Fore" Sci.. Vol. 29, No.3, 1983, pp. 469-477 Copyright 1983, by the Society of American Foresters Soil Nitrogen, Sulfur, and Phosphorus in
Relation to Growth Response of Western
Hemlock to Nitrogen Fertilization
M.
A. RADWAN
J. S. SHUMWAY
ABSTRACT.
Sixteen sites of westem hemlock (Tsuga
heteroph.vlla) located in both the coastal and
Cascade zones in western Washington were examined to determine the relationships of soil N,
S, and P to growth response of the trees to N fertilization. The sites were chosen from among
nineteen N fertilizer-test installations established in Washington in 1969 by the Regional Forest
Nutrition Research Project of the University of Washington. The sites varied in productivity, but
site index did not correlate with growth response. Concentrations of total
N
and extractable P
were much higher in the forest floors than in mineral soils to a depth of 15 cm. Amounts of total
N, mineralized N, and sulfate S were higher in mineral soils of the coastal sites than in those of
the Cascades. Concentrations and amounts of extractable P of both the forest floors and mineral
soils, however, were higher in the Cascades than on the coast. The nutrients studied and some
of their ratios were significantly related to growth response to N fertilization. Overall, the strongest
correlation was with extractable P in the forest floor (r
=
0.77, P < 0.001) and the best correlations
(r = 0.67,
0.66, P < 0.01). The data strongly suggest that low
involving nutrients in mineral soil were with the ratios of extractable P/mineralized N
P < 0.005) and extractable P/total N
(r
=
levels of P alone or in combination with high soil N may be important factors in the reported
lack of growth response of hemlock to
ADDITIONAL
KEY
WORDS.
N
fertilizers. FOREST SCI . 29:469-477.
Tsuga heterophylla.
site index, forest floor, mineral soil. nutrients. total
N, mineralized N, mineralized S, sulfate S, extractable P.
WESTERN HEMLOCK (Tsuga heterophylla (Raf.) Sarg.) is one of the most productive
conifer species in the PacificNorthwest. Increased demand and prices for hemlock
products during the past decade have spurred interest in management of this
important resource. Application of nitrogen (N) fertilizer to increase productivity
has not been consistently successful. Growth responses reported so far range from
increases of 50 percent or more to apparent reductions of about 20 percent. In
general,N fertilization has been more successful in stands located on the lowlands
west of Puget Sound and on the west slopes of the Cascade Range than in forests
of coastal Oregon and Washington where N fertilization is now believed unprof­
itable (DeBell and others 1975, Webster and others 1976, Olson and others 1980).
Presently, the causes of lack of growth response of hemlock to N fertilization
are unknown. Olson and others (1979, 1980) concluded that factors other than
gtowing space may govern the potential for response in natural stands. Similarly,
recent experiments have shown that source of fertilizer N may not be a factor
The authors are, respectively. Principal Plant Physiologist, Forestry Sciences Laboratory. USDA
Forest Service, and Soil Scientist, Forest Land Management Center, Washington Department of
Natural Resources, Olympia, Washington. They thank the PacificNorthwest Regional ForestNutrition
Research Project and the University of Washington. Seattle, for making the studied sites available for
collection of forest floor and mineral soil samples and for provjding the data for growth response and
the data used to calculate site index. Manuscript received 4 December 1981.
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affecting response of hemlock seedlings (Radwan and DeBell 1980a). On the other
hand, high supplies of soil N or low levels of extractable phosphorus (P) or sulfur
(S) seem to be likely factors. This view is based upon recent work in our laboratory
and investigations by others elsewhere. For example, we have recently reported
that foliar N was higher and foliar P and sulfate S were lower in coastal than in
Cascade forests (Radwan and DeBell 1980b). Our unpublished data also show
rapid depletion of foliar sulfate S after N fertilization. In addition, greenhouse
tests have shown positive growth response to P fertilization by hemlock seedlings
grown in coastal soil (Anderson and others 1979, Heilman and Ekuan 1980). We
conducted this study, therefore, to further examine the relationships of soil N, S,
and P to growth response of western hemlock to N fertilization.
MATERIALS AND METHODS
The Sites. -Sixteen natural stands of western hemlock on sites of different pro­
ductivity were used. Sites, approximately S acres each, were chosen from among
19 installations established in western Washington in 1969 by the Regional Forest
Nutrition Research Project (RFNRP) of the University of Washington. Stands
were 10 to 40 years old when the plots were installed, and growth responses to
N fertilization were obtained from the RFNRP. Eight sites occur in the coastal
hemlock zone within about 40 km of the Pacific coast, and the other eight are
located inland on the west slopes of the Cascade Range (Fig. 1). On average, the
coastal zone is believed to be more productive than the Cascade zone.
Estimation o/ Site Index.-Site index values are based on data from 24 dominant
trees at each site. Heights and breast-high ages of the trees were measured in 1970
by RFNRP personnel; we used this data and Wiley's tables (1978) to estimate
site index at SO-year breast-height age.
Determination 0/ Growth Response.-Seven-year radial-increment growth re­
sponse to application of 224 kg N/ha was determined by RFNRP personnel, using
a tree-pairing method. Fertilized and unfertilized trees were paired according to
similarity in size, competitive status, and past diameter growth. Percent response
was calculated using 30 pairs of-trees (Olson 1979, Olson and others 1980). Briefly,
the main findings were: (I) response was generally small and extremely variable,
ranging from -20 percent in the coastal zone to 38 percent in the Cascades; (2)
some sites in both geographic zones showed good response, but average response
in the Cascades (S.9 percent) was somewhat higher than that on the coast (3.S
percent); and (3) response was not related to site class.
Forest Floor and Soil Sampling a nd Processing.-In 1979, three 30-m transect
lines were laid out at each of the 16 sites. The starting point and direction of each
transect were selected at random from 28 grid points located in the unfertilized
strips between RFNRP plots. Samples of forest floor materials and mineral soils
were collected at 3-m intervals along the transects. Forest floors were obtained
using a 78.S-cm2 template and a cutting knife, and underlying mineral soils were
sampled to a IS-cm depth. Sampling points were moved to avoid stumps and
logs. Twigs and other material < 6 mm in diameter were discarded, and forest
floor samples included both the litter and humus. There were 10 samples per
transect, and samples of forest floors and soils were composited separately for
each transect. About 0.004 m3 of soil was collected per site, and volume of forest
floor collections varied with its thickness.
Samples were airdried at room temperature; and roots, large stem material, and
rocks were removed. Forest floor samples were weighed and ground to a fine
powder in a pulverizer equipped with ceramic plates. Soil was passed through a
470
/ FOREST SCIENCE
i
Western hemlock
•
o
Coast
Cascades
240
N
023
1090
1080
o
o
111
58
Seattle
Olympia
o
18
0117
---___
•
42
Kelso
,
30km
FIGURE I.
Approximate geographic location of study sites in western Washington. Sites 100. 15. 84.
80. 9, 4, 3, and 42 occur in the coastal hemlock zone. and sites 24. 23. 109. 108. III. 58, 18. and
117 are located on the west slopes of the Cascade Range.
2-mm sieve, and the resulting fractions were weighed. For determination of total
N, a subsample of the sieved soil was pulverized to pass through a O.S-mm sieve.
All samples were individually stored in plastic containers at -1SoC until analyzed.
Chemical Anal.1'sis.-Moisture in forest floor and soil samples was determined
by drying to constant weight at 65° and 10S°C, respectively. Total N (including
nitrate) in the sample was assayed by the micro-Kjeldahl procedure (Bremner
1965a).
Mineralizable N was determined, in duplicate, under anaerobic conditions for
soil samples and aerobic conditions for forest floor materials. Anaerobic condi­
tions were obtained by waterlogging. according to Waring and Bremner (1964).
and all incubations were run for 2 weeks at 28°C, Aerobic conditions were achieved
by mixing subsamples of forest floor with 30- to 60-mesh acid-washed quartz
sand in widemouth vials and moistening the mixture with a small amount of
VOLUME 29. NUMBER 3. 1983 I 471
TABLE I.
Zone and
installa tion
number
Approximate
geographic area
Site and stand characteristics.
Elevation
Weight
of
forest
floor
m
Soil parent
material
Site
index
at SO
years I
mTlha
m
percent
49
Sedimentary
31
.
Coastal
90
Growth
response
to N
fertilizerl
17
IS
Sappho
42
Cathlamet
360
74 Volcanic
35
16 80
Littleton Creek
240
48 Sedimentary .
30
10 84
60
28 Sedimentary .
ISO
ISO
38 Glacial
29 Neah Bay
90
40 opalis
60
22 Mt. Gunderson
Burnt Hill
9
Humptulips
4
100
3
Average
36
7
37
3
Glacial
37
3
Sedimentary
34
Glacial
33
34
ISO
I
- 20
5
Cascade
117 24
Landers Creek
900
48
Volcanic
34
38 Deming
660
82
Glacial
20
20 9
23 Bold Mountain
330
39 Glacial
35
18
Orting
480 44
Volcanic
36
5
111 Duvall
240
29 Volcanic
29
2
108
Sap Lake
120
32
Glacial
36 -2
109 Beaver Lake
900
52
Glacial
33 - 12
270 31 Volcanic
32
- 12
Echo Glen
58
488
Average
32
6
I
Based on heights and ages of 24 trees at each site measured by RFNRP in 1970.
2
Se,ven-year radial-increment growth response to a pplication of 224 kg N/ha determined by RFNRP
using a tree-pairing method (Olson 1979).
distilled water to approximate field capacity. Ammonium produced under an­
aerobic conditions was determined by steam-distillation in presence of KCl and
MgO. Similarly, ammonium andnitrate production under aerobic conditions were
assayed as ammonium by steam-distillation in presence of KCl, MgO, and De­
varda's alloy (Bremner 1965b). Mineralized N in all samples was calculated by
subtracting initial ammonium and nitrate contents from post-incubation concen­
trations.
Mineralizable S was determined in triplicate, using aerobic incubation for both
forest floors and mineral soils. Subsamples were mixed with 30- to 60-mesh acid­
washed quartz sand in widemouth plastic jars containing a small volume of
distilled water. Soils and forest floors were incubated for 2 and 3 months, re­
spectively, at 30°C (Williams 1967). Water lost by evaporation was periodically
replaced. Sulfate, extracted from incubated and unincubated samples with Ca(H2
P04h according to Fox and others (1964), was determined by the turbidimetric
method of Butters and Chenery (1959). S mineralized during the incubation period
was estimated by subtracting the initial from post-incubation sulfate concentra­
tions. Extractable P, extracted from all samples with Bray-Kurtz solution 2, was
determined colorimetrically (Bray and Kurtz 1945).
Kilograms per hectare of total and mineralized N, sulfate and mineralized S,
and extractable P were calculated from concentrations found in the samples and
dry weights of forest floors and the < 2-mm fraction of mineral soil in the 0- to
15-cm layer.
Statistical Analysis.
472 /
-
Data were subjected to analysis of variance, and means were
FOREST SCIENCE
TABLE 2.
Chemical characteristics of forestjloor materials from the study sites.·
Zone and
inst all at ion
number
Coast al
TotalN
Mineralized N
Sulfat e S
...................................................................................... kgl ha
Miner alized S Extract able P
...................................................................................... 15
391 a
I a
Oa
I a
42
616 a
12 a
Oa
5a
9 a
80
421 a
3 a
Oa
3 a
13 a
84
237 a
8a
Oa
1a
6a
9
316 a
9a
Oa
1a
5a
4
232 a
4a
Oa
I a
9 a
3a
19 a
100
345 a
9 a
Oa
5a
3
215 a
2a
Oa
2a
2a
347 x
6x
Ox
2x
8x
117
212 a
-2 b
Oa
I a
41 a
24
513 a
1b
1a
1a
25 ab
23
326 a
9 ab
Oa
2a
7 b
18
360 a
6b
Oa
3 a
6 b
111
299 a
6b
Oa
1a
8b
11 b
Average
Cascade
108
286 a
11 ab
Oa
1a
109
559 a
24 a
1a
4 a
8 b
58
246 a
6b
Oa
I a
12 b
3 50 x
8x
Ox
2x
15 y
Aver age
J Within zones, values in t he same vertical column which are followed by the same lett er, a or b,
and zone averages in t he same vertical column followed by t he same lett er, x or y, are not st at ist ically
differ ent (P < 0.05) by T ukey's t est.
compared according to Tukey's test. Correlation coefficients (r) were caJculated
according to Snedecor (1961).
RESULTS
Site Index.-Site index at 50-year breast-height age ranged from 30 m to 37 m
on the coast, and from 20 m to 36 m in the Cascades; it averaged 33 m over the
16 sites (Table I). The average site index for the coastal sites, 34 m, was somewhat
higher than that for the sites in the Cascade zone, 32 m. This result is in agreement
with earlier findings (Radwan and DeBell 1980b). Also, site index was not sig­
nificantly correlated with growth response to N fertilizer. Using 26 western hem­
lock RFNRP installations, Olson and others (1980) reported a similar result.
Total Nitrogen. Total N in the forest floor averaged 0.83 percent and 348 kgl
ha over the 16 sites (Table 2). Highest level of N, 1.07 percent, was in site 109;
and the lowest, 0.48 percent, was in site 117, both in the Cascades. Amounts of
total N ranged from 212 to 616 kglha; it averaged 347 kglha on the coast and
350 kglha in the Cascades. Differences within and between zones, however, were
not significant. Also, total N in the forest floor was significantly correlated with
mineralized S; it was n.ot significantly related to growth response to N fertilizer
(Table 3).
Concentration of total N in mineral soil (range, 0.05 to 0.55 percent) was lower
than that in the forest floor, but the average amount ofN was much higher in the
surface 15 cm of soil than in the forest floor ( l ,830 vs. 348 kglha) (Tables 2 and
4). Amount of N varied significantly within and between the two geographical
-
VOLUME
29,
NUMBER
3, 1983 / 473
TABLE 3. Simple linear regressions among soil nutrients and between nutrients
and groll'th response a/ hemlock to N/ertili=er.
Forest floor
Correlation
coefficient
Regression
(r)
.
P-valuel
Mineral soil
Correlation
coefficient
(r)
P-valuel
0.47
0.066
0.82
0.001
-0.01
0.971
-0. 56
0.023
Total N vs. mineralized S
0.62
0.011
0.83
0.001
Mineralized N vs. mineralized S
0.52
0.041
0.68
0.004
Mineralized N vs. extractable P
-0.51
0.043
-0.30
0.261
Mineralized S vs. extractable P
-0.35
0.181
-0.22
0.401
0 . 15
0.581
-0.50
0.050
-0.47
0.067
-0.43
0.093
-0.34
0.203
Total N vs. mineralized N
Total N vs. extractable P
Total N vs. growth response
Mineralized N vs. growth response
0.00
Sulfate S vs. growth response
Mineralized S vs. growth response
-0.13
0.639
-0. 60
0.014
0.77
0.001
0.44
0.084
0.68
0.004
0.66
0.006
-0.06
0.825
0.67
0.005
0.70
0.002
0.28
0.285
Extractable P vs. growth response
Extractable P/total N vs. growth
response
Extractable P/mineralized N vs.
growth response
Extractable P/mineralized S vs.
growth response
I
Correlations are considered significant at P
s
0.05.
zones, and the average total N in the soil was significantly higher in the coastal
zone than in the Cascades. Soil N was also strongly correlated with mineralized
N and mineralized S; it was negatively related to available P and growth response
to N fertilizer (Table 3).
Mineralized Nitrogen. Incubations of the forest floor under aerobic conditions
for 2 weeks resulted in net mineralization of N in 15 sites and net immobilization
of N in I site (Table 2). The N mineralized in the forest floor ranged from I to
24 kglha (11 to 440 ppm); it averaged 7 kglha. with no significant difference
between zones. Mineralized N in the forest floor was also correlated with min­
eralized S and negatively related to extractable P; it was not significantly correlated
with growth response to N fertilizer (Table 3).
More N was mineralized in the surface 15 cm of mineral soil than in the forest
floor (38 vs. 7 kglha). and mineralized N varied significantly among and within
zones (Table 4). It ranged from a low of 4 kglha (6 ppm) for installation 24 in
the Cascades to a high of 88 kglha (139 ppm) for installation 84 on the coast.
with a significantly higher average value in coastal soils than in the Cascades. Soil
N mineralized was also significantly correlated with mineralized S but not with
growth response to N fertilizer (Table 3).
-
Sulfate Sulfur. Forest floors were very low in available S as measured by levels
of sulfate S (Table 2). Sulfate was detected in only two samples of forest floor,
indicating that sulfur was present mostly in the organic form.
Mineral soil contained much higher concentrations (up to 79 ppm) and amounts
(up to 50 kglha) of sulfate S (Table 4). Also, sulfate S differed significantly within
and among zones, with greater amounts occurring in coastal soils than in those
of the Cascades. Average amount of sulfate S in the soil did not correlate with ,
growth response to N fertilizer (Table 3).
-
474 /
FOREST SCIENCE
TABLE 4.
Zone and
installation
number
Coastal
Chemical characteristics of mineral soils from the study sites.'
Total N
Mineralized N
Sulfate S
............................................................................................. kg/ha
Mineralized S
Extractable P
.............................................................................................
15
1,735 cd
15 e
42
2,390 abc
41 cd
50 a
13 a
17 a
63 a
80
1,057 d
21 de
Od
6a
33 a
22 a
4 cd
150 a
84
2,683 ab
88 a
4 cd
14 a
9
2,224 bc
43 c
15 bed
lOa
9a
4
2,422 abc
56.bc
5 bed
13 a
48 aa
100
3
Average
2,096 bc
42 cd
30 ab
15 a
lOa 2,977 a
72 ab
26 abc
20 a
16 a 2,198 x
47 x
17 x
14 x
44 x 146 ab
Cascade
606 de
I I cd
Ob
Oe
24
337 e
4d
Ob
I bc
23
1,104 ed
43 ab
Ob
16 ab
117
69 ab
213 a
18
2,962 a
54 a
17 ab
21 a
17 b
III
2,187 b
32 bc
l Ob
IS abc
36 ab
108
1,469 be
30 be
13 ab
13 abc
134 ab
109
1,597 be
28 be
13 abc
II b
58
1,442 be
23 bed
38 a
2b
I I abc
87 ab
1,463 y
28 y
l Oy
ll x
89 Y
Average
I Within zones, values in the same vertical column which are followed by the same letter. a to e.
and zone averages in the same vertical eolumn followed by the same letter, x or y, are not statistically
different (P < 0.05) by Tukey's test.
Mineralized Su/fur.-As with N, S was mineralized upon aerobic incubation of
the forest floor (Table 2). Mineralization of S, however, was much slower than
that ofN; and incubations had to be run for 3 months before sulfate levels became
easily detectable. Still, mineralized S was consistently lower than mineralized N;
amounts ranged from I to 5 kglha (15 to 128 ppm), but differences among and
within zones were not significant. In addition, mineralized S in the forest floor
was not significantly related to growth response to N fertilizer (Table 3).
Similar to N, more S was mineralized in soil than in the forest floor (average.
12 vs. 2 kglha) (Tables 2 and 4). Mineralized S varied greatly among the 16 sites,
but average amounts in the two zones did not differ significantly. Additionally.
soil S mineralized was negatively correlated with growth response to N fertilizer
(Table 3).
Extractable Phosphorus.- Extractable P in the forest floor ranged from 2 kglha
(89 ppm) on the coast to 41 kglha (746 ppm) in the Cascades (Table 2). The
average amount of extractable P in the Cascades, 15 kglha. was significantly higher
than that in the coastal sites, 8 kglha. More importantly, extractable P in the
forest floor, the ratio of extractable P/total N. and the ratio extractable P/min­
eralized S were all significantly correlated with growth response to N fertilizer
(Table 3).
Concentrations of extractable P in mineral soil (18 to 333 ppm) were much
lower than those found in the forest floor. Average amounts of extractable P in
the surface 15 cm, however, were higher than these found in the forest floor (66
vs. 12 kglha) (Tables 2 and 4). Amounts of extractable P varied significantly
VOLUME
29,
NUMBER
3. 1983
I
475
among the Cascade sites, and the average amount was significantly higher in the
Cascades than on the coast. The ratios extractable P/total N and extractable PI
mineralized N were strongly related to response to N fertilizer (Table 3).
DISCUSSION AND CONCLUSIONS
Hemlock productivity at the 16 sites used in this study varied greatly. Growth
response to N fertilization, as determined by Olson (1979), also varied consid­
erably. High-quality sites and good growth response to fertilizer occurred in both
geographical zones, but response was not significantly correlated with site index.
Similar results were recently reported by Olson and others (1979, 1980) using 26
RFNRP hemlock installations which included the 16 sites used in this study. The
data are also in general agreement with earlier findings by Webster and others
(1976).
Chemical properties of forest floor materials differed markedly from those of
the surface mineral soil. In general, concentrations of total N and extractable P
were much higher and contents of sulfate S were much lower in the forest floor
than in mineral soil. Compared with mineral soil, however, the forest floor con­
tained much lower quantities of total N, sulfate S, extractable P, and mineralized
N and S. Nevertheless, the forest floor still accounts for considerable quantities
of nutrients, and hemlock is known to have many of its "feeder" roots in the
forest floor (Ross 1932). Moreover, the forest floor recently has been suggested
to be important for P nutrition in the coastal hemlock forests (Heilman and Ekuan
1980).
Amounts of some nutrients in the forest floor and in mineral soil varied sig­
nificantly between the two geographic zones. For example, total N, mineralized
N, and sulfate S were significantly higher in mineral soils of the coastal sites than
in those of the Cascades. On the other hand, extractable P levels of both forest
floor and mineral soil were significantly higher in the Cascades than on the coast.
High levels of total and mineralizable N and low amounts of extractable P in
coastal soils could account for the reported unfavorable growth response of hem­
lock to N fertilizer on the coast. In addition, the low P status of the coastal sites
is in agreement with recent results by Heilman and Ekuan (1980).
The nutrients studied and some of their ratios were significantly correlated with
growth response of hemlock to application of N fertilizer. Overall, the strongest
significant correlation was with extractable P in the forest floor. The best corre­
lations involving nutrients in mineral soil were with ratio of extractable P/min­
eralized N, followed closely by the ratio of extractable P/total N. These data
suggest that, in general, positive growth response to N fertilizer by hemlock is
most likely to occur when extractable P is high and total and mineralizable N are
low. This conclusion is in agreement with our discussion above; it also supports
earlier results implicating P as a possible factor in the erratic response of hemlock
stands to N fertilization (Anderson and others 1979, Heilman and Ekuan 1980,
Radwan and DeBell 1980b, Gill 1981). Conversely, results with S, especially the
negative correlation between mineralized S in the soil and response, indicate that
S is probably not involved. Field experiments involving application of P with and
without N at carefully selected sites are necessary before a definite and practical
regime can be recommended to obtain a consistent positive growth response of
hemlock.
LITERATURE CiTED
ANDERSON, S. J., R. J. ZASOSKI, and S. P. GESSEL. 1979. Gr eenhouse nutritional studies of conifer
seedlings in two coastal Washington soils. Northwest Sci Abstr 1979:36.
476 I
FOREST SCIENCE
BRAY, R. H., and L. T. KURTZ.
1945. Determination of total, organic, and available forms of
phosphorus in soils. Soil Sci 59:39-45.
BREMNER, J. M. 1965a. Total nitrogen. In Methods of soil analysis, Part 2 (c. A. Black, ed). Agronomy
9: 1149-1178.
BREMNER, J. M. 1965b. Inorganic forms of nitrogen. In Methods of soil analysis, Part 2 (c. A. Black,
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BUTTERS. B., and E. M. CHENERY.
1959. A rapid method for the determination of total sulphur in
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DEBELL, D. S., E. H. MALLONEE, J. Y. LIN, and R. F. STRAND. 1975. Fertilization of western hemlock:
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Fox, R. L., R. A. OLSON, and H. F. RHOADES. 1964. Evaluating sulfur status of soils by plant and
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GILL, R. S.
1981.
Factors affecting nitrogen nutrition of western hemlock. Unpubl Ph D thesis,
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HEILMAN, P. E., and G. EKUAN.
1980. Phosphorus response of western hemlock seedlings on Pacific
coastal soils from Washington. Soil Sci Soc Am J 44:392-395.
OLSON, J.
1979. Phase I, western hemlock fertilizer response analysis. Report to Regional Forest
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OLSON, J., W. ATKINSON, and M. RINEHART.
1979. Response of western hemlock to nitrogen fertil­
ization and thinning in the Pacific Northwest.
In
Forest fertilization conf proc (S. P. Gessel. R.
M. Kenady, and W. A. Atkinson, eds), p 67-77. Univ Wash, Seattle, Wash.
OLSON, J., W. ATKINSON, and M. RINEHART. 1980. Radial increment response of western hemlock
to nitrogen fertilization and thinning. Regional ForestNutrition Research Project Tech Rep, 9 p.
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