The museum-preserve «The Kulikovo Field» is situated in the northern part of the Central Russian Upland within the forest-steppe zone near its northern border. There are a lot of abandoned fields, most of them left fallow since 1990–2000 years; the exact age of the old fields is unknown. Using the Braun-Blanquet approach as well as the method of K. Kopecký and S. Hejný (Kopecký, Hejný, 1974; Kopecký, 1992), the classification of the old-field vegetation of the central part of the museum-preserve was carried out. One derivate community, 2 basal communities (one of them with two variants), 3 communities and one association with three variants have been identified.
The derivate community Conyza canadensis [Artemisietea vulgaris/Stellarietea mediae] is dominated by annual and biennial ruderal species: Conyza canadensis, Lactuca serriola and Carduus acanthoides. The association Convolvulo arvensis–Elytrigietum repentis is heterogeneous both in its floristic and subdominant composition and it can be divided into 3 variants. The variant typica represents monodominant communities where Elytrigia repens prevails. The variant Lactuca serriola is characterized by high constancy of Conyza canadensis, Lactuca serriola and some other ruderal plants and it represents coenoses dominated by Elytrigia repens with subdominants such as Lactuca serriola, Senecio jacobaea and Pilosella spp., mainly P. bauhini. The variant Cichorium intybus is dominated by Elytrigia repens with a subdominant Cichorium intybus; some mesophilous meadow species are often present. Variants Melilotus officinalis and Sonchus arvensis of the basal communityElytrigia repens–Cichorium intybus [Artemisietea vulgaris] are dominated by Cichorium intybus, Poa angustifolia and Elytrigia repens, the first of them also by Melilotus officinalis, Artemisia absinthium, and the second by Carduus acanthoides and Calamagrostis epigeios. Unlike the foregoing syntaxa the basal community Elytrigia repens–Cichorium intybus [Artemisietea vulgaris] is characterized by relative high constancy of some species pertaining to the order Galietalia veri, namely Fragaria viridis, Galium verum, Potentilla argentea. These species and also Poa angustifolia are the first steppificated meadow plants that appear in the old field communities under investigation. The community Pilosella bauhini [Onopordion acanthii] is dominated by Pilosella spp., mainly by P. bauhini that sometimes replaced by some ruderal plants, e. g. Achillea nobilis or Cichorium intybus. The peculiarity of this community is the low constancy and abundance of Elytrigia repens. The species of the orders Galietalia veri and Arrhenatheretalia play much noticeable role in the other syntaxa mentioned below, so these syntaxa are between the natural and synanthropic vegetation. The community Leucanthemum vulgare–Galium mollugo [Onopordion acanthii/Molinio-Arrhenatheretea] is distinguished by high abundance and constancy of some mesophilous and xeromesophilous meadow plants (Leucanthemum vulgare, Galium mollugo, Phleum pratense etc.). The community Artemisia marschalliana [Onopordion acanthii/Galietalia veri] is dominated mostly by Leontodon hispidus and Pilosella spp.; its peculiarity is a relatively high constancy of species common in the local steppificated meadows. The basal community Poa angustifolia [Galietalia veri/Artemisietea vulgaris] represents monodominant communities where Poa angustifolia prevails. Annual, biennial and some perennial ruderal species are rare in this variant, unlike most of the syntaxa mentioned above. The common feature of the last three syntaxa is subdominance of Fragaria viridis.
It is known that the floristic composition of communities is changed during an old-field succession: the percentage of annual and biennial species declines and that of species pertaining to the classes of natural vegetation increases (Bonet, Pausas, 2007; Yamalov et al., 2008; Pankratova, Gannibal, 2009). Using the life-form and phytosociological spectrum of the syntaxa analysis an attempt to evaluate succession status of the described communities was made (tab. 9, 10). Based upon the results of this analysis, it is possible to suggest that the derivate community Conyza canadensis [Artemisietea vulgaris/Stellarietea mediae] is the earliest stage one can find in the investigated old fields. The variants Lactuca serriola and typica of the association Convolvulo arvensis–Elytrigietum repentis and the community Pilosella bauhini [Onopordion acanthii] are the next in the series. It seems that Pilosella spp. (P. bauhini and probably some other species of this genus) outcompete the pioneer species of initial stages, like Elytrigia repens, in some cases. The basal community Poa angustifolia [Galietalia veri / Artemisietea vulgaris] seems to be the most advanced stage: the percentage of annuals and biennials is minimal, and that of species pertaining to the syntaxa of natural vegetation of the high ranks, especially to the order Galietalia veri, increases greatly.
Species richness of the communities is minimal at the most early stage, which is the peculiarity of the old-field vegetation (Pankratova, Gannibal, 2009; Ovcharova, Yamalov, 2013). Similar phenomenon was also noticed at the succession stages where strong dominant (Elytrigia repens or Poa angustifolia) prevails, regardless of how much advanced these stages are, the fact mentioned earlier (Prach, 1985). Species richness attains maximum at those stages of succession where the communities are polydominant and contain both early- and late-successional species, that was also previously described (Meiners et al., 2007).
INDUCTION OF FLOWERING IN WILD LETTUCE (LACTUCA SERRIOLA L.). III. VERNALIZATION-DEVERNALIZATION CYCLES IN BURIED SEEDS
