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.