Altitudinal Zonality

Altitudinal Zonality


the regular change of processes and phenomena with increasing elevation in mountains. Caused by changes in the density, pressure, temperature, humidity, and dust content of the air with increasing elevation. Atmospheric pressure decreases in the troposphere by 133 newtons per sq m (1 mm of mercury) for each 11-15 m of altitude; at an altitude of 5.5 km it is approximately half of its value at sea level. Half of all water vapor is concentrated below 1.5-2 km, and the amount of dust in the air also decreases rapidly with altitude. For these reasons, the intensity of solar radiation in the mountains increases with altitude, whereas the emission of longwave radiation from the surface of mountain slopes into the atmosphere and the absorption of counterradiation from the atmosphere diminish. Under the conditions created in the atmosphere for absorption and emission of radiation and vertical exchange of air, the temperature of the atmosphere usually diminishes within the troposphere by an average of 5°-6° C for every kilometer of altitude. The conditions of condensation of water vapor in this case are such that the number of clouds concentrated mainly in the lower kilometers of the troposphere increases up to a certain altitude. This leads to the existence of a belt of maximum precipitation and to a decrease at higher levels.

Climatic altitudinal zonality also involves changes in the conditions of river discharge; the type of soils, flora, and fauna; and certain geomorphological processes—that is, al-most all components of the complex of nature. Altitudinal zonality is most clearly apparent in the vertical variability of hydrological, climatic, and soil-biology components of the landscape. Altitudinal zonality is expressed in the terrain in connection not only with differences in climatic conditions but also with the fact that areas of destruction and removal and of the effects of ancient and modern glaciation occur in the upper zones of mountains, whereas areas of accumulation of material occur at their bases. In addition, altitudinal zonality is complicated by the multilevel gradation of the terrain, which reflects various stages in the history of mountain formation, and by the preservation at various levels of vestiges of ancient leveling surfaces, separated by sharper ledges and tiers of erosion cutting.

The aggregate of altitudinal zones of the macroslope (declivity) of a mountain region or the specific slope of an individual ridge is usually called the set or spectrum of zones. In each spectrum the base landscape is the landscape of the mountain foothills that is close to the conditions of the horizontal natural zone in which a given mountain region exists. The combination of the many factors that influence the structure of the altitudinal zonality causes complex differentiation of types of altitudinal spectra. Even within a single zone, the spectra of an altitudinal zone are often different—for example, they become richer as the height of the mountains increases.

There is a certain analogy in the change of altitudinal zones within the spectrum of a mountainous region on the one hand and of the horizontal geographical zones from low to high latitudes on the other, but they are not completely identical. For example, the polar day and night, as well as a unique rhythm of hydrological-climatological and soil-biology processes, are inherent in the tundra of arctic latitudes. The high-altitude analogues of tundras in the lower latitudes, as well as alpine meadows, are devoid of such characteristics. High-mountain regions in equatorial latitudes with regular annual thermal regimens and moisture conditions have particular landscapes—the paramo (the Ecuadorean Andes, Kilimanjaro)—which have little in common with the zone of alpine meadows. The most complex spectra of altitudinal zonality occur on the slopes of high mountains at low latitudes. Toward the poles the levels of the altitudinal zones drop, and at certain latitudes the lower zones are crowded out. This is particularly well expressed on the slopes of mountainous regions that extend meridionally (the Andes, the Cordillera, and the Urals). In addition, the spectra of altitudinal zones on exposed and intermontane slopes often differ.

The complex of altitudinal-zone spectra also changes greatly in relation to the distance from the seas toward the continental interior; it is divided into oceanic, continental, and transitional sectors. The dominance of mountain-forest landscapes is usually characteristic of oceanic areas; treeless landscapes usually dominate continental areas. The complex of altitudinal-zone spectra also depends on many local conditions—peculiarities of the geological structure and the exposure of slopes in relation to direction and to the prevailing winds. For example, in the Tien-Shan the altitudinal zones of the mountain forests and forest steppe are generally peculiar to the northern—that is, shady and more humid—slopes of the ranges, whereas mountain steppe is characteristic of southern slopes at the same altitudes. In some areas the altitudinal zones are inverted, so that a landscape characteristic of higher levels also appears in a lower zone. Thus, the stunted growth that takes the place of the mountain taiga in Eastern Siberia and the Far East above the timberline is sometimes also found at the foot of the slopes. This is caused by local orographic and climatic peculiarities—stagnation of cold air at the bottom of intermontane hollows, the influence of cold currents on seacoasts, and so on.

Altitudinal zonality also influences the economic conditions of mountain regions. The growing season becomes shorter with increasing elevation, and it becomes difficult or impossible to cultivate warm-season crops; transhumant livestock raising is most common in the mountain-meadow zone. As the elevation increases there is a reduction in pressure and oxygen content, and special problems arise in the functioning of transportation, mines, and other operations. Certain physiological reactions of the human organism also change, sometimes causing altitude sickness.

Major general conclusions on the principles of altitudinal zonality belong to the German scientist A. Humboldt; however, his conclusions embrace only climate and the organic world. Modern teachings on altitudinal zonality are based on the works of V. V. Dokuchaev, who discovered the relationships between animate and inanimate nature in the principles of both horizontal and altitudinal zonality.


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