14 



TIMBER OF THE EDWARDS PLATEAU OF TEXAS. 



TABLE I. Discharge of spring rivers arising from fissure 



THE VEGETATION OF THE EDWARDS PLATEAU. 

 UENERAL DESCRIPTION. 



The character of this vegetation is very clearly a product of climatic 

 and geologic conditions. These combine to give it the stamp befitting 

 an arid or at least a semiarid environment. Its structure and habits 

 indicate that it is a xerophytic or dry climate vegetation; but though 

 this is true of it as a whole, conditions vary enough to give in some 

 places, as in well-watered and sheltered canyons, a relatively luxu- 

 riant growth, while in other situations, as upon stony arid slopes, 

 there is the scantiest vegetation, by no means sufficient to hide the 

 white glare of the chalky soil (PI. Ill, fig. 2). On the western 

 border of the region the vegetation bears a markedly desert aspect. 



TRANSFORMATION OF PRAIRIE INTO TIMBERLANI). 



It is of fundamental importance to note that the type of vegetation 

 in this region is undergoing a change. This change, broadly indicated, 

 consists in a transition from grass to woody growth. This transition 

 is very apparent even to the casual observer. Everyone has observed 

 how the mesquite captures the open pastures (PI. IV, fig. 2), and many 

 have watched the scrub oak timber occup} r uplands that formerly 

 were open prairies. 



Some of the causes of this are reasonably evident. In the first 

 place, dissection of the old plateau surface by the process of erosion 

 has favored the establishment of forests in the rougher parts. Pro- 

 gress due to this cause, however, is too slow to be apparent. The 

 presence of trees upward of 500 years old in some of the canyons is 

 an index to the length of time forestation has been in progress. As 

 one passes from the canyons and hills to the level plateau divides, the 

 timber gives place more and more to open prairie, which, until within 

 recent years, was free from woody growth of any kind. But these 

 ranges have been overpastured, and the grass has consequent! } r not 

 succeeded in maintaining a continuous sod, and so has become unable 



