NATURAL AllEAS AND RKGIONS 



607 



5. Grassland 



The great plains region, referred to 

 locally as the "cattle country" because 

 it is the great grazing range of the state, 

 is a region of grassland. It is the south- 

 ern extension of the vast temperate 

 grassland country lying to the north 

 between the JMississippi valley anil 

 the eastern slope of the rocky moun- 

 tains, and occupies a vast area in nortli 

 central Texas. To the eastward it is 

 met by the diminishing oak grove 

 savanna which merges imperceptibly 

 into it; to the south it meets the Rio 

 Grande succulent desert to which it is 

 bound by the northward extension of 

 the great mesquite country of southern 

 Texas, while its western boundar}^ is 

 the great Llano Estacado. It is a region 

 of perennial sod grassland with a mes- 

 quite {Prosopis glandulosa) overgrowth. 

 It is a region of greater elevation than 

 any of the country to the east, rising 

 from approximately 1200 ft. to nearly 

 3000 ft. The latter figure is the eleva- 

 tion at the western extremity of the 

 region, from which the semi-desert 

 broken steppe (the staked plains) rise 

 to an elevation of 400 ft. The annual 

 rainfall averages about 20 to 25 in., but 

 this rain is periodic: ordinarily a wet 

 spring and early summer, followed by a 

 dry late summer and fall, and a cold, 

 dry winter. Ecologically, the mes- 

 quite element is important as it must be 

 regarded as an invasion from the Rio 

 Grande desert, and its extension is fol- 

 lowed by floral changes which pro- 

 foundly alter the typical grassland con- 

 ditions. The mesquite invasion is 

 followed by conspicuous changes in 

 the grass composition, the so-called 

 "bearded mesquite grass" eventually 

 excluding the plains grasses because of 

 its greater shade tolerance. This is 

 followed by the appearance of otlicr 

 woody species, as the lotebush {Zizn- 

 phus) until the open country is covered 

 by a dense growth that proves of serious 

 economic importance. From the faunal 

 viewpoint, the mesquite is also of 

 importance, as we find a number of 



animaia, audi ati the liuird wood rat 

 {Ncotoiua niicropus) and the Tcxoii dia- 

 mond rattlesnake (CrolnluH a. atroz) 

 extending thtir range from the typical 

 mes(iuitc semi-desert of the south clear 

 up into the northern imrtion of the 

 plains region, following very exactly 

 the distribution of the me»(juJto. Such 

 invasions are confusing until one recoR- 

 nizcs the fact that again tlicy simply 

 represent local conditions. 



Tills is the region of the once great 

 herds of bison. Today it is the haunt of 

 the gray wolf {Cani.s (jri.-i.s(:uN) or "lobo" 

 of tlie cattlemen. Tiie kit fox (Vulpcs 

 velox) is found here, as is the coyote 

 {Canis spp.). It is the home of the 

 prairie-dog (C'l/nomi/s ! ' 'inus) 

 which formerly inhabited i jon in 



countless millions, but which is now 

 rapidly on the decline. Here they dug 

 their "towns" over great areas of sandy 

 soil. The black-tailed jackrabbit 

 (Lepus californicus subspp.) is abundant 

 over the plains, and is found in local 

 extensions of the grassland on every 

 side. Of particular interest is the dis- 

 tribution of the Richardson kangaroo- 

 rat (Dipodomijs ordii richard.'iuni) in 

 this region. This species enters Texas 

 from the plains to the north, and in its 

 distribution it completely and exactly 

 encircles the staked plains, and by so 

 doing the species marks very well the 

 western limit of the grassland. It in- 

 habits only sandy country, and extends 

 its range down river valleys when the 

 type of soil is suitable. 



6. Temperate desert grassland 



This is the region known throughout 

 Texas as the Llano Estacado — the staked 

 plains. It is a region of considerable 

 elevation averaging around lii)00 ft. hut 

 reaching 4000 ft. in places. It is a liigli, 

 arid, open stepj)e, that is characterized 

 at once by the almost total absence of 

 trees. Rainfall is never in excess of 20 

 in. and often fails to reach 15 in. The 

 conditions arc, therefore, much more 

 xcrophytic than in any of the series of 

 regions which leail up to the plain.s- from 

 the east. The area lies west of the lOlst 



