CLIMATIC AND EDAPHIC INDICATORS. 351 



from considerations of future as well as present management. This sub- 

 division of permanent forest types or of any other kind of types can be effected 

 by the use of an indicator. Indicator plants, volume growth, and height- 

 growth are means to this end. Under certain circumstances the use of 

 indicator plants may prove very useful, as experiments by Korstian and others 

 indicate. 



"The use of the current annual increment as a means of determining 

 site involves the double difficulty of securing a basis and of applying the 

 measure of the site, when found, to the identification of similar site conditions 

 elsewhere. As an exact indicator it may prove the last word in refining 

 previous site determinations in locahties where it can be employed, but as a 

 general method, suitable for immediate use, it fails to meet the requirements of 

 simpUcity and widespread utihty previously set forth. The utihty of height 

 one of the functions of volume, but far less unwieldy as an index ought 

 to be plainly evident to everyone as the logical immediate basis for sub- 

 division. Height-growth, as a matter of fact, appeals in two ways: First, 

 as an immediate means of classifying forest sites in general, and second, as a 

 guide and a short cut in arriving at a possible future classification of sites on a 

 physical or permanent type basis. 



"In conclusion, the principle of height-^owth as a guide to site has the 

 following features : 



"1. It is simple, natural, easily understood, and easily applied in the field. 



"2. It is independent of the determination of physical sites producing 

 definite permanent forms of forest; but the two are not antagonistic; both are 

 'indicators' and both demand equally a determination, more or less refined, 

 of the physical factors of site. 



"3. The sites determined by height-growth are species sites, not permanent- 

 type sites; hence they are useful with reference to short-Hved intolerant and 

 long-Hved tolerant species growing in the same stand. 



"4. By adopting one or more index species (intolerant species of wide oc- 

 currence on a variety of sites) the height-growth of other species can be gauged, 

 their relative value in each site can be determined, and this value can be ex- 

 pressed by naming the site in terms of the growth of each species present, and, 

 by analogy, of other species which do not happen to be present. 



"5. It affords a means of comparing the growth of all American species on 

 the basis of the soil and climate to which each is best suited, as well as in less 

 favorable sites. 



"6. It permits a ready comparison (a) between even-aged second-growth 

 stands in widely different regions, thereby avoiding such inconsistencies as 

 those to be found in the pubhshed yield tables for the same species in different 

 States; and (6) between second-growth and old-growth stands in the same or 

 different regions. 



"7. Since height-growth is sensitive to interferences in the natural Ufe of 

 the stand (fire, culling, changes in density, etc.) care and judgment are 

 necessary in the choice of trees to serve as the index; but except for very pre- 

 cise site determinations, the method, if used with ordinary caution, will 

 undoubtedly prove serviceable for the majority of wild-woods conditions as 

 well as for even-aged stands. 



"8. As the knowledge of the laws of growth of our species increases, the 

 refinement of site determination by height-growth can be increased." 



The correlation of height-growth with rainfall and other factors has been 

 made by Pearson (1918:688) for PiniLS ponderosa in Arizona: 



" Western yellow pine in northern Arizona makes its height-growth during 

 the period of lowest precipitation in the year. During this period of high 



