SILVICULTURAL MANAGEMENT OF BLACK SPRUCE IX MINNESOTA 43 



tality during the 5 years subsequent to logging amounted to 32 trees 

 per acre or only 6 percent of the trees over 2.5 inches d. b. h. in the 

 residual stand. In areas where all of the merchantable trees had been 

 cut ( over 75 percent of the basal area) the losses were 56 trees per acre 

 or 54 percent of the residual stand. 



Uprooting, which accounted for 24 percent of the mortality, in- 

 creased with severity of cutting but in nearly all instances was of 

 secondary importance (figure 25) . Breaking accounted for 12 percent 

 of the mortality. It was not appreciably influenced by the intensity of 

 cutting. The remainder of the mortality. 64 percent, was among trees 

 that had died standing. Such mortality increased with the degree 

 of cutting. 



40 60 



BASAL AREA CUT (PERCENT) 



Figure 25. — Losses in trees per acre in 5 years in relation to severity of cutting. 



Uprooting, of course, is caused almost wholly by wind pressure but 

 breaking can be caused by wet snow, glaze, and rot as well. Thus, 

 windthrow was responsible for something less than one-third of the 

 mortality. It can be concluded that windthrow has been overrated as 

 a cause of loss subsequent to logging in swamp spruce. 



Black spruce has shallow roots and peat does not provide very solid 

 footing, but the resilient quality of the peat apparently allows the 

 roots considerable motion without actually tearing them loose. 

 Occasional storms of exceptional violence uproot many spruce trees 

 but this occurs in uncut stands as well as thinned stands. Further 

 evidence of the relatively high windiirmness of swamp spruce was 

 supplied by the July 1932 windstorm in northeastern Minnesota. 

 Upland black spruce sustained approximately twice as great losses 

 as black spruce growing in swamps. 25 



The causes of death among the trees classified as standing dead could 

 not be completely diagnosed. In the most lightly cut areas they ap- 



25 Eyre, F. H. when a torxado strikes a forest, the superior blowdotvx of 



1932. U. S. Forest Serv.. Lake States Forest Expt. Sta. Original manuscript. 



1933. [An article bearing the same title appeared in the Minn. Conservationist. 

 July 1933, but essential information was omitted.] 



