Occurrence and differentiation of Arnoserido-Scleranthetum ( Chouard 1925 ) in the Południowopodlaska Lowland

The paper presents Arnoserido-Scleranthetum phytocoenoses at the eastern and north-eastern peripheries of their occurrence range and the results of a field study conducted in the Południowopolska Lowland in 1990–2010. The investigations were based on phytosociological relevés made by the Braun-Blanquet method. The greatest numbers of localities of the analyzed association were found in the mesoregions located in the south-western part of the Południowopolska Lowland, i.e. the Kałuszyn Upland, Węgrów Lowering, Żelechów Upland, the south-western part of the Łuków Plain, and the southern part of the Siedlce Upland. Fewer patches of this association were reported from the eastern and north-eastern part of the Południowopolska Lowland, in particular from the Podlasie Gorge of Bug. With the exception of the latter one, in all the mesoregions the patches of the analyzed association exhibited internal variability reflecting the diversity of habitat moisture conditions, which was manifested by the occurrence of hygrophilous species in the analyzed phytocoenoses. They comprised patches of Arnoserido-Scleranthetum with Illecebrum verticillatum, which are regarded as threatened with extinction in Poland and Europe. The phytocoenoses of Arnoserido-Scleranthetum with the dominance of Anthoxanthum aristatum are particularly noteworthy as a reflection of degenerative changes caused by the mass occurrence of this expansive species.


Introduction
The Arnoserido-Scleranthetum association, characterized by a sub-Atlantic character, occurs in an impoverished form in the western and central part of in Poland [1].It occupies the poorest habitats with sandy acidic soils with a pH of 4.0-5.0.Arnoseris minima is a characteristic species serving a diagnostic function.In turn, Spergula morisonii, Veronica dillenii, Anthoxanthum aristatum, and Teesdaela nudicaulis are differential species occurring with varied frequency across Poland.The association tends to retreat from the area of Poland [2][3][4][5][6][7] and Europe [8][9][10][11][12][13].The abandonment of land management on the poorest soils (land fallowing and afforestation) and changing habitat conditions through increased nutrient inputs pose a threat to the phytocoenoses of the association.The Arnoserido-Scleranthetum localities in the Południowopolska Lowland, in particular its north-eastern peripheries, are the easternmost and northernmost localities in the occurrence range of this retreating association.
The aim of the study is to analyze the Arnoserido-Scleranthetum phytocoenoses and assess the frequency of their occurrence in the mesoregions of the Południowopolska Lowland.

Material and methods
The field study involved analysis of phytosociological relevés made by the Braun-Blanquet method [14].In total, over 2000 phytosociological relevés were taken in cereal-growing fields, including 262 defined as Arnoserido-Scleranthetum communities.Phytosociological classification was accepted according to Matuszkiewicz [1].The nomenclature of species follows Mirek et al. [15].The observations were carried out in 1990-2010 in the area of the Południowopolska Lowland macroregion.The investigations were conducted in 6 mesoregions, i.e. the Kałuszyn Upland, Węgrów Lowering, Siedlce Upland, Łuków Plain, Podlasie Gorge of Bug, and the eastern part of the Żelechów Upland.The total study area covered approximately 9000 km 2 (Tab.1).The study area comprises a large proportion of light sandy soils (a poor rye complex and a very poor rye complex), particularly in the Podlasie Gorge of Bug and Kałuszyn Upland.The Południowopolska Lowland is a typical agricultural macroregion with a high proportion of extensive farms, producing mostly for their own needs.
The analysis of the distribution and frequency of the occurrence of Arnoserido-Scleranthetum phytocoenoses was based on the number of localities = locations and the proportion of association patches in potentially suitable habitats (on light acidic soils).The frequency was expressed in a 3-point scale: 1 -species covering over 60%; 2 -species covering from 40 to 60%; and 3 -species covering below 40% of the species-specific habitats.Constancy and cover factors were calculated for each species presented in the table [14].In order to compare the analyzed phytocoenoses in the mesoregions in question, the systematic group value, illustrating the share of characteristic and differential species in these communities, was calculated [16]: D = G × S / 100, where: S -mean constancy of the group, G -collective share of the group.
These parameters were calculated according to the following equations: S = (g / z × n) × 100 and G = g / t × 100, where: z -number of species in the group, n -total number of relevés in the association table, g -sum of records of species from the particular groups in the table, t -sum of records of all species in the table.
The systematic group value was calculated twice taking into account the share of Scleranthus annuus in the group of characteristic and differential species (D 1 ) and disregarding the species (D 2 ) due to its low diagnostic value caused by its frequent occurrence in other phytocoenoses in similar habitats.

Arnoserido-Scleranthetum patches
The phytocoenoses of the analyzed association were noted most frequently in the Kałuszyn Upland, where the greatest density of patches, exceeding 60%, was found on light and acidic soils in the individual localities.
A lower density of localities (40-60%) was found in the Węgrów Lowering area, where nutrient-rich soils prevail.Similar frequency of the patches of the analyzed association was reported from the Żelechów Upland, the south-western part of the Łuków Plain, and the southern part of the Siedlce Upland.
In contrast, no phytocoenoses studied were found in the northern part of the Siedlce Upland due to the absence of habitats suitable for Arnoserido-Scleranthetum.This region comprises the largest proportion of the most fertile soils of the Południowopolska Lowland.Arnoserido-Scleranthetum patches were seldom observed in the area of the Podlasie Gorge of Bug as well.A higher share of patches of the associations in question was only reported in Mielnik commune.In the other parts of the Podlasie Gorge of Bug area, small insular localities of the association, exhibiting no temporal and spatial continuity, were only found.

Analysis of the diversity of the composition and structure of the phytocoenoses of the analyzed associations
In the Południowopolska Lowland, Arnoserido-Scleranthetum patches occupy different habitats in terms of moisture.This was reflected in the differences in the composition and structure of the analyzed communities.Typical and wet variants were distinguished.This diversity was observed in the entire study area with the exception of the Podlasie Gorge of Bug.Communities with the greatest species richness were noted in the Kałuszyn Upland (a mesoregion located in the western part of the macroregion), where the average number of species per relevé was 13.4 (Tab.2).Phytocoenoses developing in the area were characterized by the highest share of Arnoseris minima.High frequency and substantial cover rates were found in the patches of the analyzed association for the differential species Teesdalea nudicaulis and Anthoxanthum aristatum as well as, although less frequently, for Veronica dillenii, Spergula morisonii, and Holcus mollis.Similarly, Scleranthus annuus was characterized by a large share in the phytocoenoses of all the mesoregions.

Systematics of the distinguished association
Floristically poorer typical Arnoserido-Scleranthetum patches were found in the mesoregions located east of the Kałuszyn Upland, i.e. in the Węgrów Lowering, Siedlce Upland, Łuków Plain, and Żelechów Upland, where the average number of species per phytosociological relevé was 10.1.In these mesoregions, there was a lower share of differential species, in particular in the Żelechów Upland and Łuków Plain.The high constancy and cover of Anthoxanthum aristatum in the Siedlce Upland is noteworthy.The greatest number of patches with dominance of the species was found in the Kałuszyn Upland, where a sub-variant with the share of this species was distinguished.Patches dominated by Anthoxanthum aristatum were floristically poorer than the typical subvariants.In total, only 32 species were reported in these phytocoenoses, and the average number of species per relevé was 11.0 (Tab.2).
The Arnoserido-Scleranthetum patches occupying the area of the Podlasie Gorge of Bug, i.e. the north-easternmost mesoregion, were substantially different in terms of their floristic composition.These communities exhibited relatively high species richness and the lowest share of differential species, as besides Scleranthus annuus only Veronica dillenii was found there.In turn, higher frequency and cover were reported for species characteristic of the alliance and order, in particular Anthemis arvensis and Centaurea cyanus.Characteristic species from the class Stellarietea mediae also represented a large proportion.Among these, the highest constancy and cover were reached by Spergula arvensis and Conyza canadensis, whereas Viola arvensis was noted frequently, but with lower cover rates.Among the accompanying species, Rumex acetosella exhibited the highest frequency and cover.Its cover was found to be the highest in the area of the Podlasie Gorge of Bug and Siedlce Upland (Tab.2).

Wet Arnoserido-Scleranthetum variant
Phytocoenoses with the share of hygrophilous species were characterized by greater species richness than the typical patches (Tab.3).Among them, the highest average number of species per relevé was found for the phytocoenoses with Illecebrum verticillatum noted exclusively in the Kałuszyn Upland (average number of species per relevé -23.7).Scleranthus annuus constituted a lower proportion in patches containing hygrophilous species than in typical patches.It exhibited high frequency and cover only in the Siedlce Upland and Łuków Plain.A high share of Anthoxanthum aristatum was observed in these phytocoenoses of some mesoregions; the species reached considerably high cover rates in the area of the Węgrów Lowering and Żelechów Upland.The share of the other differential species was similar to that in the typical patches.The lowest proportion was found in phytocoenoses with Illecebrum verticillatum in the Kałuszyn Upland and in patches with Polygonum hydropiper in the Łuków Plain (Tab.3).
The analysis of the share of hygrophilous species indicated the highest species richness in the phytocoenoses with Illecebrum verticillatum, where rare and endangered species, e.g.Illecebrum verticillatum, Radiola linoides, Centunculus minimus, Juncus capitatus, Hypericum humifusum and Peplis portula, were noted.The other analyzed phytocoenoses did not differ substantially in their hygrophilous species composition; some differences were only found in the occurrence frequency and cover rates of some species.No significant differences were reported in the occurrence of species characteristic of the alliance, order, and class.

Systematic group value
The analysis of the share of the characteristic and differential species carried out on the basis of the systematic group value indicated the highest value of the indicator for the phytocoenoses in the Kałuszyn Upland, Podlasie Gorge of Bug, Żelechów Upland, and Siedlce Upland (Tab.4).However, this value was different when the share of Scleranthus annuus was not taken into account, in which case the greatest value of D 2 was noted for the communities with Anthoxanthum aristatum.The typical phytocoenoses in the Kałuszyn Upland were characterized by high values.The significant decrease in this value observed implies a considerable proportion of Scleranthus annuus in the communities of the Podlasie Gorge of Bug and Siedlce Upland.A general twofold decrease in this value was found in the phytocoenoses of the Podlasie Gorge of Bug and Łuków Plain.A slight change was noted in the patches with Anthoxanthum aristatum in the Kałuszyn Upland.Smaller differences were found for the wet phytocoenoses characterized by a lower share of Scleranthus annuus.

Discussion
Arnoserido-Scleranthetum is a sub-Atlantic association, which usually inhabits rye fields in the poorest habitats.Its patches develop on acidic soils, most frequently with a pH of 4.0-5.0[17].It has narrow ecological amplitude in relation to soil fertility and reaction, and considerable amplitude in relation to moisture content [17].The highly specific habitat requirements determine its sensitivity to increasing intensification of agricultural production, such as liming and fertilization [18].Another threat is posed by the abandonment and habitat transformation of the poorest low-productivity habitats [19].Therefore, the association is threatened with extinction in many regions of Poland [2][3][4][5][6][7].In Europe the situation is even worse due to the long-term farming intensification processes.Arnoseris minima is regarded as extinct in many regions of Germany (in Swabia and the Alp foothills); in Baden-Württemberg, Upper Rhine, and Black Forest, the species was endangered at the end of the 20th century [8].Similarly, the species is threatened with extinction in the Czech Republic and Slovakia [20] as well as the Netherlands and France [21], while in Great Britain [11] and Switzerland the species is considered as extinct [21].
In Poland, the association has been reported from many regions; the greatest number of localities have been noted in the central part of the country [17,19,22,23] where patches of the analyzed association are best developed in terms of both species composition and species richness.Arnoserido-Scleranthetum phytocoenoses seem to be threatened to a greater extent in the south-western part of the country [2,4,6] where the climatic conditions are more favorable than those prevailing in the central-eastern Poland [24][25][26][27][28].This is related to the intensification of the anthropogenic impact in the western regions of the country and the high proportion of light soils and extensive farming in the east.The distribution of Arnoserido-Scleranthetum patches in the Południowopodlaska Lowland is associated with the presence of the poorest soils and the impact of the oceanic climate.Therefore, the highest density of the patches was observed in the localities of the Kałuszyn Upland, i.e. the south-westernmost mesoregion, whereas the lowest density was noted in the Podlasie Gorge of Bug area, which is the north-easternmost region.
Arnoserido-Scleranthetum shows no regional variation, although impoverishment of the floristic composition can be observed towards the north east as indicated by the gradual disappearance of the differential species [28][29][30][31].In the Południowopodlaska Lowland, except for the area of the Podlasie Gorge of Bug, Arnoserido-Scleranthetum phytocoenoses are well developed and they comprise all differential species.These communities do not differ in their species composition from those described in central Poland [22,23].Impoverished patches with a low share of differential species, with total absence of Teesdalea nudicaulis and Anthoxanthum aristatum, were only noted in the peripheries of the occurrence range.Until recently, these species have not been noted in the patches of the analyzed association in the area of the Podlasie Gorge of Bug despite the considerable spread of Anthoxanthum aristatum across the Południowopodlaska Lowland [32].The mass occurrence of the species in Arnoserido-Scleranthetum patches leads to changes in the structure and species composition of the analyzed phytocoenoses.The dominance of Anthoxanthum aristatum in the analyzed patches by competitive interaction results in impoverishment of the phytocoenoses, affecting adversely their biodiversity.A similar degenerative impact of this species has also been observed in other mesoregions of the country [33][34][35].
In Germany, where there are disastrous changes in the occurrence of some archaeophytes, including Arnoseris minima [13,36], field fallowing accompanied by compensation for farmers is proposed with the aim of protection under agro-environmental schemes as well as the establishment of protected sites, i.e. field belts cultivated in the traditional way without application of pesticides [18].In this context, weed diaspore seeding is a special protection method [12,37,38] for impoverished poor sandy fields.
Currently, there are no indications for special protection of Arnoserido-Scleranthetum patches in the Południowopodlaska Lowland, since they constitute common phytocoenoses in this area.Only patches of Arnoserido-Scleranthetum with Illecebrum verticillatum are threatened with extinction due to changes in the crop structure and increased nitrogen fertilization [39][40][41].Their composition comprises a large group of endangered sub-Atlantic species of the class Isoëto-Nanojuncetea, e.g.Illecebrum verticillatum, Radiola linoides, Hypericum humifusum, Juncus capitatus, Peplis portula, Centunculus minimus, and Gnaphalium luteo-album.Similar phytocoenoses with the species mentioned above were reported by Warcholińska [42,43].Their presence depends on the low content of nutrients as well as humidity and light conditions [43].For that reason, the intensification of production (melioration, fertilization, large share of arable crops) is currently a major threat to these communities.Species of the class Isoëto-Nanojuncetea belong to the most endangered in Central Europe [39,40,44,45].Therefore, effective methods of their protection should be applied in Poland [41].

Conclusions
(i) Arnoserido-Scleranthetum phytocoenoses were noted in all the analyzed mesoregions.The association occurred with the highest frequency in the western part of the Południowopodlaska Lowland.(ii) The distribution of Arnoserido-Scleranthetum phytocoenoses depends on the presence of trophically poor light acidic soils and climatic conditions.The cooler climate at the northern-east end of the macroregion limits the occurrence of the analyzed association.(iii) Impoverishment of the composition and structure of the analyzed association towards the north east was found.(iv) The increasing frequency of Anthoxanthum aristatum occurrence led to impoverishment of the analyzed phytocoenoses, as well.(v) In order to protect the biodiversity of Arnoserido-Scleranthetum phytocoenoses, especially those with the participation of the Isoëto-Nanojuncetea species, it is necessary to provide additional payments supporting traditional farming practices under agroenvironmental programs.

Mesoregion Area (km 2 )
Surface area of the investigated mesoregions.