CAUSES DETERMINING FOREST DISTRIBUTION. 13 



NATURE OF THE SOIL AND ROCK. 



The geological structure alone may account for heavy timber in 

 regions of low rainfall, simply by the supply of percolating waters 

 which it may furnish. The heavily timbered canyons of the Edwards 

 Plateau as far west as Kerr County are an example of this. On the 



other hand, upon the pine barrens of east Texas, although the rainfall 

 is heavy, the form and habit of growth of young longleaf pine shows 

 very evidently its adaptation to periods when there is lack of sufficient 

 moisture — a lack which the older trees partially overcome by sending 

 their roots deep into the sand beds, through which the rain water 

 drains away from the surface. In general, erosion canyons and the 

 broken strata of mountain masses tend to make percolation waters 

 available, and explain the relatively heavy timber in the far west of 

 the State. So also the waxy clay soils of the Black Prairie, which 

 tenaciously retain the moisture supplied by precipitation, support a 

 forest growth which on more porous soils in the same rainfall area 

 can not maintain itself. The swamp forests, the alluvial bottom 

 forests, the loblolly and hardwood, and the shortleaf and hardwood 

 forests are distinct forest types in a region of high rainfall, within 

 which soil texture and chemical make up, configuration, and elevation 

 are the determining factors. To understand the various t} 7 pes of 

 forest which the State furnishes, it is necessary to take into consider- 

 ation along with the rainfall the geological formation, the quality 

 of the soil, and the physiographic features, which not only determine 

 the minor classifications within each type, but also often broadl\ T 

 modify the types of large areas. 



Nevertheless, it is rainfall rather than the nature of the soil and rock 

 which has played the principal part in producing the main types into 

 which the forests of the State naturally divide themselves. This is 

 shown by the fact that the gradual transition from moisture-loving 

 to drouth-enduring species takes place along the line of decreasing 

 rainfall, and across the line of successive geological formations. These 

 successive formations lie approximately parallel with the present Gulf 

 coast, so that they traverse the rainfall zones nearly at right angles. 

 It has already been pointed out that the forest belts follow a general 

 north and south direction; in consequence they succeed one another 

 along each geological formation. For example, the Fayette Prairie 

 begins at the Sabine with very heavy forests of longleaf pine, which 

 are succeeded west of the Trinity by post oak as the dominant species. 

 As we advance southwestward this becomes more open and stunted, 

 until at last it disappears in the dense body of the Rio Grande 

 chaparral. 



