CHOICE OF SPECIES IN ARTIFICIAL REGENERATION 41 



vegetation with the ratio . . More recently Shreve 1 



evaporation 



soil moisture T . . c 



has used the ratio . . Livingston 2 has made use of a 



evaporation 



combination of temperature summations and evaporation data. 



A detailed examination of all the factors of the site is a very 

 complicated and difficult operation. Moreover, without a broad 

 foundation in ecology the interpretation of the vegetation in 

 terms of the site factors is often difficult, even after the factors 

 are measured. Too much importance is often assigned to one 

 factor and too little to another. Furthermore, the factors in their 

 degree of influence are rarely the same over extensive areas, but 

 often change more or less abruptly at short distances. Therefore, 

 where the site factors have been carefully measured the data can- 

 not safely be applied to other regions. 



In the silvical investigations of the future, the direct method for 

 determining the quality of the site is likely to be of far-reaching 

 importance. However, the time and expense involved in the use 

 of instruments and in laboratory analyses are so great and the 

 difficulties experienced in relating the results to the growth and 

 development of the forest are so pronounced that the use of the 

 direct method will be left largely to experienced silvicists and 

 ecologists connected with experiment stations or other scientific 

 institutions where original research is under way. 



The practicing forester must, to a very large extent, depend 

 upon the indirect method for determining the quality of the site 

 for any particular species. He must arrive at a judgment of site 

 quality very largely from the growth and character of the vegetation 

 upon or near it, the available data relating to the site factors serving 

 as an aid in the application of the indirect method. 



3. The Evaluation of the Site Factors by the Indirect 

 Method 



A crop of trees growing under normal conditions upon a given 

 site represents the result of all the site factors working together. 

 Although we do not know to what extent each of the several 



1 Shreve, F.: Rainfall as a determinant of soil moisture. (Plant World, 

 vol. VII, pp. 9-26. 1914.) 



2 Livingston, B. E.: Climatic areas of the United States as related to plant 

 growth. (Proc. Am. Philos. Soc., vol. LII, pp. 257-275. 1913.) 



