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TheImprovement ofTropical and Subtropical Rangelands

TheImprovement ofTropical and Subtropical Rangelands

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276 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS<br />

composed of a lleries of parallel ridges of declining altitude; they run<br />

from the northwest to the southeast. Folding, rather than faulting,<br />

has been the major mechanism in the formation of these ridges, <strong>and</strong><br />

as a result, they constitute a clusic series of long anticlinal ridges<br />

separated by deep synclinal valleys. The ridges are densely packed<br />

everywhere, but in the north they are generally broad <strong>and</strong> high,<br />

whereas in the south they are small <strong>and</strong> fine. Outst<strong>and</strong>ing dominant<br />

peaks are seldom found. Instead, there is a gradual increase in the<br />

altitude of the individual ridges from the south to the north, those in<br />

the south averaging about 600 m, while occasional ridge domes can<br />

be found in the north that are as high as 4,000-4,500 m.<br />

Erosion, compounded by overgrazing <strong>and</strong> by overcutting of the<br />

forest, has resulted in deep, incised, <strong>and</strong> narrow river valleys. These<br />

rivers flow parallel to the mountain ridges in a grid pattem, except<br />

where they are able to break through the mountain barriers in a<br />

series of spectacular gaps <strong>and</strong> gorges called tinag.. Through the<br />

tOftg. run the principal avenues into the interior. In certain locations<br />

streams are unable to pierce the anticline ridges <strong>and</strong> interior drainage<br />

basins, <strong>and</strong> associated swamps <strong>and</strong> shifting saline lakes are the result.<br />

Because of somewhat heavier precipitation, the mountain slopes in<br />

the northern areas are apt to have an oak forest cover, but farther<br />

south, where the rainfall is less abundant <strong>and</strong> where deforestation<br />

is more advanced, slopes of exposed rock in striking colors of red,<br />

yellow, white, gray, green, or black are common. 1S Valley floors are<br />

deep, may range from 1,200 to 2,500 m above Ilea level, <strong>and</strong> are<br />

covered by a park-like grass <strong>and</strong> shrub vegetation that is sharply<br />

contrasted with the denuded slopes above. It is upon these valley<br />

floors that the nomads of the region find the principal supply of grass<br />

along their seasonal migration routes.<br />

Falling almost entirely in the winter months, precipitation is<br />

scanty; the lowest totals, generally around 500 mm, fall in Fars<br />

province. Precipitation amounts increase markedly as one goes upslope<br />

<strong>and</strong> northward into the higher elevations. 4 Yet, except for<br />

these higher elevations <strong>and</strong> favored sites along perennial streams<br />

draining the higher mountains, aridity is the rule. This paucity<br />

of precipitation is due to the dominance of dry, hot, Indian monsoon<br />

air in the summer, <strong>and</strong> dry, cold, stable air flowing out of the<br />

Siberian anticyclone in the winter. Rain that does fall originates in<br />

the Mediterranean region in the form of cyclonic disturbances <strong>and</strong><br />

is confined almost entirely to the winter season. At irregular intervals,<br />

the dominance of the Siberian anticyclone breaks down <strong>and</strong>

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