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TIMBER QUALITY AND PRUNING: AN
ANALYSIS OF MANAGEMENT REGIMES
FOR THE SIUSLAW NATIONAL FOREST
Roger D. Fight
Thomas D. Fahey
Stuart Johnston
for regimes of about 150 trees per acre. All regimes assume
a precommercial thinning at age 11 to achieve good spacing.
These regimes were analyzed with and without prunin~.
The pruning regimes involved pruning 70 trees per acre
to a height of 18 feet in one lift. The age of pruning varied
from 17 to 22 depending on height and crown length. In
each case the pruning age was the earliest age at which the
pruning could be done without removing more than onethird of the live crown. Although regimes of pruning involving more than one lift may be preferable, the existing
recovery data are directly applicable only to regimes of one
lift. Predictions of limb sizes were made using Maguire's
(in press) limb size prediction equation.
Initial analysis of commercially thinning stands with initial stocking of 250 trees per acre showed that they produced
stands much like those with lower initial stocking and the
present net worth was lower. The results of the analysis of
these thinning regimes are not included in this paper.
ABSTRACT
The Siuslaw National Forest is a predominantly Douglasfir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii)
forest in coastal Oregon. Excluding riparian areas, the forest is 22 percent 50-year site index 130, 65 percent site index 119, and 13 percent site index 108. The current forest
plan calls for harvesting managed stands at 60 to 80 years
of age. An analysis of stocking and pruning showed that
the expected financial return to pruning on site 130 would
be very attractive. The expected returns from pruning are
greater than differences related to stocking for unpruned
stands. Although the returns to pruning on the lower sites
are less, the same general conclusion applies to those sites
as well. Pruning Douglas-fir stands with light stocking
on high sites may be the most financially attractive timber
production investment that exists in the Pacific Northwest.
INTRODUCTION
PRODUCT RECOVERY
This study of the Siuslaw National Forest provides the
first analysis of management regimes for coastal Douglasfir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii)
based on product recovery information for managed stands
(Fahey and others, in press) and pruned stands (Cahill and
others 1988). Management regimes for Douglas-fir called
for in the current forest plan are intended to result in 250
well-spaced trees per acre after precommercial thinning.
Stands are expected to be harvested at 60 to 80 years. The
purpose of our analysis was to explore a wide range of initial stocking levels and to look at the financial return of
adding pruning to each stocking level.
The volume and grade recovery of lumber from regimes
without pruning were based on the results of a mill recovery study of young-growth Douglas-fir (Fahey and others,
in press). Grade recovery is determined by knot size, log
diameter, and the percent of the log that is juvenile wood.
We used grade recovery based on machine stress rating to
ensure that the effect of juvenile wood on lumber strength
and stiffness would be recognized. Knot size limits the
strength and therefore the grade of structural lumber. The
size of knot that is permitted in a particular grade oflumber, however, is proportional to the width of the item. Because larger logs can yield a higher proportion of wide items,
these two effects offset each other to the extent that the difference in value between lumber from large logs with large
knots and small logs with small knots is minor.
Volume recovery is estimated based on log size and taper.
The geometry of sawing rectangular boards from a tapered
cylinder determines this relationship. Once a log has been
squared up there is little further loss in edging boards to
eliminate wane, therefore larger logs produce more lumber
per unit oflog volume. Logs with lower taper have less
wood lost to edgings, trim, and short pieces and therefore
yield more lumber per unit oflog volume.
Grade recovery for pruned logs was based on the results
of a mill recovery study of pruned Douglas-fir (Cahill and
others 1988). The grade recovery of pruned logs is predicted
based on the percentage of the log volume in clear wood laid
down after pruning and the size of the log. The primary
REGIMES
Regimes for sites 130 and 108 on a 50-year site index
were analyzed. Numbers of trees per acre following precommercial thinning were 100, 150,250, and 500. Yields
are from the DFSIM stand simulator (Curtis and others
1981). Although the yields are for regimes that fall outside the range of data from which DFSIM was developed,
the yields were judged acceptable. The yields are maximum
Paper presented at the National Silviculture Workshop, Cedar City,
UT, May 6-9,1991.
Roger D. Fight and Thomas D. Fahey are Research Foresters, Pacific
Northwest Research Station, Portland, OR; Stuart Johnston is a Silviculturist, Mapleton Ranger District, Siuslaw National Forest, Mapleton, OR.
93
Soil Expectation Value, Site 130
No Pruning
effect of pruning on grade recovery is to reduce the amount
oflumber in high structural grades and increase the amount
oflumber in select appearance grades. The financial effect
of pruning is therefore sensitive to the difference in price
between the high structural grades and the select appearance grades.
2500
2000
1500
LUMBER PRICE TRENDS
f?
~
0
C
The price of lumber in general has increased in real
terms over the past 200 years at a rate of about 1.5 percent
per year as shown by the producer price index for lumber
(Ulrich 1990). Projections oflumber prices from the Timber
Assessment Market Model reported in Haynes and Fight
(in press) show these trends continuing for another 20 years
and then remaining relatively constant through the year
2040. When average price for lumber increases, the difference between grades tends to increase as well.
Western lumber markets can be characterized as declining
in quality and showing increasing premiums for the highquality material. The decline in quality is most apparent
in Douglas-fir where the proportion of select lumber has
declined from about 15 percent in 1972 to less than 3 percent in 1990 (Haynes and Fight, in press; Warren 1991).
Although the evidence from markets for lumber appears to
point to a continuation or increase in premiums for quality,
there can be no guarantees. If one takes the opposite point
of view, however, the argument will have to be made on
other grounds because the market data are not consistent
with a view that premiums for quality are declining.
The lumber prices used in the analysis are derived from
those for the year 2040 reported in Haynes and Fight (in
press). They have been modified where necessary to fit the
machine stress-rating grades used in the recovery study.
They are as shown in the following tabulation:
Grade
Selects & Shops
2100f
1650f
1450f
Utility
Economy
.
1000
500
0
70
60
80
Harvest Age
-€I- 100 tpa
~ 150tpa
-+-
250 tpa
-A- 500tpa
Figure 1-Soil expectation value for regimes
without pruning on the Siuslaw National Forest.
1.5 inches, respectively, and quadratic mean diameters
breast height are 17 and 27 inches, respectively, on regimes
with initial stocking of 100 and 500 trees per acre.
The soil expectation value of stands with from 100 to 250
trees per acre is almost the same. With 500 trees per acre
the soil expectation value is considerably less because of
the cost of planting to achieve that stocking and the fact
that the cubic volume at harvest is less for stocking that
high. Figure 1 shows the soil expectation values for site
130. The soil expectation values for site 108 are somewhat
lower, but the ranking of regimes is the same.
RESULTS OF ANALYSIS
Price
(1989 Dollars)
OF PRUNING
957
569
515
408
276
144
Butt logs from the pruning regimes we analyzed were estimated to be about 70 to 90 percent clear wood. The present net value from pruning was directly related to growth
rates, because faster growing trees produce more clear wood
to cover the cost of pruning. With the cost of pruning at
$4 per tree the present net value from pruning 70 trees per
acre on site 130 varied from less than $5 per tree for initial
stocking of 500 trees per acre to more than $15 per tree for
initial stocking of 100 trees per acre. Figure 2 shows the
results for site 130.
Although these values do not reflect any mortality of
pruned trees or additional costs of keeping records on
pruned stands, it appears that rates of return would still
be substantially above the assumed 4 percent cost of capital. The present net values from pruning on site 108 are
only two or three dollars less than on site 130, thus it appears there are many regimes where pruning would be an
attractive investment opportunity with real rates of return
greater than 4 percent.
Because the returns to pruning are highest on stands
with lowest stocking, the increase in soil expectation value
of regimes with pruning increases more on regimes with
lower stocking. The soil expectation value with pruning
is somewhat higher with 100 and 150 trees per acre than
,'t
COSTS
Management costs were from recent costs experienced
on the Siuslaw National Forest. Real costs of management
were assumed constant at current levels. The cost of pruning was assumed to be $4 per tree. The real rate of interest
used in all of the financial analysis was 4 percent.
RESULTS OF ANALYSIS
OF UNPRUNED REGIMES
In general, regimes with higher stocking produced smaller
trees with smaller knots and higher quality wood, but with
about the same value of lumber per cubic foot of log. This
results because of the offsetting effects of limb size on grade
recovery and log size on both volume and grade recovery.
For example, on site 130 maximum limb sizes are 2.2 and
94
tree. This flexibility may also be helpful in designing silvicultural regimes to meet multiple forest management
objectives.
One strategy that is suggested by this analysis is to have
a target stocking of 150 trees per acre for those stands that
might be pruned. The soil expectation value if they are not
pruned is only slightly better than 100 or 250 trees per acre,
however, if they are pruned the soil expectation value is
almost the same as for 100 trees per acre and significantly
higher than for 250 trees per acre. For stands that will not
be pruned additional well-distributed trees above that target are not a problem up to 250 trees. For stands that will
be pruned there is a substantial reduction in value with additional trees. Trees in excess of 250 significantly reduce
financial return whether or not they will be pruned.
The analysis suggests that stands with as few as 100 welldistributed trees per acre should be viewed as financial opportunities rather than regeneration failures. Pruned, these
stands should have soil expectation values greater than
any unpruned stand.
In general the fastest growing stands and the fastest
growing trees in stands are the best candidates for pruning; however, many operational questions are not answered
by this analysis. Should pruning be done in one or multiple lifts? To what height should pruning be done? Which
and how many trees in a stand should be selected for pruning? At what age should pruning be done? Computer programs and data that were developed during the course of
this study can be used to make reasonable extrapolations
addressing these questions.
Present Net Value per Pruned Tree, Site 130
With 70 Pruned Trees
20~--------------------------------------~
15
III
~
'0
c
10
5r---------______
~
O~------------------~------------------~
70
60
80
Harvest Age
-&-100 tpa
-&- 150tpa
~
250tpa
-fr-500tpa
Figure 2-lncrease in present net value per tree
with addition of pruning to regimes on the Siuslaw
National Forest.
with 250 trees per acre. The soil expectation value with
500 trees per acre is substantially less. Figure 3 shows the
results for site 130. Although the soil expectation values
are somewhat less on site 108 the ranking of regimes is
virtually the same.
CONCLUSIONS
REFERENCES
This analysis suggests that silviculturists have wide
latitude within the range of 100 to 250 trees per acre to
design regimes to accommodate multiple forest management objectives because the tradeoffs in timber values
will be small. In addition, pruning is a flexible silvicultural
practice. The number of trees per acre that are pruned can
vary with minor effect on the financial return per pruned
Cahill, J.M.; Snellgrove, T.A.; Fahey, T.D. 1988. Lumber
and veneer recovery from pruned Douglas-fir. Forest
Products Journal. 38(9): 27-32.
Curtis, R.O.; Clendenen, G.W.; DeMars, D.J. 1981. A new
stand simulator for coast Douglas-fir: DFSIM user's guide.
Gen. Tech. Rep. PNW-128. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest
Research Station. 79 p.
Fahey, T.D.; Cahill, J.M.; Snellgrove, T.A.; Heath, L.S. [In
press]. Lumber and veneer recovery from intensively managed young-growth Douglas-fir. Res. Pap. PNW-RP-437.
Portland, OR: U.S. Department of Agriculture, Forest
Service, Pacific Northwest Research Station. 28 p.
Haynes, R.W.; Fight, R.D. [In pressJ. Price projections for
selected grades of Douglas-fir, coast hem-fir, inland hemfir, and ponderosa pine lumber. Res. Pap. Portland, OR:
U.S. Department of Agriculture, Forest Service, Pacific
Northwest Research Station.
Maguire, D.A.; Kershaw, J.A., Jr.; Hann, D.W. [In press].
An approach to predicting the effects of silvicultural regime on branch size and crown wood core. Forest Science.
Ulrich, A.H. 1990. U.S. timber production, trade, consumption, and price statistics 1960-88. Misc. Publ. 1486.
Washington, DC: U.S. Department of Agriculture, Forest
Service. 77 p.
Warren, D.D. 1991. Production, prices, employment, and
trade in northwest forest industries, third quarter 1990.
Resource Bull. PNW-RB-182. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest
Research Station. 99 p.
Soil Expectation Value, Site 130
With 70 Pruned Trees
2500
2000 -
f!? 1500·
..!!!
'0
c
1000
500
0
60
70
80
Harvest Age
-e- 100 tpa
-8-150 tpa
-of<- 250 tpa
-A- 500 tpa
Figure 3-Soil expectation value for regimes with
pruning on the Siuslaw National Forest.
95