12 



FOREST RESOURCES OF TEXAS. 



like. After these, on higher lands, come the pines, loblolly on the 

 sandy knolls and ridges of the Coast Plain, longleaf on the more rolling 

 sand barrens of the Fayette Prairie, and shortleaf on the higher 

 uplands of the Lignitic Belt. The second forest zone consists of oak 

 barrens, lying westward from the types of forest just mentioned, and 

 in a rainfall zone of 45 to 35 inches. Next is the central Texas hill 

 zone, with a rainfall of from over 30 to less than 25 inches, where 

 occur mountain cedar, mountain oaks (five or six species), cedar elm, 

 gum elastic, Mexican persimmon, and numerous others. Last of all 

 come the p} 7 gnry forests of chaparral, embracing mesquite, retama, 

 huisache, cat-claw, allthorn, palo-verde, and a score besides. 





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IX 





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Fig. 2.— Precipitation in the Texas region: I, over 50 inches; II, over 45 inches; III, over 40 inches; 

 IV, over 35 inches; V, over 30 inches; VI, over 25 inches; VII, over 20 inches; VIII, over 15 inches; 

 IX, over 10 inches. More extended observations since the preparation of this map show a general 

 slight reduction of the rainfall throughout this region. 



Rainfall alone, however, does not determine the limits within which 

 these species occur. There are cairyons in the region where the 

 annual rainfall is scarcely 20 inches in which may be found not only 

 oaks, hickories, and similar trees, but even the swamp-loving cypress. 

 While the moisture demands of the different kinds of trees constitute 

 the most potent of the causes which determine their distribution, it is 

 not primarily the amount of moisture which falls to the ground, but 

 the amount of moisture which the soil holds that affects them. The 

 distribution through the zones of rainfall is consequently modified 

 veiy considerably by the varying geological and soil conditions. 



