Fibril angle in young-growth ponderosa pine as related to site index, d.b.h., and location in tree

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Southwest cro } Research Note RM-492

September 1989

USDA Forest Service

Rocky Mountain Forest and
Range Experiment Station

Fibril Angle in Young-Growth Ponderosa Pine as Related
to Site Index, D.B.H., and Location in Tree

Craig E. Shuler, Donald D. Markstrom, and Michael G. Ryan‘

Young-growth ponderosa pine on two different sites in northern New
Mexico were evaluated for differences in fibril angle. Measurements were
made radially at the butt end, mid-length, and top end of the merchant-
able stem divided into multiple 8-foot log lengths. Results indicated that
the butt ends have larger fibril angles than the other two locations. Fibril
angle did not vary significantly with either site index from 55 to 100 or

d.b.h. from 9 to 14 inches.

Keywords: Ponderosa pine, fibril angle, young growth, site index

Management Implications

Loss of lumber grade from excessive warp usually
results from abnormal and/or asymmetrical shrinkage
when lumber dries. In conifers, fibril angle, juvenile
wood, compression wood, grain orientation, and knots
have been associated with excessive lumber warpage.
Longitudinal shrinkage typically ranges from 0.1% to
0.3% when wood is dried from green to ovendry condi-
tions. Longitudinal shrinkage greater than 0.3% from
green to overdry conditions for straight grain material is
considered abnormal and is related to deviation of the
microfibrils (fibril angle) from the longitudinal axis of
the tracheid. Study results indicated that the fibril angle
of young-growth ponderosa pine did not significantly
vary with site index from 55 to 100 or d.b.h. from 9 to
14 inches.

Introduction

Fibril angle of the S, layer of woody cells has been
shown to affect some properties of wood material (McMil-
lin 1973, Meylan 1972, Panshin and de Zeeuw 1980, Piirto

‘Authors are, respectively, Associate Professor, Colorado State Univer-
sity, Fort Collins; Research Wood Technologist and Biometrician, Rocky
Mountain, Forest and Range Experiment Station. Headquarters is in Fort
Collins, in cooperation with Colorado State University.

et al. 1974, Voorhies and Blake 1981, Voorhies and
Groman 1982). Variations in shrinkage and mechanical
behavior have been the properties of primary interest.
The researchers cited generally agreed on two aspects
of fibril angle: (1) large fibril angles (as measured from
the longitudinal axis) in softwood tracheids and hard-
wood fibers result in reduced mechanical properties
parallel to the grain and in increased longitudinal
shrinkage; (2) fibril angle is inversely related both to
distance from the pith and to cell length, with regions
of a tree undergoing rapid growth (i.e., earlywood,
juvenile wood, reaction wood) generally having larger
fibril angles than regions growing more slowly. The
increased longitudinal shrinkage associated with larger
fibril angles often results in warp, specifically crook and
bow, when there is a marked asymmetric distribution
of fibril angles throughout a piece of lumber.

This fibril angle study was part of a larger project in-
vestigating the development of warp during drying in
young-growth ponderosa pine from the southwestern
United States (Markstrom et al. 1984). Significant results
of that study were that lumber grade recovery and warp
were related primarily to tree and log diameters, and the
factor of height location in the tree had no effect on the
amount of warp that developed. The objective of this
research was to develop an equation that would predict
fibril angle in young-growth ponderosa pine (Pinus

ponderosa Dougl. ex Laws) using the variables of site
index, diameter at breast height (d.b.h.), relative height
location in the tree, and distance from the pith.

Methods

Forty-one trees ranging from 9 to 14 inches d.b.h. were
cut from an area with a site index of 55. Another 41 trees
with the same diameter range were cut from an area with
a site index of 100. Both sites were located in the Santa
Fe National Forest in northern New Mexico. Studs cut
from these trees were used for the earlier drying study
(Markstrom et al. 1984).

Sixty of the trees (5 for each of the 6 d.b.h. classes on
each site) were sampled for fibril angle. When the trees
were felled, a 1/2-inch-diameter increment core was taken
at the merchantable stem mid-length; and approximately
2-inch-thick disks were cut from the butt of the lower log
and from the top of the upper log. Sections were cut from
these disks and cores for determining the fibril angles.

Fibril angle was measured as follows:

1. From each disk or core, a true radial surface was
exposed from the pith to the cambial zone.

2. Sections were marked on this radial surface at 5-
year intervals from the pith to the cambial zone. For the
larger butt sections, the intervals were marked every 10
years from the thirtieth ring to the cambial zone.

3. At each marked ring, thin sections were cut by hand
using a razor blade. These sections were placed in water
on a microscope slide, covered, and observed on the
screen of a Reichert Veripan? microscope at a magnifi-
cation of 130.

4. The fibril angle was considered to be defined by the
angle formed between the longitudinal axis of the tra-
cheids and the aperature of the half-bordered pit pair in
the crossfields of the ray parenchyma and the longitudi-
nal tracheids (Shottafer et al. 1972). Angles were meas-
ured to the nearest degree using a protractor directly on
the microscope screen. Measurements were made on
earlywood tracheid crossfields.

For each thin section examined, three fibril angles were
measured. These three measurements were then averaged
to obtain the fibril angle for that location in the sample.

Results and Discussion

The relationship between the fibril angle and rings
from the pith is generally accepted to be exponential. The
data in this study, however, showed a high degree of varia-
bility (fig.1) which, from a purely statistical point of view,
indicated a linear relationship. This variability may have

The use of trade and company names is for the benefit of the readers;
such use does not constitute an official endorsement or approval of any
service or product by the U.S. Department of Agriculture to the exclusion
of others that may be suitable.

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been the result of the less well-defined characteristics
viewed in the hand sections, taking measurements on
earlywood tracheids, and/or the method of angular meas-
urement used in this study. Consequently, both a linear
and exponential model were evaluated. In either case,
only (1) rings from the pith, (2) height location in the
tree, and (3) the d.b.h. x rings-from-pith interaction were
found to be statistically significant at the 0.05 level. The

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Figure 1.—Data plots of fibril angle versus rings from pith for the
three heights; (a, butt; b, mid-stem; c, top) with linear regres-
sion curves.

contribution of interaction (3) to the correlation (R2)
value, however, was not considered sufficient to war-
rant its inclusion in the final models shown below.

FA

51.6 - 0.462G + 15.3X -1.19Y [1]
FA = e (XPH) (3.978 - 0.0114G + 0.281X
— 0.0358Y)

where FA = fibril angle (degrees);
G =rings from pith; and

X and Y =dummy variables representing loca-
tion in tree coded as:

[2]

XxX Y
Butt 1 0
Mid-stem 0 1
Top 0 0

Because the heights of the mid-stem and top varied with
the heights of the trees sampled, the data were evalu-
ated also according to actual height in the tree. Plots of
the linear curves obtained showed a distinct separation
of the butt data and two other groupings, one consist-
ing of the 17-, 33-, and 49-foot heights and the other,
the 9-, 13-, 21-, 25-, 41-, and 57-foot heights. Although
not totally consistent, these latter two groupings tended
to follow the mid-stem and top categories, respectively.
Thus, we retained the original categories.

For equation [2] the data were fit to the model using
Marquardt’s nonlinear least squares algorithm (Mar-

quardt 1963). R? for both models was 0.590, with a stand-
ard error of estimate of 9.4 degrees.

While the exponential model did not improve the fit
of the data, it still has features indicating it is more suita-
ble for describing the relationship. Figure 2 shows the
regression curves obtained from equation [2]. Two fea-
tures should be noted. First, the slopes of the curves are
more shallow through the juvenile zone than past studies
would lead us to expect. Second, the slopes of the curves
in the mature region (e.g., greater than 50 years of age)
are not as flat as might be expected. Whether these con-
ditions are the result of the regression process or truly
representative of the young-growth ponderosa pine is not
clear. The continuing decrease in fibril angle at the
greater ages is similar, however, to that shown by Voorhies
and Groman (1982). A linear relationship, on the other
hand, would imply a continued constant decrease in
fibril angle as the tree ages.

Although height location in tree was a statistically sig-
nificant factor for all three locations evaluated, it appears
that much of that significance is the result of the
appreciably larger fibril angles in the butt material. The
steeper negative slope seen in the curve for the butt
material as compared with the other two locations is
important. This could have an effect on the amount of
warp that develops in lumber.

Although our earlier study (Markstrom et at. 1984)
indicated it was not possible to correlate the fibril angle
with warp development, the possible relationship is of
enough importance to merit some discussion. Crook and
bow are warp primarily caused by differences in longitu-
dinal shrinkage on opposite sides of a piece of lumber.
A very steep negative slope in the fibril angle curve indi-
cates a relatively large difference in fibril angle would
occur on opposite sides of a piece of wood cut from that
location. This would likely result in differential longitu-
dinal shrinkage. Thus, in the cases where more crook has
been reported in lumber cut from butt logs (Blake and
Voorhies 1980, Maeglin and Boone 1983), or where the
amount of warp is related to smaller log diameters (Mark-
strom et al. 1984), a contributing factor may be the fibril

80

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Fibril angle (degrees)
.s
°

Butt
Top
Mid-stem

nm
oO

0 20 40 60 80 100
Rings from pith

Figure 2.—Regression curves for fibril angle versus rings from pith
for three X locations in ponderosa pine.

angle differential present in the juvenile wood. It must
be emphasized, however, that the impact of fibril angle
is most likely quite small. Other gross features (i.e., grain
orientation, knots, compression wood) are expected to
have more influence on the behavior of full-sized wood
members.

Conclusions

1. The fibril angle in young-growth southwestern pon-
derosa pine is related to rings from the pith (i.e., age and
diameter) and height location in the tree.

2. Differences in fibril angle related to height location
in the tree are heavily influenced by the greater fibril
angles in the butt material.

3. Site index and diameter breast height are not sig-
nificantly related to fibril angle.

Literature Cited

Blake, Bradford R.; Voorhies, Glenn. 1980. Kiln drying
of young-growth ponderosa pine studs. Ariz. For. Notes
No. 13. Flagstaff, AZ: Northern Arizona University,
School of Forestry. 14 p.

Maeglin, R. R.; Boone, R. S. 1983. An evaluation of saw-
dry-rip (SDR) for the manufacture of studs from small
ponderosa pine logs. Res. Pap. FPL-435. Madison, WI:
U.S. Department of Agriculture, Forest Service, Forest
Products Laboratory. 7 p.

Markstrom, Donald C.; Shuler, Craig E.; King, Rudy M.
1984. Warpage of studs from young-growth ponderosa

pine from New Mexico. Res. Pap. RM-257. Fort Collins,
CO: U.S. Department of Agriculture, Forest Service,
Rocky Mountain Forest and Range Experiment Station.
13 p.

Marquardt, A. W. 1963. An algorithm for least squares
estimation of non-linear parameters. Society for Indus-
trial and Applied Mathematics Journal. 2: 431-441.

McMillin, Charles W. 1973. Fibril angle of loblolly pine
wood as related to specific gravity, growth rate, and dis-
tance from pith. Wood Science and Technology. 7:
251-255.

Meylan, B. A. 1972. The influence of a microfibril angle
on the longitudinal shrinkage-moisture content rela-
tionship. Wood Science and Technology. 6: 293-301.

Panshin, Alexis J.; de Zeeuw, Carl. 1980. Textbook of
wood technology. 4th ed. New York, NY: McGraw Hill
Book Co.: 251-266.

Piirto, D. D.; Crews, D. L.; Troxell, H. E. 1974. The effects
of dwarf mistletoe on the wood properties of lodgepole
pine. Wood and Fiber Science. 6(1): 26-35.

Shottafer, J. E.; Kutscha, N. P.; Hale, R. A. 1972. Proper-
ties of plantation grown red pine related to its utiliza-
tion. Tech. Bull. 61. Orono, ME: University of Maine,
Life Science and Agricultural Experiment Station. 72 p.

Voorhies, Glenn; Blake, Bradford R. 1981. Properties
affecting drying characteristics of young-growth pon-
derosa pine. Ariz For. Notes No. 14. Flagstaff, AZ:
Northern Arizona University, School of Forestry. 27 p.

Voorhies, Glenn; Groman, W. A. 1982. Longitudinal
shrinkage and occurrence of various fibril angles in
juvenile wood of young-growth ponderosa pine. Ariz.
For. Notes No. 16. Flagstaff, AZ: Northern Arizona
University, School of Forestry. 18 p.