Botany - B.E. Yakubenko 2017

Part Three. Kingdom Plantae
Chapter XII. Fundamentals of Phytocenology. The Concept of Vegetation
12.3. The Role of Species in the Life of a Plant Community

Plant communities are extremely diverse in genetic, structural, coenotic, and ecological terms, and the species that form them vary significantly in their phytocoenotic role. This is primarily due to their ability to combine into phytocoenoses. The process of association unfolds through competition for living conditions and adaptation to coexisting with other species. As a result, each species achieves a certain degree of influence and ability to play its specific coenotic role within the community.

There are various Perspectives on evaluating a species' role in a phytocoenosis. To determine this role, researchers use the category of phytocoenotype. The analysis of phytocoenotypes helps to understand the relationships between species within a phytocoenosis. Phytocoenotypes refer to the measure and character of a species' influence on the Composition and Structure of a phytocoenosis. A phytocoenotype is defined as a group of species with a similar role in forming the phytocoenosis. Both species-level and population-level phytocoenotypes are distinguished.

G. M. Vysotsky distinguished two categories of phytocoenotypes: 1. — prevalids, represented by perennial plants that determine the very Nature of the phytocoenosis, and 2. — ingredients, which are temporary and represented by annual species that fill the gaps between the Components of the phytocoenosis.

Y. K. Paczoski likewise identified two categories of phytocoenotypes: components (consisting of persistent perennial species) and ingredients (under which he included annual species).

Later, L. G. Ramensky thoroughly developed the categorization of species-level phytocoenotypes and characterized them as follows:

a) violents, or dominant species — species that aggressively occupy an area and hold it against others, simultaneously suppressing other species through the energy of their own vital activity and the thorough utilization of environmental resources (an example is the pedunculate oak, which creates its own microenvironment);

b) patients, or tolerant species — plants that do not exhibit exceptional metabolic energy, yet are highly resilient and capable of successfully securing territory and establishing new coenoses in habitats unfavorable to violents (for example, the common reed);

c) explerents, or filler plants — species capable of rapid spread in the gaps between violents and patients, though they are quickly and easily displaced by the latter (for example, creeping buttercup in floodplain meadows, or ephemerals in steppe phytocoenoses, which develop en masse in spring among the tussocks of dominant grasses).

Phytocoenotypes at the population level are even more diverse. For instance, J. Braun-Blanquet and K. Pavillard distinguished five categories of phytocoenotypes (edificators, conservators, consolidators, neutrals, destructors), whereas G. I. Poplavskaya and V. N. Sukachev proposed the following Classification: 1. Edificators: A — autochthonous, B — degressive; 2. Asectators: A — autochthonous (subdivided into edificaturophiles and edificaturophobes), B — adventive.

Various other classifications of phytocoenotypes have also been proposed and are utilized in modern geobotany.

Class="center">Stratification

One of the Structural Features of a phytocoenosis is its division into spatially distinct layers, or strata. Stratification is caused by plants growing together in a limited area while having different ecological requirements for environmental conditions. For example, light-demanding plants grow in the upper canopy, whereas shade-tolerant and shade-loving species, which are satisfied with the filtered light penetrating the canopy of the top layer, inhabit the lower strata (Fig. 115). In nutrient-poor deep sands, pine trees develop a deep taproot system, whereas cowberries and bearberries growing beneath their canopy locate their ROOT systems in the topsoil layers. Consequently, stratification can be above-ground and underground. The number of layers and their delineation depend on The Diversity of species composition and Water/128.html">Morphology/18.html">PLANT LIFE FORMS.

Tree species that reach the greatest height form the first (uppermost) layer; those positioning their crowns beneath the first layer form the second, and beneath that — the third layer. For example, a pine tree 24 m tall forms the first layer, while an oak 18–20 m tall forms the second layer.

Plant communities formed by a single species are called monostratified (stands of cattail, reed, or wheat fields); those formed by two species are bi-stratified (such as a forest bog where pine occupies the first layer and sphagnum the second); and those with many species are termed poly-stratified or multi-layered (such as oak forests, where pedunculate oak grows in the first layer, Norway maple In the second, hazel in the third, wood bluegrass in the fourth, wood stitchwort in the fifth, and green mosses in the sixth).

Cover

Expanding with their above-ground parts, plants cover a certain area of the surface. The area of the horizontal projection created by the aerial Organs of plants is called cover. Each phytocoenosis develops its own specific cover, which is the morphological manifestation of the growth of its components' above-ground organs. Vigorously developing plants form vegetative shoots, expand in space, and possess a higher degree of cover than suppressed species.

The degree of cover is of utmost importance because its increase ensures enhanced assimilation of CO₂ and, consequently, higher photosynthetic productivity.

Cover can be basal or projective. Basal cover is formed by the bases of cut stems and tree trunks. It is particularly evident in stubble fields after harvesting, in mown grasslands, and in forest clearings after logging.

Projective cover is formed by the totality of branches, leaves, inflorescences, fruits, seeds, and so on. Projective cover can reach 100% or more if plants overlap one another. Usually, it is below 100%.

Cover can be total if it is formed by all components of the phytocoenosis. For instance, in moist pine forests, it is created by pine, bilberry, Hair-moss, and accompanying species.

Partial cover is formed by only a portion of the species composition. For example, partial cover in oak forests is formed by the pedunculate oak, the understory is formed by hazel, and so forth. Partial cover can also refer to the cover of a single species.

Cover can be stratified. It is formed by plant species belonging to a single layer. Thus, tree stands, shrubs, and dwarf shrubs form their respective layers. Individual cover is formed by the specimens of a single species.

Abundance

The interaction of abiotic and biotic factors within a phytocoenosis significantly affects the biological characteristics of its components. Consequently, some species find optimal conditions for their development and play a major role in the community, while others remain suppressed and play no significant part in the phytocoenotic composition. A species' participation in a plant community determines its abundance.

The abundance of a species depends not on the biological traits of its individual specimens, but on its capacity to achieve a high degree of specimen density. The greater the density of individuals of a single species in a phytocoenosis, the higher its abundance. Individual-based abundance in any phytocoenosis is always higher than species-level abundance. For instance, in a grass-forb community, 23 species may grow per 1 m², whereas the number of individual plants in the same area runs into the thousands. Abundance is not static; it changes alongside environmental conditions. During geobotanical studies, various Methods are used to assess abundance: numerical objective evaluation, weight analysis, volumetric analysis, visual estimation using the six-point Drude scale, etc.

Aspectness

The visual appearance of a phytocenosis and its coloration change across the GROWTH AND DEVELOPMENT phases—such as vegetative growth, flowering, and fruiting of individual plant species. Consequently, aspectness varies distinctly in both space and time throughout the growing season, depending primarily on Plant Growth and Development phases, vertical stratification (canopy layers), species composition, and other factors. The aspect of a phytocenosis is exceptionally lush during the plant flowering period. Oak forests, steppes, and meadows in early spring are uniquely beautiful, with multiple aspects shifting over the course of a week. A simple aspect is formed by a single plant species, whereas a complex aspect is formed by several. These species are often referred to as aspect species.

Aspectness can be chronological or phenological. Phenological changes in phytocenosis aspects are driven by the phenological developmental phases of plants. They shift multiple times throughout the growing season and recur every year.

Chronological changes in aspects do not manifest every year; rather, they occur periodically under METABOLISM/18.html">The Influence of weather conditions, flooding levels, periodicity of fruiting, etc.

Identifying and studying the aspectness of agricultural crops and compiling phenological maps for each farm, tailored to physiographic and soil conditions, is of great importance for scheduling agronomic management practices (such as weeding, irrigation, Fertilization, and harvesting) and ensuring the rational use of plant land resources.

Species Constancy

This is one of the Characteristic Features of a phytocenosis, determined by the frequency with which a species occurs across various plots of a given phytocenosis. For instance, in a slender sedge community, the slender sedge itself is found in all 20 studied plots of that community, the marsh bluegrass in 12, the mud horsetail in 7, and the white clover in only 2 plots. Consequently, the constancy of these species within the phytocenoses varies.

Plant species found in the majority of studied plots (over 50%) of a given phytocenosis are termed constant. Species occurring in 25% of the studied plots are called accessory (subordinate), while species with a constancy of less than 25% are designated as accidental (according to Brockmann-Jerosch). When conducting a detailed study of vegetation, the number of investigated plots should be at least 20 for each phytocenotic association.

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Species Vitality

Each plant community is characterized by a diversity of environmental conditions. Consequently, some plants develop, grow, flower, and fruit successfully. Others, by contrast, fail to find favorable conditions for their development: they may begin vegetative growth and flower, but do not produce fruits or seeds. The vitality of these plants varies. Under optimal environmental conditions, a plant species exhibits higher vitality than under deteriorating ones. Plant vitality also depends on resistance to diseases and pests, the presence of defense mechanisms (such as toxicity), The ability to coexist with other organisms, seed reproductive capacity, and other factors.

Three types of plant vitality are distinguished: 3 — the species completes its full developmental cycle; 2 — the species only vegetates under these conditions; 1 — the species exhibits weak vegetative growth or merely reaches the seedling stage before dying off.

Species Distribution

Species within a phytocenosis are distributed in various ways. Typically, most individuals of a single species are more or less evenly distributed across the entire area, whereas others occur in clustered patches, groups, or both. This species trait is driven by its biological characteristics and The Nature of habitat conditions. The MORPHOLOGICAL STRUCTURE OF the phytocenosis is closely linked to species distribution. The most commonly observed degrees of species distribution are: solitary, in groups, diffuse, confluent, and in microcenoses.

Succession of Phytocenoses

As living, dynamic systems, phytocenoses are characterized by development and change. Any community represents one of the Developmental Stages of a given territory's vegetation. In nature, the replacement of one plant community by another is frequently observed. Such changes are termed succession. The relatively stable state of the plant cover that emerges through the succession of phytocenoses is called the climax and is regarded as The final stage of successional series.