28 



CIRCULAR 



1. U. S. DEPARTMENT OF AGRICULTURE 



F-372659 



Figure 19. — A typical 

 mature swamp-grown 

 black spruce. Height 

 65 feet, d. 1). h. 10 

 inches, age about 120 

 years. 



In well-stocked stands on average sites some 

 trees reach 5 inches d. b. h.. the minimum mer- 

 chantable size, in about 50 years. Thereafter, 

 diameter growth continues at a fairly steady 

 rate for 60 to 80 years or more on the trees that 

 are not seriously crowded or suppressed by their 

 neighbors (19) . Crowded trees continually suc- 

 cumb to competition, and such trees, of course, 

 have diminishing rates of growth. 



A peculiarity of swamp black spruce is that 

 ring counts made at ground level oftentimes do 

 not reveal total age (22). This is due to the 

 continual raising of the peat surface by accumu- 

 lation of litter, which results in development of 

 adventitious roots on the stem above the original 

 root crown. Sometimes several successive strata 

 of roots can be detected. The number of annual 

 rings that are "buried" in this manner is vari- 

 able, but may amount to 20 years on the slower 

 growing trees. It accounts for some of the 

 seeming variation in ages in stands that are 

 actually even-aged. 



Growth in Relation to Site Quality 



Classifications of site quality based upon age- 

 height ratios for black spruce stands have been 

 prepared by several workers. The earliest of 

 these site classifications, constructed by Averell 

 and McGrew (2) , dealt only with swamp forests. 

 As a consequence, some of the very best sites — 

 swamp border and upland — were not taken into 

 account and many sites too poor to produce mer- 

 chantable stands were included (fig. 20). 

 Nevertheless, the trends conform with those 

 found by Fox and Kruse (12) for merchantable 

 black spruce stands in northeastern Minnesota. 



Bowman (4) pooled data for balsam fir, white 

 spruce, and black spruce. Hence, the growth 

 trends do not necessarily reflect accurately the 

 behavior of black spruce. The somewhat faster 

 diminution in rate of growth after 60 to SO years 

 suggests that balsam fir. a shorter-lived and 

 more quickly maturing species, had considerable 

 influence on the form of the curves. 



In Millars 17 classification, which was devel- 

 oped in Ontario, the plots were segregated into 

 Type I. "Spruce Hardwood Association" : Type 

 II. "Spruce Flats and Swamp Association'': 

 and Type III. "Spruce Muskeg Association." 

 The upper line plotted in Millar's curves (fig. 

 20) is his Type I. the lower line Type III. Type 

 II. not shown in figure 20, is only slightly lower 



17 See footnote 1.*?. p. 15. 



