Plant Geography
Plant geography
The study of the spatial distributions of plants and vegetation and of the environmental relationships which may influence these distributions. Plant geography (or certain aspects of it) is also known as phytogeography, phytochorology, geobotany, geographical botany, or vegetation science.
A flora is the collection of all plant species in an area, or in a period of time, independent of their relative abundances and relationships to one another. The species can be grouped and regrouped into various kinds of floral elements based on some common feature. For example, a genetic element is a group of species with a common evolutionary origin; a migration element has a common route of entry into the territory; a historical element is distinct in terms of some past event; and an ecological element is related to an environmental preference. An endemic species is restricted to a particular area, which is usually small and of some special interest. The collection of all interacting individuals of a given species, in an area, is called a population.
An area is the entire region of distribution or occurrence of any species, element, or even an entire flora. The description of areas is the subject of areography, while chorology studies their development. The local distribution within the area as a whole, as that of a swamp shrub, is the topography of that area. Areas are of interest in regard to their general size and shape, the nature of their margin, whether they are continuous or disjunct, and their relationships to other areas. Closely related plants that are mutually exclusive are said to be vicarious (areas containing such plants are also called vicarious). A relict area is one surviving from an earlier and more extensive occurrence. On the basis of areas and their floristic relationships, the Earth's surface is divided into floristic regions, each with a distinctive flora.
Floras and their distribution have been interpreted mainly in terms of their history and ecology. Historical factors, in addition to the evolution of the species themselves, include consideration of theories of shifting continental masses, changing sea levels, and orographic and climatic variations in geologic time, as well as theories of island biogeography, all of which have affected migration and perpetuation of floras. The main ecological factors include the immediate and contemporary roles played by climate, soil, animals, and humans. See Island biogeography
Vegetation refers to the mosaic of plant life found on the landscape. The vegetation of a region has developed from the numerous elements of the local flora but is shaped also by nonfloristic physiological and environmental influences. Vegetation is an organized whole, at a higher level of integration than the separate species, composed of those species and their populations. Vegetation may possess emergent properties not necessarily found in the species themselves. Sometimes vegetation is very weakly integrated, as pioneer plants of an abandoned field. Sometimes it is highly integrated, as in an undisturbed tropical rainforest. Vegetation provides the main structural and functional framework of ecosystems. See Ecosystem
Plant communities are an important part of vegetation. No definition has gained universal acceptance, in part because of the high degree of independence of the species themselves. Thus, the community is often only a relative social continuity in nature, bounded by a relative discontinuity, as judged by competent botanists. See Ecological communities
In looking at vegetation patterns over larger areas, it is the basic physiognomic distinctions between grassland, forest, and desert, with such variants as woodland (open forest), savanna (scattered trees in grassland), and scrubland (dominantly shrubs), which are most often emphasized. These general classes of vegetation structure can be broken down further by reference to leaf types and seasonal habits (such as evergreen or deciduous). Geographic considerations may complete the names of the main vegetation formation types, also called biomes (such as tropical rainforest, boreal coniferous forest, or temperate grasslands). Such natural vegetation regions are most closely related to climatic patterns and secondarily to soil or other environmental factors.
Vegetational plant geography has emphasized the mapping of such vegetation regions and the interpretation of these in terms of environmental (ecological) influences. Distinction has been made between potential and actual vegetation, the latter becoming more important due to human influence. See Vegetation and ecosystem mapping
Some plant geographers point to the effects of ancient human populations, natural disturbances, and the large-herbivore extinctions and climatic shifts of the Pleistocene on the species composition and dynamics of so-called virgin vegetation. On the other hand, it has been shown that the site occurrence and geographic distributions of plant and vegetation types can be predicted surprisingly well from general climatic and other environmental patterns. Unlike floristic botany, where evolution provides a single unifying principle for taxonomic classification, vegetation structure and dynamics have no single dominant influence.
Basic plant growth forms (such as broad-leaved trees, stem-succulents, or forbs) have long represented convenient groups of species based on obvious similarities. When these forms are interpreted as ecologically significant adaptations to environmental factors, they are generally called life forms and may be interpreted as basic ecological types.
In general, basic plant types may be seen as groups of plant taxa with similar form and ecological requirements, resulting from similar morphological responses to similar environmental conditions. When similar morphological or physiognomic responses occur in unrelated taxa in similar but widely separated environments, they may be called convergent characteristics. See Plants, life forms of
As human populations alter or destroy more and more of the world's natural vegetation, problems of species preservation, substitute vegetation, and succession have increased in importance. This is especially true in the tropics, where deforestation is proceeding rapidly. Probably over half the species in tropical rainforests have not yet even been identified. Because nutrients are quickly washed out of tropical rainforest soils, cleared areas can be used for only a few years before they must be abandoned to erosion and much degraded substitute vegetation. Perhaps the greatest current challenge in plant geography is to understand tropical vegetation and succession sufficiently well to design self-sustaining preserves of the great diversity of tropical vegetation. See Biogeography, Ecology, Rainforest
Geography, Plant
(phytogeography), a branch of botany and physical geography that studies the geographic distribution of plants. The main objects of plant geography are ranges of species and larger taxonomic units, and floras (aggregates of plant species occupying a given territory). Phytochorology is a branch of plant geography specifically concerned with ranges. Floristic plant geography studies floras. The relationship between plant distribution and environmental conditions is investigated by ecological plant geography. However, it is not necessary to consider this as a separate branch because the study of ranges and floras inevitably includes the consideration of ecological matters. In a broad interpretation of plant geography, ecological plant geography also includes the study of plant communities and their distribution on the earth, which is now the object of an independent botanical discipline known as geobotany. A separate field is historical, or genetic, plant geography, which is concerned with the history of floras, settlement of plants, and so forth in relation to the general evolution of the plant world and the history of the earth.
The elements of plant geography (information on the distribution of plants in different countries) appear in the writings of the ancient scientists (for example, Theophrastus). They were described more specifically in the 18th century, in the works of such naturalists as the French botanist J. P. de Tournefort, the Swedish scientist C. Linnaeus, and the Russian academician P. S. Pallas. Plant geography took shape as a separate branch of knowledge at the end of the 18th century and was associated with the names of the German naturalists C. Willdenow and A. Humboldt. The work of the Danish scientist J. Schouw (1822) laid the foundation for the classification of floras. The Swiss botanist Alphonse de Candolle (1855) made an important contribution to the study of ranges and floras with an analysis of the ecological factors responsible for their development. The work of the German scientist A. Grisebach was chiefly ecological and geographic in character. C. Darwin used the geographic distribution of organisms as one of the proofs of their evolution, thereby establishing a new theoretical basis for dealing with the history of floras and faunas. Using various examples, he showed possible solutions to specific phytogeographic problems. Darwin’s ideas were used in the botanical and geographic studies of the English scientist J. D. Hooker and the American botanist A. Gray. Of great importance in introducing the historical-genetic method into plant geography was the work of the German botanist A. Engler, who examined the development of the ranges of plants and floras from a historical standpoint in relation to the geologic history of the earth.
The geographic distribution of plants and floristic complexes in the European USSR and the Caucasus, Middle Asia, Siberia, and the Far East was largely explained by the studies of Russian (prerevolutionary) and Soviet scientists: A. F. Middendorf, F. I. Ruprekht, K. I. Maksimovich, S. I. Korzhinskii, A. N. Krasnov, P. N. Krylov, N. I. Kuznetsov, and V. L. Komarov, and later N. I. Vavilov, I. M. Krasheninnikov, A. N. Krishtofovich, E. V. Vul’f, A. A. Grossgeim, M. G. Popov, and M. M. Il’in. These scientists substantiated the botanical-geographic regionalization of the USSR and, in more detail, its individual parts. They traced the changes in botanical-geographic relations throughout the Cenozoic. The work of Russian scientists such as A. N. Beketov, A. N. Krasnov, and N. I. Kuznetsov were particularly useful in the dissemination of knowledge about plant geography.
The study of the patterns of the geographic distribution of plants is of great value in familiarizing people with the laws of evolution of the plant world, because this evolution is inseparably connected with geographically differentiated environmental conditions. Plant geography is of practical use in that it broadens the variety of plants of value to man, solves problems of the introduction and adaptation of useful plants, and directs the search for new plants to be exploited.
It is important to study the ranges of plants in order to determine the relationship between their distribution and modern conditions and to recreate the history of the settlement of species and the formation of floras. The features of the range of each species are determined mainly by climatic conditions; the details of its distribution depend on soil conditions and on the plant’s ability to adapt to the conditions prevailing in its plant community (for example, plants of taiga forests or high moors). The study of the ranges of genera (especially those containing many species) shows the species to be unevenly distributed within the ranges of their genera. The part of the range where the largest number of species is concentrated is often called the center of distribution of the genus. In some cases, this center may coincide with the territory in which the genus originally developed (the center of origin). In other cases, a multiplicity of species indicates that the genus began to flourish relatively recently because of favorable conditions (in secondary centers). Thus, it is important to investigate the ranges of genera and taxonomically larger groups in order to understand their history.
The study of floras requires above all that they be inventoried, that is, that account be taken of all the species (practically, the species of the higher plants, seed-bearing and fern) growing in a region whose flora is selected as the object of study (a continent, island, country, part of a country, or botanical-geographic region). The process of taking inventory reveals the total number of floral species and their distribution among different taxonomic groups. The total number of plant species indicates the richness of a flora in comparable regions. Because it is impossible to compare the floras in regions that differ sharply in size, several formulas have been suggested for computing the coefficient of richness of a flora based on the number of species and the physical size of the country or region. Some botanists use the data on botanical-geographic regions based on minimum surface areas (elementary floras) for purposes of comparison. In high arctic regions the number of species of elementary floras varies from 20 to 90-100. There are 450-700 species in the taiga zone, 1,000 in the broad-leaved forest zone, and 1,300-1,500 along the shores of the Mediterranean and in Transcaucasia. The number of species rises to 2,000 in heavily wooded tropical countries and to 3,000 in some parts of Brazil. It decreases appreciably on oceanic islands and in high mountains (often in combination with a very peculiar species composition of the floras).
Besides taking inventories of floras, plant geographers use botanical-geographic analysis, which consists of dividing each flora into its elements: geographic, combining species with a similar geographic distribution and the same type of range, and genetic, combining species similar in origin and florogenetic relations. The botanical-geographic analysis also includes the division of floras into autochthonous elements, which developed and are developing in the region whose flora is under study, and allochthonous elements, which became part of the flora as a result of settling somewhere, that is, immigration. The correlations between these elements to a large extent characterize the age of different floras: a flora of a recently settled area (for example, after the sea regressed or after a glacier retreated) is always characterized by the predominance of allochthonous elements (sometimes as much as 100 percent of the composition). Such floras are sometimes called migrational. Richness in autochthonous elements always indicates that the flora originated quite long ago and its living conditions have been quite stable. Endemic elements (species, genera, and so forth) peculiar to a flora throw light on its history. The distinctiveness of a flora is indicated by the relative number of endemic species (usually expressed as a percentage), especially by the presence of endemic genera or, less commonly, families.
Each flora includes species differing in the time they came into being, the time they appeared in a given space, and the position they occupy. Some species by their nature are not completely able to manage in modern conditions of existence and are dying out; species representing survivals of obsolete floras are called relicts. In contrast to these, there are progressive elements of the flora, species that recently developed in an area or recently entered it and are in the process of settling there. A third category is composed of conservative species, plants that long ago firmly established themselves in an area (wherin they resemble relicts) but by their nature are fully able to manage in modern conditions and are therefore thriving (wherein they resemble progressive elements). They often dominate the vegetation. Floras rich in relict elements are sometimes called relict floras.
The analysis of a flora and the comparison of the ranges of the species and genera that constitute it, with account taken of the paleobotanical data whenever possible, serve as the basis for florogenetic research aimed at explaining the formation of floras, the transformations of their composition, and the correlations between floras that change in the course of the earth’s history. This research relies on the data of historical geology, and in some cases it is used to correct geological hypotheses (for example, in matters concerning ancient connections between the continents).
Floristic regionalization of the earth’s surface is the result of the comparative study of floras and of the history of floras and ranges.
Plant geography is studied in the USSR in the V. L. Komarov Botanical Institute of the Academy of Sciences of the USSR, the N. I. Vavilov Scientific Research Institute of Plant Growing, and the botany departments of several universities and other institutes of higher learning. Several universities (such as Leningrad and Tomsk) have special research laboratories of plant geography. In other countries, plant geography is studied chiefly in botany departments of universities and in botanical institutes, mainly specialized ones like institutes of plant classification and geography (or special botany). A number of botanical journals also deal with the subject.
REFERENCES
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Alekhin, V. V., L. V. Kudriashov, and V. S. Govorukhin. Geografiia rastenii s osnovami botaniki, 2nd ed. Moscow, 1961.
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Engler, A. Versuch einer Entwicklungsgeschichte der Pflanzenwelt, insbesondere der Florengebiete seit der Tertiärperiode, vols. 1-2. Leipzig, 1879-82.
Cain, S. A. Foundations of Plant Geography. New York-London, 1944.
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A. I. TOLMACHEV