APPENDIX C 

 DEVELOPMENT OF DIAMETER GROWTH MODELS 



Prior Findings 



Diameter growth is influenced by several factors including productivity. A measure of 

 productivity is site index (the height obtainable by free growing trees at a base age) . 

 Minor's (1964) site index was developed for even-aged stands in northern Arizona, and its use 

 is recommended for the area.^ The application of site index to uneven-aged stands is compli- 

 cated by the difficulty of finding suitable free growing site trees. This complication is 

 reduced to some extent in the uneven-aged stands because they have relatively low stocking 

 levels; however, this problem still causes difficulties. Another problem with site in the 

 study area is the previously mentioned narrow range of site indices computed for the plots, 

 which may make the inclusion of a meaningful site measure difficult. 



Another factor related to productivity that also influences diameter growth is rainfall. 

 Fritts and others (1965) found that diameter growth in southwestern ponderosa pine is influenced 

 by precipitation and temperature of the autumn, winter, and spring prior to the growth period. 

 Pearson and Wadsworth (1941) found that gross 5-year volume increment appeared to be related 

 to the amount of precipitation in the period. Finally, Pearson (1936) hypothesized that a 

 fall off in 5-year diameter growth in the second period after thinning was due to lower precipi- 

 tation in that period. 



Total stand density can also influence diameter growth. For uneven-aged stands in the 

 Southwest, Krauch (1940) and Pearson (1950) both reported that a reduction in stocking through 

 cutting increased diameter growth. After evaluating growth in young, even-aged clumps of 

 uneven-aged stands. Cooper (1961) found that average tree diameter was "...largely determined 

 by stand density." For even-aged ponderosa pine stands in the Southwest, the effect of density 

 upon growth has been widely reported (Pearson 1936; Hornibrook 1936; Krauch 1949b; Gaines and 

 Kotok 1954; Cooper 1960; Larson and Minor 1968; Schubert 1971, 1974; and Myers and others 

 1976). In all cases, an increase in density reduced growth. 



The correlation of diameter class size with growth has also been reported in the Southwest. 

 Some have found that, following cutting in uneven-aged stands, diameter growth was not as 

 closely correlated with diameter class size as might be expected (Pearson 1933, 1950; Pearson 

 and Marsh 1935). Others, however, have found that diameter growth did vary markedly between 

 classes in cut stands (Krauch 1937) and in uncut stands (Pearson 1950) . For even-aged stands, 

 both Pearson (1936) and Krauch (1949a, 1949b) reported differences in average diameter growth 

 between diameter classes. 



One feature mentioned frequently in the literature is the difference in growth rates 

 between "yellow" pine and "blackjack" pine. In all cases, diameter growth was greater in 

 blackjack pine than in yellow pine (Pearson 1933, 1950; Pearson and Marsh 1935; and Krauch 

 1934, 1937). 



IVhile not mentioned specifically in the literature for the Southwest, one additional 

 factor that does affect diameter growth is the position of the diameter class in the stand 

 (that is, how much competition a given stand receives from above and below it). A measure of 

 this type can be thought of as an index of the competitive structure, and it is intimately 

 related to stand structure. A competitive structure index might, therefore, explain differences 

 between even-aged and uneven-aged growth rates. Competitive structure was one of the critical 

 factors for meeting objective number two of this study. 



^Personal communication with Gilbert H. Schubert, principal silvilculturist with USDA For. 

 Serv., Rocky Mountain Forest and Range Experiment Station, Flagstaff, Ariz.; retired. 



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