Species composition of phytocenoses. Abstract: Phytocenoses Hidden fluctuations occur

Composition and structure of plant communities (synmorphology)

Phytocenosis, just like any other plant object, can be considered as a system. According to L. Bertalanfii (Bertalanfii, 1956), a system is a complex of elements that interact. Any system can be characterized by its composition, i.e. the totality of all elements of the system, structure - the spatial relationship of the elements of the system with each other, and functional structure - the set of connections that arise between the elements of the system. Therefore, considering the phytocenosis as complex system, in its organization one should distinguish:

· composition of phytocenoses;

· the structure of phytocenoses (their spatial structure) - the distribution of above-ground and underground plant organs that are part of the phytocenosis;

· functional structure of phytocenoses - a set of connections between elements of the phytocenosis;

Composition of plant communities

Vegetation cover is a collection of plant individuals. But in nature it is impossible to find a phytocenosis that would be composed of absolutely identical plants. Almost any plant community consists of coenopopulations of different species belonging to different life forms and ecological groups, playing different roles in nature. And within the same cenopopulation, individuals most often differ in age, degree of development or depression, and so on. Therefore, to characterize the composition of phytocenoses, such features as the floristic composition of phytocenoses, the composition of life forms, the population composition, the composition of ecomorphs and the quantitative ratios of species in the phytocenosis are of primary importance. We discussed the last two characteristics in detail above when considering abiotic environmental factors (composition of ecomorphs) and when considering the specificity of species in their impact on the environment (quantitative ratios of species in a phytocenosis). Therefore, below we will dwell in detail on the first three features characterizing the composition of phytocenoses.

Floral composition

Floral composition – it is the complete collection of plant species found within a particular plant community. Floristic composition is the most important constitutional feature, largely determining the structure and functions of the community. This is a very informative sign that speaks about the environmental conditions in which the community is located, its history, the degree and nature of its disturbance, etc.

The floristic composition is characterized by a number of indicators. The first is species richness, that is, the total number of species characteristic of a phytocenosis. This indicator can vary from 1 (monodominant single-species communities) to 1000 or more species (some tropical forests). According to the witty remark of R. Margalef (Margalef, 1994), species richness can in any case be placed between two extreme situations: the “Noah’s Ark” model - there are a lot of species, but each is represented by only one pair of individuals, and the “Petri dish” - a microbiological culture , in which a huge number of individuals of one species are represented. Species richness is the simplest measure of alpha diversity, that is, biotic diversity at the phytocenosis level.

With all the interest in the indicator of the degree of species richness, it is obvious that its use in comparative analytical constructions is in many cases incorrect. So, for example, a small swamp and a section of tropical forest are incomparable in terms of species richness. Therefore, in geobotany it is much more often used species richness index– number of species per unit area. But here it should be noted that in order to determine the species richness of a phytocenosis, it is necessary in any case to know its species richness.

If species richness is identified using square or round areas of increasing size inscribed within each other, then, as a rule, as the area of the recording unit increases, the number of species identified in the phytocenosis will increase. If you construct a curve from the obtained values, it will fairly well reflect the dependence of the increase in the number of species on the size of the counting area. As a rule, such a curve will initially rise sharply, and then gradually reach a plateau. The beginning of the transition to the plateau will show that on an area of this size the overwhelming number of species in the phytocenosis have already been identified. As a rule, the richer the phytocenosis is in species, the smaller the size of the area at which the curve reaches a plateau.

Rice. 1. Curves “number of species / area” for the desert (a), as well as for desert (b) and meadow (c) steppes;

point * corresponds to the minimum range (Mirkin et al., 2002).

The size of the area at which the “turn” of the curve occurs (Fig. 1) (although it should be noted that it is not clearly expressed in all cases) is called the minimum area (area-minimum). Due to the strong correlation of floristic and physiognomic characteristics of a phytocenosis, the minimum area very often coincides in area with the coenoquantum - an area of homogeneous phytocenosis, sufficient for, in addition to the species richness of the phytocenosis, to statistically reliably assess the projective cover of all species in it. Very similar in meaning to these two terms, but somewhat broader, is the concept of detection area introduced by L. G. Ramensky. Detection area –

the size of the registration area on which all the essential features of the phytocenosis are revealed (floristic composition of the phytocenosis, its structure and quantitative ratio of species; in forest communities, in addition, the wood supply and the distribution curve of trees by diameter classes).

The size of the recording area is a very important factor influencing the species richness of the phytocenosis. So, for example, on a small scale, alvar meadows found in Estonia and Sweden are characterized by the greatest species richness.

These meadows are formed on shallow soils on carbonate rocks, so they are formed by small-sized plants and even in such a small area as 1 dm 2 up to 40 different species fit. In the Kursk steppes, V.V. Alekhin counted up to 100 species per 1 m2. On a large scale (hundreds of square meters), tropical forests are the richest in species, where up to 2000 species of trees, vines and epiphytes can grow on an area of 400 m2.

The factors that determine the species richness of a phytocenosis are numerous and interact in complex ways. That is why species richness is one of the most difficult to predict characteristics of a phytocenosis. So, for example, M. Palmer (Palmer, 1994) gives 120 hypotheses explaining the species richness of the phytocenosis.

Let us consider the main factors influencing the species richness of the phytocenosis. This is the set of species from which species can be selected to form a particular community. For natural and most semi-natural communities, this factor is decisive in the formation of communities. But, at the same time, this fact has a relatively weak effect on ruderal communities that arise under conditions of intense and constant disturbances, since they are based mainly on adventive species that have a wide range, sometimes even almost or completely cosmopolitan.

Possibility of diaspora arrivals. Following R. Sernander, any part of the plant that serves for its propagation is called diaspora. The supply of diaspores depends, on the one hand, on the composition of the local flora. On the other hand, the possibility of the entry of diasporas is very strongly influenced by the probability of their introduction from other regions, which, in turn, depends on the activities of transfer agents and the absence of barriers to the entry of diaspores. This factor especially strongly influences the species composition of isolated communities, such as, for example, high-mountain meadows or clearings remote from each other in a large forest. The intensity of diaspore entry into such communities is associated with the number of seeds produced by different species and, as a consequence, with the likelihood of their introduction into such an isolated habitat. At the same time, dominant species that form seeds in large numbers have a greater probability of forming full-fledged populations than rare species with poor seed productivity.

Ecotop. This is the ecological volume of habitats, which is determined by the favorable conditions for the growth of plants that form the phytocenosis. As stated earlier, each species is ecologically distinct and has a unique range of tolerance to each environmental factor. This leads to the fact that a specific habitat can be inhabited only by those plant species whose tolerance ranges overlap with the boundaries of the conditions of a given ecotope. If the habitat is favorable, the soils are sufficiently moist, rich in mineral nutrition elements and have a neutral environmental reaction, and the climate is mild, then such a habitat has a large ecological volume, that is, many species can potentially grow in one phytocenosis. In extreme conditions (desert, salt marsh, arctic desert, etc.), only a small number of patient species that are specially adapted to such conditions can potentially grow.

Variability of environmental regimes. In a number of cases, fluctuations in the environmental conditions of the ecotope are an important factor in the coexistence of species and an increase in species richness. At the same time, as a result of fluctuations, the range of ecological conditions of a particular ecotope greatly increases. The fact is that the processes of competitive exclusion in plant communities proceed rather slowly, which allows a large number of species, quite different in ecological niches, to coexist in one place. It should be noted that this is true mainly for short-term fluctuations, when species that find themselves in unfavorable conditions, are oppressed, but not yet completely driven out of the community. The importance of this factor is indirectly confirmed by the fact that variability in environmental factors is characteristic of many ecotopes characterized by high species richness, for example, meadows and steppes.

Strategic spectrum of species. This is a factor that very much depends on habitat conditions. If the conditions are harsh and, therefore, the ecological volume of the habitat is small, then the phytocenosis will be dominated by patients. If environmental conditions are favorable, then with a certain degree of probability there may be a violent in the phytocenosis. In its presence, species richness decreases sharply, since the powerful violent almost completely uses the resources of the environment. An example of this could be beech and spruce forests, reed thickets in river floodplains, etc. If there is no violent, then environmental resources can be divided between different types due to the differentiation of their ecological niches. This leads to high species richness in such communities. Such conditions are created, for example, in tropical rainforests, steppes, and meadows with moderately dry but fairly rich soils.

As one moves away from the area of favorable environmental conditions to the area of their pessimal values, the number of species in phytocenoses decreases. This decline is most pronounced where some species are capable of absolute dominance, as this limits the growth of other species. An example can be the results of observations in the alpine belt of the Eastern Carpathians, where in places where excrement accumulates (camps), thickets of powerfully developed alpine sorrel are formed, where, in addition to it, only 1-2 species of higher plants grow. At the same time, in similar conditions, but on poor soil without accumulations of excrement, phytocenoses with a predominance of white beetles are formed, including more than 30 species.

Violation mode. A moderate regime of violations can slightly, and sometimes quite significantly, increase species diversity community, but only if it prevents the strengthening of the role of violents. So, for example, the grass cover of a broad-leaved forest moderately visited by vacationers is richer in species compared to a reserved forest, where most of the niche space is taken over by squirrels. The species composition of floodplain meadows used as hayfields is always much higher than that of unmown meadows, where communities of several dominant species are formed and the rest are displaced. But if the load of the disturbing factor is high, then the species richness of the phytocenosis will sharply decrease, and with periodic action of the factor (plowing, the passage of machinery with disturbance of the vegetation cover, etc.), the explorers will predominate, and with constant disturbance (intensive grazing), the patients will prevail.

The possibility of coexistence of many plant species increases under the influence of earth-moving animals. Their activity leads to increased heterogeneity of the environment. The formation of disturbed places with a sharply reduced intensity of competition provides the opportunity for the growth of species with low competitive ability, including annuals. The appearance of spots characterizing various stages of vegetation restoration after its disturbance provides the opportunity for the formation of floristically richer phytocenoses. In a heterogeneous environment, individuals of individual species can be distributed among different microhabitats. In this case, individuals of species within the same community may not interact with individuals of some other species, since they are confined to different microhabitats.

Anthropogenic factor. Under human influence, the floristic composition of phytocenoses undergoes very strong changes, both in the direction of increasing species richness and, more often, in the direction of its depletion.

Thus, people often create new phytocenoses by sowing or replanting plants, often alien to the local flora. An example of this is potato fields in Belarus, forest plantations of North American conifers in Western Europe, New Zealand, etc.

Often, people deliberately introduce new species into existing phytocenoses, sometimes imported from other regions. An example of this would be overseeding Lupinus polyphyllus And Sarothamnuscoparius in our pine forests.

With the direct participation of humans, accidental introduction of plants from other places often occurs, and these plants begin to successfully penetrate local phytocenoses. Thus brought into the territory of Belarus Acoruscalamus(Central Asia), Elodeacanadensis And Conyzacanadensis from America, etc.

Sometimes plants are first introduced into gardens and parks as ornamental or economically useful crops, from where they successfully and often spread en masse into local phytocenoses. An example of this in Belarus could be North American species: Amelanchierspicata, which is currently actively introducing itself into forest cenoses, and Echynocystislobata, often growing en masse in river floodplains.

Very often, diaspores of weeds enter natural phytocenoses from fields, which, as a rule, can be transported over long distances by wind or water.

Often, a person deliberately destroys plants that he considers undesirable, but weed control, as a rule, only leads to a reduction in the number of individuals of such species, and not to their complete exclusion from the composition of phytocenoses. The use of meadows as hayfields may cause the extinction of species that reproduce exclusively by seeds if the timing and frequency of mowing interfere with their seeding. Human impact on the ecotope (drying, irrigation, liming, fertilization) leads to limiting the growth of some species and creating conditions favorable for others. Of great importance in determining the floristic composition of phytocenoses is such a factor as livestock grazing, especially intensive. This usually leads to a sharp reduction in the number of species, since very few of them are able to exist in such conditions.

Time (age of the community). Time is a universal factor that manifests itself in any community. However, the significance of this factor can vary greatly during the formation of different phytocenoses, just as the time scale can be different. For example, in ruderal communities formed mainly by explorer species, species richness increases on a scale of months and years, while in natural climax communities it increases on a geological time scale. An example is the species richness of analogous communities on serpentine soils in the mountains of North America, studied by R. Whittaker. These communities are located in areas that were and were not subject to glaciation. As it turned out, the species richness of communities in areas that were not influenced by the glacier was 2 times higher than that of similar communities that formed in areas subject to glaciation. This is primarily due to the fact that as the duration of the existence of a phytocenosis increases, the chances of diaspores of more plant species entering it increase.

The concept of phytocenosis age, introduced by L. G. Ramensky in 1924, is very closely related to the concept of floristic completeness and incompleteness of phytocenoses. By floristically incomplete phytocenoses he understood communities that do not include all plant species that can exist in them. Ramensky identified phytocenoses as absolutely complete, natively complete, practically complete and clearly incomplete. The completeness or incompleteness of phytocenoses can be accurately established only by conducting experiments with sowing seeds of species not included in their composition. Absolutely complete phytocenoses probably do not exist in nature, but it is impossible to verify this, since it would be necessary to reseed all plant species capable of growing in the conditions of a given ecotope. The introduction of plants accidentally introduced by humans from other regions into phytocenoses, as well as the deliberate introduction of many species into natural communities (for example, multileaf lupine in pine forests) give reason to talk about the wide distribution of floristically incomplete phytocenoses.

At the same time, many long-established phytocenoses are natively full-membered, that is, they include all types of local flora that can grow in the given conditions. To identify floristic incompleteness, long-term observations are necessary, since often individuals of a species, accidentally introduced or deliberately introduced by an experimenter, exist for only 1-2 years and then die, since the habitat in a given phytocenosis is unfavorable for them. It must also be taken into account that some species in certain conditions are represented only by individuals in a state of dormancy (viable seeds, dormant underground organs). The incompleteness established in relation to such species is, therefore, apparent (the so-called false inferiority or hidden completeness of phytocenoses). Most often it is a temporary phenomenon. In this case, resting individuals switch to an active state as soon as favorable conditions are created for this. This sometimes happens periodically or episodically, and sometimes only with a complete or local disturbance of the phytocenosis as a result of a strong deviation from average meteorological and hydrological conditions, as well as with the mass reproduction of shrews.

Can be distinguished primary And secondary, or anthropologically caused inferiority. The primary incompleteness of a phytocenosis occurs during its formation and is gradually eliminated as the community structure develops and becomes more complex. An example of anthropologically determined incompleteness may be incompleteness associated with the lack of seeding of some plant species that reproduce exclusively by seeds during the transition from single-cut to double-cut use of meadows. It is worth noting that the absence of seeding of plants can occur both without changing ecotopic conditions (during haymaking) and when they change (for example, during grazing).

In addition to floristic, there is also the so-called phytocenotic incompleteness, that is, the state when in a phytocenosis some species are present in quantities less than the minimum possible to ensure their seed reproduction. So, for example, cross-pollinating plants can be present in such a small quantity in a phytocenosis and be so sparsely located that the probability of their pollination will be close to zero. As a rule, the phytocenotic incompleteness of phytocenoses after some time turns into floristic, since the cenopopulations of such plant species simply die out.

Floristic and phytocenotic incompleteness of phytocenoses can have a large practical significance. Thus, the absence in phytocenoses of species that could potentially be included in their composition (or if they are present, they could potentially be in much greater quantities) and thereby increase their productivity or improve the quality of products, gives us the opportunity to introduce them into communities. An example would be overseeding legume seeds to improve meadows or lupines in pine forests. And vice versa, if in phytocenoses there are no plant species of low value or harmful from a human point of view that can grow in these conditions, then measures must be taken to prevent the introduction of such species into the community.

All of the listed factors in the formation of species richness interact, which explains the difficulty of predicting this characteristic of communities. However, if we ignore the details and consider the general trends in changes in species diversity on a global scale, we can talk about a certain main diversity gradient. R. Whittaker defined it as changes in communities from the high latitudes of the Arctic to the tropics on the plains and from the highlands to the plains. The richest communities are tropical forests and savannas, while the poorest are communities of alpine and arctic deserts.

It is clear that adjustments to the gradient on the plain need to be made taking into account the continentality of the area, that is, its distance from the ocean and, accordingly, changes in the amount of precipitation and the nature of temperature changes in the annual cycle. Heat without moisture, like moisture without heat, cannot serve as a source of improving conditions and increasing the physical hyperspace of resources, and therefore alpha diversity. For this reason, at low latitudes, if it is a desert, alpha diversity will be low. A similar picture is observed in the mountains. A gradient of increasing species diversity will be observed only if in the area where the mountain system is located, the ratio of heat and moisture is optimal, that is, if it is an area of the humid tropics or subtropics. If, say, a mountain system is located in a desert, then the change in species diversity will be described by a parabolic curve with a maximum in the middle part of the gradient. So, at first it will increase, that is, the desert will be replaced by steppe or savanna, and only then it will decline. Thus, Whittaker's claims about a major diversity gradient should be taken with caution.

Whittaker's conclusion about the known independence of changes in the richness of communities by species belonging to different life forms is very interesting. Thus, along the north-south gradient (that is, from the Arctic to the tropics), the number of tree species increases, but the number of grasses decreases. This precisely reflects the success of Raunkier’s system of life forms and makes it possible to derive the so-called “normal spectra” of life forms of different variants of zonal vegetation.

Subject : Diversity of phytocenoses.

Goals: to form an idea of the diversity of plants in natural communities. Develop the ability to find relationships in a community and recognize plants by external signs.

Lesson type: Discovery of new knowledge based on activity approach technology using ICT

Educational Resources: presentation [Electronic resource].

Technical support: computer, multimedia projector and screen, herbarium specimens of plants.

Software security : Microsoft Power Point, Microsoft Word

Planned educational results:

Subject (scope of development and level of competence): learn to give examples of the relationships between plants and environmental factors; will have the opportunity to learn to evaluate their actions in relation to nature and talk about them; make assumptions and prove them; understand learning task lesson and strive to fulfill it; work in groups, using the information presented to gain new knowledge; answer final questions and evaluate your achievements in class.

Metasubject (components of cultural competence experience/acquired competence): use various ways search (in reference sources and textbooks), collection, processing, analysis, organization, transmission and interpretation of information in accordance with communicative and cognitive tasks; determine a common goal and ways to achieve it; be able to negotiate the distribution of functions and roles in joint activities; exercise mutual control in joint activities; adequately assess your own behavior and the behavior of others.

Personal: the formation of a holistic, socially oriented view of the plant world in its organic unity and diversity of nature, respect for other opinions; development of ethical feelings, goodwill and emotional and moral responsiveness, development of motives educational activities and personal meaning of teaching; mastering the logical actions of comparison, analysis, synthesis, generalization, classification according to generic characteristics of plants; willingness to listen to the interlocutor and conduct a dialogue, recognize the possibility of the existence of different points of view and the right of everyone to have their own, express their opinion and argue their point of view and assessment of events.

Universal learning activities(UUD):

Cognitive: general educational – extracting the necessary information during development new topic on plant diversity;brain teaser - addition and expansion of existing knowledge and ideas about the plant world.

Communicative: know how to exchange opinions, listen to each other, construct understandable speech statements; accept a different opinion and position, allow for the existence of different points of view.

Regulatory: orientation in the textbook and workbook; accept and maintain the learning task; evaluate the results of their actions; predict the results of the level of mastery of the studied material.

Personal: understand the importance of knowledge about natural communities for humans and accept it.

Methods and forms of training: partially search, group, individual.

During the classes.

IMotivation for learning activities

IIUpdating knowledge.

1...Hello guys!!! We have words scattered on the board, we need to create a logical diagram from them so that all the words are connected to each other.

2...This diagram tells us the topic of our lesson. What theme? (Diversity of phytocenoses). Today we will have unusual lesson, lesson-journey. We will go on a journey through natural communities.

3. ..And what do we need to learn along the way???

We will find out what plant levels there are in different natural communities, what plant world is typical for them, and how it is connected by invisible threads with factors of inanimate nature.

4..

So, let's go on a journey.

You, my friend, don’t let us down!

Promise to be truthful and kind!

Don't hurt either the bird or the cricket,

Don't buy a butterfly net.

Love flowers, forests, the expanse of meadows, fields -

Everything that is called your homeland.

IIIInclusion in the knowledge system and repetition.

TEST: (5 min)

IVLearning new material.

Practical work with herbarium specimens of plants.

Now let's conduct practical work. We will work in groups. Each group has its own plant herbariums corresponding to the phytocenosis.

The class is divided into groups according to the name of the biogeocenosis: “Spruce forest”, “Meadow”, “Pine forest”, “Oak grove”, “Swamp”. "Field and Garden".

Select a team captain who will lead the group.

The team captain distributes tasks.

Pupils work in groups.

They protect their phytocenoses by responding according to plan. Herbarium specimens are shown. Answer students' questions.

VPhysical education minute (exercises for the eyes and to improve cerebral circulation)

A set of gymnastics exercises for the eyes

1. Blink quickly, close your eyes and sit quietly, slowly counting to 5. Repeat again.

2. Close your eyes tightly (count to 3), open your eyes and look out the window (count to 5). Repeat 2 times.

A set of exercises to improve cerebral circulation

Starting position – sitting on a chair. 1–2. Smoothly tilt your head back, tilt your head forward without raising your shoulders. Repeat 3-4 times. The pace is slow.
2) Starting position – sitting, hands on your belt. 1. Turn your head to the right. 2. Starting position. 3. Turn your head to the left. 4. Starting position. Repeat 3 times. The pace is slow.

VI.Summing up the lesson. Assessing students' work in class.

Our journey has come to an end.

Let's remember where we went today? (By groups)

How are plants located in any natural community? (by tiers)

Name these tiers.

Why do plants of different natural communities differ from each other? (different biotope)

Team captains hand over achievement sheets to the teacher.

VII. Homework assignment

By the next lesson you should be goodknow:

1. The name of the organisms that make up the natural community;

2. Tiers of natural communities.

Mustbe able to :

1. Distinguish between plants.

2. Establish their relationship in nature.

3. Always be careful about the environment.

Plan:

1 Development of views on the nature of phytocenosis
2 Formation of phytocenosis
3 Factors of phytocenosis organization
- 3.1 Interactions of organisms in phytocenoses
  - 3.1.1 Direct (contact) mutual influences
  - 3.1.2 Transabiotic interactions
  - 3.1.3 Transbiotic interactions
4 Influence of phytocenosis on the environment
5 Structure of phytocenosis
6 Influence of phytocenosis on the environment
7 Dynamics of phytocenoses
8 Classification of phytocenoses
9 Territorial structure of vegetation cover

Introduction

Forest phytocenosis

Phytocenosis(from Greek φυτóν - “plant” and κοινός - “general”) - a plant community existing within one habitat. It is characterized by relative homogeneity of species composition, a certain structure and system of relationships of plants with each other and with the external environment. According to N. Barkman, phytocenosis is the essence of a specific segment of vegetation in which internal floristic differences are less than differences with the surrounding vegetation. The term was proposed by the Polish botanist I. K. Pachoski in 1915. Phytocenoses are the object of study of the science of phytocenology (geobotany).

Phytocenosis is part of the biocenosis along with zoocenosis and microbiocenosis. The biocenosis, in turn, in combination with the conditions of the abiotic environment (ecotope) forms a biogeocenosis. Phytocenosis is the central, leading element of biogeocenosis, as it transforms the primary ecotope into a biotope, creating a habitat for other organisms, and is also the first link in the cycle of substances and energy. The properties of soils, microclimate, the composition of the animal world, such characteristics of biogeocenosis as biomass, bioproductivity, etc., depend on vegetation. In turn, the elements of phytocenosis are cenopopulations of plants - a collection of individuals of the same species within the boundaries of the phytocenosis.

1. Development of views on the nature of phytocenosis

At the dawn of the development of geobotany, the idea of a phytocenosis as a really existing discrete unit of vegetation cover took shape, which at that time seemed quite appropriate, since the identification of individual phytocenoses significantly facilitated the task of studying vegetation as a whole. However, at the beginning of the 20th century, a diametrically opposite point of view was expressed, according to which the vegetation cover was considered continuous, and its division into individual elements - phytocenoses - was artificial. The absence of sharp boundaries between plant communities and the presence of transition zones between them contributed to the emergence of the doctrine of continuity of vegetation cover, based on individualistic concept:

Each plant species is individual in its requirements for environmental conditions and has characteristic environmental amplitudes for each environmental factor
Environmental factors change gradually, both in space and time
The transition from one combination of cenopopulations to another occurs continuously: some species gradually decrease their abundance and disappear, others appear and increase.

Extreme supporters of the concept of continuity of vegetation cover considered not a phytocenosis, due to its artificiality, but an individual plant as an object of study of geobotany. Extreme supporters of the idea of discreteness postulated a clear distinction and delineation of individual phytocenoses.

Based on the synthesis of both concepts, the idea of a combination of both discreteness and continuity in the nature of vegetation was put forward. This was presented as one of the manifestations of the inconsistency inherent in the material world as a whole. According to this idea, the vegetation cover has the property of continuity, but it is not absolute, but relative. At the same time, it also has the property of discreteness, but this is not absolute, but relative. These properties are organically combined, not excluding, but complementing each other.

2. Formation of phytocenosis

Primary free area earth's surface after a volcanic eruption

The formation of phytocenoses can be considered both in a dynamic aspect (change of communities) and in terms of their formation on free areas of the earth's surface.

Distinguish primarily vacant areas, which in the past were not populated by plants and do not contain their rudiments. Phytocenoses can form on them only when diaspores are introduced from the outside. Such areas include rocky outcrops, fresh river and sea sediments, the exposed bottom of reservoirs, areas freed from glaciers, lava fields, etc. In general, they occupy insignificant areas on Earth.

Secondary vacant areas are formed in places where vegetation previously existed, but was destroyed due to the influence of some unfavorable factor. Examples include burnt areas, scree, unseeded arable land, areas of phytocenoses eaten away by pests or livestock. In most cases, soil and diasporas are preserved on them, and the formation of phytocenoses occurs much faster than in initially free areas.

The formation of a phytocenosis is a continuous process, but can be conditionally divided into stages:

according to V.N. Sukachev:

according to A.P. Shennikov:

Pioneer group- coenopopulations are small in number, there are no relationships between them
Group-thicket community- coenopopulations are distributed in clumps in which interaction between plants occurs
Diffuse community- coenopopulations mix, a system of interspecific interactions is developed

according to F. Clements:

Migration- introduction of diasporas
Ecesis- consolidation of the first settlers
Aggregation- formation of groups of offspring around mother plants
Invasion- mixing of coenopopulations
Competition- development of competitive relations due to a sharp increase in crowd density
Stabilization- formation of a sustainable closed community

E. P. Prokopyev, summing up the various division schemes of the process of formation of a phytocenosis, proposes to distinguish three stages in it:

Receipt of primordia into a free area. The species composition of the emerging phytocenosis will depend on the species composition of plants in the surrounding area and the nature of the distribution of their diaspores, and the main role will be played by the rudiments of allochoric species, mainly anemochores.
Ecotopic (abiotic) selection. Not all diasporas that land on a free plot will take root on it: some will not germinate, and some of those that have sprouted will die in their young state due to an unfavorable combination of abiotic factors. Established plants will be pioneers for this territory.
Phytocenotic selection. Due to the reproduction and settlement of pioneer species throughout the site, they will begin to influence each other and change the ecotope, forming a biotope (habitat). The primary abiotic environment of the ecotope turns into a secondary biotic - phytoenvironment. Under the influence of the phytoenvironment and the mutual influence of plants, some pioneer species that are not adapted to it fall out. This may occur, for example, due to shading or allelopathy. At the same time, new species, already adapted to the given phytoenvironment, are established on the site.

3. Factors of phytocenosis organization

Factors in the organization of a plant community can be divided into four groups: characteristics of the environment (ecotope), the relationship between plants, the influence of heterotrophic components (animals, fungi, bacteria) on vegetation and disturbances. These groups of factors determine the combination and characteristics of cenopopulations of species in a phytocenosis.

Ecotop is the main factor in the organization of phytocenosis, although it can be significantly transformed by the biotic influences of plants or disturbances. Abiotic factors influencing community organization include:

climatic (light, heat, water regimes, etc.)
edaphic (particle-size and chemical composition, humidity, porosity, water regime and other properties of soils and soils)
topographic (relief characteristics)

Plant relationships are divided into contact And mediated : transabiotic- through abiotic environmental factors and transbiotic- through third organisms.

Influence on the organization of phytocenoses heterotrophic components biogeocenoses are extremely diverse. The influence of animals is manifested in pollination, eating, spreading seeds, changing the trunks and crowns of trees and associated characteristics, loosening the soil, trampling, etc. Mycorrhizal fungi improve the supply of plants with mineral nutrients and water, and increase resistance to pathogens. Nitrogen-fixing bacteria increase the supply of nitrogen to plants. Other fungi and bacteria, as well as viruses, can be pathogens.

Violations, both anthropogenic and natural origin can completely transform the phytocenosis. This occurs during fires, felling, livestock grazing, recreational load, etc. In these cases, derivative phytocenoses are formed, which gradually change towards the restoration of the original one if the impact of the disturbing agent has ceased. If the impact is long-term (for example, during recreation), communities are formed that are adapted to existence at a given level of stress. Human activity has led to the formation of phytocenoses that did not previously exist in nature (for example, communities on toxic industrial waste dumps).

3.1. Interactions of organisms in phytocenoses

The presence of a system of relationships between plants is one of the main signs of the existing phytocenosis. Studying them, due to their large overlap and strong influence abiotic factors is a difficult task and can be implemented either in the form of an experiment during which the relationships between two specific species are studied, or by isolating such relationships from a complex of others using methods of mathematical analysis.

3.1.1. Direct (contact) mutual influences

Trees affected by Mistletoe ( Viscum album)

Pink trumpet (Lactarius torminosus) forms ectomycorrhiza with birch

Symbiotic relationship manifest themselves in the coexistence of plants with fungi and bacteria (including cyanobacteria). Accordingly, they distinguish mycosymbiotrophy And bacteriosymbiotrophy.

The ecological meaning of the formation of mycorrhiza is that the fungus receives carbohydrates and some vitamins from the plant, and the plant receives the following benefits:

Plants that form mycorrhiza can be divided into two groups according to their requirements for the presence of mycosymbiont:

obligate mycosymbiotrophs - incapable of development without a mycosybiont (family Orchidaceae)
facultative mycosymbiotrophs - capable of existing without a mycosymbiont, but developing better in its presence

Bacteriosymbiotrophy is the symbiosis of plants with nodule bacteria (Rhizobium sp.). It is not as widespread as mycosymbiotrophy - about 3% of the plants of the world flora enter into symbiosis with bacteria (mainly the legume family (about 86% of the species of the family), as well as some species of the family Poaceae, Birchaceae, Suckeraceae, Buckthornaceae). Nodule bacteria play the role of nitrogen fixers, converting atmospheric nitrogen into forms accessible to plants. There are root and leaf forms of interaction. In the root form, the bacteria infect the roots of the plant, causing intense local cell division and the formation of nodules. Leaf bacteriosymbiotrophy occurs in some tropical plants and is still poorly studied.

One of the three-field culture methods is based on the ability of legume family plants to enter into symbiosis with nodule bacteria.

Tropical epiphytic fern Staghorn ( Platycerium bifurcatum)

Epiphytes, settling on plants - phorophytes they use the latter only as a substrate, without entering into physiological interactions with them. Epiphytic forms are found in groups of angiosperms, ferns, mosses, algae and lichens. Epiphytes reach their greatest diversity in tropical rainforests.

In ecological terms, the relationship between epiphytes and phorophytes is usually represented by commensalism, but elements of competition may also appear:

epiphytes partially intercept light and moisture from phorophytes
by retaining moisture, they contribute to the decay of the phorophyte
By shading the phorophyte, epiphytes reduce its effective photosynthetic surface
growing profusely, can cause deformation or breakage of phorophytes

Lianas, bringing their leaves closer to the light during growth, receive benefit from cohabitation with the supporting plant, while the latter is mainly harmful, both direct - due to the mechanical impact of the liana and the breaking / death of the supporting plant, and indirect - due to the interception of light by the liana, moisture and nutrients.

Lianas also reach their greatest diversity in tropical rainforests.

3.1.2. Transabiotic interactions

The influence of plants on each other, mediated by abiotic environmental factors. They arise due to the overlap of phytogenic fields of neighboring plants. Divided into competition, allelopathy And code interactions.

Competition develops either due to the initial limitation of habitat resources, or as a result of a decrease in their share per plant due to overpopulation. Competition leads to a decrease in resource consumption by the plant and, as a consequence, a decrease in the rate of growth and storage of substances, and this, in turn, leads to a decrease in the quantity and quality of diaspores. Distinguish inside- And interspecific competition.

Intraspecific competition affects the fertility and mortality rates in a coenopopulation, determining the tendency to maintain its numbers at a certain level, when both values balance each other. This number is called maximum density and depends on the amount of habitat resources. Intraspecific competition is asymmetrical - it affects different individuals differently. The total phytomass of the cenopopulation remains constant over a fairly large range of density values, while average weight of one plant, when thickened, begins to steadily decline - law of constant harvest(C=dw, where C is the yield, d is the coenopopulation density and w is the average weight of one plant).

Interspecific competition is also widespread in nature, since the vast majority of phytocenoses (except for some agrocenoses) are multispecies. The multispecies composition is ensured by the fact that each species has an ecological niche characteristic only of it, which it occupies in the community. Moreover, the niche that a species could occupy in the absence of interspecific competition is fundamental, tapers to size implemented. In a phytocenosis, differentiation of ecological niches occurs due to:

different plant heights
different depths of penetration of the root system
contagious distribution of individuals in the population (in separate groups/spots)
different periods of growing season, flowering and fruiting
unequal efficiency of plants' use of habitat resources

With weak overlap of ecological niches, coexistence of two cenopopulations can be observed, while with strong overlap, the more competitive species displaces the less competitive species from the habitat. The coexistence of two highly competitive species is also possible due to the dynamism of the environment, when one species or another gains a temporary advantage.

Allelopathy- the influence of plants on each other and on other organisms through the release of active metabolites into the environment, both during the life of the plant and during the decomposition of its remains.

3.1.3. Transbiotic interactions

4. Influence of phytocenosis on the environment

5. Structure of phytocenosis

Depending on the specifics of the research, V.V. Masing (1973) identifies three directions in the concept of “biocenosis structure” that he developed for phytocenoses.

1. Structure, as a synonym for composition (specific, constitutional). In this sense, they talk about species, population, biomorphological (composition of life forms) and other structures of the cenosis, meaning only one side of the cenosis - composition in the broad sense. In each case, a high-quality and quantitative analysis composition.

2. Structure, as a synonym for structure (spatial, or morphostructure). In any phytocenosis, plants are characterized by a certain affinity to ecological niches and occupy a certain space. This also applies to other components of the biogeocenosis. Between the parts of the spatial division (tiers, sinusia, microgroups, etc.) you can quite easily and accurately draw boundaries; you can plot them on a plan, calculate the area, and then, for example, calculate the resources of useful plants or food resources of animals. Only on the basis of data on the morphostructure can one objectively determine the points at which certain experiments were performed. When describing and diagnosing communities, the spatial heterogeneity of cenoses is always studied.

3. Structure, as a synonym for sets of connections between elements (functional). The basis for understanding structure in this sense is the study of relationships between species, primarily the study of direct connections - the biotic connex. This is the study of chains and nutrition cycles that ensure the circulation of substances and reveal the mechanism of trophic (between animals and plants) or topical (between plants - competition for nutrients in the soil, for light in the above-ground sphere, mutual assistance).

All three aspects of the structure of biological systems are closely interrelated at the coenotic level: species composition, configuration and placement structural elements in space are a condition for their functioning, that is, life activity and production of plant mass, and the latter, in turn, largely determines the morphology of cenoses. And all of these aspects reflect the environmental conditions in which the biogeocenosis is formed.

The phytocenosis consists of a number of structural elements. There are horizontal and vertical structure of phytocenosis. The vertical structure is represented by tiers, distinguished by visually determined horizons of phytomass concentration. The tiers consist of plants of “different heights”. Examples of layers are 1st tree layer, 2nd tree layer, ground cover, moss-lichen layer, understory layer, etc. The number of layers may vary. The evolution of phytocenoses is moving in the direction of increasing the number of tiers, as this leads to a weakening of competition between species. Therefore, in more ancient forests temperate zone In North America, the number of layers (8-12) is greater than in similar younger forests of Eurasia (4-8).

The horizontal structure of the phytocenosis is formed due to the presence of tree canopies (under which an environment is formed, somewhat different from the environment in the inter-canopy space), terrain heterogeneities (which cause changes in groundwater levels, different exposures), and the species characteristics of some plants (reproducing vegetatively and forming monospecies “spots” , changes in the environment of one species and the response of other species to this, allelopathic effects on surrounding plants), animal activities (for example, the formation of patches of ruderal vegetation on rodents).

Naturally repeating spots (mosaics) in a phytocenosis, differing in the composition of species or their quantitative ratio, are called microgroups(Yaroshenko, 1961), and such a phytocenosis is mosaic.

Heterogeneity may also be random. In this case it is called variegation.

6. Influence of phytocenosis on the environment

7. Dynamics of phytocenoses

dynamic processes: reversible (daily, seasonal, fluctuations) and irreversible (succession, evolution of communities, disturbances of communities). Fluctuations are year-to-year changes associated with unequal conditions for the existence of plants in different years. the composition does not change, the size and age composition of the population may change. Succession is a gradual change in phytocenoses that is irreversible and directional. caused by internal or external reasons in relation to phytocenoses, reasons. primary and secondary successions. Primary successions begin on lifeless substrates, while secondary successions begin on substrates on which the vegetation has been disturbed.

8. Classification of phytocenoses

9. Territorial structure of vegetation cover

Notes

Barkman N. Modern representations about the continuity and discreteness of vegetation cover and the nature of plant communities in the phytosociological school of Braun-Blanquet. - Botanical Journal, 1989, vol. 74 No. 11
Ramensky L. G. On the comparative method of ecological study of plant communities // Diary of the Congress of Russian Naturalists and Doctors - St. Petersburg, 1910
Greason H. A. The structure and development of the plant association // Bul. Torrey Bot. Club - 1917
Negri G. Le unita ecologiche fundamentali in fitogeografia - Roy. Acad Sc. Torino, 1914
Lenoble F. A propos des associations vegetales // Bull. Soc. Bot. France - 1926
Aleksandrova V.D. On the unity of continuity and discreteness in vegetation // Philosophical problems of modern biology. - M.-L.: Science, 1966
Aleksandrova V.D. Classification of vegetation. - L.: Science, 1969
Sukachev V.N. Dendrology with the basics of forest geobotany. - L.: Goslestekhizdat, 1938
Shennikov A.P. Introduction to geobotany. - L.: Leningrad State University Publishing House, 1964
Weaver J. E., Clements F. E. Plant ecology. - N.Y.-London, 1938
1 2 Prokopyev E. P. Introduction to geobotany. Tutorial. - Tomsk: TSU Publishing House, 1997. - 284 p.
1 2 Rabotnov T. A. Phytocenology. - M.: Moscow State University Publishing House, 1978
Rabotnov T. A. History of phytocenology. M.: Argus, 1995
G. N. Tikhnova Ficus stranglers from the island of Borneo - bio.1september.ru/2001/27/1.htm

Literature

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This abstract is based on an article from Russian Wikipedia. Synchronization completed 07/12/11 13:38:08
Categories: Phytocenosis.
Text is available under a Creative Commons Attribution-ShareAlike license.

In biology there is such a thing as a biotope. This is a homogeneous but limited territory of a geopolitical area (for example, some kind of body of water or a piece of land). This territory occupied by a biocenosis of a certain species (a biocenosis can be defined as a system of living organisms (various animals, plant units or microorganisms)) that live in a certain area of space.

Within the framework of one biotope, discussed above, a set of plants of different species conducts their life activities. But, nevertheless, these species are quite homogeneous, have a single structure and certain relationships, as well as connections with the world around them. To put it simply, a phytocenosis is a grouping of plants that are most similar to each other when compared with the flora of the environment.

This definition was first voiced by Joseph Konradovich Pachosky, a botanist who lived in Poland. The term appeared in 1915. Phytocenosis appeared within the framework of a science called phytocenology (geobotany is its middle name).

As noted above, biocenosis is the name of the entire system, which consists of microbiocenosis, zoocenosis and phytocenosis, discussed in this article. The element discussed here is considered the central link of the biogeocenosis, which is formed by the biocenosis in combination with the circumstances of the abiotic habitat. There is a logical explanation for this dominant role: it creates a habitat for other living organisms. Thus, phytocenosis plays a role entry level in the ecological cycle (its elements are substances and energy).

In simple words, from flora many factors depend. Among them are the quality and nutritional value of soils, the species structure of the animal world, microclimatic conditions, as well as bioproductivity and biomass.

Within one phytocenosis, several cenopopulations of plant elements can be distinguished. This means that individuals of a single species are collected in coenopopulations, of which, in turn, the phytocenosis consists.

What factors are necessary for phytocenosis to take place?

In order for a set of plants to turn into a phytocenosis and function properly, certain factors are necessary. These can be divided into several groups:

The state of the ecotope can be defined as a central factor. These include climate conditions, soil or soil properties (saturation, humidity, nutritional value, etc.), the structure of the terrain;
The impact of heterotrophic elements of biocenoses includes the influence of organisms of animal origin (it is comprehensive: eating plants, their pollination and even dispersal of seeds), fungi (feeding the soil with minerals and water), nitrogen-fixing bacteria (feeding vegetation with nitrogen);
Interactions between individuals can be indirect (organisms of the third level act as intermediaries) and contact (abiotic factors as helpers);
All kinds of disturbances have the power to completely change the phytocenosis. Among these are fires, recreational loads, land development for pastures, and deforestation. In this situation, the mediator is a phytocenosis of derivative purposes, which over time transforms and turns into a root one. A similar “trick” occurs only if the influence of the offending object has been stopped. It should be noted that people, sometimes without noticing it themselves, have formed thousands of new phytocenoses. This can be proven by the change natural conditions a certain area for the promotion of recreational activities or the creation of industrial production (in the latter case, the phytocenosis is more likely to disappear than to be replaced by another).

How is a phytocenosis formed?

This process can occur in two ways:

Formation of a phytocenosis in a primarily unoccupied area. Such areas have never had vegetation before. An important condition here is that the formation of a new group of individuals occurs only when diaspores arrive from a neighboring phytocenosis (diaspora is a seed or spore of a plant, a certain part of it). Most often, primary free areas include sediment from lakes or seas, areas freed from glaciers, rock outcrops and other areas;
Creation of a phytocenosis in a place previously inhabited by a certain type of vegetation. These individuals could disappear after being exposed to unfavorable factors (natural disasters, fires, deforestation). Secondary free territories include various screes, arable lands (which for some reason are not used by humans), places where livestock are located (they feed on vegetation, as a result of which the territory becomes deserted).

Botanists have identified four stages at which the formation of a biocenosis occurs:

There is no phytocenosis! IN in this case elements of the plant environment do not interact with each other and have completely different species composition;
Open type phytocenosis. Coenopopulations weakly interact with each other, their structure is unstable;
The phytocenosis is of a closed type, but not yet developed. The species composition is more stable, but allows the presence of individual species of other structural groupings; the separation of tiers is noticeable;
The phytocenosis is of a closed type, and it has completely developed. The species structure is stable; therefore, species not related to a given phytocenosis will no longer be able to penetrate this set. In this case, all cenopopulations closely interact with each other, and the tiers have clear outlines.

What classification does the diversity of phytocenoses have?

As noted earlier, in nature there is an indescribably large number of different plants, united in phytocenoses. But how to distinguish between these populations of individuals? Phytocenosis is the smallest element in the entire system. The association is in second place in terms of seniority (it unites several phytocenoses), gluing similar phytocenoses into a single whole. This similarity can manifest itself in the species composition of the group, its tiers, or the number and habitat of plants in the group. Each association has a connection with certain factors, be it climate or soil.

Elements that are similar in characteristics are combined into a group of associations. Next (in order from smallest to largest) comes the class of associations and formation. Then comes the formation group and his class. The final step is the type of vegetation.

What is the purpose of phytocenosis?

Approaching the conclusion of the article, it is necessary to consider the last important aspect in the issue of biocenoses - their purpose. After all, you can know with precision how they are formed and what they are, but have no idea what role they play in the life of nature and society.

The entire set of elements showing that phytocenoses are of great benefit can most clearly be proven with examples:

Trees are capable of creating shade for their fellows: shrubs, various kinds of mosses, mushrooms, and so on. The fact is that many plants just need such conditions, and trees come to their aid;
Needles or foliage that falls from trees creates a protective and, at the same time, nutritious layer over the soil. It becomes soil humus;
If a strong wind breaks out, then the phytocenosis has a much greater chance of overcoming it than an individual plant;
During the day, phytocenoses tend to reduce the amount of carbon dioxide in the air;
Elements of phytocenosis are capable of creating and then releasing substances of volatile origin that can destroy an individual plant;
The soil temperature in the forest largely depends on the phytocenosis. Air temperature also applies here;
These sets of plants have the power to control the amount of precipitation, as well as moisture in the area (spruce leaves a huge part of the moisture on the crown, while beech forest is different in that water flows down the trees and touches the ground);
Cenopopulations have a great impact on the environment. If we take a plant such as horsetail as an example, and imagine that it is not widely distributed, then we can say: individuals influence each other very weakly. If you imagine a thick, and, moreover, mixed, cover, then its units will influence each other to a very strong degree. The result of such a huge influence will be the replacement of some (the weakest) plants with others (those that are the majority in the cenopolulation). The changes will also affect the soil.

All the examples given prove only one thing: environment-forming relationships are present in all phytocenoses without exception.

Constitutional structure of phytocenoses

Concept constitutional structure phytocenosis proposed by T.A. Rabotnov (1965), reflects its composition in a broad sense, including species, population, ecological and biological composition, composition of phytocenotypes and coenogenetic groups, etc. Below we briefly discuss the main features of the constitutional structure of phytocenoses.

Species composition of phytocenoses

Each phytocenosis is characterized by its own specific species composition. Its complexity or simplicity is determined by the indicator in id (floristic) saturation, which is understood as the number of species per unit area of the phytocenosis. The dependence of the value of this indicator on the metering area is determined by the regression curve, which initially goes sharply upward and then becomes flatter (Fig. 12). The nature of such a curve indicates that in order to identify the species composition of a phytocenosis, the counting area should not be less than a certain threshold value (S), which very much depends on the size of the plants forming the cenosis. Therefore, when establishing the species composition of communities of different types, we use trial plots(registration areas) of unequal sizes, for example, forest phytocenoses are usually described on a sample area of 0.25 hectares, herbaceous ones - 100 sq.m., moss and lichen - 1 sq.m. m.

According to the value of the species richness index, phytocenoses can be divided into three groups: a) floristically simple, consisting of a small number of species (up to one to two dozen), b) floristically complex, including many dozens of species, c) phytocenoses occupying an intermediate position in species saturation . However, when determining species richness, only higher plants and macrophytic lichens are usually taken into account. If we take into account that phytocenoses also include species of algae, fungi, bacteria, the number of which is usually several times higher than the number of higher plants, then we must admit that in nature, apparently, there are no floristically simple phytocenoses, as evidenced by the following table 3.

The species diversity of phytocenoses is influenced by a number of factors. A certain role in this regard is played by general physical-geographical and historical conditions, on which the species richness of the flora of each specific area depends. And the richer the flora of the area, the more candidate species there will be that can settle in each specific phytocenosis. For example, the species richness of humid phytocenoses tropical forests, formed in conditions of exceptionally rich tropical flora, is estimated to contain hundreds of species of higher plants, and the species richness of Siberian taiga forests, formed against the background of poor boreal flora, varies, as a rule, between 15-30 species.

Table 3

Complete species composition of phytocenoses of the deserted steppes of Kazakhstan (Rabotnov, 1978)

Phytocenose numbers
Plant groups	Number	%	Number	%	Number	%	Number	%
Flowering		12,5		10,5		10,5		12,0
Mosses		0,8				0,5
Lichens		4,6		6,0		9,0		7,0
Seaweed		9,9		17,0		11,5		4,5
Microscopic mushrooms		32,3		34,5		34,0		39,0
Bacteria and actinomycetes		39,9		32,0		34,5		38,0
Total

The floristic diversity of phytocenoses also depends on habitat conditions: the more favorable they are, the more complex the species composition, and, conversely, floristically simple phytocenoses are formed in unfavorable habitats. For example, in the undisturbed phytocenoses of meadow steppes of the European part of Russia, associated with moderately moist fertile chernozem soils, there were (Alekhine, 1935) per 100 sq. m. up to 120 or more species of higher plants, while in the same areas along the banks of reservoirs with unfavorable, heavily waterlogged soils or on salt marshes, phytocenoses of 5-10 species of higher plants are formed.

Thus, the unfavorable factors of the ecotope exclude the possibility of many species growing in it and create the so-called ecotopic closure phytocenosis, which in this case determines the simplicity of its composition. Therefore M.V. Markov (1962) quite rightly considered the species richness of a phytocenosis as an indicator of the ecological capacity of a habitat.

In addition to ecotopic factors, the species diversity of phytocenoses is also influenced by cenotic conditions, which is manifested, firstly, in the competitive exclusion of some plant species by others and, secondly, in the formation in communities of a specific internal environment that prevents the introduction into them of species adapted to a given environment. For example, in place of destroyed dark coniferous forests in the taiga zone of the Western Siberian Plain, derivative birch forests are formed, which over time are replaced by dark coniferous tree species. Together with it, some of its companions disappear from the lower tiers and are subsequently absent; they cannot tolerate the significant shading of abundant coniferous litter and increased soil acidity created in dark coniferous forests. This phenomenon, named A.K. Kurkin (1976) cenotic isolation phytocenoses, widespread in nature. At the same time, the ecotopic and coenotic aspects of closure are interconnected and jointly determine the overall environmental isolation of the phytocenosis and limit the capacity of its habitat.

The species diversity of phytocenoses can also be influenced by animals and humans. Thus, long-term intensive grazing of domestic animals leads to a significant simplification of the species composition of the initially floristically rich meadow and steppe phytocenoses. A person sometimes deliberately destroys plants that are undesirable for him in phytocenoses and thereby simplifies their species composition. For example, this is what is done when clearing weeds from agrophytocenoses. In other cases, on the contrary, people introduce new useful plants into natural phytocenoses, complicating the species composition. This is done when improving the composition of natural pastures by sowing valuable forage grasses. Most often, a person consciously or unconsciously influences the habitat, changing it and thereby causing a change in the species composition of phytocenoses: fertilizing meadows, draining swamps, irrigating natural phytocenoses of steppes and deserts.

In addition, the species diversity of a phytocenosis may depend on the regime of entry of plant primordia into its territory from the outside, which is primarily determined by the landscape position of the phytocenosis. The more diverse and in greater quantity plant rudiments from outside enter the phytocenosis, the higher the likelihood that this phytocenosis will be floristically rich. And although this pattern is not always realized in nature, the tendency for its manifestation remains constant. Back in the early twenties, L.G. Ramensky (1971) drew attention to the fact that in natural phytocenoses there are not always viable rudiments of all those plant species that could grow in them. Taking this into account, he introduced the concept floristic completeness And floristic incompleteness phytocenoses and proposed to distinguish between two similar categories of phytocenoses.

Floristically full-membered phytocenoses include all species that can grow in them. Later it was established (Rabotnov, 1978) that absolutely complete phytocenoses apparently do not exist in nature, since in any case there will be species from other floristic regions that can grow in one or another specific phytocenosis. This is evidenced by the facts of widespread distribution and introduction into natural phytocenoses of species accidentally introduced by humans from other regions. Therefore, we can only assume the existence of native full-membered phytocenoses, which include all types of local flora that can grow in them. Such phytocenoses should be sought among the stable "ecological-phytocenotic closed"(Kurkin, 1976) communities.

Obviously not full-membered phytocenoses do not contain all types of local flora that can grow in them. When germs arrive naturally or with human help, these species take root in these phytocenoses. Apparently, most natural phytocenoses are floristically incomplete (Vasilevich, 1983), as evidenced, in particular, by the facts of the successful artificial introduction of useful plants into many natural phytocenoses.

In general, the question of the completeness or incompleteness of phytocenoses is not easily resolved, since it is based on long-term observations of the fate of sown or planted new species in order to verify the results of their establishment. At the same time, this issue is of great practical importance, since the presence of floristic incompleteness of phytocenoses is associated with the possibility of consciously introducing plants useful to humans into natural phytocenoses, successfully controlling weeds, increasing the productivity of natural hayfields and pastures, and solving a number of other problems of rational use of vegetation.

These are the main factors determining the species diversity of phytocenoses. In addition, there are some other reasons that influence species richness, for example, sharp variability of ecological regimes in a number of habitats, some ecological and biological characteristics of species, etc.

Cenopopulations

Each species in a phytocenosis is almost always represented by a more or less significant number of individuals, which together form cenopopulation. The concept of coenopopulation was developed in the forties and early fifties of TA. Rabotnov (1945, 1950a, etc.), and the term coenopopulation was introduced into geobotany later by V.V. Petrovsky (1960) and A.A. Korchagin (1964). Subsequently, the doctrine of coenopopulation was intensively developed by A.A. Uranov and his students, who published many interesting works, including the two-volume monograph “Plant Cenopopulations” (1975,1989).

To date, the idea of cenopopulation as one of the main elements of the composition of phytocenosis has been formed, and phytocenosis is often defined as a system of coenopopulations associated with each other and with the environment. Each coenopopulation occupies its own ecological niche. When species grow together in a phytocenosis, the niches of cenopopulations partially overlap, but their centers are always differentiated (Mirkin and Rosenberg, 1978). Cenopopulations are a heterogeneous formation. It consists of individuals that differ in age, size, life status, reaction to external influences, etc. It is believed that internal differentiation of the coenopopulation is a factor of stability. Due to the continuity of vegetation cover and the weak discreteness of natural phytocenoses, coenopopulations of one species from neighboring phytocenoses turn out to be connected by gradual, continuum-like transitions and are not clearly delimited from each other (Mirkin, Rosenberg, 1983).