MESA AND FOOTHILL VEGETATION 35 



mesa region is clearly a tension line; conditions here are severer for 

 the pine. From a study of the factors affecting its distribution on the 

 mesas, one may arrive at some conclusions regarding the factors limiting 

 its general distribution. The optimum conditions for the growth of any 

 plant as a rule are found near its center of distribution and most adverse 

 conditions prevail at its limits. At the limits there is an accumulation 

 of adverse conditions, all of which operate to prevent it from extending 

 its range. This is well shown in the case of the Rock Pine. 



As seen in the study of distribution on the mesas, moisture is an 

 important factor. The soil is almost too dry for the seedlings to get 

 a start and they can only attain a foothold in the most favorable situa- 

 tions during seasons in which there is unusual rainfall. Farther east 

 on the plains the moisture is still less than it is on the mesas and probably 

 under the most favorable conditions is too little for the growth of the 

 seedling. However, it is sufficient in many places to support a pine 

 of good size with its extensive root system. This is shown by the con- 

 tinued growth of such trees transplanted to the plains regions or of 

 seedlings carried past the danger period under cultivation. Clearly 

 it is the effect of the dryness upon the seedling and not upon the large 

 tree, which prevents further extension of the Rock Pines. 



Competition with the grassland formation is another important 

 factor limiting the distribution of the trees. This again has its relation 

 to the seedling and not to the older tree. The fine-grained soil of 

 the plains is adapted to the growth of grass which on the edges of the 

 pine formation prevents the pine seed from reaching the soil. The 

 coarser soil of the mesas, being less favorable for a dense growth of 

 grass, allows the pine to get a foothold in the mesa region. This explains 

 in part the good growth of trees on the top, and especially on the rocky 

 crests of the mesas. Also it accounts for their almost complete absence 

 on the lower parts of the mesa slopes, especially toward the east 

 end, where the fine-grained soil supports a dense growth of 

 grass. 



It has been pointed out in the account of the climatology, p. 29, that 

 daily extremes of temperature are considerably greater on the plains 

 than on the mesas. This may be of considerable importance in checking 



