The ecological effects in the formation of the floristic composition of plant communities that occur under the influence of certain natural and anthropogenic phenomena are demonstrated on an example of the floodplain oak forests of the Upper Dnieper basin. The analysis of coenoflora of 12 variants of the ass. Filipendulo ulmariae–Quercetum roboris Polozov et Solomeshch in Semenishchenkov 2015, which is widely distributed in the European Russia, is carried out in the syntaxonomical space following the J. Braun-Blanquet (1964) approach. This syntaxon belongs to the alliance Fraxino–Quercionroboris Passarge 1968, order Alno–Fraxinetalia excelsioris Passarge 1968 within the class Alno glutinosae–Populetea albae P. Fukarek et Fabijanić 1968 (Semenishchenkov, Lobanov, 2019). The syntaxonomy is based on 143 relevés from the southwestern regions of Russia (Bryansk, Kaluga, and Smolensk).
The specificity and diversity of environmental conditions in the habitats of floodplain oak forests are manifested in several effects.
The high values of floristic diversity can be explained by the ecotone effect (increasing biodiversity at the border of contrast communities, landscapes, natural zones). However, the ecotone should not be confused with independent formations of the listed types with a complex of ecological and, on a zonal scale, also climatic, conditions. In the studied forests, high biodiversity is determined not only by their location at the conditional boundaries of the massifs or by the habitat fragmentation, but also by their structural features.
Fragmentation, natural sparseness, and the above disturbances create conditions for high ecological diversity, which is reflected in the coenoflora of syntaxa, which are composed of species affine to several classes of vegetation. This leads to the effect of ecological heterogeneity of coenoflora (co-existence of species of different ecological groups)
There is a low uniformity of species abundance in poor-species communities, and the Shannon–Weaver index values becomes higer with an increase in floristic saturation. The effect of dominance (reduction of species richness in the presence of dominants) takes place after anthropogenic disturbances and also is observed in natural and semi-natural communities of different types.
In the process of the establishment of variants (small within association units), the following two effect were revealed. The group significance effect of species is that the general characteristics of the syntaxon are summarized by taxa close in individual ecological preferences, although each of them has a low, but significant at the syntaxonomical level, frequency. When DCA-ordination is carried out, the communities of the typical variant (subass. typicum, var. typica), which is a kind of basic type for the entire phytocoenotic diversity within the association, tend to the most mesophytic conditions among other syntaxa. Communities of the another variants are sequentially located on the gradients of the environmental factors. For this phenomenon, the term effect of ecological balance in syntaxonomic space is proposed. The man impact, which leads to the effect of meadow transformation and incompleteness, is associated with the effect of the formation of floodplain oak forest communities (forest cultures become, to a large extent, similar in composition and structure to natural forests with the same tree species in analogous habitats). Anthropogenic impact often increases species richness, that is why numerous meadow, fringe and synanthropic light- and thermophilic species appear in the coenoflora after moderate grazing, destruction of undergrowth and lightening during felling. However, disturbances can also lead to a decrease in species richness with the formation of communities in which some vegetative mobile species get a local advantage. With dominants in the herb layer the total species richness, as a rule, is lower, that corresponds to the effect of dominance. The incomplete communities are classified as small units (var. inops) within the association. The reasons for the existence of floodplain oak forests with «depleted» floristic composition, compare with the typical units (subass. typicum, var. typica), are trampling, grazing, anthropogenic and natural fires, the activity of burrowing animals (wild boars). The «invasibility» of floodplain oak forests is due to their high availability for the penetration of alien species, which is can be explained by the community incompleteness.
The botanico-geographical features of the coenoflora of floodplain oak forests demonstrate the valley effect (Semenishchenkov, 2018), that make possible to clarify the syntaxonomical decisions in the classification of forest vegetation. The class Alno glutinosae–Populetea albae is assigned to the group of «azonal» vegetation (Mucina et al., 2016); however, some changes in the floristic composition of floodplain forests are observed along the gradient of increasing continental climate. This is manifested in the presence and change in the abundance of both trees (Acer campestre, A. tataricum, Alnus glutinosa, A. incana, Fraxinus excelsior, Picea abies, Tilia cordata, Ulmus minor) and herbaceous species; changes in the spectrum of geoelements with a decrease in the proportion of boreal and subboreal species. In contrast to polyzonal species, the distribution of some zonal marker ones and communities with their presence allows to identify significant geographical features of floodplain oak forests on a latitudinal gradient. If consider the river valley as a focus of specific conditions against the plakor habitats, the valley effect echoes the V. V. Alekhin’ «rule of replacement» by: «one factor can be replaced by another in whole or in part».
Analyzing the Biological and Structural Diversity of Hyrcanian Forests Dominated by Taxus baccata L.