Vegetation changes of the Molinio-Arrhenatheretea class in the Bystra valley , eastern Poland

The study objective was to analyze and assess the vegetation changes of the meadows (Mo linio-Arrhenatheretea class) located in the valley of the Bystra River in eastern Poland after a period of 38 years, and to identify the direction of plant communities succession. The studies were conducted in the years 1973 and 2011 on grasslands belonging to farmers. The floristic diversity was identified based on the phytosociological structure and mean number of species calculated based on the number of species in the particular relevés of the phytocenosis under study. The highest frequency of occurrence was demonstrated by the Poa pratensis-Festuca rubra community within which more than half of the patches persisted over the 38-year period. New communities (Scirpetum sylvatici, Alopecuretum pratensis, Lythro-Filipenduletum) appeared after the withdrawal of habitats of the Phragmitetea class, due to the lowering of the groundwater level. A portion of the meadows and pastures were transformed into arable fields, gardens or building plots. Among ecological indicators, the greatest differences were observed in the mean soil moisture values between the two years compared, both for all meadows and the predominant community, which indicates an increase in habitat humidity.


Introduction
The valleys of rivers and smaller streams are an integral feature of agricultural landscape.They are characterized by varied habitat conditions both in terms of water content and fertility of the soil, which is reflected in the diversity of semi-natural communities occurring there.Most of them were or still are used agriculturally, mainly by cutting and, to a lesser extent, grazing of animals.Besides the habitat conditions, both the manner and intensity of land use have an impact on the persistency and floristic diversity of these communities.Interference with the habitat conditionsthrough regulating the hydrologic regime, limiting the intensity of use or its abandonment -leads to changes in the species composition and size of the area covered by plant communities [1][2][3].Modern agricultural trends result in increasing the area of land excluded from use, including grassland [4].The discontinuation of the use of meadow communities leads to their fast degradation [5] and frequently irreversible habitat changes [6][7][8].Due to draining and land use changes, many habitats situated in river valleys in Poland have been subject to considerable transformation resulting in the extinction of certain plant species and disappearance of the previously occurring plant communities [9,10].Development of meadow communities depends upon flora composition and edaphic conditions of the habitat [2].Herbaceous vegetation is also one of the most sensitive indicators of landscape changes [11].One of the areas subjected to various changes is the valley of the Bystra River, a right-hand tributary of the Vistula, flowing across the Lublin Upland in eastern Poland and emptying into the Małopolska Vistula Gap at Bochotnica [12].The aim of the study was to (i) analyze the vegetation changes of the Molinio-Arrhenatheretea class in the valley of the Bystra River in eastern Poland after a period of 38 years and to (ii) identify the direction of plant communities succession.

Characteristics of the Bystra catchment
The Bystra River starts in Czesławice (vicinity of Nałęczów) and flows from the east to the west, emptying into the Vistula at Bochotnica (downstream from Kazimierz Dolny) in eastern Poland (Fig. 1).It is fed by several streams, the largest of which is the Czerka River.The Bystra catchment, covering about 299 km 2 , encompasses the eastern part of the Kazimierz Plateau composed of Cretaceous rocks covered by post-glacial materials (sands and gravels as well as layered clays) of varying thickness, overlain by a loess layer [13].The thickness of the loess cover depends on the intensity of erosion that commonly occurs in the Bystra catchment [14].A large network of gullies dissecting both sides of the Bystra valley are a characteristic feature of the catchment [13].
Most of this area is used agriculturally (about 90%, including 1.6% of grasslands), while the remainder is covered by forests and shrubbery.Erosion causes the accumulation and sedimentation of soil material, which results in the formation of alluvial soils.Alluvial fans develop at the mouths of the gullies (the sediment layer is up to 100 cm thick) and, in some places, constitute a barrier along the river channel, thus restricting the free drainage of flood waters, which causes the periodic or permanent paludification of the valley below them.The hydrologic regime in the valley is also shaped by processes of bottom and riverbank erosion.The Bystra has high gradients (up to 3%) in its upper and lower reaches and a lower gradient (1%) in its middle reaches where engineering works were carried out (mainly straightening of the river channel) aimed at accelerating the drainage of flood waters.This has resulted in increased intensity of river bottom erosion, controlled by damming facilities that were damaged by flood waters [13,14].The biological structure in the form of Salix species occurs along most stretches of the river, which considerably reduces bottom erosion, particularly along the previously straightened stretches of the river channel [15].The course of the river and layout of the valley bottom are characteristic of typical lowland rivers.The meadows in this valley are frequently flooded both during spring thaw and after heavy rainfall in the summer.The groundwater level varies, which has an impact on the plant communities.

Field study
The studies were conducted in the years 1973 and 2011 in the valley of the Bystra River, on grasslands with an area of approx.20 ha belonging to farmers from the following localities: Nałęczów (2.45 ha), Łąki (2.66 ha), Wąwolnica (4.90 ha), Marecz ki (3.04 ha), Rogalów (2.33 ha), Zawada (1.55 ha), and Bartłomiejowice (3.31 ha).Twenty-nine phytosociological relevés were performed in 1973 and 38 in 2011 according to the Braun-Blanquet [16] method.All relevés presented in this study were assigned to plant communities of the Molinio-Arrhenatheretea class.The same area of grassland was taken into account in both years.In 2011, several mosaic alliances were observed and relevés were performed for the most distinct patches covering a minimum of 25 m 2 ; the patches were marked with letters (a, b, c, etc.).The floristic diversity after a period of 38 years was identified based on the phytosociological structure and mean number of species calculated based on the number of species in the particular relevés of the phytocoenosis under study.Owing to the large number of relevés, the predominant Poa pratensis-Festuca rubra community was presented in Tab. 1 and Tab. 2, using degrees of constancy (S) and cover-abundance (D) scales.

Data analysis
Changes of the edaphic conditions were assessed using ecological indicator (F -moisture, R -acidity, and N -nitrogen content) values by Ellenberg et al. [17].This study presents the range (minimum and maximum) of ecological indicator values and the mean values for all relevés and for the predominant Poa pratensis-Festuca rubra community.The pragmaTax (Cortex Nova, Bydgoszcz, Poland) program was used to carry out the numerical classification for all relevés based on the quantitative share of species.The weighted pair group method of arithmetic averages (WPGMA) was used.
A comparison of the dendrograms obtained in the classification made it possible to include groups of relevés at alfa scale 0.5 -similar in terms of community species composition -in the phytosociological tables.The qualitative similarity between the vegetation after a period of 38 years at the level of class (ChCl.)and order (ChO.) was calculated using Jaccard's index [18].Phytosociological taxonomy is based on Matuszkiewicz [19], and the species names are provided according to Mirek et al. [20].Based on the studies conducted in 1973, when 29 phytosociological relevés were performed, five phytocoenoses were distinguished: two associations (Arrhenatheretum elatioris and Lolio-Cynosuretum) as well as three communities (Poa pratensis-Festuca rubra, Phleum pratense, and Polygonum bistorta) (Tab. 1, Fig. 2).In the Bystra valley, the Poa pratensis-Festuca rubra community was recorded most frequently; it was represented by 19 phytosociological relevés.In 1973, this community showed a high degree of constancy and cover-abundance for P. pratensis and F. rubra; it was lower for the other differential species: Alopecurus pratensis and Holcus lanatus.Species characteristic of the Arrhenatheretalia order (nine species) were also recorded in this community: Heracleum sphondylium and Leucanthemum vulgare occurred most frequently.Species characteristic of the Arrhenatherion alliance occurred rarely and with low cover-abundance (Tab.1).The most numerous group was constituted by species characteristic of the class (15 species), among which Festuca pratensis stood out with its high degree of constancy and considerable cover-abundance.Constant components of the meadow (S = V) were also: Ranunculus acris, Trifolium pratense, Rumex acetosa, and Lathyrus pratensis as well as Cerastium holosteoides, P. pratense, and Plantago lanceolata (S = IV).Lychnis flos-cuculi was recorded most often among the seven species characteristic of the Molinietalia order.

Phytosociological
The other communities identified were represented by a small number of phytosociological relevés (Tab. 1, Fig. 2).The Lolio-Cynosuretum association was identified based on four relevés where Trifolium repens predominated.The share of other species characteristic of this association (Bellis perennis and Leontodon autumnalis) was small.No species characteristic of the Arrhenatherion alliance and a few taxa of the Arrhenatheretalia order were found in this community.The Arrhenatheretum elatioris association was distinguished by the presence of Arrhenatherum elatius and Geranium pratense (Tab.1).Characteristic species of the Arrhenatherion alliance (Galium mollugo, Crepis biennis, Knautia arvensis) and the Arrhenatheretalia order (H.sphondylium, L. vulgare, Taraxacum officinale, Lotus corniculatus, Daucus carota, Bromus hordeaceus, Dactylis glomerata, and Rhinanthus alectorolophus) were also recorded in this community.Species characteristic of the Molinio-Arrhenatheretea class were also highly represented (14 species).The association colonized the driest parts of the valley, i.e., typical dry grasslands, in contrast to the community Polygonum bistorta that occupied the most humid habitats, at the boundary with communities of the Phragmitetea class.This community was characterized by high abundance of P. bistorta and occurrence of Cirsium rivulare (Tab.1).One patch (Relevé 50) was a transitional community corresponding to hay meadows as evidenced by the high cover-abundance of P. pratensis and species characteristic of the order (six species), particularly H. sphondylium (Fig. 2, Tab. 1).
Communities of the Arrhenatheretalia order were characterized by a considerably greater diversity (mean number of species in relevé -26) in comparison with Molinietalia (18).They were distinguished by a larger share of species characteristic of the Arrhenatheretalia order with higher degrees of constancy on the one hand and, on the other hand, by a smaller share of taxa of the Calthion alliance and Molinietalia order and lower degrees of constancy.These communities were also dominated by species characteristic of the class, with high cover-abundance and a high degree of constancy in comparison with communities of the Molinietalia order (Tab.2).
Tab. 1 Floristic composition of plant communities in 1973.Within the class discussed, the Poa pratensis-Festuca rubra community, occupying large areas in the Bystra valley -mainly in its middle reaches -was the most numerous.This community was distinguished by a high degree of constancy and coverabundance of P. pratensis and other distinguishing species: H. lanatus, A. pratensis, and F. rubra.Species characteristic of the Arrhenatherion (four species) and Cynosurion alliance (three species) were found in the meadows.The Arrhenatheretalia order was represented by eight species, among which H. sphondylium, T. officinale, D. glomerata, and L. vulgare were the most frequently recorded.The most numerous group comprised species characteristic of the Molinio-Arrhenatheretea class (14 species), among which Festuca pratensis was the most frequently recorded.Following species, R. acris, T. pratense, P. pratense, and P. lanceolata occurred often, too.The Molinietalia order (16 species), including nine characteristic species of the Calthion alliance, was also highly represented.The most frequently recorded were D. caespitosa, L. flos-cuculi, Equisetum palustre, and C. rivulare.It is worth noting the occurrence of species of the Phragmitetea class, namely Carex gracilis and Phalaris arundinacea, which indicates periodically greater humidity of a portion of the habitat.In addition, two communities with a predominance of D. glomerata and P. pratense were also found within the Arrhenatherion alliance.The first community corresponded to hay meadows with a large share of species characteristic of the Arrhenatheretalia order (six species) and Molinio-Arrhenatheretea class (11 species).The community with Phleum pratense occurred in a periodically humid habitat, which was indicated by the occurrence of species characteristic for the Molinietalia order as well as a small number of humid habitat species, such as C. gracilis and P. arundinacea (Tab.2).The community remained very much the same over the years and its species composition was similar to that in 1973.The last community of this alliance was the Lolio-Cynosuretum association that, in comparison with 1973, survived as a single patch where traditional goat grazing had been conducted.This association was characterized by a predominance of Lolium perenne and T. repens and a considerably smaller share of B. perennis.

Lolio-Cynosuretum
In 2011, a considerably greater number of communities of the Molinietalia order was recorded.One of them was the Lythro-Filipenduletum ulmariae association, characterized by a predominance of F. ulmaria and L. salicaria and high cover-abundance of species characteristic of the Filipendulion alliance and the order (Fig. 3, Tab.2).Besides the predominant species mentioned above, L. flos-cuculi and Lysimachia vulgaris also occurred frequently.Within the Calthion alliance, the Cirsietum rivularis association was distinguished; it was characterized by a predominance of the characteristic species and considerable cover-abundance of species of the order (nine species) and class (eight species).Scirpus sylvaticus and Symphytum officinale were also found to have high cover-abundance.The Scirpetum sylvatici was characterized by the predominance of the characteristic species and occurrence of the Calthion alliance species (Tab.2).Within the alliance, the D. caespitosa community was also distinguished, characterized by the predominance of that species and high cover-abundance of the species characteristic of the Molinietalia order (seven species).
Within the Alopecurion alliance, the Alopecuretum pratensis association was distinguished, with the predominance of A. pratensis and a certain share of S. officinale.Species characteristic of the Molinietalia order (six species) and Molinio-Arrhenatheretea class (eight species) were also recorded in the sward of these meadows.The community H. lanatus also showed similar characteristics.This community is distinguished by high cover-abundance of H. lanatus and species characteristic of the Molinietalia order.High cover-abundance was characteristic for F. ulmaria, while D. caespitosa and L. salicaria had low cover-abundance.No species characteristic of the Arrhenatherion alliance and Arrhenatheretalia order (except Achillea millefolium) were found.
The phytosociological studies conducted in the valley of the Bystra River in 2011 showed a greater diversity of the species composition of communities of the Molinio-Arrhenatheretea class in comparison with 1973.The number of communities within this class in the study area rose from 5 (1973) to 10 (2011).The Poa pratensis-Festuca rubra community occurred most frequently in both years (58% of relevés).More than half of the patches of this community persisted for the 38-year period with few changes in their species composition.The cover-abundance of the two differential species for this community, i.e., P. pratensis and F. rubra, decreased, while the cover-abundance of A. pratensis and H. lanatus increased.Furthermore, there was a considerable increase in the cover-abundance of Festuca pratensis, which became a dominant species in this community, and of other species characteristic of the Molinio-Arrhenatheretea class.Patches of the Lolio-Cynosuretum association and community with Phleum pratense also persisted (Tab.2, Tab.3).Some phytocenoses of the Poa pratensis-Festuca rubra community changed into mosaic complexes where the analyzed community was accompanied by patches (over 100 m 2 ) with H. lanatus and D. caespitosa or, in a local depression, the Cirsietum rivularis association.Some communities changed and showed considerable syntaxonomic similarity: the Poa pratensis-Festuca rubra community changed into the community with D. glomerata, which may have resulted from the use of resowing; Arrhenatheretum elatioris changed into the Poa pratensis-Festuca rubra community, while a community with Polygonum bistorta changed into the Cirsietum rivularis association characterized by similar species composition (Tab.3).The Arrhenatheretum elatioris association was not recorded in 2011.Most of the new communities recorded in 2011, belonging mostly to the Molinietalia order, also appeared in habitats after the withdrawal of communities of the Phragmitetea class, due to the lowering of the groundwater level.Some meadows and pastures were changed into arable fields, gardens or building plots (Tab.3).
An important indicator of changes in communities was the total number of species in the study area, number of species characteristic of the particular syntaxonomic units, and the degree of their constancy.Within the Molinio-Arrhenatheretea class, 85 herbaceous species were recorded in 1973 and 100 in 2011.However, the average number of species per relevé was higher in 1973 (24) than in 2011 (23).In 2011, communities of the Arrhenatheretalia order showed greater species richness in comparison with the Molinietalia order, within which communities of the Calthion alliance, particularly Scirpetum sylvatici, had the poorest species richness (Tab. 1, Tab. 2).In comparison with 1973, the constancy and number of species characteristic of the Arrhenatheretalia order and its alliances decreased in 2011, but increased in the case of the Molinietalia order and Calthion alliance (Tab.4).The most numerous group comprised species characteristic of the class.In 1973, a greater number of these species showed the degree of constancy IV and V.In 2011, species characteristic of the Phragmitetea class, particularly C. gracilis and P. arundinacea, were recorded in the meadow.Due to decreased humidity, communities of this class partially transformed into communities of the Molinietalia order.Jaccard's indices revealed remarkable changes in the species composition after a period of 38 years, especially at the level of the order Arrhenatheretalia (J = 0.26) and Molinietalia (J = 0.34).A larger similarity at the level of the class (J = 0.54) was observed (Tab.4).These habitat changes are also confirmed by the ecological index numbers [17].The paper does not include climatic indicators because they are usually subject to the smallest fluctuation.Edaphic indicators, particularly the moisture index (mean F = 5.5 for year 1973, mean F = 6.3 for 2011), showed greater changes.The habitats of communities of the Molinietalia order (Cirsietum rivularis F = 7.3-7.9and Scirpetum sylvatici F = 7.8-8.0)had the highest humidity index.However, changes in the moisture index were also observed within Poa pratensis-Festuca rubra communities (5.5 for year 1973, 5.9 for 2011), which resulted from the appearance of humid habitat species (Tab. 1, Tab. 2).The mean values of acidity indicator decreased only slightly.On the other hand, the nitrogen content index decreased (5.8 for year 1973, 5.3 for 2011), which may have resulted from the gradual impoverishment of the habitat (Tab.5).The decrease in the values of this indicator also applies to the Poa pratensis-Festuca rubra communities (5.8 for year 1973, 5.5 for 2011).

Discussion
The plant communities of the Molinio-Arrhenatheretea class described in this study are typical of valleys of smaller and larger rivers in Poland [21][22][23][24][25][26].In the Bystra valley, the Poa pratensis-Festuca rubra community was recorded most frequently, which resulted from the systematic cutting of meadows, frequently accompanied by fertilization, as indicated by the considerable cover-abundance of agriculturally valuable plant species (F.pratensis, P. pratensis, P. pratense, and Trifolium pratense).This vegetation grew mostly in habitats of flood-meadows subjected to periodic short-lasting flooding, as evidenced by the presence of humid (Molinietalia order) and wet habitat species such as Carex gracilis (Phragmitetea class) which belongs to flood-tolerant species and has a wide range of plasticity to adjust its physiology resulting in metabolic, morphological, and anatomical acclimation [27].The consequence of waterlogging is usually anoxia and wetland species characterized by tolerance to this phenomenon [28].On the other hand, Poa pratensis-Festuca rubra meadows are Natura 2000 habitats (6510-2) but, according to Korzeniak [29], these habitats only include floristically rich meadows whose species composition is similar to psammophilous grassland.However, cultivated meadows, with a predominance of grasses with a high fodder Mean number of species (min-max) 24  23  value, may not be included in this habitat.Typical meadows with a large share of grasses, especially F. pratensis, P. pratensis, Anthoxanthum odoratum, D. caespitosa, F. rubra, or Agrostis gigantean, and a small share of dicotyledons develop in more humid habitats [30].An analogous community, with a very similar species composition, was described in another river valley in the Lublin Upland [31][32][33][34].
The Lolio-Cynosuretum association was recorded in dry grasslands formed at the mouths of the gullies, along which sediment-carrying water flowed from the adjoining areas into the valley [14].These phytocoenoses have developed under the influence of intensive grazing, in this case -goat grazing, in contrast to the other association Arrhenatheretum elatioris which is mainly based on cutting.
In 2011, a considerably greater number of communities of the Molinietalia order was recorded.One of them was the Lythro-Filipenduletum ulmariae association.Meadows of this type are anthropogenic or partially natural herbaceous communities occurring along streams [19].The existence of Molinietalia meadows depends on the water regime of river valleys [1].They are characterized by the colorful aspect of blooming plants.The Scirpetum sylvatici association developed as an ecotone within some associations of the Phragmitetea class.It usually occupies small areas in local depressions, often in places fed by seeping water.According to Kucharski [21], the other meadow association, Alopecuretum pratensis, is one of the most agriculturally important types of wet meadows in Poland, maintained thanks to adequate fertilization and cutting.Neglecting such measures causes the degradation of these meadows, which was observed in the studies.Many Alopecurus meadows transformed into Deschampsia ones.Meadows with the predominance of D. caespitosa develop due to the lack of proper maintenance on humid grasslands and are characterized by poor fodder value [19].These grasslands are characterized by low biodiversity, but tussocks of D. caespitosa are reservoirs of meadow species even at advanced successional stages and may play a significant role in the process of long-term species turnover [35].The other type of degraded meadow is the Holcus lanatus community, regarded by Matuszkiewicz [19] as a degenerative phase caused by the excessive drying of the upper soil layers or the abandonment of fertilization, which is suggested by the occurrence of A. pratensis (Tab.2).These patches corresponded to the humid meadow variants occurring in Poland, as described by other authors [22,36,37].
In 2011, a greater diversity of the species composition of communities of the Molinio-Arrhenatheretea class in comparison with 1973 was noted (Tab.3).The number of communities within this class in the study area increased from 5 (1973) to 10 (2011), but the Poa pratensis-Festuca rubra community was the predominant in both years.
The Arrhenatheretum elatioris association was not recorded in 2011, as it is locally threatened with extinction due to its strong dependence on appropriate grassland management [21,38,39].One of the reasons is also the gradual disappearance of characteristic species due to the land management changes [1].The differences in the species diversity of communities from the Arrhenatheretalia and Molinietalia order are confirmed by the results of studies by other authors [38].
Habitat changes are also confirmed by the greater changes in ecological indicator values [17].Ecological indicators perform a very important role in assessing changes Tab. 5 Changes in the values of edaphic indicators [17]. of the vegetation cover and habitat conditions occurring in several homologous communities replacing one another in the course of ecological succession.Phytocoenoses include sets of species which are relatively uniform, relatively constant and instantly recognizable, which can be helpful in attempting an assessment of vegetation transformations [40,41].Moisture conditions are the most various indicators on the grassland sites [42].Changes in moisture conditions influence the changes in the vegetation and even cause the formation of new plant communities (Tab.3).Vegetation changes in flooded meadows in rivers valley also depend on flooding duration and alluvial processes which cause changes in the soil seed bank [43], which is considered an important factor influencing plant community changes.The reserve of propagules in the soil facilitates regeneration of plant communities [44].At the level of succession changes of plant communities within different phytosociological classes, the soil humidity index has a medium indicative strength.Among the plant communities analyzed in this study, communities of the Arrhenatherion elatioris alliance occur in the driest habitats, while those of the Filipendulion ulmariae, Calthion palustris, and Alopecurion pratensis alliance -in the most humid [41].It should also be noted that in meadows of the Molinio-Arrhenatheretea class, indicator-species of humid and wet habitats predominate [17], which has an impact on the succession changes of grasslands [40].On the other hand, a gradual impoverishment of the habitat in the study area was observed as shown by the decreasing values of the nitrogen content indicators (Tab.5).

Indicator
A reduced nitrogen content in the soil was observed not only for all communities but also within the dominant Poa pratensis-Festuca rubra community, which resulted from the changes in species composition, the appearance of indicator species with a lower value of that indicator, and a decreasing share of indicator species with a higher value.It should be noted that communities of the Arrhenatherion alliance are among the most fertile in the class [41].This phenomenon is also observed within the same plant communities, which shows a considerable reduction in application of mineral fertilizers by farmers or their abandonment [4,5].Changes in the vegetation of grasslands in the Bystra valley were primarily caused by the extensification of management, including reduced mowing and absence or reduction of mineral fertilization.The vegetation transformation also resulted from the changes in land use.Furthermore, the directions of grassland succession were caused by the changes in the soil environment, particularly increased humidity and decreased fertility of habitats.Succession resulting from decreased humidity (drying) was also observed in a few cases.

Tab. 4
Changes in the number of characteristic species and their constancy degree.

of the community in particular years (relevé number) 1973 2011
3ab.3Directions of changes in plant communities of Molinio-Arrhenatheretea class.