No. 89 

1959 

SELECTION OF STAND VARIABLES IN SOUTHERN MAINE 

FOR MAKING VOLUME ESTIMATES FROM AERIAL PHOTOS 

Aerial photographs are used widely in forest inventories. But there is a continuing need for improving the 

techniques of photo interpretation and making more efficient 

use of photographs. When the number or intensity of sample 

ground plots is controlled by airphoto classifications, a 

reliable stratification of the timber area is a must. 

Three measurements are commonly used on photos of 

forest inventory plots--total tree height, tree-crown diameter, and crown-closure percent (stand density) . In the 

Forest Survey of the Northeast, these stand variables are 

used to classify photo plots according to gross volume per 

acre. Field measurements are then made on sample plots from 

each volume stratao1 The number of field plots required 

depends in part on the accuracy of the photo-volume stratif icat ion. 

Little basic data are available in the Northeast to 

indicate which stand variables are most closely correlated 

with timber volume. So, a study was designed to determine 

the value of tree height, crown diameter, and crown-closure 

measurements for predicting gross cubic volume per acre. Onthe-ground measurements of these variables were obtained on 

28 one-fifth acre plots in Hancock County, Maine. Gross 

plot volumes included hardwood and softwood species, 5 inches d.b.h. and larger. 

Correlation coefficients were computed and statistically tested to determine the usefulness of each variable. 

As indicated by table 1, the highest single correlation with 

l~ickford, C. Allen. Increasing the efficiency of airphoto forest survey by 

better definition of classes. Northeast., Forest Expt. Sta., Sta. Paper 58. 9 

pp. 1953. 

1 
gross volume (+0.6827) was shown by crown-closure percent. 

The addition of average crown diameter and tree height produced a higher correlation coefficient (+0.7521), But tests 

showed that this increase was not significant at the 5-percent level. It was concluded that reasonable predictions of 

gross cubic volume could be made in stands similar to those 

sampled by measuring crown closure alone. 

Gross cubic volumes were then plotted over corresponding measurements of crown-closure percent. A linear regression,of the form Y = a + bx, was fitted to the data by 

the method of least squares. The resulting prediction equation was: Y = 45.0 + 4.356X, where Y = gross cubic-foot volume of all trees 5 inches d.b.h. and larger on a 1/5-acre 

plot, and X = crown-closure percent of all trees in the 

stand at least 20 feet tall. 

From this relationship, an airphoto stand-volume 

table was constructed for spruce-fir-hardwood forests in 

southeastern Maine (table 2). This table was compiled for 

crown-closure classes of 40 to 100 percent, the approximate 

range encountered on the field plots. 

Table 1.--Correlations of stand variables with gross cubic volumes1 

Ground measurements of Gross or multiple 

independent variables correlation coefficients 

H (Average height, 3 tallest trees) + 0.2078 3 

H (Average height, 5 tallest trees) .2494 

H10 (Average height, 10 tallest trees) .4375* 

CD (Average crown dia., 3 tallest trees) .2975 

CD (Average crown dia., 5 tallest trees) .2 862 

CDIO (Average crown dia., 10 tallest trees) .I723 

Cqo (Crown closure percent) .6827** 

H5 x CD5 .3464 

CD, x Cc% .7187** 

Cc% x H5 .7317** 

H5 X CD5 X Cqo .7521** 

H10 x CDIO x Ca .7417** 

l~ased on 28 one-fifth-acre plots in Hancock County, Maine. * 

Significant at the 5-percent level. ** 

Significant at the 1-percent level. 
Table 2.--Airphoto stand-volume table for 

spruce-f ir-hardwood f orestsJ 

Percent 

40 

50 

60 

7 0 

80 

90 

100 

Crown 

closure 

Gro s s 

cubic feet 

219 

2 63 

306 

3 50 

3 93 

43 7 

481 

Gross 

cubic feet 

153 - 286 

211 - 315 

265 - 348 

312 - 388 

350 - 437 

382 - 492 

411 - 550 

Volume per 

1/5 acre 

'~ased on ground measurements from 28 one-f if th-acre 

plots in Hancock County, Maine. 4 plots predoniinately conifers (spruce, fir, hemlock); 12 plots predominateiy hardwoods (birch, aspen, red maple); 12 plots mixed hardwoods 

and conifers. Correlation coefficient: +0.6827. 

95-percent 

confidence band 

It was assumed that 10-percent classes represented 

the most refined estimates of crown closure that could be 

consistently made with aerial photographs. On this basis, 

it seemed unrealistic to use photo-volume stratifications 

with ranges smaller than those given in table 2. If desired, 

volume entries and corresponding confidence bands shown may 

be converted to a per-acre basis by multiplying by 5. 

--EARL J. ROGERS, GENE AVERY, and ROY A. CHAPMAN 

Division of Forest Economics Research 

Forest Service, U. S. Dept. Agriculture 

AGRICULTURE-FOREST SERVICE-UPPER DARBY