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FERTILIZER RESPONSE IN MATURE STANDS
OF 'DOUGLAS-FIR
Richard E. Miller and Steve R. Webster
ABSTRACT
We compiled published and unpublished response data from fer­
tilizer trials in mature stands of Douglas-fir (Pseudotsuga menziesii
[Mirb.) Franco) in western Washington and Oregon. Stand age
ranged from 60 to 120 years. We analyzed response in gross cubic
volume per acre, separately for individual locations having replicated
plots or combined for numerous locations having no replicated plots.
We conclude: (1) Nitrogen fertilization increased volume growth by 9
to 60 percent in these 60- to 120-year-old stands, (2) sizeable gains
occurred in the first decade after fertilization, (3) the economics of
high stumpage values and short investment periods make fertilizing
mature Douglas-fir economically attractive.
INTRODUCTION
Large acreages of mature Douglas-fir (Pseu dotsugamenziessi
[Mirb. ] Franco) exist in Western Washington and Oregon.
Recognizing the potential for accelerating growth in such
stands, several organizations have tested the effects of N fer­
tilization in 60- to 120-yr-old stands at scattered, west-side
locations. We will first summarize cubic volume growth and
growth response in these stands. Then we will discuss some
implications of the results to forest managers and some needs
for future research.
stated the differences as "apparent." We will present gross
growth per acre; i.e., growth of trees that were present at the
start of the measurement period and describe these trials in
order of their increasing stand age.
RESULTS AND DISCUSSION
WEYERHAEUSER EMPIRICAL TRIALS
The Trials-Weyerhaeuser personnel applied varying
amounts of urea to unreplicated plots at 21 locations in western
Washington and Oregon (Figure 1). Rates of 0, 100, 300, and
500 lb N/acre were applied to 60- to 120-yr-old stands on sites
I through IV. The following results are for five growing sea­
sons after treatment.
Response and Initial Volume When data from the 300-N
dosage were used as an example, apparent response were
closely related to the ratio of the initial stand volume of the
-
I
/
Figure I, Location of Weyerhaeuser empirical fertilizer trials.
METHODS
We compiled published and unpublished data on fertilizer
trials in mature stands of Douqlas-fir. Stand ages ranged from
60 to 120 yr, site quality from I to IV, and response period
from 4 to 14 yr. Standard-sized field plots and general pro­
cedures were used to measure qrowth. We analyzed response
in cubic volume per acre separately for individual locations
having replicated plots or combined for numerous locations
having no replicated plots.
Where statistical tests were made by other authors or by us
for unpublished data, we judged differences as statistically sig­
nificant--{)r true when p
0.10. Where no statistical tests of
differences were available or when we believed the experimen­
tal design was not adequats for valid tests of significance, we
126
•
•
••• •
•
• •
•
•
Coos
•
LongView
•
=-----;
Springfield
..
fertilized and the control plot at each location (Figure 2). Rela­
tive to growth on control plots, the annual change in volume growth on the 300-N plots ranged from -130 ft3 to + 115 ft3/acre. Note, however, that so-called "negative response" occurred when the initial volume of the fertilized plot was less than 90% of that of the control plot. In the subsequent analysis
of these data we used covariance analysis to adjust for differ­
ences in initial volume. Response and Nitrogen Dosage After growth at all loca­
tions was adjusted to a common level of initial volume, aver­
age response to each N dosage generally increased with
increasing amount of fertilizer applied (Figure 3). The linear
equation fit the four adjusted means with better precision than
did a quadratic equation (p < 0.003 vs. 0.150). Because of the
limited number of points and because responses in younger
stands (Gessel et al. 1969, Miller and Pienaar 1973) have
proven to be curvilinear, we speculate that a rsduction of fer­
tilizer efficiency at higher N dosages is more likely.
Rather than using linearly or curvilinearly smoothed means .
-
e
n
/
Figure 2. Apparent average annual response to 300 lb N related to
initial volume, per acre basis.
L 150
c
til
g
11 ld
s
)N
re
100 >.
­
til 0.
(")
.
to compare the response to each dosage, we used the
unsmoothed adjusted means (Table 1). Depending on N dos­
age, the average total gain over the 5-yr period ranged from 10
to 150 ft3/acre. With 300 lb N/acre, average gain was 10% or
0.5 ft3/lb/N applied (Table 1).
Response and Initial Age or Site Index Recall that apparent
response to 300 IbIN/acre was strongly dependent on the dif­
ference in initial volume between the fertilized and unfertilized
plots. To screen the data for relationships between response
and initial age or site index, we restricted the data to that from
13 locations where initial volume of the fertilized plot was
within 10% of that of the control plot.
-
Data plotting and simple correlation analyses of this reduced
sample showed no clear relationship between response to 300
Ib/N/acre and initial stand age. Similarly, we found no statisti­
cally confirmed relationship between response and site index. Moreover, absence of any pattern in the data when plotted in three dimensions suggested that multiple correlation analyses, which would include both age and site index as dependent vari­
ables, were not likely to provide additional insight. We there­
fore concluded that response was either independent of age and
site index or that our sample of 13 locations was insufficient to
detect any definite relationship.
Table 1. Average gains in gross growth at 21
Weyerhaueser locations, S-yr period, per acre
basis.a
. .
+50
-tJ
.
.
. .
-
o
Sl
•
(lb)
8. -50
VI
til
a::
Ie
100
150
Average gain
p.a.i. Per ear
--- Total --­
----- (ft ) ----(%)
N
o
0
100
300
•
-----L--------------O.G
0. 3 0.9
1.2
Initial volume ratio
(300 N
289
291
319
312
SOO
1.5
2
30
23
10
ISO
US
1
10
8
0
O.S
0.2
ON)
a
periodic annual increment (p.a.i.) was
adjusted by covariance for initial differences
in stand volume among the 21 locations.
Figure 3. Average annual growth during a 5-yr period and amount of
applied N at 21 Weyerhaeuser locations.
'I
,....,
<!)
L
u
0
320
0
(Y)
+' '+.!: +'
0
L
VOIGHT CREEK EXPERIMENTAL FOREST
0
--
300
0
0
t9 280 0
100
300
Dosage (Ib N/acre)
The Tria l Two hundred pounds N/acre as urea were applied
in unthinned, site II-IIl60-yr-old Douglas-fir stands in the gla­
ciated foothills of the western Cascades in Washington (Figure
4).
The soils in this 230-acre experimental forest range from a
gravelly sandy loam developing on glacial outwash material to
a deep, silt loam developing on glacial lake sediments. This
range of soils was sampled by five pairs of plots; one plot of
each pair was randomly assigned for fertilization. Results from
covariance analysis of growth using volume prior to fertilza­
tion as the covariate are presented here.
-
127
'
Figure 4. Location of additional fertilizer trials in mature stands of 2.8 ft3/1b of N applied (Table 3). During the 8-yr period fol­
Douglas-fir.
lowing refertilization, total gain averaged 552 ft3 or 37% over
the control plots (p < 0.03). Unfortunately, we could not
detennine how much of this gain was from the original fer­
tilization or the refertilization because all plots were refertil­
ized.
Response in Thinned Stands-At the same time that the
un thinned stands at these three locations were refertilized, 150
Ib of N/acre as urea was applied for the first time in nearby,
recently thinned portions of the same stands.
During the 8-yr period after this single fertilization, the total
gain was 416 ft3 (p < 0.04) or 2.8 ft3/1b/acre applied (Table 4).
* Voight Creek
Mollala
McCLEARY EXPERIMENTAL FOREST
Fa ll Creek
The Trial-The effects of urea versus ammonium nitrate
were tested in an 80-yr-old, site II Douglas-fir stand near
McCleary, Washington (Miller and Harrington 1979) (Figure
4). A dosage of 200 Ib of N/acre was applied with each fer­
tilizer.
« N. Umpqua River
Response-Total gain during the 7-yr period after applica­
tion of 200 Ib of N averaged 224 ft3/acre or 9% over control
growth (Table 2); however, the average difference in growth
between fertilized and control plots was statistically nonsignifi­
cant (p < 0.28).
Table 3. Average gain in gross growth at three locations after fertilization (6-yr period) and refertilization (8-yr period), North Umpqua River, per acre basis. a Period
NORTH UMPQUA RIVER
The Tria ls-Two applications of N fertilizer were made to
unreplicated plots in three mature stands in the North Umpqua
River drainage (Figure 4). All stands were site IV and 60-yr
old at the time of the first fertilization. Although 150 Ib N/acre
were supplied at both applications, ammonium nitrate was
used in the first treatment (with some P and S) and urea in the
second application which occurred 6 yr later.
Response in Unthillned Stands- Response during the 6-yr
period following the first fertilization averaged 414 ft3/acre at
the three locations (p < 0.08). This represents a 37% gain or
o
200
Average gain
p. a.'!'
Per ear
-­ Total
(%)
-----­
(ft ) -----370
402
32
224
9
Average Gain
Per year -- Total --- (ft3)
(%)
b
1964-1970
1970-1978
NPS
N
164-1970
Both
Per lb N
(ft3)
69
69
414
552
37
37
2. 8
69
966
37
3.2
?
I
a
Growth adjusted by covgriance for initial
differences in site index.
Nitrogen and varying
amounts of phosphorus (p) and sulfur (S).
Table 2.
Average gain in gross growth during a
7-yr period after fertilization with 200 lb N at
Voight Creek, per acre basis.a
N
(lb)
Treatment
Table 4. Average gain in gross growth at three
locations during an 8-yr period after fertiliza­
tion in thinned stands, North Umpqua River, per
acre basis.a
Treatment
Per lb N
(ft3)
Thinned
(T)
T+N 150
1.1
Average gain
p.a.i. Per year --- Total --­
----- (ft3) -(%)
-
218
270
52
416
24
2. 8
a
periodic annual increment (p.a.i. ) was
adjusted by covariance for initial differences
in site index.
eriodic annual increment (p. a. i. ) was
adjusted by covariance for differences in pre­
fertilization volume.
128
R esponse and Nitrogen Sour ce-During the 4 yr after treat­
ment. the average gain from both fertilizers was 20% or
'lpproximately 1.4 ft3/lb of N applied (p < 0.04). Although the
IVerage response to ammonium nitrate was slightly more than
that to urea (292 vs. 268 ft3/aere), this difference was not
stati stically significant (Table 5).
CROWN ZELLERBACH
lJ. rea fertilizer was applied by helicopter to a 90­
yr-old stand near MoHala, Oregon (Figure 4). Dosages of 0,
200, and 400 lb N/acre -were compared.! Each dosage was
tested on two downslope strips. Each experimental unit (strip)
was subsampled with twelve O.1-acre plots; thus a total of 24
plots sampled each dosage. Although there was some commer­
cial thinnin g throughout the area during the seven growing sea­
sons after treatment, we believe this thinning was equally
applied to all treatments and did not confound our interpreta­
tion of the data.
Response and Nitrogen Do/age-Apparent total gain over
the 7-yr period averaged 154 and 245 ft3/acre for the 200- and
400-lb N dosages, respectively (Table 6). The differences
among the treatment means, however, were not statistically
sign ificant (p < 0.135). At the conventional 200-lb N/acre
dosage, the apparent gain over control growth was 11 % or 0.8
ft31lb of N applied.
The Trial
-
fertilizer on four plots. The authors statistically analyzed gross
basal area growth using analysis of covariance to adjust for dif­
ferences in initial basal area after thinning. No statistical analy­
sis was made of estimated volume growth because the sample
of tree heights was considered inadequate for a sensitive mea­
sure of volume and volume qrowth.
Response and Nitrogen Source-During the 4 yr after treat­
ment, gross basal area growth increased after fertilization (p <
0.05); moreover, growth with ammonium nitrate was signifi­
cantly more than growth with urea. Estimated gain in cubic
volume growth after ammonium nitrate application was 296
versus 184 ft3/acre after urea application. The estimated gain
with ammonium nitrate represented a 59% gain over the
untreated plots or 2.0 ft3/lb N applied (Table 7).
CONCLUSIONS ABOUT ALL LOCATIONS
All trials showed positive response to N fertilizer- provid­
ing comparable stand conditions existed on both treated and
control plots prior to fertilization. During the 4- to 8-yr period
after fertilization with 150 to 300 lb N/acre, gains averaged
FALL CREEK
The Trial-The effects of urea and ammonium nitrate fer­
tilizer were compared in a recently thinned (Harrington and
Miller 1979) 110-yr-old, site II stand east of Eugene, Oregon
(Figure 4). A dosage of 150 lb N/acre was supplied by each
Table 6. Average gains during a 7-yr period
after fertilizing a 90-yr-old stand, Mollala
Tree Farm, per acre basis.
N
(lb)
o
200
400
Average gain
p.a.i. Per ear
--- Total -­
------- (ft ) ------(%)
19 3
215
227
22
35
154
245
11
18
0.8
0.6
I We thank Dr. Robert F. Strand and Crown Zellerbach Corporation for sup­
plying these data.
Table 5. Average gains during a 4-yr period I
after fertilization with 200 lb N at McCleary,
per acre basis.a
Average gain
p.a.i. Per year
--- Total --------- (ft3)
(%)
N
-
o
Urea
A.N.b
342
409
415
67
73
-
268
292
19
21
Table 7. Average gains during a 4-yr period after
150 lb of N were applied to 110-yr-old stand, Fall
Creek, per acre basis. a
Per lb N
(ft3)
p.a.i. Per year
------- (ft3)
N
o
1.3
1.5
Ure
A.N.
125
171
199
46
74
Average gain
--- Total -----____
184
296
(%)
Per lb N
(ft3)
37
59
1.2
2.0
a
Estimated periodic annual increment (p.a.i.)
was adjusted by covariance for initias differences
Ammonium
in basal area before fertilization.
nitrate.
eriodic 'annual increment (p.a. i.) was
adjusted by covariance for initial differences
in basal area growth before fertilization.
b Ammonium nitrate.
129
between 150 and 416 ft3/acre (Table 8). These represented 9%
to 59% more growth on the fertilized plots.
MANAGEMENT IMPLICATIONS
The results from these trials provide practical il1}plications
for forest managers.
VOLUME YIELDS
Volume yields can be increased by fertilizing mature stands
in the 60- to 120-yr-old age classes. In fact, 200-yr-old Doug­
las-fir reportedly responded to N fertilization on Vancouver
Island (Crossin et aI. 1966). In the mature stands that we inves­
tigated, application of 150 to 200 Ib N/acre in the form of urea
or ammonium nitrate increased volume growth within a short
time period after treatment and to about the same amount as
reported for younger stands (Turnbull and Peterson 1976).
In summary, the results from our sample of 'nature stands
parallel those in younger stands of Douglas-fir. Therefore,
growth of Douglas-fir can probably be increased by N fertiliza­
tion over a wide range of sites and stand ages in western Wash­
ington and Oregon.
VALUE YIELDS
The extra volume produced in mature stands by fertilization
has higher value than equal volume produced in younger
stands; therefore fertilization of older stands is potentially
more profitable. This higher profitability has three bases:
I . Response to fertilization apparently occurs just as imme­
diately in older stands as in younger. Under intensive manage­
ment regimes, fertilization of young stands will probably not
start before age 15 and the earliest commercial thinning will
usually occur between ages 25 and 35 yr. This gives an investTable 8.
ment period of 10 to 20 yr. Where commercial thinning fol­
lows fertilization of mature stands, however, some of the
accrued volume gains can be removed within the same decade.
Where subsequent commercial thinning is not feasible or
desired, accrued volume gains can be harvested much more
rapidly in older stands because of the shorter wait until final
harvest.
2. A greater proportion of the volume gain is merchantable
volume, because the ratio of merchantable:total volume
increases with tree size.
3. Stumpage returns per unit of volume are greater in
mature stands, because higher prices are paid for larger trees
and harvesting costs are lower. These and other factors which
influence the economic returns of fertilization have been dis­
cussed elsewhere in greater detail (Regional Forest Nutrition
Project 1977, Miller and Fight 1979). A key figure from the
latter publication shows price-quantity combinations that are
needed to offset a cumulative $98 per acre. cost for fertilizing
(Figure 5). This cost equates to compounding at 5% interest for
10 yr, a current fertilizing cost of $60 per acre. Notice that the
amount of extra volume needed to cover treatment costs
depends on stumpage price; lesser volumes are needed when
stumpage prices are hiqh. In this example, the stumpage price
for trees averaging 8 in. dbh at harvest was estimated at 25¢
per ft3, and for trees averaging 16-in. dbh a stumpage price of
60¢ per ft3 was used. These represent conventional stumpage
prices of $90 and $140 per thousand board feet, respectively.
Using these stumpage prices the break-even volume from 8­
"
in. trees is 392 ft3. In contrast, only 163 ft3/acre yield is needed
from fertilization if this volume is harvested from 16-in. trees.
C
las­
que,
at a
mer
inc
wil
nee j
tion
GO
tiiI
wo,
rna
3
me
attr
j
ing
ha
Pro
ROTATION LENGTH IN MANAGED FORESTS
I
The current trend in industrial forest management is to max­
imize profits by shortening rotation length. Thus stands are
usually cut before they attain maximum mean equal increment,
and this results in some sacrifice of total potential yield.
Response in various mature stands of Douglas-fir to 150 to 400 lb N, per acre basis.
Trial
Weyerhaeuser
Voight Creek
No. Umpqua River
McCleary
Crown Zellerbach
Fall Creek
Areas
21
1
3
3
1
1
1
Replications
0
5
0
0
4
2
6
b
Thinned
Age
(yr)
No
No
No
Yes
No
Yes
Yes
60-120
60
60
68
80
90
110
Site
index
class
I-IV
II-III
IV
IV
II
III-IV
II
Growth
period
(yr)
5
7
6
8
4
7
4
a
-------------- Gain
Per lb
Per
year Total
of N
(ft3) ------- Percent
-
d
30
32
69
52
70
22
46
74
d
150
224
414
416
280
154
184
296
d
0.5
1.1
2.8
1.4
0.8
1.2
2.0
--
c
lO
9
c
37
24
20
11
37
c
59
b
eplicated plots er treatment.
Gain in rowth of the fertilized stands compared to unfertilized
g
§
over the same period.
With ammonium nitrate.
Average response to 300 lb N/acre at 21 locations.
130
j
l
2
I
I
I
I
Our compilations clearly show that mature stands of Doug­
las-fir responded quickly and well to N fertilization. Conse­
quently , mean annual increment would increase and culminate
at a later age. Moreover, if fertilization is followed by com­
me rcial thinning, financial returns from fertilization will be
inc reased and financial maturity and final harvest of the stand
will be shifted to a later age. These considerations indicate a
need to reexamine the potential impacts of fertilization on rota­
tion length of current mature stands.
(lI­
the
dc.
o[
(lrc
nal
hie
mc
In
Cl'S
ieh
Ii.\­
Ion
the
are
ing
for
the
Ists
len
CONCLUSIONS
.
We conclude from these trials that:
\. Nitrogen fertilization increased volume growth by 9% to
60% in mature, 60- to 120-yr-old Douqlas-fir stands. 2. Sizeable qains occurred during the first decade after fer­
tilizing . With 150 to 400 Ib N/acre, 150 to 600 extra ft3 of
wood were produced. The average response was approxi­
mately 300 ft3/acre.
3. The economics of high stumpage values and short invest­
periods make fertilization of mature stands economically
ent
m
I
.
attractive. ice
:S¢
of
1ge
Iy.
S­
le d
RESEARCH NEEDS
Fertilizing X Thinning Interactions-Profits from fertiliz­
ing are clearly greatest when followed by a commercial or final
harvest that recovers fertilization costs shortly after they occur.
Profits from fertilization would be further enhanced if a posi-
tive fertilizer X thinning interaction occurs, meaning that vol­
ume gained from fertilization is higher in thinned than in
unthinned stands. Evidence for such interaction was presented
in an earlier chapter in this volume. In order to confirm this
reported interaction, future experiments in mature stands
should test fertilization and thinning in factorial combinations.
Nutrient Cycling-Fertilization of mature stands could
potentially increase the rate of decomposition in the forest floor
and thus the rate of nutrient cycling. Past research has shown
that soil N availability decreases with age because N accumu­
lates in unavailable forms in the stand and forest floor (Gessel
et al. 1972). Although mature stands apparently compensate
for decreased supplies of available N in the soil by more con­
servatively recycling N already in the crowns of trees in the
stand, the response to N in these fertilizer trials reveals that this
cycling is not adequate to meet the needs of the tree for N.
Fertilization could increase the supplies of available N
directly or indirectly by accelerating rates of natural decompo­
sition. Increased decomposition should prolong the response
from fertilization. Additionally, a moderately increased rate of
decomposition should reduce the amount of nutrients in the
forest floor and make them less vulnerable to loss during sub­
sequent harvest and site preparation. Long-range productivity
of the site could be enhanced.
Therefore, basic studies should be imposed in mature stands
to clarify the effect of thinning and fertilizing treatments on
organic matter decomposition, nutrient balance, and nutrient
cycling. Moreover, the implication of these effects on long­
term site productivity should be examined.
s.
Figure 5. Price-quantity combinations to offset a $98 per acre cumulative cost of fertilizing (from Miller and Fight 1979).
r-...
+>
lX­
lre
nt.
0
0
"t-
U
.0
:J
U
-
..t}
(\)
U
L
(L (\) O'l
o
0..
E
:J 0.80
0.60
I
I
I
I
I
I
I
I
I
OAO
if)
+>
c
L
:J
U
I
____ ---L------_>-_
0.20 .
I
I
I
I
I
I
163
'f
I
I
I
I
I
-----$98
392
01:---L.-200--L....,,-...L.-�4CX)�-L.-�600L----l--8:')().,..I--::--...J.-�10:-'-OO--I
O'O°
Increased Yield (cubic feet)
131
LITERATURE CITED
Crossin, E. C., J. A. Marlow, and G.L. Ainscough.
1966. A progress report on forest nutrition studies on Vancouver
Island. For. Chron. 42(3):265-284.
Gessel, S. P., T. N. Stoate, and K. J. Turnbull.
1969. The growth behavior of Douglas-fir wjth nitrogenous fer­
tilizer in western Washington. The second report. 1 19 p. Institute
of Forest Products, Univ. Washington, Seattle.
Gessel, S. P., D. W. Cole, and E. C. Steinbrenner.
1972. Nitrogen balances in forest ecosystems of the Pacific North­
west. Soil BioI. Biochem. 5: 19-34.
Harrington, C. A., and R. E. Miller.
1979. Response of a 1 10-year-old Douglas-fir stand to urea and
ammonium nitrate fertilization. USDA For. Servo Res. Note
PNW-336, 7 p., iIIus. Pac. Northwest For. and Range Exp. Stn.,
Portland, OR.
/.
Miller, R. E., andL. V. Pienaar.
1973. Seven-year response of 35-year-old Douglas-fir to nitrogen
fertilizer. USDA For. Servo Res. Pap. PNW- 165, 24 p., illus. Pac.
Northwest For. and Range Exp. Stn., Portland, OR.
Miller, R. E., and R. D. Fight.
1979. Fertilizing Douglas-fir forests. USDA For. Serv. Gen. Tech.
Rep. PNW-83, 56 p. Pac. Northwest For. and Range Exp. Stn.,
Portland, OR.
Miller, R. E., and C. A. Harrington.
1979. Response to urea and ammonium nitrate fertilization in an
80-year-old Douglas-fir stand. USDA For. Servo Res. Note
PNW-330, 5 p., iIIus. Pac. Northwest For. Exp. Stn., Portland,
OR.
Regional Forest Nutrition Research Project. VI.
1977. An economic analysis of Douglas-fir response to nitrogen
fertilizer. IN Regional Forest Nutrition Research Project Biennial
Report. 1974--1975, p. 27-35. Coil. For. Resour., Univ. Washing­
ton, Seattle.
Turnbull, K. J., andC. E. Peterson.
1976. Analysis of Douglas-fir growth response to nitrogenous fer­
tilizer. Part 1: Regional trends. For. Resour. Tech. Note, Institute
of Forest ProductsContribution No. 13, 15 p., Univ. Washington,
Seattle.
I
/
132