,;@“1 United States

Department of

Agriculture

@ill I’%

(71 .u s

Forest Service Research Note

w Southern Forest SO-277

Experiment Station September 1981

Evaluation of Bole Straightness in Cottonwood

Using Visual Scores

D.T. Cooper and R.B. Ferguson

SUMMARY

Selection for straightness in natural stands of cottonwood can be effective in improving straightness of openpollinated progeny. Straightness appears to be highly

heritable, but it is subject to imprecise evaluation. This

can be largely overcome by repeated application of an

imprecise scoring system using a minimum of two views

per tree separated by 90 degrees.

Additional keywords: Clone, crook, heritability.

INTRODUCTION

Bole straightness is an important character which must

be considered in development of genetically improved

trees for sawlog and veneer production. However, it is

difficult to quantify. Use of subjective techniques is the

only practical way to evaluate large numbers of trees

(Shelbourne 1966). Done properly, the evaluation of

straightness correlates with actual yield of lumber (Miller

1975). In the present study, several scoring observations

per tree were made to examine the advantages of multiple observations per tree in a replicated cottonwood clonal test.

MATERIALS AND METHODS

Straightness was scored in a 7-year-old plantation in

which tree heights averaged 22 m and diameters at

breast height (dbh) averaged 20 cm. The study initially

consisted of 80 clones, 5 replications, and 4-tree linear

plots at 3 x 3 m spacing. A randomized-complete-block

design was used. Alternate row thinning at age 3 removed two trees per plot. The poorer of the two remaining

trees was removed at age 5. One clone was poorly

adapted and died. The remaining clones included a

select group of 37 clones, a random group of 40 clones

and two other clones which did not fit into either group and

were eliminated from consideration. The select group

averaged 1.5 cm greater diameter than did the random

group and consisted of from 1 to 8 cloned open-pollinated

progeny from each of 15 female parent trees. Parents had

been chosen from natural stands on the basis of phenotypit superiority in a number of characters, including

straightness. Progeny were chosen on the basis of fast

growth as seedlings and as clones in l-year tests. The

random group consisted of cloned young trees randomly

selected from six stands in the general vicinity of the

select parents.

A 1 to 9 straightness score was used with 1 being very

straight, 9 very crooked, and 5 near the average. Two

scorers, A and 6, having 3 and 8 years cottonwood research experience, respectively, examined several trees

and attempted to standardize their scoring procedure,

lumping crook, sweep and lean into a single score. Each

then scored the 385 trees 4 times, first from the west, then

from the east, north, and south, standing 9 m (3 planting

spaces) from the base of the tree and observing approximately the lower 10 m of the tree. Scores were called off

to assistants, enabling scorers to devote their full attention to the trees and remaining unaware of clonal identity.

The two scorers stayed far enough apart to avoid hearing

each other’s scores. Rest breaks were scheduled beSouthern Forest Experiment Station/T-10210 U.S. Postal Services Bldg., 701 Loyola Avenue, New Orleans, La. 70113

Forest Service, U.S. Department of Agriculture.

Serving Alabama, Arkansas, Louisiana, Mississippi, Eastern Oklahoma, Tennessee, Eastern Texas.

1
tween directions of scoring. Scoring each tree from each

of 4 sides by each scorer resulted in 8 observations per

tree.

Each of the 8 sets of observations was separated into

select and random groups. The resulting 16 sets of data

and various combinations of sets, consisting of the mean

of two or more observations per tree, were subjected to

analysis of variance. The limitations of analysis of

variance in dealing with subjective “score” data were

ignored. Analysis was according to the following form:

Source d f WS)

Replications r-l a; + ca:

Clones c-l IT; + ru:

Rep x Clone (r-l)(c-1) 2

UC3

The proportion of variation due to the clonal component

(4) was then computed. Simple correlations between

various observations were also computed.

RESULTS

Clone straightness scores computed over directions

and scorers averaged 3.9 for the select group and ranged

from 1.7 to 6.5. The random group was significantly more

crooked (.05 level), averaging 4.7 and ranging from 2.0 to

7.1.

Scorer B scored the select group 0.2 units more severely and the random group 0.6 units more severely than

did scorer A. The difference among scorers was statistically significant at the .05 level in both groups. Scorer B

also utilized approximately 1.5 units greater spread in

scoring. Both error and clonal components of variance for

B’s data were greater than for A’s data. The proportion of

total variation attributable to clones was greater for scorer

B than for scorer A, particularly in the select group (table

1). Scorer B’s correlations between observations from

different directions were consistently about 0.10 higher

than those for scorer A (table 2).

A single observation per tree would have been sufficient to detect significant differences among clones within groups. Multiple observations increased the proportion

of total variation attributable to clones. On the average,

clonal variance computed from a single set of scores

represented 37 percent of the total variation, while analy

sis of the mean of two scores per tree from directions

differing by 90 degrees increased this percentage to 53,

and averaging the scores from all 4 sides further increased the percentage to 58. Utilization of information

from 4 directions from a second scorer further increased

clonal variance to 63 percent of the total within-group

variance. Clonal variance as a proportion of total variance

is equivalent to broad sense heritability on an individual

tree basis over the entire area (5 replications) of the

experiment (~~/a~ + I$ + a:)).

Table 1 .-Proportion of variation in straightness attributable to clones

Direction

West

East

North

South

R

W+E

N+S

x

W+N

E+S

x

W+E+N+S

W+E+N+S

(Combined over observers)

A

.26

.25

.25

.36

ii

.30

.39

.34

.39

.41

.40

.45

Select

.58

B A

.43 .44

.35 .32

.35 .34

.40 .40

.36 .38

.44 .44

.44 .43

.44 .43

.55 .59

.55 .54

.55 .56

.60 .62

Random

.68

B

.47

.44

.42

.37

.43

50

.45

.47

.62

.56

ii3

.64

Cooper is Plant Geneticist, Ferguson is Associate Plant Geneticist at the Forestry Sciences Laboratory, maintained at Starkville, Miss., by the Southern

Forest Experiment Station, Forest Service-USDA.

2
Table 2.-Correlations between directions for straightness

Scorer

Direction A B

West with east .70 .82

North with south .65 .75

West with north .35 .46

East with south .38 .44

W + E with N + S .45 50

CONCLUSIONS

Selection in natural stands for straightness appears to

be effective, since the clones derived from openpollinated progeny of phenotypically superior female parent trees averaged nearly one full unit straighter than did

the random clones.

Although straightness is highly heritable in cottonwood, imprecise evaluation effectively reduces heritability. This can be overcome by making more than one

observation per tree. Apparent straightness depends on

the side of the tree viewed. Two 90 degree opposed views

substantially improve assessment of straightness. Additional observations make only minor contributions. Precision may differ sufficiently among scorers to justify comparison of samples of their work prior to launching a

large-scale effort to score straightness.

LITERATURE CITED

Miller, R.G.

1975. Visual assessment of stem straightness in

radiata pine. Aust. For. Res. 7(1):4!5-56.

Shelbourne, C.J.

1966. Studies on the inheritance and relationship of

bole straightness and compression wood in southern

pines. Ph.D. Thesis, N.C. State Univ., Raleigh, N.C.

235 p.

3

*U.S. GOVERNMENT PRINTING OFFICE: 1981-771-081/5