1922] Forest Types 185 



separation of types 



In deciding what shall be regarded as constituting a permanent 

 forest type or tree society in such a complex as obtains at the southern 

 end of the Appalachians, only such societies have been considered 

 as are most sharply defined and well marked either by composition 

 or, in the case of pure stands (at least 66% pure), by marked differ- 

 ences in the height of the dominant trees and in the volume of wood 

 (quality of stand). The intergrading or transitional stages have 

 been neglected, as well as temporary types. In such societies the 

 quality of the stand, which is an expression of the possibility of yield 

 of the tree society, is considered as important a concept as is varying 

 proportion of species : for example, practically pure stands of chest- 

 nut occur and are widely distributed from Connecticut to north- 

 eastern Alabama, but the chestnut in the pure stands of this species 

 on different sites may vary greatly in height from not to exceed 40 

 feet along dry and wind-swept upper slopes and crests of ridges, to 

 exceeding 110 feet on sites most favorable for its growth. The same 

 condition is observed in other species, and for this reason to obtain 

 an adequate expression of the forest site the height of the trees in the 

 mixture should be considered as a feature as well as the intermixture 

 of species. With this variation in the height of the overwood there 

 is a concomitant change in the character of the undershrubs and her- 

 baceous associates. 



In the division of pure stands into types, the height of the domi- 

 nant trees (Appendix 4) in the mature forest has been regarded as 

 the criterion, with an interval of 25 feet (Appendix 5) between the 

 different types. It has been suggested (Appendix 6) that this inter- 

 val be standardized on the basis of 20 feet at 100 years for Appa- 

 lachian species, which for stands of old timber practically conforms to 

 the interval of 25 feet which is being employed. This interval in 

 height results in a difference exceeding 2,000 cubic feet of wood 

 per acre in stands of average height (100 feet), and indicates a great 

 difference in the average amount of available soil moisture in the 

 root zone on soils having the same mechanical condition and depth 

 (Appendix 7), i. e., a difference in the number of critical (dryest) 

 periods and a corresponding difference in the maximum depth of the 

 water table ; or a marked difference in transpiration and evaporation 

 of soil moisture. 



