MEDICAL BOTANY - A.G. Serbin - 2003

SECTION 4. ECOLOGY, PHYTOCENOLOGY, PHYTOGEOGRAPHY

ELEMENTS OF PHYTOCENOLOGY

Phytocenoses

Phytocenology, or geobotany, is The Study of the Structure and spatial distribution of diverse plant communities across the Earth. As defined by Academician V.N. Sukachev, "a plant community, or phytocenosis, is an assemblage of plants growing together in a uniform area, characterized by a specific composition, structure, constitution, and by reciprocal interactions among the plants themselves as well as with environmental conditions. The Nature of these interactions is determined, on the one hand, by the biological—or ecological—Properties of the plants, and on the other hand, by The properties of the habitat, i.e., the climate, soil, and The impact of humans and animals."

A phytocenosis represents a historically developed community comprising both higher and lower plants with diverse relationships to environmental factors. Examples of phytocenoses include forests, meadows, and steppes. A forest is a plant community consisting of woody plants beneath whose canopy herbaceous plants grow. A meadow is a community composed of perennial herbaceous mesophytic plants, which develop in sufficiently moist habitats. A steppe is a phytocenosis of arid climate zones, consisting of perennial herbaceous xerophytic plants.

In nature, plants are intimately connected with animals, which form the zoocenosis. The combination of a phytocenosis and a zoocenosis constitutes a complex community of living organisms known as a biocenosis. A biocenosis includes both micro- and macroorganisms of plants and animals. Together with the abiotic environment, a biocenosis forms a complex system—a biogeocenosis—characterized by intricate interactions and interdependencies among its constituent parts. Green plants produce organic matter and provide food for animals, which in turn modify the living conditions of green plants (by aerating and fertilizing the soil, etc.). Fallen leaves, branches, dead roots, and other plant debris serve as food for achlorophyllous organisms (Bacteria, Fungi, and other microorganisms). Concurrently, through the breakdown and Mineralization of organic matter, microorganisms establish favorable conditions for the soil and mineral Nutrition of green plants. Plants significantly buffer the mechanical and destructive forces of the inanimate environment—wind, Temperature, and Water—while simultaneously shaping soil structure and fertility. Thus, a continuous interrelation between biotic and abiotic components takes place within the plant community. As a historically evolved community of organisms, a biogeocenosis exists in a state of continuous flux and development.

Each phytocenosis is characterized by a set of diagnostic features that reflect its constitution and structure. The primary features include: species (or floristic) composition, vertical stratification, Abundance (number of individuals), quantitative and qualitative species ratios, frequency, cover, vitality, and habitat characteristics.

The character of phytocenoses changes not only seasonally, but also in response to altered external conditions. For instance, As a result of land drainage or heavy grazing, a given community may disappear and be replaced by a new one. Species often coexist successfully if their ROOT systems occupy different soil horizons: the roots of some species inhabit the upper layer, those of others penetrate to considerable depths as taproots and thus do not compete with the former, while a third group occupies vacant intermediate spaces. If the roots of many species overlap within the same horizon, interspecific competition intensifies.

Within a community, various species are arranged in both above-ground and underground tiers (layers) (Fig. 4.4). Some species occupy the uppermost tier—these are light-demanding plants characterized by intensive growth; others form the middle tier; while a third group consists of shade-tolerant species that thrive in the understory without being suppressed by the upper layers.

Class="center">Fig. 4.4. Underground and above-ground stratification in a herbaceous community

Multi-tiered phytocenoses can be observed, for example, in deciduous forests of the forest-steppe zone: the forest canopy, the shrub understory, several herbaceous layers (tall, low, and prostrate), moss layers, mycorrhizal fungi, Lichens, bacteria, and Algae. The uppermost tier exerts a strong influence on the lower ones. For instance, if the trees (upper tier) in a forest are felled, the resulting surge of direct sunlight may destroy understory species; conversely, waterlogging of the forest floor by a dense moss cover can lead to the demise of canopy-forming species.

According to their origin, communities are classified as natural and anthropogenic (i.e., those whose structure results from human intervention). Anthropogenic communities also include cultivated phytocenoses such as shelterbelts, windbreaks, field crop plantings, and sown meadow-pasture mixtures.

Every phytocenosis undergoes a succession of aspects throughout the year. In early spring, it is characterized by the blooming of certain species while others are just awakening; in late summer, conversely, the early species are no longer visible (having shed their leaves and seeds), whereas other species enter full bloom. Phytocenoses are dynamic; they lack rigid forms and undergo qualitative changes during their development, with new species continually appearing. Annual variations in growth patterns and seed productivity, as well as the impacts of climate, weather fluctuations, and soil processes, all influence the longevity of phytocenoses.

The economic significance of phytocenoses is immense. Detailed studies of these communities form the basis for determining the industrial and agricultural value of forest lands, meadows, pastures, hayfields, aquatic plankton (as fish food resources), peatlands, and other natural assets.