68 



The Plant World. 



Distribution of Deciduous Trees and Shrubs on the Mesas, by 

 W. W. Robbins. Topographic and soil maps, together with 

 maps showing the distribution of several conifers and some 

 eighteen of the more important shrubs and deciduous trees, 

 with a number of photographs, elucidate the text. 



The method of carrying out the studies of local distribution 

 is illustrated by the case of the' rock pine {Finns scopulorum) , 

 which is the dominant conifer of the foothill region, though its 

 extreme limits of range in altitude at this point are approxi- 

 mately from 6,000 to 10,000 feet. Its local distribution depends 

 not upon conditions affecting large and well established 

 trees, but upon those which the germinating seeds and 

 seedlings encounter. There is a greater growth of this pine 

 on the north than on the south slopes, and seedlings are 

 often found growing at the side of a large rock, this 

 choice of habitat corresponding, apparently, to a greater 

 abundance of soil water. On the other hand, its limits 

 below the mesa appear to be largely a matter of competi- 

 tion. The fine-grained soil of the plains is adapted to the growth 

 of grass, which prevents the pine seeds 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. Extremes 

 of temperature, which are considerably greater on the plains 

 than on the mesas, are very likely of importance in checking 

 the advance of this pine on the former. It is very probable 

 that there the seedlings are killed by late frosts. 



By careful comparison of the local distribution of the rock 

 pine with the prevailing atmospheric and soil conditions it 

 seems, therefore, that the factors determining distribution are, 

 first of all, soil moisture, but that temperature is also potent, 

 and that competition, where soil conditions favor other species, 

 is distinctly a limiting influence. 



It is a distinct advantage to be able, by the critical study 

 of restricted areas to designate, as is done in the case of the 

 deciduous trees and shrubs, the actually dominant factor by 

 which local distribution is determined. With this satisfactorily 

 established the next step, namely, the determination of the 

 percentages of soil moisture which favor or admit of the growth 



