56 BULLETIN 544, U. S. DEPARTMENT OF AGRICULTURE. 
while for 3 and 2 thinnings 5 years will be saved in bringing the stand 
to maturity. The severity of each thinning is gauged so as to leave 
approximately the same number of trees as are shown by the yield 
table to be capable of growing under naturally competitive condi- 
tions at the end of the thinning period. Thus, in the case of the 
series of 4 thinnings, the- first one in the twenty-fifth, year reduces 
the number of trees from 1.316 to 1.097, which not quite approxi- 
mates the condition which would be brought about by natural 
selection in the succeeding 5 years. The normal number of trees 
in unthinned stands (column 2) does not enter into the calculation 
but is included only for comparison to show how far each thinning 
will eliminate competition during the period. 
All first thinnings, according to the table, show no remunerative 
yield. They are accordingly made light to reduce as far as possible 
the expense involved in making them. The maintaining of a reason- 
able density is also a consideration in making the thinning light, 
thus improving the form and quality of the final yield. With four 
thinnings the number of trees is not sufficiently reduced in any one 
thinning so that a normal density will not be restored within 5 years. 
With the 3 thinnings the number of trees is reduced in each case 
somewhat below the normal for a fully stocked stand of 5 years 
greater age. For example, the first thinning at 25 years reduces the 
number of trees to 1,054, while the normal density for 30-year stands 
is 1,062. With 2 thinnings the cut is heavier still. In each case, 
however, there is little doubt that the crown cover will be reestablish d 
in 5 years. 
In calculating the volume of each thinning the volume of the aver- 
age tree in the yield table has been taken as a basis. The assump- 
tion introduces a plus error, since doubtless the average of the trees 
taken out will not in all cases be the same as the average of the 
stand. Yet any error which may arise from this cause will be more 
than offset by the fact that in addition to the indicated number of 
dominant and intermediate trees taken an unestimated amount will 
be yielded by suppressed and dead trees taken at the same time. The 
final yield also is based exclusively on that to be obtained from dom- 
inant and intermediate trees, while the suppressed and dead will 
doubtless yield several cords additional at the final cutting. 
In the absence of graded mill tallies for second-growth spruce it 
is not possible to determine the rotation yielding the highest qualitv 
increment. Table 20, however, attempts to show the intermediate 
and final yield in board feet for spruce which has started under a 
light hardwood cover and been managed under the wide strip system 
outlined elsewhere. 
