were about 220 years. Evidently for both species the growth potential 

 for trees within this age range is similar. 



Because of no real difference between the two broad age groups 

 all trees of a given species were combined into one age group and sorted 

 by vigor classes. The regressions of 20-year growth on diameter were 

 then tested by three vigor classes: good, fair, and poor. Correlation 

 coefficients for all regressions were significant. By presenting the 

 results in general vigor terms they may be interpreted for the several 

 tree classification systems in use. 



Board-foot growth for 120- to 220-year-old trees of both species 

 by initial diameter and vigor was greater on the medium than on the 

 light cutting as shown in table 2. Among the factors accounting for 

 the better growth in the medium cutting are greater growing space and 

 a higher percentage of trees in the better-vigor classes. Greater 

 growing space proved to be the major factor. Reserve trees had more 

 growing space where 78 percent of the volume was cut than where 41 

 percent was cut and they responded accordingly. For example, a 20- 

 inch good-vigor ponderosa pine grew 175 board feet in 20 years on the 

 medium cutting compared with 99 board feet on the light cutting. 



Growth of both ponderosa pine and Douglas-fir good-vigor trees 

 was not significantly different from the growth of fair-vigor trees 

 on the medium cutting. Therefore, the data for the two vigor classes 

 were combined in a single regression to compute the values recorded in 

 table 2. On this cutting there were insufficient poor-vigor trees to 

 include in the analysis. 



Under the greater competition existing on the light cutting, 

 trees of both species grew according to vigor (table 2). The ad- 

 justed mean growth of good-vigor trees significantly surpassed that 

 of the fair trees, and fair trees in turn surpassed poor-vigor trees. 

 However, with increased size the trend of growth was toward equality 

 between good- and fair-vigor classes. Growth of poor-vigor trees was 

 consistently lower in all sizes. 



CONCLUSIONS 



The specific growth values reported here, of course, are ap- 

 plicable only to partial cuttings in similar stands under like condi- 

 tions. However, certain growth relationships such as differential 

 response by vigor classes dependent on growing space may have broader 

 application. 



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