74 



as classified by the Soil Survey, but, with a single exception, separate 

 studies were not made for growth rates when the same soil type was 

 found on unglaciated and glaciated areas, or on areas of different periods 

 of glaciation. The growth rates for certain acidulous upland soils of the 

 lower Illinoisan area of glaciation were found to be so markedly lower 

 than for similar soil types elsewhere that a special grouping of studies on 

 these soils is made under the title Illinoisan. 



The studies are incomplete in that the investigation of the growth 

 rates for a given species was not made on each soil type upon which the 

 species grows, nor were sufficient data collected to determine with finality 

 the varying degrees to which growth is influenced by soil and site condi- 

 tions, but the studies do show the general growth relations for the various 

 commoner species of the state on the common soil types. 



A diameter of 10 inches inside the bark on the stump is adopted as 

 the minimum diameter at which trees will be harvested for sawlogs or for 

 railroad ties. Such a tree will produce but one first-class tie, and in saw- 

 log operations a 12" stump D. I. B. more nearly represents the average 

 cutting limit. Comparison of the periods required to attain this mer- 

 chantable size (Table 1, pp. 78-79) brings out the facts that (1) on the 

 same soil type, trees grown in even-aged stands require a shorter period 

 than those grown in all-aged stands, that (2) there may be a very great 

 difference in this period for diflferent species on the same soil type, and 

 that (3) the difference in this period for the same species growing on 

 different soil types is not so marked. 



(1) That trees grown in even-aged stands require a shorter period 

 than those grown in all-aged stands to attain such a relatively low diam- 

 eter as 10 inches is shown by the following tabulation. 



These six species are the only ones on which studies have been 

 made for trees grown in both even and uneven aged stands on the same 

 soil type. With the exception of black oak, the trees grown in all-aged 

 stands had not yet made up for the period of initial suppression and over- 

 come the lead of the trees grown in even-aged stands. The fact that ash 

 on yellow-gray silt loam attained a merchantable size in 50 years grown 

 in even-aged stands while it required 78 years to attain the same size in 



