The distribution of plants over the aboveground layers is determined by uneven illumination, which leads to differences in temperature regime and humidity mode.

In the same tiers there are plants of the same height, similar or different in their ecological characteristics (for example, conifers and deciduous species), but having approximately the same need for lighting.

Allocate tiered aboveground and underground. Due to the tiered arrangement of plants in the community, natural conditions (light, heat, soil) are most fully used. The quality of life depends on belonging to a certain tier, since environmental conditions in different tiers are not the same.

In forests, tiers are formed by plants of individual life forms (according to Serebryakov), the following tiers are distinguished:

Tier A - tree stand (tier of trees);

Layer B - undergrowth (shrub layer);

Tier C - herbaceous (tier of herbaceous plants);

Layer D - moss-lichen layer.

One of the important indicators of the tree layer is the degree of crown density.

The degree of crown closure is the ratio of the area occupied by crowns to the total area of ​​the site being described. This indicator is evaluated visually, expressed in tenths of a unit (or as a percentage).

For herbaceous communities and herbaceous forest layer, one of the characteristics is the aspect - this appearance phytocenosis (its physiognomy, color), changing in accordance with the alternation of the phases of plant development and the season.

Abundance is an external characteristic - this is the number of individuals of a certain plant species within a given vegetation cover of a trial plot. Determining the number of individuals can be done by direct counting or by using a subjective eye estimate. It is customary to use the five-point scale of the German scientist O. Drude to determine the abundance.

In some meadows, tiers can also be distinguished - tall grass, medium grass and short grass.

The presence of tiers is not obligatory sign phytocenoses and is mainly characteristic of forest phytocenoses.

coexistence different types and life forms in the plant community leads to their spatial isolation. This is expressed in the horizontal and vertical division of the phytocenosis into separate elements, each of which plays its role in the accumulation and transformation of matter and energy.

Vertically, the plant community is divided into tiers - horizontal layers, strata, in which the above-ground or underground parts of plants of certain life forms are located.

This layering is especially pronounced in forest phytocenoses. There are usually five or six tiers here: tree tiers, shrubs, grass-shrub, moss or lichen, litter (leaf litter). Along with such diverse phytocenoses as forest, there are so-called low-tiered communities - meadow, steppe, swamp - have only two or three tiers. Plants have one type of nutrition: the diet of almost all species includes a solution of minerals, carbon dioxide and sunlight. However, their ecological niches are differentiated.

Plants of different tiers influence each other. The plants of the upper aboveground layers are more photophilous than the plants of the lower layers, and are better adapted to fluctuations in temperature and humidity. Under their crowns, they create conditions of low light and stable temperature and humidity. Therefore, the lower tiers are formed by plants in which the need for light is less.

In turn, the plants of the lower tiers affect the plants of the upper tiers. So, for example, a layer of mosses in a spruce or fir forest accumulates a significant amount of moisture; the grass cover of the forest is involved in the process of soil formation, forming litter, etc.

A canopy (Sukachev, 1930) is a temporary layer formed by young plants or plants oppressed by factors outside the phytocenosis (for example, felling).

Not all tiers are the same. Some of them, formed only by trees or only by shrubs and shrubs, are permanent and retain a system of trunks and branches, and in some cases even leaves, all year round. Others are fickle. They are formed by herbaceous plants, the aerial parts of which completely or partially die off during an unfavorable period of the year.

When selecting tiers, two (or three) tiers of trees, one or two tiers of shrubs, three tiers of herbs, one tier of ground cover are distinguished

Plants that develop their foliage in different tiers are called intertier (or extratier) plants.

The underground layering of phytocenoses has been studied worse than the above-ground layering. The distribution of plant roots in underground layers is determined by the change in the degree of soil moisture with depth, its richness nutrients and a decrease in the degree of soil aeration with depth.

The underground tiers, as well as the above-ground ones, influence each other. Roots that form the upper underground layer can intercept rainwater from plants that have roots in deeper layers. In turn, the roots that make up the deeper underground tiers intercept the rising capillary water from the roots of the upper underground tiers.

The underground layering of phytocenoses has been studied worse than the above-ground layering. The distribution of plant roots in underground layers is determined by the change in the degree of soil moisture with depth, its richness in nutrients, and the decrease in the degree of soil aeration with depth.

Underground tiers are distinguished on the basis of the depth of penetration of the roots into the soil and the location of the active, i.e., capable of absorbing water and nutrients, part of the root systems, usually equipped with root hairs. In forests, one can often observe three to six underground tiers. For example, in a broad-leaved ash-oak forest, a tier of occurrence of rhizomes and roots of shallow-rooted herbs, a tier of roots of more deeply rooted herbs, one or two tiers of shrub roots (more superficially and more deeply rooted) are distinguished, and these tiers can coincide and then combine with underground tiers of grasses, two tiers of tree roots (the upper one is formed by ash roots, the lower one is deeper oak roots).

The underground tiers, as well as the above-ground ones, influence each other. Roots that form the upper underground layer can intercept rainwater from plants that have roots in deeper layers. In turn, the roots that make up the deeper underground tiers intercept the rising capillary water from the roots of the upper underground tiers.

It is generally accepted that the root systems of plants included in one or another underground layer use the moisture and nutrients of those soil horizons in which this layer is located. However, in phytocenoses with insufficient moisture, thin active, so-called ephemeral roots are formed in the near-surface layers, which appear very quickly and die just as quickly when these layers dry out.

In communities with a predominance of drought-resistant plants, the mass of roots is often many times greater than the mass of above-ground plant parts.

Often, root systems close in such communities where the above-ground parts of plants are separated. Sometimes, for example, on gravelly slopes, differentiation of root systems occurs: absorbing and attaching roots are formed.

The concept of layering was formed in the late 19th - early 20th century in the works of phytocenologists who studied the boreal and broad-leaved forests of the temperate zone. So, for the first time, layering was described in 1863 by the Austrian scientist A. Kerner. In a spruce forest, he identified a tree layer, a fern layer, and a moss layer.

Then the Swedish scientist R. Gult identified 7 tiers in the forests of northern Finland:

• 1. upper tree layer;
• 2. lower tree layer;
• 3. undergrowth;
• 4. upper grass layer;
• 5. medium grass layer;
• 6. lower grass layer;
• 7. ground layer.

LEVELING UNDERGROUND, see art. Community tiering.

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"STRING UNDERGROUND" in books

UNDERGROUND ROAD