Communities of the Bidentetea class of small coastal river valleys of the Western Pomerania (Poland)

The aim of work was to investigate the diversification, structure, physiognomy and floristic richness of riverside terophytes from Bidentetea tripartitae class in selected coastal river valleys. The investigates area encompassed the lower and middle section of the river valleys of Rega, Parsęta, Wieprza, Słupia, Łupawa  in the Western Pomerania (Poland). 68 phytosociological records were made according to the system of Braun-Blanquet. A database was created with the help of the software Turboveg for Windows.The hierarchy classification with the MVSP package was used in order to arrange and group the collected phytosociological data and to single out the types of plant communities. In order to evaluate similarity of samples with respect to species composition the Bray Curtis was calculated and the Unweighted Pair Group Method with Arithmetic Mean (UPGMA) was applied. Three plant communities belonging to one alliance Bidention tripartitae were identified: Bidenti-Polygonetum hydropiperis (Miljan 1933) Lohmeyer in R.Tx. 1950 nom invers., Bidentetum cernui Kobendza 1948, community with Polygonum minus, and one plant community belonging to the alliance Chenopodion glauci: Chenopodietum rubri Timar 1947.Given the natural origin of most of the patches, their structure, and the relatively small share of synanthropic species, including alien species, it should be emphasized that they constitute a spontaneous stage of succession of riparian vegetation. The presence of therophyte communities in the valleys of the coastal rivers, as well as their structure and physiognomy, testify to the preservation of their natural character.


Introduction
River valleys are often distinguished by abundance of flora and vegetation and are centers of biodiversity on the background of the surrounding landscape (Ward et al. 2002). Riparian zones are under the influence of natural disturbance such as flooding, erosion and sediment deposition, which create dynamic environmental conditions this specific heterogeneous habitat (Naiman & Decamps 1997). Communities of summer-annual pioneer vegetation are one of the components of the riparian zone. The habitats of riverside therophytes are protected as part of the European ecological Natura 2000 network (code 3270) (Borysiak 2004). These are pioneer communities of natural or anthropogenic character which occurs in wet and nutrient-rich habitats such as river banks, muddy alluvial sediments, desiccating oxbows, wet ditches and other man-made habitats (Šumberová & Lososová 2011). The habitats they occupy develop in river valleys during long periods of low water as a result of natural erosion and accumulation processes and are naturally characterized by rapid changes (Borysiak & Stachnowicz 2000). Their optimum occurrence is associated particularly with the low and middle course of large and medium rivers. River regulation combined with straightening and deepening of the river channel and human activities in the river valleys such us land use, recreation, logging, grazing, trampling eliminates habitats of riverside therophyte. Pollution, salification and hyperfertilisation of the habitats results in the recession of particularly sensitive communities, as well as the reshaping of the structure and physiognomy of the remaining phytocenoses (Brzeg & Ratyńska, 1983, Mucina et al. 1993, Richardson et al. 2007). In addition, riverside habitats are highly prone to invasion by alien plants, because of their dynamic hydrology and anthropogenic disturbances (Hood & Naiman 2000). For the full understanding and protection communities of riverside therophytes is necessary to examine the influence of habitat conditions on their state and species composition.
The aim of work was to investigate the diversification, structure, physiognomy and floristic richness of riverside therophytes from Bidentetea class in selected coastal river valleys in Western Pomerania.

Study area
The research was carried out within the valleys of coastal rivers of West Pomerania (Figure 1)the Rega (length 168 km, basin area 2725 km 2 ), Parsęta (length 139 km, basin area 3145 km 2 ), Wieprz (length 112 km, basin area 683 km 2 ), Słupia (length 139 km, basin area 1620 km 2 ) and Łupawa (length 99 km, basin area 924 km 2 ). They flow mainly through the Szczecin Coastland and Koszalin Coastland (Pobrzeże Szczecińskie and Pobrzeże Koszalińskie) geographic macroregions in the South Baltic Coastland (Pobrzeże Południowobałtyckie) subprovince and the mesoregions of the Łobez and Polanów Uplands (Wysoczyzna Łobeska and Wysoczyzna Polanowska) in the West Pomeranian Lakeland (Pojezierze Zachodniopomorskie) macroregion of the South Baltic Lakeland (Pojezierze Południowobałtyckie) subprovince (Kondracki 2000). The rivers are classified as small (Bajkiewicz-Grabowska & Mikulski 2017) and carry their water directly to the Baltic Sea. They flow from the high slopes of ground and terminal moraines and lake regions, with long stretches taking on the character of foothill rivers with well oxygenated, cold water and a bottom of stones or of stones and sand. Only near the estuaries where they empty into the sea do they acquire the features of lowland rivers with a bottom of sand and silt. Their characteristic feature is an uneven slope, sometimes reaching 2-3‰, and even 5‰ in short stretches. The catchment areas of the coastal rivers are mainly moraine plateaus used for agriculture, built on the surface of moraine clays of low permeability. Compared to rivers of other regions of Poland, fluctuations in water levels and flow are relatively small, which is linked to the high retention capacity of the land, in particular the abundance of lakes and endorheic basins and significant forest cover. The highest flow is usually noted in spring (20-60% more water than the yearly average) and the lowest in July and August, but they do not fall below half the average annual flow (Borówka et al. 2004).

Material and methods
Data sampling. The studies were conducted in summer and autumn in the years 2009-2012. The investigated area encompassed the lower and middle sections of the valleys of small coastal rivers -Rega, Parsęta, Wieprza, Słupia, Łupawa and their tributaries in Western Pomerania (Poland).
Nomenclature of plant species and plant communities. The classification of plant communities was taken from Brzeg & Wojterska (2001). The higher syntaxonomic units (alliances, orders, classes) were done according to Mucina et al. (2016). Vascular plant nomenclature was taken from Mirek et al. (2002) and bryophyte nomenclature was taken from Ochyra et al. (2003).

Results
In the research conducted on the banks of the coastal rivers of West Pomerania, 68 patches of summer therophytes of the class Bidentetea were identified. Based on the numerical classification (Figure 2) (Table 1, 2) Physiognomy and structurewas exercised by Polygonum hydropiper (dominated species). Among characteristic species belonging to the class and alliance, very low constancy characterised the following: Polygonum minus, P. mite, P. lapathifolium, Bidens tripartita, B. cernua, Alopecurus aequalis, Rumex palustris, Rorippa palustris. In some patches these phytoceonoses are dominated by the neophyte Bidens frondosa. Moss layer was not usually observed in the investigated patches. Group Da -The phytocenoses exercised by Bidens frondosa (dominated species). Among characteristic species belonging to the class and order alliance, very low constancy characterized the following: Polygonum lapathifolium subsp. lapathifolium, Rorippa palustris, Polygonum hydropiper, Polygonum minus, Rumex palustris, and Ranunculus sceleratus with high constancy. There was observed a significant share of species belonging to the class Isoëto-Nanojuncetea (Juncus bufonius, Plantago intermedia, Gnaphalium uliginosum). Species from the Artemisietea vulgaris class also play an important role in patches of Group Da (such us: Urtica dioica, Epilobium roseum, Eupatorium cannabinum). These species pass from adjoining phytocenoses. This phytocenoses was observed only in one river valley (Rega). Habitatson a flat, thin, sandy shoal covered with aggradate mud which developed on the edges of riparian forests, only one patch was found next to the rush communities complex. Group Db -Meadow species have significant share in these phytocoenoses such as Deschampsia caespitosa, Agrostis stolonifera, Ranunculus repens. Among characteristic species from class and alliances almost only Polygonum hydropiper with high coverage was recorded. Habitatsthe patches were observed in shallow depression within anthropogenic coniferous forest in contact with mid-forest meadow fitocoenoses. Group Dc -A low share of characteristic species of Bidentetea class was observed in patches from this group. Species from adjoining phytocoenoses penetrated into the inside of pathes (especialy from Phragmito-Magnocaricetea and Molinio-Arrhenatheretea class). Habitats -The phytocenoses from this group have been observed in several types of habitat. Large group of patches developed at the edge of a footpath trodden by people and in watering places visited by animals. The other group of patches accompanied the outflow of subsoil water and were developing on a very soggy, watery surface. Several patches also developed in shallow terrace depressions filled with the stagnant water. Group Dd -Patches are characterized by considerable share of Bidens frondosa. A relatively high share of species from the class Molinio-Arrhenatheretea was observed (such as Ranunculus repens, Poa palustris, Equisetum palustre, Myosotis palustris). Habitatson flat slimy river banks, within land depression behind natural levee, and along meander scroll. Group De -Patches belongs to this group are characterized by significant share of characteristic species from Bidentetea class in comparison to other groups. Habitatson flat river banks of meanders where silt formation process was observed in the soil (black colour of soil surface) Group Dfhas an anthropogenic character, patches were found on the ground road with stagnant water in contact with meadow fitocoenoses -hence the significant share of meadow species in patches. Characteristic species from class and alliances are sparsely represented. Floristic richnessfloristically rich. In total, 153 plant species were identified in the investigated patches. The number of species in particular relevés varied only from 7 to 29 (average 15 species). Only 6 species belonged to the constancy classes III-V (Polygonum hydropiper, Urtica dioica, Glyceria fluitans, Phalaris arundinacea, Ranunculus repens, Veronica beccabunga). Sizefrom 1m 2 to even 70 m 2 . Adjoining phytocenosesrush communities, especially Phalaridetum arundinaceae and Caricetum acutiformis, and less frequently to Glycerietum maximae, Phragmitetum communis, Caricetum gracilis, Typhaetum lalifoliae, Caricetum paniculatae; tall herbs communities, mainly Urtico-Calystegietum sepium, less frequently Eupatorietum cannabini and comm. with Rubus caesius; riparian forests (Alnion incanae); meadow communities (Calthion palustris).

Discusion
Phytocoenoses of summer therophytes observed by various authors in river valleys have been both anthropogenic and natural (Geißelbrecht-Taferner & Mucina 1993, Ratyńska 2001, Spałek 2008. Most of the patches (75%) of therophyte communities formed in the study area were of natural origin and were typical of riparian vegetation. They were most often formed on the flat banks of rivers and within alluvia, where there was a layer of fertile silt of varying thickness on the surface of the sandy material deposited by the river. They were dominated by patches of the Bidenti-Polygonetum hydropiperis association. They have been reported from the banks of small watercourses and streams, e.g. by Kępczyński (1965) from the Dobrzyń Upland (Wysoczyzna Dobrzyńska) and by Stuchlikowa (1972) from the valley of the River Skawa.
An important element of the structure of the patches was species penetrating them from neighbouring phytocoenoses, which was often aided by their small area. Species of the class Phragmiteto-Magnocaricetea played a particularly important role, and to a lesser extent species of the classes Molinio-Arrhenatheretea and Artemisietea vulgaris. Authors such as Kępczyński, Peplińska (1998), and Felzines & Loiseau (2005) have observed a large contribution of species from neighbouring phytocoenoses.
Compared to the communities described from large rivers, such as the Oder (Borysiak 2002), Warta (Brzeg & Ratyńska 1983), and Elba (Kiesslich et al. 2003), the patches observed were relatively poor in synanthropic species. In addition, only one patch on the edge of a dirt road was of the alliance Chenopodion rubri, which is represented in large numbers near large rivers. Similarly, Stępień (2010) observed patches of communities belonging only to the Bidention tripartitae alliance in the valleys of small rivers and streams of the Wałcz Plain (Równina Wałecka).
Only 25% of the patches owe their origin to human interference. They have developed in the ruts of roads and on the edges of wooded roadsides, in whose depressions water had accumulated after flooding of rivers in the spring, as well as after rainfall. Therophyte communities accompanying roads have been noted by Wawer (1981) and by Fijałkowski & Adamczyk (1990). Therophyte patches have also developed in depressions and paths trodden by people, mainly anglers, near riverbeds and in peatlands.
They were also observed on animal trails leading to watering holes and areas rooted by wild boar, where the destruction of the plant cover and the formation of a slight depression relative to the ground surface enabled longer water retention and the appearance of patches of summer therophyte communities (8.8% of the patches were created due to animal activity). Therophyte communities developing at watering holes have also been observed by Harasim (1978).
Many authors note the contribution of alien species in patches of summer therophyte communities, especially Bidens frondosa, which has become a common element of therophyte phytocoenoses, influencing their physiognomy and structure. Its occurrence has been reported for years from large rivers (Borysiak 1994, Tokarska-Guzik 2005, Myśliwy 2019), but also from the valleys of small rivers. For example, Ratyńska (2003) observed it on the River Główna, which directly flows into the Warta. The presence of this species in the valleys of coastal rivers is particularly pronounced in the valley of the Rega and its tributaries. Other alien species were rarely recorded.
Given the natural origin of most of the patches, their structure, and the relatively small share of synanthropic species, including alien species, it should be emphasized that they constitute a spontaneous stage of succession of riparian vegetation. The presence of therophyte communities in the valleys of the coastal rivers, as well as their structure and physiognomy, testify to the preservation of their natural character.

Conclusions
Contrary to communities from alliance Chenopodion rubri belonging to the class Bidentetea which are situated in the valleys of large rivers or places with anthropogenic pleasure, in the valleys of the small coastal rivers almost exclusively patches belonging to the alliance Bidention tripartitae were recorded. Very important components of the communities surveyed include the species from contact phytocoenoses, especially rush and meadow species.
Contrary to the phytocoenoses described from river valleys with stronger antropopressure, synanthropic species as well as alien species played a marginal role in the most of the patches, which proved the natural origin of these phytocoenoses. The presence of Bidens frondosa is particularly pronounced only in the valley of the Rega and its tributaries.
The presence of therophyte communities in the valleys of the coastal rivers, as well as their structure and physiognomy, testify to the preservation of their natural character.