vegetation(redirected from Vegetation structure)
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the total mass of plant associations (phytocoe-noses) inhabiting the earth or individual regions. Vegetation is distinguished from flora as concerned not so much with species diversity as with the number of individuals, the combination thereof, and ecological relations.
Vegetation includes all plant species—most of which are autotrophic. Because its autotrophic representatives store solar energy, vegetation plays a vital role in the primary synthesis of organic matter. It is also a very significant factor, along with the animal population, in the cycle of matter on earth. A major component of the biosphere, vegetation is closely associated with the distinctive features of climate, water regime, soil, topography, and other elements of the natural environment. It and all these elements form biogeocenoses, that is, ecological systems.
Modern vegetation is the product of the long process of plant evolution, which originated at the same time as the evolution of the animal population and the development of the geographic shell as a whole.
Structure. Terrestrial vegetation and marine vegetation differ markedly from each other in structure, habitat, history of development, floristic composition, and role in the cycle of matter. Terrestrial vegetation comprises 20 to 30 phyla, which differ from each other in the predominance of particular life forms that evolved historically but reflect adaptation to modern conditions of existence (trees, shrubs, grasses).
In the 19th century the study of vegetation consisted mainly in determining which life form was predominant in various plant communities. Since 1950 equal importance has been placed on the geographic and ecological conditions of plant communities: the water regime (hydrophytic, mesophytic, xe-rophytic, and other communities), the temperature (microthermal, megathermal, and other communities), and the salinity (halophytic and oxylophytic communities).
Important characteristics of vegetation include the division into stories, synusial structure, and seasonal rhythms. The seasonal rhythms usually conform to the water and heat regimes of a biotope (tropical evergreen vegetation, tropical vegetation leafy in the rainy season, broad-leaved forests leafy in summer, early-spring ephemeral and ephemeroid desert vegetation).
The vegetation of a particular region may be identified not only by means of its species composition and phytocoenotic characteristics but also by means of the spatial patterns of its distribution, which vary with the ecological and geographic factors acting on global, regional, and local scales. Actions on a global scale are responsible for the principal differences in the earth’s plant cover. The regional characteristics of vegetation are manifested within geobotanical regions and provinces, and local factors operate within limited areas, such as forests, where in conformance to the macrorelief, microrelief, microclimate, and soil characteristics the vegetation consists of diverse ecological series of associations.
Classification. Study of the diversity of vegetation and its structural, ecological, and other differences has led to the elaboration of classifications that, in reflecting the existing multistage coordinate phenomena, follow in most cases a hierarchical principle. Of particular importance is the universal classification system, in which the subdivisions of vegetation are regarded as dynamic systems that evolved historically and that change spontaneously or as a result of man’s activities.
Of major importance are the largest subdivisions, or types: tundra, taiga, steppe, savanna, and so on. These vegetation types are classified into groups, or suites: the northern extra-tropical suite, the tropical suite, and the southern extratropical suite. The suites are based on the most general geological and ecological conditions. The earth is also divided into floristic realms, or regions (holarctic, pantropical, and holantarctic dominions). The vegetation types are subdivided into plant formations, which are in turn divided into associations. Intermediate taxa are often used: groups and classes of formations and associations.
A vegetation is classified according to species diversity and the range and ecological relationship of individual species in the communities. Considerable significance is attached to species that dominate the plant cover (seeDOMINANTS). The ecological and geographic classification of vegetation permits the use of a plant association as an indicator of various environmental characteristics, such as the potential soil fertility, groundwater depth, soil salinity, and presence of minerals. Special classifications are created for certain purposes, for example, in reclamation work, for the utilization and improvement of the feed supply, and in forestry. Of great value also are maps compiled from universal or specialized vegetation classifications. The maps clearly show the subdivisions of vegetation of different ranks.
Modern vegetation formed gradually over a very long period of time, and hence its subdivisions vary in age. Some formations of tropical rain forests existed in their present habitat as far back as the Miocene. Modern tundra and taiga formations date to the Quaternary; they generally are older than the associations included in them.
Distribution of types. The differences between vegetation types and their formations and associations lie in the size of the biomass that they produce. For example, the associations of arctic deserts and tundras and those of the tropical deserts are the least productive; tropical rain forests are the most productive. Great possibilities exist for increasing the production of the biomass wherever a spontaneously developing vegetation has been preserved.
The spatial characteristics of vegetation are clearly revealed by geobotanical zonation. Such zonation is important in the evaluation of land by means of its vegetation, and it reflects the main patterns of distribution of vegetation types and plant formations and associations. Geobotanical belts divided into geobotanical regions represent the range of suites of vegetation types. The regions within a belt are subjected to continental conditions or oceanic influences. The northern extratropical expanses, the largest land masses on earth, are clearly divided into three parts: the continental, Atlantic, and Pacific regions. The vegetation of these parts, each of which consists of several geobotanical regions, shares several characteristics owing to the history of the development of existing plant associations and to ecological factors that are influenced by continental and oceanic conditions.
In general, the zonal distribution of plant formations is particularly evident on plains within continental geobotanical regions. For example, the following succession of types of vegetation and classes of formations can be traced from north to south on the West Siberian Plain, the largest in Eurasia: arctic tundras, subarctic tundras, sparse tundra forests, northern larch-fir taiga, central cedar-bog taiga, southern spruce-cedar-fir taiga, subtaiga birch-aspen forests, meadow steppes, forb-sod steppes, and dry fescue-feather grass steppes. Similar patterns of zonal distribution exist on the East European (Russian) Plain and the plains of North America. However, each large geobotanical region on a plain has its own zonal characteristics. Vegetation zones are divided into provinces, which are in turn divided into geobotanical districts and regions. Often they are also divided into subzones.
Vegetation belts in mountains are almost always similar to vegetation zones on plains. Every mountainous country has its own typical vertical distribution of plant associations in accordance with its natural characteristics.
There are different approaches to vegetation zonation. Zonation aimed at determining the spatial combination of plant formations as integral territorial systems within which the formations are related to the totality of geographic and ecological factors has general scientific and practical value.
Dynamics. Vegetation is a dynamic component of a landscape, reacting to changes in the surrounding natural conditions and, especially, to human activity. Vegetation unaltered by man is called indigenous. Under man’s influence, vegetation often changes substantially and comes to embrace other associations (derivative vegetation) widespread over a large area and frequently highly characteristic of a particular locality. For example, mostly birch forests occupy the former sites of pine forests, and in the tropics large expanses formerly occupied by forests destroyed by fires and other external influences become savannas. About 17 percent of the land area of the earth is occupied by plant associations that are used as hayfields and pastures; the vegetation has been altered by man to various degrees. For example, in Europe meadow vegetation, with few exceptions, develops on the sites of forest stands felled in the remote past. In the absence of interference by external agents, a derivative vegetation regenerates and assumes the appearance of an indigenous vegetation or something close to it. There can be a succession of plant associations, even without man’s influence, where the topography, moisture, and other conditions change.
Transformation and conservation. Optimization of the structure of vegetation is important not only to boost productivity and increase the number of useful raw-material and industrial plants but also to enable the vegetation to alter the environment in a desired direction. Vegetation as a factor favorable to man from the standpoint of sanitation by improving the local climate, retarding soil erosion, and regulating the flow of rivers and thus preventing flooding deserves to be protected in every possible way. It must, however, be transformed in order to eliminate natural focal diseases and to eradicate mosquitoes and other insects that make it difficult to develop new regions in different parts of the world (taiga, tropical forest). The aesthetic and healthful properties of vegetation dictate that it be taken into consideration when organizing recreational activities and vacations.
Vegetation is studied by geobotanists and ecologists. In some countries, scientists have elaborated a special science concerning the plant cover.
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V. B. SOCHAVA