately, were neither processed nor, certainly, published and apparently partially lost and partially stored unnamed in the VLA herbarium in Vladivostok. The collections of liverworts from that area are completely unknown to us. In the circumstances of potentially rich flora, the Ayan bryological "white spot" (Bakalin 2013b) existence looked very attractive. Indeed, its continuing existence is caused simply by the difficulty in accessing the site, due to the considerable distance from any federal car or rail roads (the nearest year-round operated road is 720 km west of Ayan in the Republic of Yakutia) and the only regular passenger communication operated from Khabarovsk City to Ayan by small aircrafts, which is extremely dependent on weather conditions. The latter is the only means at the disposal of the botanist who wants to visit Ayan in the summer season.
The geological diversity, described in the Material and Methods section, identifies high plant taxonomic diversity that may not be confined to vascular plants and likely appears in bryophytes.The high plant diversity in the area besides geology may be also foreshadowed by: 1) climate variation from subcontinental (locally suboceanic) in areas adjacent to the Sea of Okhotsk (also with frequent fogs shrouding the coast), to continental in the axial part of the Dzhugdzhur Range; 2) the sub-meridional stretch of the Dzhugdzhur Range axis linking the mountain flora of the south of eastern Siberia (the Stanovoy Range), which, in turn, is in contact with the mountains of East Asian Floristic Region, such as the Sikhote-Alin Mountain system. Northwards, the Dzhugdzhur Range contacts with the North-East Asia mountains, for example, the Kolyma Upland and Verkhoyansky Range. The listed factors determine the landscape diversity and influence community composition from the historical perspective and are a prerequisite for the discovery of a rich liverwort flora.
With specially supported field research in Ayan in 2019, we have visited this area and travelled from the sea coast to the ridge line in Dzhugdzhur Range collecting several hundred liverwort specimens within three weeks. The main goals of the present work are: 1) to describe the liverwort taxonomic diversity of this land; 2) to find the phytogeographical connections between floras in amphi-Pacific North-East Asia and 3) try to reveal the factors determining the diversity.

Study area
North Okhotiya is the informal name for a huge area of land surrounding North-West coast of the Sea of Okhotsk and geologically belongs to adjacent edges of the Eurasian and Pacific Geological Plates (Seno et al. 1996). If the geological map of the west coast of the Sea of Okhotsk is looked at, a mosaic of bedrocks of different composition and age would be very prominent at the 56th degree of the northern latitude, just southward of the contact zone of two plates (http://atlaspacket.vsegei.ru/#c9d3d204b97b3b163). This geological diversity 'hot spot' covers the vicinity of the Ayan Settlement and stretches westwards to the axial line of Dzhugdzhur Range -one of the great ranges orographically connecting Northeast Asia and East Asia. The rocky outcrops in this area vary from Archean time (observed mainly on the axial part of Dzhugdzhur Range), to younger rocks, up to the Cretaceous (in areas adjacent to the seacoast). This great age variation is combined with a variety of compositions: Archean rocks belonging to anorthosites, gabbroanorthosites, gabbros, gabbroamphibolites, norites, metagabbro, metapyroxenites, metaperidotites, various gabbroids, granites etc.; Proterozoic rocks represented by gabbros and dolerites; Paleozoic bedrocks represented by Devonian granites, granite-porphyry, diorites, dioriteporphyrite and gabbros. The youngest rocks are from the Mesozoic Cretaceous. Noticeable geological diversity may likely portend to a high taxonomic diversity in the flora.
The surroundings of Ayan is the area that is hard to access. In the vicinities of the settlement, there is only one all-year-round operated road of 12 km length that connects the settlement itself with the local airport. Another way goes to Nelkan Village, located 200 km west from Ayan and can be called the 'road' very conditionally, since it is passable only for three-axle heavy-wheeled vehicles and all-terrain (crawler) vehicles and only for two months per year (July and August), with the greater part of the track going directly by the riverbed. These circumstances make the exploration of the area very difficult and, before 1990-th, the works were mostly conducted using helicopter -a practice almost abandoned in the current floristic research in Russia. The maximal distance between localities studied in the present paper is about 60 km (by direct line). The studied localities were visited in the course of pedestrian routes combined with heavy vehicle transportation. The study was undertaken over a period of three weeks at the end of June and the beginning of July of 2019. In total ca. 400 specimens of liverworts and 350 specimens of mosses (not covered in the present account) were collected. The explored localities could be combined into three main areas: 1.
The area immediately adjacent to the Ayan Settlement (3-7 km from the centre of the settlement); 2.
Low and middle altitudinal belts of the northern part of Pribrezhnyi Range (Unych'ya River catchment); 3.
Axial part of Dzhugdzhur Range, in other words, the watershed of the Pacific and Arctic Oceans, mainly in the upper reaches of the Tugorma River (more precisely, in one of its left tributaries).
For more details, the localities are shown in Table 1 and Fig. 1. The mosses were air-dried immediately, whereas the liverwort specimens were kept in the fridge (in anabiosis conditions) until they arrived at the Cryptogamic Biota laboratory in VBGI where they were identified alive by morphological methods and partly photographed (mostly for oil bodies data) and only then dried. Since the main goal of the work was to reveal the liverwort diversity, the main efforts were assigned to their collection.. In the collection process, we tried to cover all types of vegetation communities and landscapes present in the studied area. In practice, this could not be achieved, due to both the inaccessibility of some geological formations and the time limit allocated for research. Nevertheless, we studied both presumed acidic rocks (meaning the pH of the water extract of the sedimentary products), for example granites, as well as presumed alkaline rocks, from various kinds of metamorphic rocks to limestone. Therefore, we expect the core of the liverwort flora to be revealed. The collecting localities, numbered in accordance with Table 1.
There are few data on the climate of Ayan surroundings. The weather station in Ayan Settlement provides data on the climate at low elevation. However, the climate parameters in the upper elevation and at the distance from the sea coast should be noticeably different. The Ayan weather station (https://en.climate-data.org/asia/russian-federation/ khabarovsk-krai/ayan-718644/) is located at 24 m a.s.l., at a distance less than 500 m from the coast of Ayanskaya Bay of the Sea of Okthotsk. The mean annual temperature there is -3.26°C, the warmest month is August, as it commonly occurs in amphi-oceanic North Pacific areas (thus, not July), when the average temperature reaches 13.0°C, the coldest month is January with temperature average -19.6°C. The mean annual amount of precipitation is 856 mm per year, with a distinct late summer maximum from July to September, when the amount of precipitation exceeds 100 mm per month. The winter precipitation is quite scanty, represented by snow and does not exceed 33 mm per month from November to March. Taking account of available information, the Ayan surroundings climate may be called subarctic suboceanic that indeed corresponds to the dominance of subarctic vegetation in the coastal area, as described below. Going inwards from the Asian mainland, the climate transforms to continental, similar to that widely distributed in the south-eastern flank of the Republic of Yakutiya (bordering Ayano-Maysky administrative district westwards), but the character of changes remains unclear. The climate parameters in the nearest Nelkan Village (314 m a.s.l.) that is behind the main axis of Dzhugdzhur Range westwards (https://en.climate-data.org/asia/russian-federation/khabarovsk-krai/ nelkan-296379/) has only 414 mm of precipitation per year (i.e. less than half that of Ayan), the mean annual temperature -6.7°C, with the warmest month being July, 14.4°C and the coldest one being January, -30.4°C. Therefore, the Nelkan Village climate is much more continental than that observed in the Ayan Settlement.
The vegetation in the study area is difficult to describe in distinct definitions of altitudinal belts. In fact, the dominant vegetation is represented by deeply interpenetrating complexes, such as forests composed of Picea ajanensis or Larix cajanderi Mayr, crooked forests (krummholz) composed of Pinus pumila (Pall.) Regel. and Alnus fruticosa Rupr. and mountain tundra and tundra-like communities. Indeed, the area just near to the seacoast provides a variation of communities from tundra (or, at least, almost indistinguishable analogue) at the slopes with severe wind conditions to true boreal dark-coniferous forests in the narrow valleys protected from the dominant winds ( Fig. 2a). At elevations 300-400 m a.s.l., all abovementioned vegetation types occupied almost equal proportions. Azonal complexes are represented by vegetation of ice glades, rocky fields (gravelly barrens), swamps and floodplains. Rocky fields (kurum), depending on the topography, may be observed from the seashore, although they begin to dominate at elevations exceeding 500-700 m a.s.l. (Fig. 2b, c). The ice glades are occupied by willow-grass communities, sometimes with some mosses dominating in the cover, but commonly entirely liverworts free. Swamps are very local; we observed a true swamp (with peat deposits production) only once (although, it was quite peculiar due to sluggishly-flowing waters above a limestone sub-base) (Fig. 2d). Therefore, the use of terms such as boreal, subalpine and alpine belt bears only a conditional sense and reflects rather the nearest environment where the specimen was collected. Roem. (the latter also often penetrates to the virgin birch forests), are almost confined to these communities. Pure Betula lanata (Regel) V.N. Vassil. stands are quite rare and are usually confined to the gentle slopes moistened by percolating waters near the base of hills and high slopes (Fig. 3b). The ground cover in birch forest is usually grassy, more rarely shrubby. Pinus pumila -the main dominant plant in the area investigated (by total coverage) -forms well-defined stands (Figs 3b, 4). It almost does not penetrate to the spruce forests, but is widespread as an understorey in Larix forests and commonly so (although with much lesser regularity) in Betula lanata scattered forests. The virgin stands of Pinus pumila start at elevations of 100-200 m a.s.l. and usually remain widespread on the ridgelines of the spurs in the middle elevation, near 500-700 m a.s.l. Above these elevations, as far as to elevations exceeding 1000 m a.s.l., Pinus pumila clumps become more and more scattered, plants become smaller and more certainly better developed in the hollows protected from the wind, with thicker snow cover. Communities of Duschekia fruticosaanother dominant of crooked forests in the Hemiarctic Pacific Asia -are much rarer and likely confined to those places close to the surface or open percolating waters, as well as to the slopes of narrow mountain valleys where a lot of snow is deposited in winter. In addition, shrubby Betula fruticosa clumps sporadically occur near the axial part of Dzhugdzhur Range and form small-sized communities composed of dwarf modifications of the species and show a transition to the true mountain dwarf shrub tundra. Besides communities where Betula fruticosa is a dominant, that species commonly occurs in larch forests, where it reaches a height of 1 m or slightly more. In humid sites near the seacoast, where larch forests are replaced by mixed Picea ajanensis -Betula lanata forests, a continuous morphological transitional series of hybrids between B. fruticosa and B. lanata are observed, these hybrids commonly having a height between 1.5 and 3 m.
Pinus pumila communities are not so much interrupted by mountain tundra (we rarely observed sizable mountain tundra communities), but they are mostly disconnected by rock fields (kurums), both on steep and gently sloping surfaces (Fig. 2b, c). In general, the water deficit (due to the absence of a closely lying waterproof horizon -in other words, when all the moisture after snowmelt or rains almost immediately seeps into the substrate), is the main reason for the absence of closed vegetation at heights exceeding 500-1000 m a.s.l. longer and are located at the places of (or near) percolating water openings. The general organisation of alpine communities is usually exhausted by the following scheme: Sphagnum lenense tufts form the central patch, then they are surrounded by the perimeter (sometimes quite wide strip) of lichen-dwarf shrub (sometimes including dwarfy Pinus pumila) communities and then grade into rocky fields (Fig. 5a). Pure lichen tundra communities ( Fig. 5b) are distributed somewhat wider than moss and moss-shrub communities, although also are not common. The special conditions for the development of wet dwarf shrub-moss tundra are observed at the base of slopes at heights exceeding 1100 m a.s.l., in places of firns (dense snow strata lying sometimes over summer) distribution and/or groundwater openings. The severe wind regime on the seacoast, especially in the saddles between the hill-shaped uplifts, leads to the local distribution of tundra-like communities even at altitudes less than 50 m a.s.l. (Figs 5c,d,6). At a distance from the seacoast, such communities are found everywhere above 300 m a.s.l. and are confined to the most well-drained axial parts of small spurs with clearly blown snow in the winter.
In general, the vegetation of the study area can be identified as subarctic montane. The spruce forests do not dominate even in the lower altitudes and, as a rule, have a great admixture of hem-iarctic Betula lanata. Larix forests have an understorey of Pinus pumila and commonly do not have closed canopy. The larger areas are covered by the 'subalpine' shrub Pinus pumila that occurs as understorey or small-sized clumps from lower elevations starting almost from the seacoast and then stretching to the elevations slightly exceeding 1000 m a.s.l., as dwarf and strongly disintegrated patches. The subarctic nature of the flora is also determined by the climate characteristics briefly described above.

Data analyses
To evaluate the position of the studied flora within Northeast Asian floras, we have used the Detrended Correspondence Analysis (DCA), based on the matrix that is largely the same as that used in the identification of the position of bryophyte flora of northern Iturup Island . Besides Ayan flora, the data on East-Manchurian Mountains, united floras of northern part and the southern part (instead of flora of southern Sikhote-Alin) of Sikhote-Alin Mts. were added too. The comparison involves 25 floras (Table 2, Fig. 7) and is based on a matrix (Suppl. material 1) where each species was marked as 1 (presence) or 0 (absence). These data were tested using multivariate analysis (using Past ver. 4.03c (Hammer et al. 2001)). The hierarchical clustering was based on Ward's method (Ward a b c d Figure 5.    The floras involved in the comparative analysis (abbreviation are explained in Table 2).

Annotated Checklist of Liverworts
One hundred and seventeen species and one subspecies were revealed in the flora of Ayan surroundings, as was defined in the present study. All identification data were input into the electronic herbarium database of the Botanical Garden-Institute FEB RAS (http:// botsad.ru/en/herbarium/). All taxa are listed below in alphabetical order. Nomenclature follows Söderström et al. (2016), with the exception for: 1) Solenostomataceae, where the narrow genus concept was adopted; 2) acceptance of Pseudolophozia distinct from Barbilophozia; 3) new conception of Schistochilopsis genus according to Bakalin et al. (2020b) and 4) the narrow species concept in Blepharostoma (the last following Bakalin et al. (2020a)). As only a few specimens of Blepharostoma species (only in pure mats) were checked by DNA analysis, clear taxonomical status of the vast majority of Blepharostoma species found as accompanying taxa (sometimes in small amounts) to other liverworts is not known definitely. In one case (stressed in the checklist), the name Blepharostoma sp. from Kh-58-4-19 means new cryptic taxon discussed in Bakalin et al. (2020a). Cladopodiella is accepted in the old sense, as the genus is different from Odontoschisma. Each taxon is annotated with data on: 1) numbers of collecting localities in accordance with Table 1 and Fig. 1, numbers are given in bold font; 2) altitudinal diapason in the explored area, m above sea level; 3) description of habitat in area treated and accompanying taxa and 4) selected specimens examined field numbers. The subsections are divided by dash.   .  Hoffm. -17, 18 -82-154 m -Open mesic cliff crevice in large riverside Nfacing cliffs surrounded by scattered Betula lanata. With Mannia cf. sibirica. . Sauteria alpina (Nees) Nees -18 -129-154 m -Open moist cliff crevices also those filled with humus in large N-facing conglomerate riverside cliffs surrounded by scattered Betula lanata. In pure mats and with Eremonotus myriocarpus and Solenostoma obscurum . . Bryhn -7, 9, 18, 22 -119-1484 m -Open to partly shaded moist to wet cliffs and their crevices, rarely moist humus on slope, only along watercourses (although sometimes at a distance from the running water); in alpine belt with scattered vegetation, rarely in open places on riverside rocky outcrops in scattered forest. In pure mats or with Diplophyllum taxifolium, Frullania subarctica, Gymnomitrion corallioides, Herbertus aff. buchii, Plagiochila arctica, Pseudolophozia sudetica, Ptilidium ciliare and Trilophozia quinquedentata. .

Taxonomic diversity and ecological traits
The total number of revealed taxa (118 species, including one criptic taxon and 1 subspecies) is quite high for relatively brief studies of the local hemiarctic flora. This is likely a consequence of co-action of habitat diversity (of geological, orographic and climatic origin) and historical reasons (the position of explored area at the migration route between Northeast Asia and East Asia). Meanwhile, not all revealed taxa are listed in the checklist. There are several specimens which were not identified convincingly. These doubtfully identified specimens require a molecular-genetic study to find the name (some of them may belong to undescribed taxa). These unlisted taxa may potentially provide certain additional peculiarities of the studied flora.
Relatively few in number are 'forest' taxa, represented here by boreal and hemiboreal species (distributed mostly in corresponding zones of the Northern Hemisphere). Several of them were found on decaying wood in Picea ajanensis forests (less in number are epixylous taxa in Larix cajanderi forests): Fuscocephaloziopsis leucantha and Mylia verrucosa are locally abundant, Crossocalyx hellerianus is rare, Anastrophyllum michauxii, Calypogeia suecica and Radula complanata (also recorded from the cliff) were found only once. Bazzania denudata is rare and found on rocky substrates, only in the localities near the seacoast. Frullania bolanderi, apparently, is quite frequent in floodplain communities, on the trunks of Salix udensis and Betula lanata. Rarely observed are Lophocolea heterophylla (floodplain forests only) and Lophozia ventricosa (also found in Picea-Betula forest).
Two taxa complete the epiphytes in the area: Frullania bolanderi and Ptilidium pulcherrimum. The latter is widespread on tree trunks and thick branches in tall and crooked forest communities. Some taxa, commonly not limited to the forest communities in other hemiarctic floras, are exclusively forest taxa in the explored area: Fuscocephaloziopsis lunulifolia, Lepidozia reptans, Lophocolea minor, Lophozia lantratoviae, L. silvicola, Scapania irrigua, S. mucronata, S. rufidula and Solenostoma obscurum. Douinia plicata is found in all belts and is abundant in true forests and krummholtz. Larix cajanderi forests are poor in taxa (excluding liverwort flora of watercourses and other habitats only slightly related to the larch forest itself). Their dryness predict occurrence of meso-xerophytes, like Barbilophozia barbata. The species previously identified as Blepharostoma brevirete (Bryhn & Kaal.) Vilnet & Bakalin  in the annotated checklist) was found in open Larix forest and was reidentified by genetic methods as a different, not yet described (tentatively cryptic) hybrid of B. trichophyllum and an unknown taxon (Bakalin et al. 2020a). It is morphologically similar to B. brevirete.
The krummholtz (Pinus pumila and Duschekia fruticosa thickets) houses a few liverwort taxa and have no specific flora. Besides Ptilidium pulcherrimum that is the commonest species in these communities, a whole series of species are found here, descending into crooked forests from the alpine belt along watercourses, i.e. by intrazonal habitats. On the contrary, examples of taxa that would join forests and krumholtz, but would be absent in alpine belt are few: Lophozia silvicoloides, Ptilidium ciliare and Scapania mucronata -their absence from the alpine belt may be the simple result of undercollecting, since all those listed are common inhabitants of alpine belts in other parts of the eastern Paleoarctic.
The arctic-montane acidophilous (or neutrotolerant) species were quite numerous. Widely distributed, though not confined to, but more frequent in the alpine belt, were Anthelia juratzkana, Gymnomitrion corallioides, Pleurocladula albescens, Pseudolophozia sudetica, Scapania microdonta, Sphenolobus saxicola and Tetralophozia setiformis. Other taxa were sparse and occurred locally in upper elevations: Cephaloziella varians and Gymnomitrion concinnatum. Several species were restricted to the high altitudes in the upper reaches of the axial part of Dzhugdzhur Range: Gymnomitrion commutatum, Prasanthus suecicus and Protochilopsis grandiretis were all very rare in the explored area. Isopaches bicrenatus (that is not, strictly speaking, purely arctic-montane species) was found only on solifluction spots in the axial part of Dzhugdzhur Range. Neoorthocaulis binsteadii was very common on Sphagnum cushions in the axial part. Riccardia palmata that was rather expected to be found on decaying wood in lower altitudes, was found only in the alpine belt in tundra environments. Similarly, mainly swampy Scapania paludicola and Schljakovia kunzeana were found in moist moss-dwarf shrub tundra near streams. Rare were Diplophyllum sibiricum, Jungermannia afoninae, Marsupella boeckii and M. emarginata. Scapania sphaerifera, expected in the axial part, was found at the highest elevations in Pribrezhny Range in the coastal part of the area only.
A limited number of species were common in all altitudinal belts: Calypogeia integristipula, Lophozia guttulata (the specimens referred to this species were gathered in mountain tundra and may represent another taxon not yet described), L. lantratoviae, Scapania crassiretis and Sphenolobus minutus. Sparsely, although occurring across all belts, were Metzgeria pubescens, Plagiochila porelloides, Preissia quadrata, Scapania parvifolia, S. rufidula (restricted to the axial part of Dzhugdzhur Range), Schistochilopsis incisa, Sphenolobus minutus, Trilophozia quinquedentata and Tritomaria exsectiformis. S. gymnostomophila. Conocephalum salebrosum should be conditionally referred to the group of basiphilous taxa in the investigated area since it was found only in areas of distribution of limestone and neutral metamorphic rocks. Mesoptychia heterocolpos is also not the obligate basiphilous taxon; however, it was found only in the area where basic rocks are distributed. Three species, Cryptocolea imbricata, Plagiochila arctica and Radula prolifera although, were found in the axial part of Dzhugdzhur Range that is predominantly acidic, but restricted there to the places with a pronounced neutrophilic flora (including from vascular plants, for example, Salix berberifolia Pall.) and developed near basic reaction percolating water openings.
The rocky outcrop liverwort flora in lower altitudes usually is not liverwort-rich, with one striking exception. In the course of our works, we found a large riverside cliff-like massif on the right bank of the Unych'ya River in its middle course (56°25'07.3"N 137°56'54.7"E) that is very peculiar in its geological and ecological respect (Fig. 8a). The cliff area has a length of about 100 m along the riverbed and has a height of up to 50 m from the riverbed to the top. These are conglomerate rocks that unite both basic and acidic metamorphic 'pebbles' (expected only due to the bryophyte inhabitants of the rocky particles as no special chemical analyses were conducted there) (Fig. 8b). Ecologically, these cliff walls also provide a variety of moisture conditions (from very wet to very dry) combined with variations in illumination and various niches and caves. Besides, these cliffs are covered by scattered Betula lanata trees -in this case, by enriching potential habitat diversity by shading as well. The liverwort diversity occurred there covers about 40% of the entire explored flora, although with the vast majority of taxa found also in other complexes. Bazzania tricrenata, Frullania davurica and Herbertus dicranus were very common there, while Jungermannia afoninae and Metacalypogeia cordifolia were sparse. Several taxa, including Erimonotus myriocarpus, Frullania ignatovii, F. subarctica, Lejeunea alaskana, Peltolepis quadrata, Porella vernicosa, Reboulia hemisphaerica, Sauteria alpina, Solenostoma rossicum and Tritomaria scitula were found only in the mentioned cliff-like massif. Moreover, for some of the listed species (Frullania ignatovii, F. subarctica and Lejeunea alaskana), this location is very unusual, taking into account the low elevation that formally belongs to the forest belt. The striking trait of the liverwort complex in the described massif is the occurrence within the same locality of the species belonging to different flora elements: it is especially pronounced in the 'pair' Metacalypogeia cordifolia and Eocalypogeia schusteriana occurring in adjacent patches of the same cliff. The chemical composition diversity results in an almost artificial mixture of joint occurrence of acidophilous and basiphilous taxa, like Bazzania denudata growing together with Tritomaria scitula. Taking into account the total diversity and peculiarity of taxonomic composition, this cliff riverside complex deserves protection as a natural monument of regional significance.
The formerly (and on-going) migration of liverworts by the mountains along the western coast of the Sea of Okhotsk in view of possible connections between East Asian and North-East Asian floras may be well illustrated by the occurrence of mainly Beringian Cryptocolea imbricata, Eocalypogeia schusteriana, Frullania ignatovii, Frullania subarctica and Fuscocephaloziopsis pachycaulis found in the area. All of them were recorded also south of the explored area. 'Southern', mainly temperate East Asian elements were represented by a lesser number of taxa, including Metacalypogeia cordifolia, Porella a b vernicosa and Mylia verrucosa. For the first two taxa, the studied locality is the northernmost outpost of worldwide distribution. Calypogia neogaea -mainly East Asian-North American taxon, occurring in the areas adjacent to the oceanic coast -had in Ayan surroundings the most 'inland' and the most northern occurrence in the Asian mainland.
Describing characteristic traits of the liverwort flora in the area treated, the absence of several taxa, forming 'negative' peculiarity, should be also taken into account. A large number of boreal 'forest' taxa were not found in the area: Bazzania trilobata, Nowellia curvifolia, Neoorthocaulis attenuatus and Scapania apiculata. For all listed, it was quite unexpected, because they are known not only southwards, but also north of Ayan surroundings. The same should be noted on the taxa commonly not limited by forest communities and absent from the explored area: Chyloscyphus polyanthos, Geocalyx graveolens and Harpanthus flotovianus. Aneura pinguis occurs very locally. Calypogeia muelleriana is very rare. Even so widespread in Holarctic, Cephalozia bicuspidata is quite rare. Very infrequent were Diplophyllum taxifolium, Gymnocolea inflata and Mylia anomala.
Once found were Marchantia polymorpha and Pellia neesiana. Extremely rare (and observed in the axial part of the Dzhhugdzhur Range only) was Nardia geoscyphus. Certainly, the 'absence' of some taxa may be caused by the imperfections of the study, but for some taxa, another explanation may be necessary. It is difficult to expect that we have overlooked large-sized plants such as Chiloscyphus or so widespread in Northeast Asia, Harpanthus flotovianus. The rarity of Cephalozia bicuspidata and Diplophyllum taxifolium, as well as several other taxa is also surprising. The possible explanations may be in the wide distribution of basic and neutral rocks (blocking wide distribution of acidophilous taxa) and the wide distribution of neutrophilous and neutrotolerant taxa, with the latter also able to occupy the habitats potentially suitable for acidophilous taxa. A similar phenomenon was observed in the boreal zone in Sakhalin Island in the area with the evident dominance of limestone outcrops and neutral rocks ).
The vast majority of taxa belong to the arctic-montane flora elements, although boreal taxa are also quite common in the flora. Therefore, the liverwort flora, in general, possesses a subarctic character (thus being the same as the vegetation). Using Whittaker (1960) terminology, the relatively high gamma diversity of the explored area is associated with low level of alpha diversity: each small site by itself possesses a few species, but the difference in the inhabitants' composition of different sites (e.g. basic and acidic rocks) is quite strong. The combination of various flora elements and taxa of different 'acidophilia' results in taxonomically quite rich liverwort flora. Nevertheless, although our assumption on the taxonomic richness of the liverwort flora in the studied area was confirmed, the revealed diversity cannot be regarded as the final calculation. Moreover, taking into account the short duration of the studies, the data obtained by us should be considered as preliminary only and may be supplemented with many new findings in the further studies.

Phytogeographic implications
The DCA showed results in graphical form in Fig. 9, whereas Normalized numerical results are in Table 3. The hierarchical clustering in Fig. 10 (Ward's method, Euclidean distance) has shown more or less similar results. Following the latter, the most related floras to Ayan surroundings are those in South and North Sikhote-Alin (coresponding to SSA and NSA). Geographically, they are not the nearest to Ayan (e.g. the floras in northern Sakhalin Island are two times nearer and are on more similar latitudes). This cluster is united then with NH (the northernmnost part of Hokkaido Island) and then with the cluster including East-Manchurian Mountains (VMG), Changbai Mts. (CHAN) Table 3.

Normalized values of DCA for each compared flora
This unexpected relationship to the southerly situated floras inspired us to check the analysis results by its comparison with the mean distances from Ayan flora revealed by DCA. Since the results of DCA analysis are points in three-dimensional space, it is convenient to use the Euclidean distance metric to compare the minimal distance between points. The normalized Euclidean distance between the Ayan flora and the nearest other Compared flora distribution in DCA bubble chart (third axis is the colour gradient from deep blue to deep red).

Figure 10.
Hierarchical clustering (Ward's method, Euclidean distance). flora is shown in Table 4 (the distance to the nearest flora is taken as a unit). The most closely-related flora is the flora of North Sikhote-Alin (NSA, over 800 km south from Ayan), then (as is quite understandable) the flora of Lanzhinskiye Mts., situated in North Okhotiya (430 km by direct line from Ayan). The next is the mountain flora of Nabilsky Range in the middle Sakhalin (NAB). The latter is followed by South Sikhote-Alin (SSA), Shiretoko in North Hokkaido (NH), Schmidt Peninsula in Northern Sakhalin (SCHM) (geographically the nearest, slightly over 400 km from Ayan) and Ol'skoye Basalt Plateau (OLS) in Kolyma Upland (over 1000 km distance).  Table 4.
The mean distances between Ayan flora and other floras involved in the analysis. The font in bold indicates that distances are less than the mean value in the data samples.
As it could be seen from above placed description and also evident from Figs 9, 10, Ayan occupies the intermediate position between hemiboreal floras of continental mainland (Sikhote-Alin System) and hemiarctic continental and subcontinental floras in North Okhotiya and Kolyma Upland. The latter is a unique phenomenon of the flora equi-related to the conditionally 'northern' and 'southern' floras. Meantime, one more common feature may be revealed if looking precisely for the floras most related to Ayan: 1) Some of them: OLS, NSA and NAB provide the habitats for basiphilous plants (limestone or other basic substrates, like basalts). The Annachag Range (ANN), although nearer to Ayan than Ol'skoye Basalt Plateau (OLS), does not show these distinct relationships despite being entirely acidic (and the flora is composed by purely acidophilous plants). The relationships shown between Ayan and North Hokkaido (NH) may be an aberration we cannot explain definitely, although high elevations in the Island are the habitats for many arctic-alpine plants.

Conclusion
Ayan flora is the phytogeographical link between East Asian hemiboreal and Northeast Asian hemiarctic floras and shows relationships to both. Ayan surroundings were liverwort 'terra incognita' before the present study. Therefore, the conducted research is our attempt to reduce 'blank spots' and 'lost diversity' in liverwort flora of East Asia . Moreover, two additional arguments in favour of such kind of research are noticeable: 1) the collected material may be the basis for taxonomic revisions which rely on an integrative approach and understanding of the plant genetic diversity in the land; 2) the study of floras occupying an intermediate position between two vegetation zones (like Ayan) confirms the gradual changes from subarctic to hemiboreal floras -the fact commonly estimated, but infrequently tested in bryological researches.