all-aged stands does not necessarily mean that the yields per acre are 

 greater for the even-aged than for the all-aged, because during this initial 

 period of suppression the area in all-aged forest is producing two crops, 

 whereas the even-aged stand has full possession of the soil from the be- 

 ginning. The importance of this study is rather in the fact that there is 

 established a standard period required to produce merchantable sawlog 

 or tie material when trees are grown under the more uniform conditions, 

 such as prevail in fully stocked, even-aged stands. 



(2) That there may be a very great difference in the interval re- 

 quired to attain merchantable size for different species on the same soil 

 type is shown by the following tabulation. 



Soil type 



Yellow fine sandy silt loam 



Yellow-gray silt loam 



Yellow silt loam 



Sand 



Bottomland gray fine sandy loam. 

 Bottomland deep gray silt loam. . . 

 Bottomland drab clay 



It so happens that, of all the species studied, the fastest and the slow- 

 est diameter growth up to a 10-inch diameter was made on the same soil 

 type. The cottonwood on bottomland gray fine sandy loam attained this 

 average diameter in the remarkably short period of 8 years, and elm re- 

 quired 101 years. This contrast is modified somewhat by the fact that 

 the cottonwood was in an even-aged group while the elm had grown in 

 an all-aged group — yet both grew in the same stand. In the case of 

 bottomland deep gray silt loam, pin oak and hickory grew in the same 

 all-aged stand, yet the hickory required twice the period of pin oak to 

 attain a merchantable size. It is apparent that in general two to three 

 crops of the fastest growing trees come into merchantable size in the 

 period required to grow one crop of the slowest growing trees ; and the 

 waste of permitting these slow growing trees to monopolize the site be- 

 comes more apparent when it is seen that these fast growing trees pro- 

 duce also the more valuable crops, rated on a board foot basis. 



Although a minimum stump D. I. B. of 10 inches is used as a stand- 

 ard to measure the period required for a species to attain a merchantable 

 size, the relative rating of trees for a given soil should include both diam- 

 eter and height growth. The two are expressed in cubic contents, and 

 the cubic contents grown for each 20-year period for all different species 

 studied on a given soil type are shown in the tabulation, on pp. 72-SO. 



