YEGETATIOX OF THE EDWARDS PLATEAU. ±5 



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

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

 the periodic burning off 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 

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

 uplands, and finally passing 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 timbe-^ 

 passes from the big. heav}^ growth of the watered canyons to the 

 stunted though continuous- forest of the hills and bluff's (PL 11. fig. 1) 

 and the scant tree growth of the loose, ston}' slopes in the eastern part 

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

 scattered chaparral, and finally the unique Aegetation of the Sotol 

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



THE SPECIES OF TREES AXJ) THEIR GEOGRAPHIC AFFINITIES. 



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

 so to speak — belong chiefly to the Atlantic t^^pe of forest, although 

 Rocky ^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 by elms, live oak, post 

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

 walnut, pecan, sycamore, green ash, hickory', soapberry, and others. 

 From the southern Rockv ]\Iountain region are the piiion pine, two 

 or three cedars, and sevei-al oaks, and from a more southerly range 

 the mesquite, madrofia, anaqua (knackaway), lignum-vitae, and othere- 



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

 the most abundant species of al]. the mountain cedar {Sahina mhi- 

 noUles)^ is practically confined to the central Texas limestone country. 

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

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

 all representacives of species ranging farther east in moister 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 ^lexican 

 walnut, whose nuts are tiny balls scarcel}^ 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 l)y the Mexican persimmon, whose fruit is a dark blue-black: 

 Canadian redbud is here also a characteristic ''Judas-tree.-' but of a 

 different species. I'he same is true in the case of buckeye, mulberry, 

 hackberrv, and still others. 



