PERMANENT CHANGES IN VEGETATION. 15 



however, never had water except during a rain. In 1908 this aster- 

 like plant was supplanted by Bouleloua aristidoides, Aristida bro- 

 moides, and other weedy annual grasses. Besides these, there was a 

 thin and very irregular growth of such valuable perennials as Bou- 

 teloua rotlirockii, Bouteloua bromoides, Leptoehloa dubia, and Bou- 

 teloua vestita. This area was in the 590-acre inclosure "(marked 

 U. S. R. on the/ map, fig. 1). The changes were more noticeable 

 than in the large field because the small field was more completely 

 denuded of its vegetation. It is quite probable that the productive 

 capacity of the small field is much below the average of similar 

 portions of the other field. 



But such differences in vegetation, comparing one year with 

 another, are very striking. There has not been one year in ten when 

 there was such a crop of Plantago fastigiata throughout the region 

 (see PI. V, fig. 2) as in the spring of 1901. In the large field, even 

 with similar rainfall, there occurs an ascendency of one plant one 

 year and of another plant another year, and this is more especially 

 true of the growth of the spring season, which is more truly annual. 

 One year the Arizona lupine (Lupinus arizonicus) gives its charac- 

 teristic color to the landscape. Another year may witness the same 

 with reference to Orthocarpus purpurascens palmeri. Lotus liumis- 

 tratus may be the abundant species another year, and Pectocarya 

 linearis another. In the upper portion of the large inclosure at the 

 present time there is a good stand of perennial grasses. These grow 

 in the relatively humid summer season. During the summer there 

 is a very sparse growth of annuals. In the spring, however, this 

 region which grows grass in summer has its complement of annuals, 

 and the relative abundance of the species varies about the same as it 

 does on the lower portion of the field, where the perennials do not 

 occur. 



So far as known, no one has ever offered an explanation for these 

 yearly variations of annual vegetation. The change in the aspect 

 and balance of the species occurs in the annuals only. There has 

 been no change, so far as anyone can see, in the areas of Bouteloua 

 rothrockii, Bouteloua bromoides, Aristida divaricata, or Aristida cali- 

 fornica in the large inclosure since the beginning, excepting a gradual 

 thickening up of the respective species and a consequent shutting out 

 of the annuals. (See Pis. II and III.) 



PERMANENT CHANGES IN VEGETATION. 



The rancher in this region is only half wrong when he asserts, as 

 he commonly does, that the six-weeks' weedy and low-quality grasses 

 have recently taken the range and driven out what he calls "root 

 grasses," or the more valuable perennials. It is only the last portion 

 of the statement that is false. The perennials, or more valuable 



177 



