Agrocenoses, Agrobiocenoses, Agrophytocenoses

Components of Agrophytocenosis

Agrophytocenosis is made up of cultivated and weed plants. Cultivated plants in crops may be represented by one species — single-species crops — or by several — multi-species crops (vetch–oat, a mixture of clover with timothy, a mixture of sunflower with oilseed radish, etc.).

Weed plants are represented in the composition of an agrophytocenosis most often by 10–15 species, less often by 20–30. For example, in Moscow Oblast, in the agroassociation of winter wheat, there are from 22 to 40 weed species, in barley, from 17 to 32, and in annual grasses, from 14 to 26. The weed component of an agrophytocenosis also includes weed diaspores present in the soil (fruits, seeds, root suckers, rhizomes, tubers, root collars, bulbs, and other organs of renewal), collectively called the seed bank.

The idea that weeds have higher viability and resilience than cultivated plants is mistaken. The evolution of field communities over many centuries occurred thanks to human selection of the most valuable plants in terms of biological productivity and economic value from wild, including weed, species, creating favorable conditions for their growth through a system of agronomic measures.

Due to the advanced development and predominance of cultivated plants in the agrophytocenosis, a competitive advantage over weed vegetation is created and, as a consequence, the formation and state of the internal phytocenotic environment of the field community is determined by the cultivated component, which also acts as the environment-former, or edifier, of field phytocenoses. The powerful stand of cultivated plants has a suppressive effect on many weeds. At the same time, the amount of organic matter created by them rarely exceeds 2–5% of the total mass of the agrocenosis. Only through weakening of the crop for various reasons: sparse seedlings, untimely sowing, severe damage by pests, etc., can conditions be created for the competitive advantage of weed vegetation.

Various algae are also part of agrophytocenoses, but the mass of organic matter synthesized by them annually does not exceed 100–150 kg/ha.

Formation of Agrophytocenosis

The formation of the agrophytocenosis of a particular field is determined by the agricultural crop and begins at the moment of its sowing. During crop development, the intensity of mutual influence and interaction between cultivated plants and weeds increases. If another crop is placed on a given field, the expression, direction, and character of relationships between the components of the agrophytocenosis take a different form, which entails the formation and establishment of an already different plant community with features of composition and structure peculiar only to it.

The composition of weed populations depends on the time of the last soil tillage. If the last soil tillage occurs in late autumn or early spring, weeds characteristic of cereal crops will appear, even if they are to be grown in the field that year. If soil tillage is carried out in May or June, conditions will be created for the appearance of so-called “hoe weeds” characteristic of row crops. The reason for this behavior is the different temperatures of dormancy and germination of weed seeds.

The properties of the corresponding soil also affect weed reserves. Soil reaction, nutrient and moisture supply, and soil type are especially important. With the activation of vital and growth processes, the interactions and mutual influences of agrophytocenosis components become stronger and more stable. The appearance of fresh organic matter and the formation of a stable internal environment of the agrophytocenosis, favoring the habitation and reproduction of macro- and microorganisms, enriches the field community with organisms, including phytopathogenic and harmful insects. As a result, the agrobiocenosis becomes enriched in life forms and species abundance and more complex in the qualitative manifestation of relationships among its constituents.

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

However, a low level of organism diversity, including harmful organisms, leads to the inevitable loss of even the minimal capacity of the agrobiocenosis for self-regulation. The functioning of the agroecosystem in the desired regime is possible with a constant additional input of energy from outside, which is implemented in the process of various agricultural works and agronomic measures, such as land reclamation, liming, fertilization, pesticide use, soil tillage, cultivation of certain crop varieties, etc. The intensity of production impact on the agrobiocenosis reduces the capacity for self-regulation and increases the productivity of the cultivated component. In addition, the character and expression of these relationships are influenced by living organisms inhabiting the soil.

Forms of Relationships Between Components of Field Communities

One of the factors determining the productivity of agricultural crops is the interaction of the cultivated and weed components of the agrophytocenosis. The diversity of forms of direct and indirect interactions is determined by plant species, their biological properties, agroecological conditions, variability during the season, and the habitat environment.

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

• Parasitism and semi-parasitism.
• Mechanical pressure on the roots and aboveground part of one plant by another climbing and clinging plant (field pea (Pisum sativum), cleavers (Galium aparine), black-bindweed (Fallopia convolvulus), field bindweed (Convolvulus arvensis)), by a strongly branching plant (large-flowered hemp-nettle (Galeopsis speciosa), white goosefoot (Chenopodium album), wild radish (Raphanus raphanistrum), coltsfoot (Tussilago farfara), etc.) due to a powerful developed root system (annual bluegrass (Poa annua), couch grass (Elytrigia repens), field horsetail (Equisetum arvense), etc.).
• Allelopathy (biochemical influence) — the phenomenon of influence of one plant (donor) through the release of active substances — colins, which inhibit (suppress) or stimulate the vital activity of another plant (acceptor) or living organism. Active substances formed during the decomposition of dead plants or their parts are called miasmins.
• Competition for plant life factors (light, water, mineral nutrition elements, etc.).

Indirect forms of interaction:

• Phytogenic influence — the environment-forming influence of plants on the formation and state of the internal environment of a field agrophytocenosis. The essence of the influence consists in the formation by the plant dominant in number and, above all, in organic matter mass, of the physical state and chemical composition of the surface atmospheric layer. Phytogenic influence can be unilateral and mutual. Thus, the dominant plant creates in the surface layer the conditions most suitable for itself: illumination, temperature, humidity, air movement (physical state), oxygen concentration, volatile substances, radiation activity of compounds, ionic composition (chemical composition). For other plant species of the field community, these conditions become external factors determining their life.
• Edaphic factors — influence through soil conditions. The primary set of soil conditions (ecotope), determined by the physical-geographical properties of the environment, changes under the influence of plants of the field community in accordance with the needs of the plants (biotope). As a result of plant vital activity, the quantitative (nutrient content, pH) and qualitative (mobility of macro- and microelements, soil moisture, humus composition) state of the soil undergoes changes.

The responsiveness of plants to external influences is determined by the variability of climatic, biogenic, and anthropogenic factors associated with human production activity.

The direction of influence also determines the composition of the agrophytocenosis. For example, if any form of interaction is negative, the phytocenotic role in the community increases for those species that react weakly to these negative influences or quickly recover from damage.

Conversely, if an external influence positively affects the vital activity of components, the phytocenotic role of species that are most responsive to the acting factor increases. For example, application of a complete mineral fertilizer (NPK) in crops of fiber flax and barley sharply strengthens the role of the crop in community formation. In contrast, application of only nitrogen fertilizers greatly increases the phytocenotic importance of wild radish (Raphanus raphanistrum), charlock (Sinapis arvensis), pale persicaria (Persicaria lapathifolia), which leads to an increase in the weed component of the agrophytocenosis.

Evolution of Agrophytocenoses

Agrophytocenoses are subject over a long time to evolutionary processes associated primarily with the systematic change of farming systems, the emergence of new agricultural technologies, plant protection products, etc. As studies by German scientists have shown, over the past 80 years the composition of weed vegetation in the fields of Germany has constantly undergone changes.

List of Sources

1. Bazdyrev G. I., Loshakov V. G., Puponin A. I., et al. Agriculture. Textbook for universities. Moscow: “Kolos” Publishing House. 2000.