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Agrocenoses, agrobiocenoses, agrophytocenoses

Agrocenosis (from Greek ἀγρός, agros – “field”, κοινός, koinos – “common”) is a system created by man and includes a community of living organisms and related habitat factors within one territory, united between themselves by the cycle of substances and energy flow.

Agrobiocenosis (agroecosystem) – a field community of living organisms of agrocenosis. A field community includes cultivated and weedy plants (agrophytocenosis), consumers of organic matter (herbivores, various plant parasites, heterotrophic microorganisms, etc.), as well as decomposers of plant and animal residues (soil-dwelling animals, fungi and bacteria).

Agrophytocenosis: a plant community of agrocenosis, including weeds and cultivated plants, formed on cultivated relatively ecologically homogeneous agricultural land. The formation of agrocenosis, its floristic and quantitative composition is influenced by many factors:

  • agroclimatic conditions of the habitat;
  • soil conditions;
  • thermal regime and moisture supply;
  • fertilizer system;
  • stocks of different plant species (seed bank);
  • floristic diversity of surrounding areas;
  • agro-techniques of crops cultivation;
  • species and variety composition of cultivated plants;
  • level of cultivation culture.

The diversity of factors leads to the formation of dissimilar agrophytocenoses on individual fields with special interactions of plants with each other.

At the same time, similar agrocenoses, called agroassociations, are formed on the fields with similar factors, especially agrotechnics and types of cultivated plants.

Weed management system should be based not on general concepts, but on detailed knowledge of biological features and ecological preferences of weeds, specific values of floristic composition and quantitative abundance of weeds. Understanding of the peculiarities of formation of agrophytocenoses, as well as specific values of species and quantitative abundance makes it possible to conduct more effective, including economically justified, weed control.

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Agrophytocenosis components

Agrophytocenosis consists of cultivated and weedy plants. Cultivated plants in the crops may be represented by one species – one-species crops, or several – multispecies crops (vetch-oats, a mixture of clover with timothy, a mixture of sunflower with oilseed radish, etc.).

Cultivated crops, depending on the crop rotation, can occupy the field during one growing season or several seasons in a row.

Weeds are represented in the agrophytocenosis most often in the number of 10-15 species, and less often in the number of 20-30. For example, in the Moscow region there are 22 to 40 weed species in the agro-association of winter wheat, 17 to 32 in barley, and 14 to 26 in annual grasses.

The weed component of agrophytocenosis also includes diaspores of weeds in the soil (fruits, seeds, rootstocks, rhizomes, tubers, root necks, bulbs, and other organs of renewal), generally referred to as a seed bank.

The notion that weeds are more viable and resilient than cultivated plants is mistaken. The evolution of field communities for many centuries occurred due to the human selection of the most valuable plants by biological productivity and economic value from wild, including weed species, creating favorable conditions for its growth through a system of agronomic measures.

Due to the advanced development and predominance of cultivated plants in agrophytocenosis, a competitive advantage over weeds is created and, as a consequence, the formation and condition of the internal phytocenotic environment of the field community is determined by the cultural component, which also acts as a medium, or edificator, of field phytocenoses.

Strong stems of cultivated plants have a depressing effect on many weeds. At the same time, the amount of organic mass created by them rarely exceeds 2-5% of the total mass of the agrocenosis.

Only due to the weakening of the culture for various reasons: weakened seedlings, untimely sowing, severe damage by pests, etc., conditions for competitive advantage of weed vegetation can be created.

It is also worth noting that among the autotrophs various algae (green, blue-green, diatoms, etc.) are included in agrophytocenoses; however, the mass of organic matter synthesized by them annually does not exceed 100-150 kg/ha.

Formation of agrophytocenosis

The formation of agrophytocenosis of a particular field is determined by the crop and begins at the time of its sowing. In the process of crop development (swelling of seeds, formation of roots, emergence of cotyledons and true leaves) the intensity of mutual influence and interaction between cultivated plants and weeds increases.

If another crop is placed on this field, the manifestation, orientation and nature of the relationship between the components of agrophytocenosis acquires a different form, which entails the formation of another plant community with only inherent features of composition and structure.

The composition of weed populations depends on the time of the last tillage (plowing, cultivation, harrowing, husking or herbicide application). If the last tillage occurs in late fall or early spring, weeds characteristic of cereal crops will appear, even if cereal crops will not be grown in the field that year. If, on the other hand, tillage is done in May or June, conditions will be created for the emergence of so-called “hoe weeds”, characteristic of row crops. The reason for this behavior is the different dormancy and germination temperatures of the weed seeds.[1]Heinz Ellenberg: Vegetation Mitteleuropas mit den Alpen in ökologischer, dynamischer und historischer Sicht (= UTB für Wissenschaft. Große Reihe. Band 8104). 5., stark veränderte und verbesserte … Continue reading

Soil properties also influence weed stocks. Soil reactions, nutrient and moisture supply, and soil type are particularly important.

With the activation of vital and growth processes, interactions and mutual influences of agrophytocenosis components are strengthened and stabilized. The emergence of fresh organic matter and the formation of a stable internal environment of agrophytocenosis, which promotes the habitation and reproduction of macro- and microorganisms, enriches the field community with organisms, including phytopathogenic and harmful insects. As a result, agrobiocenosis is enriched by life forms, abundance of species and becomes more complex by qualitative manifestation of relationships between its components.

Agrobiocenosis includes cultural and weed components of agrophytocenosis, as well as all the diversity of living macro- and microorganisms, which significantly complicates the interaction and mutual influence within agrobiocenosis. Obviously, the lower this diversity, the lower the losses of organic matter and the higher the productivity of crops in the agrobiocoenosis.

However, the low level of diversity of organisms, including harmful ones, leads to the inevitable loss of even minimal ability of agrobiocoenosis to self-regulation. Functioning of agro-ecosystem in the desired mode is possible with a constant additional input of energy from outside, which is realized in the process of various agricultural works and agrotechnical activities, such as land reclamation, liming, fertilization, pesticide application, tillage, cultivation of certain crop varieties, etc. Intensity of production impact on agrobiocenosis reduces the ability to self-regulation and increases the productivity of the cultural component.

Additionally, the nature and expression of these relationships are influenced by living organisms living in the soil (fungi, nematodes, bacteria, earthworms, larvae and adults of insects, etc.).

Forms of relationships between components of field communities

One of the factors determining the productivity of crops is the interaction of cultivated and weed components of agrophytocenosis. The variety of forms of direct and indirect interactions is caused by plant species, their biological properties, agro-ecological conditions, variability during the season, habitat environment.

The following direct forms of interaction between plants of a field community are distinguished:

  1. Parasitism and semi-parasitism.
  2. Mechanical pressure:
    • on the roots and above-ground part of one plant by another plant that is climbing and clinging, e.g., pea (Pisum sativum), cleavers (Galium aparine), wild buckwheat (Fallopia convolvulus), field bindweed (Convolvulus arvensis);
    • a strongly branching plant, e.g., (Galeopsis speciosa), lamb’s-quarters (Chenopodium album), wild radish (Raphanus raphanistrum), (Tussilago), etc.);
    • due to a powerful developed root system, e.g., annual bluegrass (Poa annua), couch grass (Elytrigia repens), field horsetail (Equisetum arvense), etc.
  3. Allelopathy (biochemical impact) is the phenomenon of the influence of one plant (donor) through the release of active substances – colins that inhibit (depress) or stimulate the vital activity of another plant (acceptor) or living organism. The active substances produced by the decomposition of dead plants or plant parts are called miasmines.
  4. Competition for plant life factors (light, water, mineral nutrition elements, etc.).

Indirect forms of interaction:

  1. Phytogenic influence is the medium-forming influence of plants on the formation and state of the internal environment of the field agrophytocenosis. The essence of the impact lies in the formation of the physical state and chemical composition of the surface layer of the atmosphere by the dominant in number and, above all, in the mass of organic matter, plant. Phytogenic impact may be unilateral and reciprocal. Thus, the dominant plant creates in the surface layer the conditions most suitable for itself: illumination, temperature, humidity, air movement (physical state), concentration of oxygen, volatile substances, radiation activity of compounds, ionic composition (chemical composition). For other plant species of the field community, these conditions become external, determining their life factors.
  2. Edaphic factors – influence through soil conditions. The primary set of soil conditions (ecotope), determined by the physical and geographical properties of the environment, changes under the influence of the plants of the field community in accordance with the needs of the plants (biotope). As a result, the quantitative (nutrient content, pH) and qualitative (mobility of macro- and microelements, soil moisture, humus composition) soil conditions undergo changes in the process of plant activity.
  3. Responsiveness of plants to external influences is conditioned by variability of climatic (lack of heat, prolonged rains, high temperatures, drought, hail damage, soil erosion, etc.), biogenic (activity of microorganisms, insects, birds, animals and other living organisms), anthropogenic, related to human production activities.

The direction of impact also determines the composition of agrophytocenosis. For example, if any form of interaction is negative, the phytocoenotic role in the community increases for those species which react weakly to this negative impact or quickly recover from the damage caused.

Conversely, if the external influence has a positive effect on the vital functions of the components, the phytocoenotic role of the species that are most responsive to the acting factor increases. For example, the application of total mineral fertilizer (NPK) in the long-flax and barley crops dramatically increases the role of the crop in community formation. On the contrary, the application of nitrogen fertilizers only strongly increases the phytocoenotic importance of wild radish (Raphanus raphanistrum), wield mustard (Sinapis arvensis), dock leaf knotweed (Polygonum lapathifolium), and spikenard (Galeopsis), which leads to the growth of the weed component of the agrophytocoenosis.

Evolution of agrophytocenoses

Agrophytocenoses are subject to long-term evolutionary processes associated primarily with systematic changes in farming systems, the emergence of new agricultural technologies, plant protection products, etc. As studies by German scientists have shown, the composition of weeds in the fields of Germany has constantly changed over the past 80 years.[2]Joachim Hüppe, Heinrich Hofmeister: Syntaxonomische Fassung und Übersicht über die Ackerunkrautgesellschaften der Bundesrepublik Deutschland. In: Berichte der Reinhold-Tüxen-Gesellschaft. Band … Continue reading

Sources

Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 с.