Physical Geographic Zones

natural land zones; large subdivisions of the geographic (terrain) cover of the earth, regularly and systematically alternating with one another depending on climatic factors, especially the correspondence between heat and moisture. In conjunction with this, there is alternation of zones and belts from the equator to the poles and from the oceans to the heart of the continents. They generally extend in sublatitudinal directions and do not have sharply pronounced boundaries. Each zone is characterized by representative and distinctive features of its natural components and processes (climatic, hydrological, geochemical, geomorphological, soil and plant cover, and animal), its own form of historically evolved interrelation-ships between these processes, and the dominant form of combinations of them—zonal natural territorial complexes. Designations of many physical geographic zones are assigned according to the clearest indicator—the type of vegetation reflecting the most important and distinctive features of the majority of natural components and processes (for example, forest zones, steppe zones, savanna zones). The designations of these zones are frequently extended to individual components as well, as with tundra vegetation, tundra-gley soils, semidesert and desert vegetation, and desert soils. Narrower subdivisions, or physical geographic subzones, are generally distinguished within zones that occupy extensive areas. For example, a zone of savannas in its entirety is marked by the seasonal rhythm of development of all its natural components because of the seasonal arrival of atmospheric precipitation. Depending on the amount of precipitation and the length of the rainy period, there are subzones of damp tall-grass savanna, typical dry savanna, and desert savannas within the zone. Dry and typical steppes are found in a zone of steppes; and in a zone of forests of the temperate belt there are sub-zones of taiga (sometimes considered an independent zone) and of mixed and broad-leaved forests.

Physical geographic zones, if they are formed under more or less similar geologic-geomorphologic (azonal) conditions, are repeated in terms of general features in various land areas with analogous geographic locations (latitude, location in regard to oceans). Therefore, there are types of zones distinguished that are typological units of territorial classification of geographic cover (for example, tropical western preoceanic deserts). At the same time, distinctive local features of various territories (relief, rock composition, paleo-graphic development) impart individual features to each zone, in conjunction with which specific physical geographic zones are viewed as regional units (for example, the Atacama Desert, the Peru coastal desert, the Namib Desert, and the western coastal Sahara). Thirteen geographic belts are identified on the basis of B. P. Alisov’s climatic classification in the Physical Geographic Atlas of the World (1964). They consist of an equatorial belt and two (one in each hemisphere) subequatorial, tropical, subtropical, temperate, subpolar, and polar belts. (Proponents of the thermal factor being basic to the formation of zones identify only five and sometimes only three belts.) It is possible to delineate subbelts, or regions within the belts.

Each belt and large longitudinal section or sector (preoceanic, continental, or transitional) is characterized by its own zonal systems; it has its own framework, a defined continuity, and an expanse of horizontal zones and subzones in plains areas, as well as its own set (spectrum) of elevated zones in mountain regions. Thus, a forest-tundra zone is found only in a subpolar (subarctic) belt; a taiga subzone, in a moderate belt; a “Mediterranean” subzone, in a western preoceanic sector of a subtropical belt; and a subzone of monsoon mixed forests, in its eastern preoceanic sector. Forest-steppe zones exist only in transitional sectors. A forest-tundra spectrum of elevated zones is characteristic only of a moderate belt, and a gilea-paramo spectrum only of an equatorial belt. Depending on their position within a certain sector or in a particular morphostructural foundation, smaller taxonomic units in zones and subzones may be identified. Such units may be typological (like western preoceanic dark-coniferous taiga and continental light-coniferous taiga) or regional (west Siberian taiga, Central Yakutian taiga, West Siberian forest-steppe).

Insofar as physical geographic zones are principally defined by the relationship between heat and moisture, this correspondence may be expressed quantitatively. (In 1956, A. A. Grigor’ev and M. I. Budyko were the first to formulate physical and quantitative bases for zoning.) Various hydrothermal indicators are used for this purpose (most frequently humidification indexes). Use of them primarily helps in the working out of theoretical questions of zoning, discovering general laws, and objectively identifying characteristics of zones and the boundaries of zones. For example, for values of the Budyko radiation index of dryness under 1 (excessive humidification) damp zones of forests, forest-tundras, and tundras predominate; for values greater than 1 (insufficient humidification), dry zones of steppes, semideserts, and deserts predominate; and for values close to 1 (optimal humidification), zones and subzones of forest-steppes, broad-leaved and light forests, and damp savannas predominate. The determination and further identification of quantitative indexes have considerable practical significance as well—for example, in applying various agricultural practices in different sectors, zones, and subzones. It is very important to take into account not only the similarity of summary indexes, but also which values in given circumstances are used to determine them. Thus, establishing a “periodic law of zoning,” A. A. Grigor’ev recorded the periodic repetition of identical values for the radiation index of dryness in zones of different belts (for example, in the tundra, subtropical semigilea, and equatorial forest swamps). However, given a common index, both the annual radiation balance and the annual precipitation in these zones sharply differ, as all natural processes and complexes differ from each other in their entirety.

Along with zonal factors, a number of azonal factors (in addition to the original distribution of dry land and oceans, which explains to a certain extent circulation, currents, and the transfer of moisture) have a considerable influence on the formation and structure of zonal systems. First, polar asymmetry is found in the terrain cover of the earth, expressed not only in the significant oceanic structure of the southern hemisphere, but also in the existence, for example, of a unique subtropical subzone of semi-gilea and, conversely, in the absence of numerous zones and subzones of the northern hemi-sphere (such as tundras, forest-tundras, taigas, and broad-leaved forests). The configuration and size of a land area at certain latitudes also play an important role (for example, the extensive distribution of tropical deserts in North Africa, Arabia, or Australia, and the limited territory of such deserts in tropical belts occupying smaller land areas in North America or South Africa). The character of prominent relief features is also extremely important. High-altitude meridional ridges of the Cordilleras and Andes reinforce continentality and result in the presence of semidesert and desert zones in the inner plateaus of subtropical and tropical belts. The Himalayas facilitate the immediate proximity of high-altitude deserts in Tibet and a damp forest zonal spectrum of the southern slopes; the Patagonian Andes are the primary cause of a temperate belt of the semidesert zone in the east. In general, regional factors only reinforce or weaken overall zonal regularities.

It goes without saying that zonal systems have undergone considerable changes during the process of paleogeographic development. Belt and sector differences have been determined for the end of the Paleozoic. Changes took place later in the distribution of dry land and oceans, macroforms of relief, and climatic conditions. In conjunction with these changes certain zones in zonal systems that had been formed disappeared and were replaced by others; the course of zones was modified. Modern zones are of various ages. As a con-sequence of the enormous role that Pleistocene glaciation played in their formation, zones of high latitudes are the youngest. Moreover, reinforcement of temperature contrasts between the poles and the equator during the Pleistocene increased the number of physical geographic zones and significantly complicated their system. Human activities have also had a considerable effect, particularly on the borders of zones.

A tectonic map of the world graphically illustrates the distribution of zones by belts and sectors and shows differences in development of zones for high and median latitudes of the northern and southern hemispheres. Relatively small changes in the correspondence between heat and moisture and almost universally excessive humidification are observed in high-latitude belts (polar, subpolar, and the northern part of the northern temperate belt—the boreal subbelt, which is not found on dry land in the southern hemisphere). Natural differentiation is principally associated with changes in thermal conditions, that is, with increases in the radiation balance during decreases in latitude. Consequently, even zones of polar deserts, tundra, forest-tundra, and taiga run sublatitudinally, and sector differences are weakly pronounced. (Glacial deserts in the Atlantic sector of the arctic have resulted mainly from distinctive regional features.) At the same time, polar asymmetry of zonal spectra, caused by contrasts in the distribution of dry land and oceans in the two hemispheres, is sharply pronounced. The role of moisture also increases in subboreal subbelts with an intensification of heat. The increase in moisture is determined by the prevalence of western winds; and in the east, by extratropical monsoons. Humidification indexes change considerably, in terms of both latitude and longitude; this results in a diversity of zones and subzones and differences in their lengths. Preoceanic sectors are covered by damp forests; transitional, by forests, forest-steppes, and steppes; and continental, primarily by semideserts and deserts. The clearest example of such distinctive zonal features is seen in subtropical belts, inside of which latitudinal differences for radiation conditions are still great, and where moisture arrives from both the west (only in the winter) and the east (mainly in the summer). In low-latitude belts (tropical, subequatorial, and equatorial), the asymmetry of the hemispheres is smoothed over; the radiation balance achieves its maximum indicators with latitudinal differences weakly expressed. The role of moisture becomes predominant in the changes in the correspondence between heat and moisture. In tropical (trade-wind) belts, moisture comes only from the east. This explains the presence of the relatively damp zones (tropical forests, savannas, and sparse forests) that run submeridionally in the eastern sectors and the semideserts and deserts that fill the continental and western sectors. Subequatorial belts receive moisture principally from equatorial monsoons; that is, the amount of moisture rapidly declines from the equator to the tropics. Accordingly, there is practically no pattern of sectors, but the zones and subzones of forests and savannas are numerous and sublatitudinal. Conversely, moisture and heat are constant, widespread, and plentiful in the equatorial belt. Only a gilea zone is manifested in this belt.

The phenomenon of zoning was well known even to scholars of ancient Greece. Physical geographic zones for both plains and mountains were recorded by A. Humboldt. In 1898, V. V. Dokuchaev was the first to effect a scientifically based division of the earth’s dry land into zones and to formulate a planetary law of zones. A number of scientists, mainly Russian, participated in the further elaboration of Dokuchaev’s teaching; among them were the Russian scientists A. I. Voeikov, N. M. Simbirtsev, G. N. Vysotskii, A. N. Krasnov, G. I. Tanfil’ev, L. S. Berg, I. M. Krasheninnikov, A. A. Grigor’ev, and A. I. launputnin’. Non-Russian scientists working on this problem included E. von Drygalski (Germany), O. Nordenskjóld (Sweden), and C. Troll (Federal Republic of Germany).

In the USSR the questions of physical geographic zones are being studied at geography departments at Moscow, Leningrad, Voronezh, and other universities (A. M. Riabchikov, S. V. Kalesnik, A. G. Isachenko, and F. N. Mil’kov). Certain discrepancies, which have resulted from different approaches to individual aspects of the problem under examination, are noted for a number of authors in regard to the delineation of zones and subzones.