32 
BULLETIN 55. U. S. DEPARTMENT OP AGRICULTURE. 
spruce, has no distinct heartwood and sapwood. Its narrow pith 
rays of a pale or whitish, color are scarcely risible. Air-dry wood of 
balsam fir is light, weighing 24 pounds per cubic foot, as compared 
with 28 pounds for spruce. When completely dry. it has an average 
density of 0.38, and loses about 4 per cent of its volume in seasoning. 
COiLPARATTVE LENGTH OF VTOOB FIBERS OF BALSAM FIR AST) SPRUCES. 
Table 7 gives the average, maximum, and minimum lengths of 
the wood fibers of balsam fir and the northeastern spruces. 
Table 7. — Average, maximum, and minimum lengths of fibers of balsam fir and the 
northeastern spruces. 
Speeies. 
Length of wood fiber (millimeters). 
Average. 
MaTimnm, 
Ifinmmm 
Balsam fix {Abie-? balsamea) 
2.518 
3.556 
3.233 
2.599 
4. 704 
4. 15S 
3. 738 
1.680 
2.520 
1.S90 
2. 142 
White spruce (Picea canadensis) 
Red spruce (Picea rubens) 
Black spruce (Picea mariana)^. 
GROWTH. 
Balsam fir is a fairly rapid growing tree, though not as rapid as 
tamarack and white pine. 
HEIGHT GROWTH. 
Balsam fir has a period of comparatively slow growth, which, tinder 
favorable light conditions, lasts only for the first five years of its life : 
a period of rapid growth then sets in and continues until the tree is 
60 years old. From then on the growth in height begins to decline, 
and at SO years the growth is practically at a standstill. At 150 
years it stops altogether. The most rapid growth in height takes 
place between the twentieth and fortieth years. 
The slow growth of balsam fir for the first five or six years is an 
inherent characteristic of the species, and occurs even under the best 
light conditions. Beneath the shade of other trees, however, the 
period of slow growth is often extended to 25 years or more because 
of the retarding effect of the shade. 
