ASSOCIATIONS AND COMMUNITIES OF CEREAL CROPS OF THE Ł UKÓW PLAIN PART I . LIGHT SOIL ASSOCIATIONS

The characteristics of phytocenoses developing on light soils in cereal crops of the Łuków Plain are presented in the paper. The studies were carried out between 2003 and 2006 in 182 localities. Cereal crops on light soils were frequently occupied by patches of Arnoserido-Scleranthetum. The association reaches the eastern limit of its geographic range in the mesoregion. The phytocenoses were noted under various moisture conditions and were the floristically poorest cereal communities in the studied area. The paper presents new data on the occurrence of Arnoserido-Scleranthetum on its eastern distributional limits. Quite frequently, the patches of Vicietum tetraspermae scleranthetosum, especially of its variant with Juncus bufonius and Vicietum tetraspermae typicum variant with Rhinanthus serotinus, were also frequently noted in the studied mesoregion. Small patches of Papaveretum argemones were recorded very seldom.


INTRODUCTION
The floristic composition of segetal communities is evidently formed under the influence of numerous biotic and anthropogenic factors.The most important are these related to habitat characteristic, e.g.soil type, its reaction, and content of nutrient elements.These factors can be considerably modified by the intensity of applied agricultural technology, especially by widespread application of herbicides and nitrogen fertilizers.The use of certified seed material and new, more fertile cereal varieties, forming dense crop stands, also visibly affect the physiognomy of these communities.
The Łuków Plain is an interesting mesoregion, because in its area Atlantic species reach their eastern geographic range.
The aim of the work was to make a phytosociological classification and floristic analysis of communities developing in cereal crops of the Łuków Plain.

METHODS
Field studies on segetal communities were carried out between 2003 and 2006 in 186 localities situated in the agricultural areas of the Łuków Plain.In total, 304 phytosociological relevés according to the Braun-Blaquet method were made in cereal crops.The phytosociological material was divided into three parts: material related to light soil communities, compact soil communities as well as transitional and impoverished communities.The present work was concerned with light soil associations and was based on the analysis of 90 phytosociological relevés.The results were summarized in phytosociological tables, of which only one, a summary table of the analysed communities, is included in the present paper.Moreover, the phytosociological constancy and cover-abundance index were calculated for each species.The classification of communities was based on the system presented by Matuszkiewicz (2007).The species nomenclature follows Mirek et al. (2002).The studied region is shown in Fig. 1.Patches of Arnoserido-Scleranthetum were frequently found in the area of the Łuków Plain.They occupy large areas, especially in the south-western and western part of the mesoregion; this is a result of the Sub-Atlantic character of the community (F ij a ł k o w s k i , 1991; W a r c h o l i ń s k a , 1995).In the eastern part of the mesoregion (the eastern limit of its geographic range), patches of Arnoserido-Scleranthetum were observed quite rarely.Very seldom was the association noted in the area of the Podlaski Przełom Bugu mesoregion, bordering on the Łuków Plain The phytocenoses of Arnoserido-Scleranthetum develop in rye fields on light, acidic in reaction, pseudo-podzolic, brown leached and boggy soils, included in weak and very weak rye and weak cereal-fodder complexes (Table 1).Due to high floristic differentiation of the association (the effect of diverse moisture conditions), the typical and wet variants of the association were distinguished.Floristic differentiation of Arnoserido-Scleranthetum is rarely observed in Poland (W a r c h o l i ń s k a , 1995).It has been reported by the following authors: W a r c h o l i ń s k a (1997), S o b i s z (2000), R a t u s z n i a k and S o b i s z (1999).A low degree cover-abundance of Arnoseris minima and Scleranthus annuus (species characteristic for the association) was observed in patches of the variant with Polygonum hydropiper.Apera spica-venti was noted more frequently and more abundantly.The occurrence of a numerous group of hygrophilous species, e.g.Polygonum hydropiper, Gnaphalium uliginosum, Mentha arvensis, and Spergularia rubra, was also noted; however, their degree of cover was low.Differential species, e.g.Veronica dillenii, Anthoxanthum aristatum, Teesdalea nudicaulis, Spergula morisonii and Holcus mollis, were seldom recorded in the analyzed plots.Only an occasional occurrence of these Sub-Atlantic species can be a result of the process of impoverishment of the patches of the association, which was described by B i e l s k a (1989) and B ł o c k i (2001).

RESULTS AND DISCUSSION
The occurrence of patches of the association with dominance of Anthoxanthum aristatum is especially noteworthy.That expansive plant is considered rare in the study area, however strongly degenerated patches of Arnoserido-Scleranthetum with mass occurrence of this species were observed.They were distinguishable by species poverty and low cover degree as well as vitality of other plants.Data on dominance of Anthoxanthum aristatum in field associations and on transformation of communities under the influence of the weed plant have also reported from the adjacent areas by S k r z y c z y ń s k a et al.The analyzed association was the floristically poorest cereal phytocenosis in the Łuków Plain mesoregion.On average, 11 species were noted in one relevé in typical patches and 17 in patches with Polygonum hydropiper.In total, the occurrence of as much as 89 species was observed in 47 patches of the association, 55 in typical patches and 68 in patches with Polygonum hydropiper.A large number of species noted in the community is a result of the frequent occurrence of the association in the studied area (a large number of relevés) and the occurrence of numerous apophytes coming from adjacent psammophilous grasslands.

Papaveretum argemones (Libb. 1932) Krus. et Vlieg. 1939
It is a very rare association in cereal crops of the Łuków Plain mesoregion.Small areas of phytocenoses (8 patches) were observed exclusively on field margins (Table 1).The process of withdrawal of characteristic species from crop field margins and roadsides under the influence of anthropopressure seems to be observed.Dense stands of intensive crops limit their occurrence due to the worsening of light and thermal conditions.This rare association, similarly like in the Lublin Upland (F i j a ł k o w s k i , 1991), develops on sandy pseudo-podzolic soils of slightly acidic and acidic reaction included in the weak rye complex.Only 52 species were noted on these soils, on average 17 species in one relevé.Apart from Papaver argemone and early spring differential species (Arabidopsis thaliana and Veronica triphyllos), such plants as Erophila verna, Myosotis stricta, and Arenaria serpyllifolia were also frequently noted (Table 1).A large proportion of acidophilus species, e.g.Scleranthus annuus, Anthemis arvensis, was also observed.Moreover, a few species characteristic for higher syntaxonomic units, e.g.Patches of Vicietum teraspermae scleranthetosum and the typicum variant with Rhinanthus serotinus developed on light soils (Table 1).Vicietum tetraspermae scleranthetosum was diversified internally as a result of different moisture condition of habitats.Two variants: the wet variant with Juncus bufonius and the typical one were distinguished.The wet variant developed on various soil types, usually on sandy soils, formed on clayey sand and light clay, included in cereal--fodder and good rye complexes.Typical patches were noted on pseudo-podzolic and brown leached soils of weak and good rye complexes.Vicia tetrasperma and other species distinguishing the association and subassociation reached a higher cover degree in the typical variant than in the patches with the participation of hygrophilous species (wet variant) Bromus scalinus was observed sporadically in both communities, however it reached a higher cover in the patches with Juncus bufonius.The largest proportions of Juncus bufonius, Gnaphalium uliginosum, Polygonum hydropiper, Potentilla anserine, and Spergularia rubra were noted among hygrophilous species.Patches with Juncus bufonius were dominated by Apera spica-venti, typical patches -by Vicia hirsuta.In the analyzed subassociation, the occurrence of 92 species in total was noted, of which 48 in typical patches and 78 in wet ones.The number of species was respectively 14 (from 10 to 20) and 22 (from 17 to 29).
The Vicietum tetraspermae typicum variant with Rhinanthus serotinus was observed on various soil types included in good rye and week cereal--fodder complexes.These species-poor phytocenoses were dominated by Rhinanthus serotinus (Table 1).In total, 62 species were noted in them, on average 21 species in one relevé.Vicia tetrasperma and other species characteristic and differential for the association were frequently observed.Bromus secalinus was most frequently observed on light soils, however it never reached a high cover degree there.Among species characteristic for Aperion spicae-venti and Centauretalia cyani, a large share of Apera spica-venti was noted.Acidophilus species were noted seldom and not abundantly.On the other hand, hygrophilous species, e.g.Juncus bufonius, Gnaphalium uliginosum, Polygonum hydropiper, Polygonum amphibium, and Sagina procumbens, were noted frequently and with a large cover.The occurrence of similar phytocenoses of Vicietum tetraspermae has been reported by S k r z y c z y ń s k a and S k r a j n a (2004) from the Kałuszyńska Upland, by S k r a j n a et al. ( 2009) from the Mazowiecki Landscape Park, and by W ó j c i k (1965) from the Mazowsze region.

Fig. 1 .
Fig. 1.Study area (S k r z y c z y ń s k a and R z y m o w s k a , 2005).In north-eastern Poland, the community has been observed in the regions of Pojezierze Iławskie and Pojezierze Olsztyńskie (P o l a k o w s k i et al. 1989; B ł o ck i , 2001).It has never been noted in the Polesie region (F i j a ł k o w s k i et al. 2002).
(2004) and S k r z y c z y ń s k a et al. (2010).Similar observations from other regions of Poland have been published by S z m e j a (1994, 1996), W a r c h o l i ń s k a (1997), B ł o c k i (2001), and Kozak (2002).
Apera spica-venti and Centaurea cyanus, were noted with high cover.The phytocenoses of Papaveretum argemones were more frequently recorded in the adjacent mesoregions of the South Podlasie Lowland (S k r z yc z y ń s k a , 1994; S k r z y c z y ń s k a and S k r a jn a , 2004; S k r z y c z y ń s k a and R z y m o w s k a , 2005).This association is common in the Wielkopolska region (J a c k o w i a k et al. 1994) as well as in southern (A n i o ł -K w i a t k o w s k a , 1990; K oz a k , 2002; W ę g r z y n e k , 2003) and central Poland (W a r c h o l i ń s k a , 1974, 1997, 1998).Vicietum tetraspermae Krus.et Vlieg 1939 Vicietum tetraspermae is the most common association in the studied area, similarly like in the other mesoregions of the South Podlasie Lowland (S k r z yc z y ń s k a , 1994; S k r z y c z y ń s k a and S k r a jn a , 2004; S k r z y c z y ń s k a and R z y m o w s k a , 2005) and in whole Poland (W ó j c i k , 1965; S i c i ńs k i , 1974, 2003; W a r c h o l i ń s k a , 1974, 1997, 1999; K o z a k , 2002; W ę g r z y n e k , 2003).It is also the most internally differentiated cereal phytocenosis in the area of the Łuków Plain.The identified subassociations and variants reflect the mosaic of trophy-moisture conditions of the studied area and the level of agriculture.