VEGETATION OF THE EDWARDS PLATEAU. 15 



to wage an equal light against the shrubs which form the vanguard of 

 the timber vegetation (PI. IV, fig. 2). Again, settlement has stopped 

 the periodic burning otV of the grasses, which, while it left these in a 

 condition to recuperate effectually, drove out the shrubs and prevented 

 the timber from gaining on the prairie. 



Nevertheless, in general, the Edwards Plateau is a timbered region 

 only in the deeply eroded portions, becoming priarie on the level 

 uplands, and finally pacing into the great grass plains which stretch 

 northward into Canada. One must however, distinguish many 

 degrees of forestation, according to the relative amount of available 

 moisture. Through a gradual dwarfing and thinning out the timbc 

 passes from the big, heavy growth of the watered canyons to the 

 stunted though continuous forest of the hills and bluffs (PI. II, fig. 1) 

 and the scant tree growth of the loose, stony slopes in the eastern part 

 of the area (PL III, fig. 2), until at the west there remains only 

 scattered chaparral, and finally the unique vegetation of the Sotol 

 Country, in which the sotol, cactus, yucca, and agave reign supreme. 



THE SPECIES OF TREES AND THEIR GEOGRAPHIC AFFINITIES. 



The trees which make up the forests of central Texas the personnel, 

 so to speak belong chiefly to the Atlantic type of forest, although 

 Rock} r Mountain species and semitropical Mexican species occur also, 

 and in increasing numbers and importance as one goes farther west and 

 southwest. The Atlantic type is represented })y elms, live oak, post 

 oak, black jack oak, overcup oak, and basket oak, black cherry, black 

 walnut, pecan, s} T camore, green ash, hickory, soapberry, and others. 

 From the southern Rocky Mountain region are the pinon pine, two 

 or three cedars, and several oaks, and from a more southerly range 

 the mesquite, madroiia, anaqua (knackaway), lignum- vitee, and others- 

 Many of the trees of this region are its own peculiar product. Thus, 

 the most abundant species of all, the mountain cedar (Sabina sabi- 

 //"/Vr.s), is practically confined to the central Texas limestone country. 

 So also in the case of mountain oak, Mexican persimmon, and so on. 

 Further, while these species are peculiar to the region, the}^ are after 

 all representatives of species ranging farther east in rnoister climate, 

 and they indicate how the new and more trying environment has 

 practically given origin to new species. The eastern red cedar becomes 

 here the mountain cedar; black walnut is represented by the Mexican 

 walnut, whose nuts are tiny balls scarcely half an inch in diameter. 

 Texas oak becomes mountain oak. The common live oak becomes a 

 new form in its mountain habitat. The common persimmon is repre- 

 sented by the Mexican persimmon, whose fruit is a dark blue-black; 

 Canadian redbud is here also a characteristic u Judas-tree," but of a 

 different species. The same is true in the case of buckeye, mulberry, 

 hackberrv, and still others. 



J 5 



