SILVICULTURAL MANAGEMENT OF BLACK SPRUCE IN MINNESOTA 31 



The ages given above for the culmination of mean annual growth, 

 100 years, 140 years, and 160 years respectively, for good, medium, and 

 poor sites should not be regarded as rotation ages at which stands 

 necessarily should be clear cut and regenerated. There are several 

 reasons why recommendations for fixed rotations should be guarded 

 against: (1) the possibilities of making partial cuttings leading to 

 the selection system of silvicultural management; (2) the varying 

 conditions of individual stands: and (3) standards followed in the 

 utilization of products. These considerations are discussed more fully 

 later. The chief significance of the mean annual growth figures shown 

 in the yield table is the evidence they supply as to the growth poten- 

 tialities of undisturbed even-aged natural stands. They show that 

 such stands can produce from 0.11 to 0.42 cord per acre per year de- 

 pending upon site quality. 



Experience has shown that comparatively few stands cut out vol- 

 umes as large as those given in the yield table for ages greater than 

 80 to 100 years. As stated previously, the actual average volume of 

 cordwoocl stands in one of the better districts is only 9.7 cords per acre. 

 These low actual yields reflect many factors including patchy distri- 

 bution of the trees, and understocking resulting from poor initial 

 reproduction or more recent injury by wind. snow, glaze, insects, and 

 diseases. 



Yields of Understocked and Uneven-Aged Stands 



Black spruce, particularly in swamps, has a decided tendency to 

 become uneven-aged (fig. 21). The records for the stands on which 

 table 8 is based included many that had younger trees in the under- 

 story, although the overstories were even-aged (12). Hence, the 

 growth predictions given are generally applicable to any fully stocked 

 stand that has an even-aged overstory. 



When the stand is understocked or the dominant trees vary con- 

 siderably in age. future growth can be only roughly estimated. Un- 

 derstocked stands, of course, tend to increase in density. Bowman ( 4 ) 

 has reported that the average rates of increase in density for uni- 

 formly distributed understocked stands of black spruce, white spruce, 

 and balsam fir are about as follows : 



Increase per year 

 in density of 

 understocked 

 Site index 1 at 50 years (height in feet) : stands {percent) 



39 0.9 



33 . 7 



26 .5 



1 These site indices were chosen to correspond with good, medium, and poor 

 sites — records shown in table 8. 



For example, a stand on medium site only 50 percent stocked (in terms 

 of total basal area) at present probably will be 57 percent stocked 10 

 years hence (50 percent plus 0.7 percent times 10 years equals 57 

 percent) . For stands where distribution of trees is patchy, the annual 

 rates of increase are cut in half — 0.45 percent, 0.35 percent, and 0.25 

 percent for good medium, and poor sites, respectively. 



