Shrub - the life form of plants ; perennial woody plants 0.8–6 meters high  , unlike trees that do not have a main trunk in adulthood, but have several or many stems, often existing side by side and replacing each other. Life expectancy of 10-20 years.
Most often located on the border of forests (shrub steppe , forest-tundra ). In forests usually form undergrowth .
Representatives: raspberries , hawthorn , barberry .
Fruit and berry shrubs are of great economic importance: currants , gooseberries and others.
In the system of life forms, Raunkier belongs to phanerophytes .
According to the system of life forms I.G. Serebryakova shrubs are divided into four classes:
- With fully lignified elongated shoots.
- Succulent-stem leafless.
- Semi-parasitic and parasitic.
Shrubs with fully lignified elongated shoots
The vast majority of shrubs belong to this class. Skeletal branches are orthotropic (vertically upward), plagiotropic (horizontal or inclined) or lianoid, form a greater or lesser number of vegetative branches, elongated internodes of annual shoots.
The class is divided into three subclasses:
- Semi-prostrate and creeping.
Elevated axes grow vertically upward (ototropic), with a weak development of their plagiotropic underground parts form a more or less dense bundle. In the presence of long underground lignified stems ( xylopodia ) or an extensive root system with root offspring, they are loose, forming clumps of several tens of square meters. The most species-rich subclass. Distributed in various climatic areas. They are divided into aeroxyl and hydroxyl.
Unable to underground branching. New axes underground arise only as root offspring. Branching of shoots above-ground, begins near the surface of the earth by the formation of several vertical or inclined axes. Transitional forms between trees and bushes.
Loose aeroxyl shrubs are quite close to sparse (bushy) trees, such as fluffy birch. Examples of loose shrubs aeroxil - mountain ash , bird cherry , raved (in adverse swampy habitats of the taiga zone of the European part of Russia).
Pillow- shaped aeroxyl shrubs are characterized by insignificant, identical for all shoots, annual growths and a dense arrangement of skeletal axes and branches. The tops of the branches form a more or less even surface of the pillow. Examples are Bupleurum fruticosum, Poterium spinosum, species of the astragalus genus from the Tragacantha section.
Geoxyl (real shrubs)
Plants with underground branching of axes forming thick and durable underground lignified axes (xylopodia), from which thinner and less durable aboveground shoots depart. Compose the majority of shrubs of the European flora. Examples are species of the rosehip genus, common hazel, and common barberry . Many bamboo species of the tropical and subtropical zones also belong to this group.
Aeroxyl and geoxyl shrubs can be divided into subgroups of vegetatively motionless and vegetatively mobile (through root offspring and stolons). Examples of root sprouting aeroxyl shrubs are thorns , buckthorn buckthorn . Examples of vegetatively moving geoxyl shrubs are Philadelphus coronarius , common lilac , White Snowdrop.
Semi-transparent and creeping shrubs
The main axis and lateral branches are recumbent, rooting, rising at the apex. The system of the main root dies or is preserved (with weak rooting of the axes). Most semi-transparent ones, apparently, only branch aboveground. Rarely found in arid regions, dominate in the subalpine and subarctic regions, forming crooked forests (for example, Krummholz in the Alps from mountain pine and mountain alder ; overgrown willows in the mountains of Scandinavia ; shrub species of juniper in the mountains of Europe).
Skeletal axes are incapable of independent orthotropic growth and use neighboring woody plants as a support. They differ from lianoid trees with thinner stems (with a diameter of not more than 5-8 cm). Mostly distributed in tropical humid and subtropical forests, less commonly in savannah and thorny forests and shrubs. Primitive tree creepers are also included here. Classification within vines is based on climbing methods.
Shrub shoots with
Elevated axes with shortened internodes and rosettes of large leaves. Rare enough. Examples are shrubby palm Bactris major , Dracophyllum fiordense , Brachyglottis stewartiae . There are transitional forms between rosette shrubs and shrubs with elongated shoots - Olearia oporina , Olearia lyallii .
Succulent-stem leafless shrubs
The skeletal axis of fleshy consistency perform water storage and assimilation functions. Upright forms prevail. An example is Opuntia species and other cacti, Euphorbia spp .
Semi-parasitic and parasitic shrubs
They live on other woody plants, sticking to them with haustoria. The vast majority have green leaves and are capable of photosynthesis. Examples are species of Loranthaceae, mainly in the tropics, in the temperate zone - Viscum album (mistletoe) and some Santalaceae (Osyris alba from the Mediterranean region).
More environmentally adaptive than trees, shrubs are wider. They dominate and achieve the greatest diversity where conditions are unfavorable for trees - in the arid regions of the tropical and temperate zones, at the polar border of the forest belt, at the upper border of mountain forests. An increase in the phytocenotic role of shrubs is accompanied by an increase in their species and ecological-morphogenetic diversity. At the same time, plants of the same species can pass from the life form of a tree to a shrub.
Within the tropical zone, shrubs are found from tropical rainforests to deserts.
Shrubs are found throughout the distribution of rainforests. Here they reach a considerable height (up to 6-8 m and higher), shoots branch weakly, the leaves are large, usually hygrophilic. Many of the bushes are aeroxilic, without branching long-term underground stems. In the undisturbed rainforest, they do not play a significant phytocenotic role. At clearings or conflagrations, shrubs can cover large areas, forming secondary impenetrable thickets. Floristically moist tropical forests are relatively poor in shrubs.
With an increase in climate aridity, the role of shrubs in tropical phytocenoses increases. Thorny forests of Africa, India, Central and South America are often called "shrub" forests. Systematically, “shrub forests” are composed of various species of acacia , and other genera of legumes with paired-pinnate leaves. Occasionally woody with simple cirrus leaves dominate - Adina , Harrisonia , some types of root , burserium , bignonium . Most shrubs of this formation are equipped with thorns. Shrubs of thorny and savannah forests are pronounced xerophytes. Geoxyl shrubs appear here. Their underground shoots are developed more than aboveground, have a greater thickness, are more durable. In thick underground axes, moisture is stored. Floristically shrubs also prevail here.
In general, the role of shrubs in the tropical zone increases with increasing aridity and lengthening of the dry period. Along with the increasing role of shrubs in the phytocenoses of the tropics, their species diversity increases, and geoxyl shrubs appear and spread instead of the aeroxyl shrubs prevailing in the humid tropics.
Shrubs are widespread in subtropical climate biocenoses. As in the tropics, the greatest floristic diversity and the leading role in plant communities are confined to arid areas, especially with dry, hot summers and winter precipitation. In the Mediterranean and Cape region, shrubs form specific communities in such a climate where they absolutely dominate.
In humid subtropical forests, shrubs form a dense, often impassable undergrowth, but still play a subordinate role and have significantly less species diversity. The dominance of shrubs here is possible only at the border of the forest or in places of disturbed forest cover.
In evergreen hard-leaved forests, with their arid climate, the undergrowth is composed of many evergreen sclerophilic shrubs. The continuous felling of trees by humans led to the formation of specific shrub biomes here - maquis, garriga , freegans.
The height of the shrubbery of maquis is one and a half to four meters. The soil and climate diversity of the Mediterranean leads to the fact that in each of its regions its species prevail. At the same time, a number of shrubs is widely distributed over all Mediterranean maquisis - common myrtle on warm coasts, strawberry tree , Cnidian daphne, broad-leaved phylaea , three- grain knorum , species of the genus Erica , mastic tree , turpentine tree , Spanish gorse , rosehip evergreen .
In the Cape region, a close type of vegetation, Finbosh, is formed by various shrubs of the genera Erica, Sumy , Levkadendron , Proteus and other genera of heather and proteus.
In the steppes and semi-deserts of Australia, shrubs form thickets of undersized scrub .
California chaparral , in spite of its extreme proximity to maquis, appeared, apparently not as a result of re-exploitation of forests by humans, but due to exclusively climatic reasons - they occupy strictly the area where less than 20% of annual rainfall occurs in summer.
Shrubs form a developed undergrowth in the taiga forests of Canada, the USA ( vesicle , rosehip , viburnum , currant , jamesia , maple naked , bird cherry , dogwood ) and Russia ( willow , wild rose , gray alder , mountain ash ). In the north of the taiga, a shrubby form of dwarf birch appears.
The shrub layer is well developed in deciduous and mixed forests. As the climate cools, thermophilic dogwood , scam , scoop , guelder-rose , hawthorn are replaced by cold-resistant hazel , buckthorn , honeysuckle , euonymus , and daphne .
Thickets of xerophytic shrubs often form in the steppes and semi-deserts, forming a bush steppe dominated by thorns , steppe almonds , steppe cherries , and spirea .
In the desert regions of the temperate zone, the diversity of shrubs reaches a maximum, especially in dry mountains.
Shrubs dominate the subarctic and alpine climates. In the subarctic zone, a subzone of shrub tundra stands out. It is formed by dwarf willow , birch and alder . Shrub belt is also expressed in the mountain tundra .
The lower height of the bushes, compared with trees, is associated with the fragility of their axes, a sharp reduction in their life cycle. This reduction is accompanied by an increase in shoot speed. The annual growth of a number of shrubs exceeds 60 - 70 centimeters, sometimes reaching 140 - 160 cm. That is, shrubs are not an oppressed form of plants with a slow development, but, on the contrary, fast-growing.
The duration of the growth period of the aerial axes of shrubs is on average 10-15 times less than for trees. The culmination of growth in shrubs occurs in 3-5 years of the life of the axis. The rapid culmination of growth causes an early weakening and termination of the apical growth of the axes and, as a result, an early opening of the surviving sleeping kidneys at the base of the axes, which weakened or stopped apical growth.
The main axis and the renewal axes replacing it complete their life cycle from the appearance above the surface of the soil to the cessation of apical growth within 10-15 years, sometimes 20-30, rarely more. In general, the life span of shoots in some types of shrubs is close to that of trees (30 years or more in the turn , warty spindle tree ), and in others, the duration of the cycle of herbaceous plants ( raspberries , shoots of which end in 2 years). Moreover, the duration of the life cycle varies widely, not only for plants of the same family, but even within the same genus, for example, rubus , cinquefoil , gravilate , astragalus .
A sharp reduction in the lifetime of the main axis and the renewal axes replacing it gives rise to the second bilabial feature of shrubs - the simultaneous presence of several or many above-ground axes. When replacing a dead main axis, one or more kidneys farthest from the apex are revealed. They are located very close to the root system and are in a sleeping state. With their disclosure, tillering occurs. The main feature of the bush is the simultaneous functioning of the elevated axes of several generations (usually two or three) in the general system.
The formation of overground stems that are less durable than that of trees is accompanied by their differentiation into overground stems, providing assimilation and fruiting, and more durable and thick lignified underground stems - xylopodia, which perform the functions of vegetative regeneration and storage organs.