The ideas of Sukachev as a geobotanist were close to those of Clements, although Sukachev never recognized phytocoenosis as an organism motivated by the fact that, unlike an organism, elements and parts of phytocoenosis and biogeocoenosis can exist out of the whole. However, he also did not accept the individualistic concept of Glizon-Ramenskii concerning the plant cover organization. According to the author's definition, on a specific area of the Earth's surface, biogeocoenosis is a combination of homogenous natural phenomena (atmosphere, rocks, vegetation, animal and microorganisms, and soil and water conditions). These components possess specific types of interactions and a definite type of interchange of matter and energy occurs between them and with other natural phenomena, thus representing an internally contradictory dialectical unity, being in constant movement and development. N.V. Timofeev-Resovskii determined biogeocoenosis as a biochorologi-cal unit, within which there exist no biocoenotical, geomorphological, hydrological, climatic, or pedologi-cal-geochemical boundaries. Biogeocoenosis is implied as an integral discrete elementary natural cell of the biosphere that realizes the function of matter and energy transformation. Although the boundaries of each biogeocoenosis may be distinguished according to any of its components, practically it is better to accomplish it using boundaries of the best-observed component, namely vegetation, that is, according to the boundaries of phytocoenosis. Different biogeocoenoses interact with one another in space forming the biogeocoenotic cover. Sukachev did not consider specially the spatial dimension of the biogeocoenotic cover, but as it follows from the context, it corresponds to a rather vast territory commensurable with a floristic district or area. Sukachev discussed in detail the correlation of his concept with Tansley's concept of an ecosystem, different variants of its definition, and the concept of landscape and its morphological units, mainly in the interpretation of the adherents of the Russian school. He paid attention rightly to the fact that an ecosystem is considered (according to Tansley) as an abstract physical system uniting organisms with their environment. It is worthwhile to recall that Tansley actively objected to Clements's holistic concept of organism and considered ecosystem as a set of relations within different spatial-temporal intervals and at different hierarchical levels rather than a reality. Later on, this methodological content of the ecosystem concept disappeared almost completely, and ecosystem has been considered as a natural unit representing a totality of biotic and abiotic elements and as a functional system. Nevertheless, the concept of ecosystem maintains its general meaning along with its traditional interpretation as a chorological unit. Sukachev insisted that the concept of biogeocoeno-sis as a strictly territorial unit was more definite than the uncertain concept of ecosystem. One can accept this to be true to some extent, but the history of development of science showed that precisely some uncertainty inherent to the concept of ecosystem ensured its viability and incorporation into the general scientific basis. In the light of general system concepts, biogeocoenosis may be considered as a kind of ecosystem which possesses relatively spatially homogenous or stable (random or specific quasi-regular variation) properties in terms of its components within the framework of their observed boundaries. At the same time, the reality and commonness of the distinguished boundaries are not proved specially, but accepted a priori, assuming that these boundaries are relatively gradual.

Comparing the concept of biogeocoenosis with the modern concepts of landscape, it is worthwhile to note that the latter are interpreted differently. The concept of biogeocoenosis is most likely to be close to that of units accepted in the Canadian and Australian schools. However, a unit in landscape science is a functional unit rather than an operational one. In American forest science, the notion of biogeocoenosis is comparable territorially with that of stand.

Sukachev, who fully accepted the concept of the biosphere proposed by Vernadsky, regarded biogeocoenosis as an elementary cell of the biosphere.

Researchers who accepted the concept of biogeocoe-nosis differentiated between the spatial structural elements of biogeocoenosis: vertical layers and horizontal occasionally or quasi-regularly alternating parcels (parts), which are usually distinguished by the shrub, grass, and moss layers commensurable with microassociations. The genesis of parcels was mainly related to the heterogeneity of the tree layer, and they may be associated with gaps. Different parcels are often related to different pedons of soil. Sometimes, parcels are determined by the initial pattern of the nanorelief and soil-forming rocks.

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