SILVICULTURAL MANAGEMENT OF BLACK SPRUCE IN MINNESOTA 33 



Gevorkiantz and Duerr have suggested another method for esti- 

 mating the approach toward normality of understocked stands. 19 It 

 calls for the following procedure : ( 1 ) Determine the percent stocking 

 of the area; (2) ascertain from the tabulation below what percentage 

 of normal growth the stand can be expected to grow during the subse- 

 quent 10-year period; (3) find in the yield table (table 8) the 10-year 

 growth of a normal or fully stock stand of the same age; and (4) 

 apply the percentage found in following step 2. 



10-year growth 

 ~ . „ , , . ,. compared with 



Density of Stand (percent) : normal {percent) 



10 19 



20 36 



30 51 



40 64 



50 75 



60 84 



70 91 



80 96 



90 99 



100 100 



In estimating future growth in stands that do not have a reasonably 

 even-aged overstory, the best solution for practical purposes is to 

 assume annual rates of growth the same as those for even-aged stands 

 on comparable sites. Of course, appropriate allowances should be 

 made for degree of understocking, and the presence of cull trees, 

 diseased trees, and other discernable conditions that have the effect of 

 materially reducing the area available for production of merchantable 

 wood. 



Longevity 



Most black spruce trees, even when not crowded by larger neighbors, 

 succumb to various destructive agents long before they attain their 

 maximum possible age of 250 years or more. One of the principal 

 causes of the loss of otherwise vigorous trees is butt rot. Such rot 

 causes little loss in volume because it seldom extends upward more 

 than 2 or 3 feet but by weakening the bases of the trees it makes them 

 susceptible to breakage by the wind (19). In a study made in a medium 

 site swamp in Koochiching County the following relationship between 

 age and presence of advanced stages of decay was found : 



Trees with ad- 

 vanced decay at 

 base of stem 

 Age class (years) : (percent) 



46-65 



66-85 



86-105 34 



106-125 41 



126-145 47 



146 and over 71 



These data do not tell the whole story because undoubtedly many of 

 the older trees already had been broken off. The decay appeared to 

 begin weakening the trees appreciably at about 120 years. 



Trees beyond the age of 100 or 120 years in swamp stands usually 

 are noticeably less thrifty in appearance than younger trees. The 



19 Gevorkiantz, S. K., and Duerr, W. A. Methods of predicting growth of 

 forest stands. U. S. Forest Serv., Lake States Forest Expt. Sta. Econ. Note 9, 

 59 pp. 1938. [Processed.] 



