SILVICULTURAL MANAGEMENT OF BLACK SPRUCE IN MINNESOTA 29 



100 120 



AGE (YEARS) 



Figuke 20. — Height-over-age site classifications for even-aged well-stocked black 

 spruce stands. (Bowman's classification is an average for black spruce, white 

 spruce, and balsam fir. Averell and McGrew's report indicated the range 

 in height for each quality of site rather than the averages or midpoints as 

 shown by other authors. Midpoints for best and poorest sites were obtained 

 by interpolation and plotted in this figure. ) 



than Type I. Beyond the age of 80 years the data are in remarkably 

 close agreement with those of Fox and Kruse (12). Since the two 

 classifications were worked up independently, the close agreement lends 

 strength to both. The marked differences in the younger age classes 

 cannot be accounted for. However, various observations indicate that 

 the Fox and Kruse growth rates for younger age classes are more repre- 

 sentative of conditions in Minnesota. Therefore, it may be concluded 

 that the site classification and growth rates reported by Fox and Kruse 

 are the best estimates available for black spruce in Minnesota (table 8) . 



Fully stocked stands on good sites will yield 42 cords per acre from 

 trees over 5.5 inches d. b. h., at 100 years, the age when mean annual 

 growth is at a maximum (table 8). The corresponding values for 

 medium sites are 35 cords at 140 years ; for poor sites 17 cords at 160 

 years. When yields of all trees over 3.5 inches d. b. h. are considered, 

 a somewhat different pattern of growth is shown, in which mean 

 annual growth culminates at an earlier age. However, the yields 

 based upon a minimum tree size of 5.5 inches d. b. h. are more realistic 

 in view of current cutting practices. 



The maximum yield shown in the table for trees over 5.5 inches 

 d. b. h. is 54 cords at 180 years. It is interesting to note that the largest 

 volume per acre of spruce pulpwood that has been recorded in Minne- 

 sota in a well-authenticated report is 63.5 cords. 18 A deviation from 

 the yield table of such magnitude for a single limited area is not at all 

 improbable. In fact, it tends to demonstrate that the average value 

 in the yield table is about right for the average of good sites. 



18 Area and volume measured by William Heritage, February 2, 1910. Township 

 158 North, Range 26 West. 



