36 Guayule. 



BIOTIC RELATIONS. 



COMPETITION. 



The relation of guayule to the other plants with which it is com- 

 monly found associated is of great importance, especially if forestry 

 methods are contemplated. Both the mutual effect of each element in 

 the vegetation upon the guayule and the relative rate of growth must 

 be understood in order to judge what the final effect in the struggle for 

 existence is likely to be. To do this, however, involves a very consider- 

 able amount of sustained observation by means of the quadrat method, 

 first devised by Clements. Following is a census of the more important 

 plants found growing in association with the guayule in quadrats 5 and 

 6, Station 2. 



Table 17. 



Also the following, from Station 10: 



Table 18. 



With few exceptions, these constitute the dominant vegetation of 

 the foot-slopes and the low ridges, though of course a number of other 

 species may be found in other localities, and indeed may be more impor- 

 tant elements elsewhere than has been observed to be the case in North 

 Zacatecas. 



