point in class 1 and for a 640-acre segment (1 sec- 
tion) lying northeast of each central point in classes 
2 and 3. The sum total of these segments con- 
stituted the sample. 
Each sample segment was examined on _ the 
ground. All forest areas of each sample segment 
were Classified and mapped by commercial character, 
forest type, stand size, stocking, age, and site classes, 
either on a vellum overlay or directly on the photo- 
graphs. In the office the area of the various cover 
classes within the samples was determined by dot 
or square counting, summarized by sample cate- 
gories, and multiplied by the appropriate factor. 
Volume 
For the area covered by complete survey, board- 
foot estimates for saw-timber trees in saw-timber 
stands were derived from: (1) national-forest 
cruises, (2) other public and private cruises ad- 
justed to a common standard by factors based on 
check cruises, and (3) ocular estimates made by 
field men while mapping cover types. Cubic-volume 
estimates of saw-timber trees in saw-timber stands 
were calculated from board-foot volumes by the 
use of ratios based on wood-scale studies. 
The volume of pole timber in saw-timber stands 
was derived from mean-acre volumes by types, based 
on check-cruise tallies. The board- and cubic-foot 
volumes in stands other than saw timber were cal- 
culated from modified normal yield tables. Yield- 
table modifications were based on 41] 1/4-acre 
samples measured throughout these stands at 2-mile 
intervals along roads and trails. All board-foot 
volumes, initially calculated in terms of Scribner 
rule, were converted to International 1/4-inch rule 
by means of factors derived from mill-scale studies. 
For the sampled area, total volume was derived 
from mean-acre volumes and area by sample class, 
forest type, stand-size, and stocking classes. Mean 
volumes were based on tree tallies on 2,713 1/5-acre 
permanent sample plots—a cluster of 2 or 3 plots 
in each sample-area segment containing commer- 
cial forest land. Three-plot clusters were measured 
in sample class 1 and two-plot clusters in sample 
classes 2 and 3 (see Area discussion). Within a 
cluster, plots were spaced 5 chains apart, the initial 
plot being 2 chains in a random direction from 
the control point of that sample-area segment. All 
plot centers were staked and referenced to 2 scribed 
Forest Resources of Montana 
witness trees to facilitate future location and re- 
measurement. 
Growth 
For the area covered by complete survey, only 
net growth was determined. Stands were divided 
into two categories: (1) “Nongrowing stands” in 
which increment was assumed to be offset by mor- 
tality and for which no growth was calculated. 
This category consisted of all virgin stands of 
ponderosa pine, larch, and Douglas-fir older than 
200 years, and virgin stands of all other species 
older than 160 years. (2) “Growing stands’ in 
which experience indicated that increment gen- 
erally exceeded mortality. ‘This category included 
uncut stands of ponderosa pine, larch, and Doug- 
las-fir up to and including 200 years, uncut stands of 
other species up to and including 160 years, and 
partially cut stands of all types regardless of age. 
Average net growth per acre by type, stand size, 
age, stocking, and site classes was derived from 
modified normal growth tables. Modification of 
normal tables was based on actual net growth as 
determined from increment borings and mortality 
tallies on 375 1/5-acre sample plots covering the 
range of age, stocking, and site classes within types. 
Gross and net growth for the area sampled were 
derived from increment borings of sample trees 
and mortality tallies on each sample plot measured 
for inventory purposes (see discussion of Mortal- 
ity). Size of the sample measured for growth varied 
from 1/500 acre for trees 5.0 to 10.9 inches d.b.h., 
1/50 acre for trees 11.0 to 24.9 inches d.b.h., 
to 1/5 acre for trees 25.0 inches d.b.h. and larger. 
Radial growth was determined for the 10-year 
period preceding the date of field examination. 
Radial growth was converted to volume growth by 
subtracting the reconstructed volume of sample 
trees 10 years ago, taking into account changes in 
height and bark thickness as well as diameter, from 
current sample-tree volume. 
Sample trees were divided into two classes: (1) 
ingrowth trees, those that either developed from 
sapling into pole size or grew out of pole size into 
saw-timber size, and (2) main-growth trees, those 
that remained either of pole size or of saw-timber 
size during the growth-study decade. ‘Total in- 
growth was derived by multiplying the mean in- 
growth per acre by the appropriate area by type, 
stand-size, and stocking class. Total main growth 
57 
