Mar. 15,1925 Mortality of Trees Attacked by the Spruce Budworm 547 
From the data collected and from 
general observations made, certain con¬ 
clusions are drawn concerning bud- 
worm mortality and silvicultural char¬ 
acteristics of balsam fir, white spruce, 
and red spruce. These, since they are 
based on limited areas, are presented as 
tentative and with the object of in¬ 
viting criticism and further investiga¬ 
tions from the foresters and entomolo¬ 
gists familiar with the budworm in¬ 
fested regions. It is fully realized that 
with so many factors to consider it is 
practically impossible to make com¬ 
parisons of conditions alike except for 
one factor and that the results brought 
out in this paper are largely sugges¬ 
tive rather than conclusive. 
There is a certain correlation be¬ 
tween vigor of the stands (as expressed 
sistance of E. N. Munns, but no cor¬ 
relation was found. Percentage of bal¬ 
sam in the mixture was likewise con¬ 
sidered, but this bore no relation to the 
mortality of the spruce. 
The correlation between rate of 
growth and mortality for balsam is not 
at all regular for the Bathurst series 
(figs. 1 and 2, Tables IV to IX), owing 
to the severe infestation in that region, 
resulting in complete defoliation of 
many trees. From 75 to 100 per cent 
defoliation will kill even very vigorous 
trees. The correlation for the white 
and red spruces is much more regular. 
These trees are more uniformly de¬ 
foliated, only the new growth being 
eaten for three to four years; that for 
combined red and white spruce is still 
more regular, the explanation being 
Worfa/ZZy /?er cenf Z^7or/&//Zjz—/ :> €>r ce*r?Z 
Fig. 1.—Diagram illustrating rate of growth of 
spruce and mortality resulting from spruce bud¬ 
worm defoliation as recorded in Tables V, VII, 
and IX 
Fig. 2.—Diagram illustrating rate of growth of fir 
and mortality resulting from spruce budworm 
defoliation as recorded in Table XI 
by increment at breastheight) at the 
time of defoliation and budworm mor¬ 
tality such that it can be said the more 
rapid the rate of growth the lower is 
the mortality resulting from defoliation. 
This expression of the probable effects 
of budworm feeding is a more tangible 
quantity than any of the other factors 
heretofore considered, and should serve 
as a practical basis for preventive 
measures through proper silvicultural 
practices which maintain rapid growth. 
There may be different minimum 
rates of growth, entailing immunity for 
different sites, types, and age classes, 
indicating the necessity of further in¬ 
vestigation in conditions other than 
those discussed here. 
All the data bearing on density and 
basal area were plotted with the as¬ 
doubtful unless it is because of the 
greater number of trees averaged. On 
the Metis area (Tables X and XI, 
fig. 2) the correlation between rate of 
growth and mortality for balsam is 
quite regular; that for white spruce 
shows very little correlation. Here the 
dead spruce trees are all small sup¬ 
pressed trees of the understory (see 
Table XVII, mortality by diameter 
classes) as compared with the Bathurst 
spruce and Metis balsam. In fact, so 
few spruces are dead that the entire 
mortality in these trees is considered to 
be that of a normal forest and gives a 
series of plots for contrast to conditions 
at Bathurst. 
The diameter class tallies by species 
as well as by observation on the stumps 
of many felled trees indicate that 
