89
SITE INDEX AND SELECTED SOIL PROPERTIES IN RELATION TO
RESPONSE OF DOUGLAS-FIR AND WESTERN HEHL08K TO NITROGEN FERTILIZER
H. A. Radwan and J. S. Shumwayl
Abstract.--Twenty-five sites of Douglas-fir (Pseudotsuga
menziesii (Hirb.) Franco) in western Oregon and Washington and
sixteen sites of western hemlock (Tsuga heterophylla (Raf.)
Sarg.) in western Washington Jere examined to determine rela­
tionships of site index and soil N, P, and S to growth response
of the trees to N fertilizer. All sites were selected from
among the fertilizer-test installations of the University of
Washington Regional Forest Nutrition Research Project (RFNRP) .
Site indices and growth responses were based on RFNRP data.
For Douglas -fir, site index, total N, and mineralizable N were
the only variables correlated (negatively) with growth response.
For hemlock, the strongest correlations were with extractable
P, extractable P/mineralizable N, and extractable P/total N.
Total N and mineralizable S in mineral soil were highly corre­
lated and showed moderate negative correlations with growth
response of hemlock, but site index was not correlated with
response. Results suggest that site index and soil N seem
promising as indicators to predict the response of Douglas-fir
to N fertilizer. In addition to N, extractable P, especially
in the forest floor, also seems to have potential as a predic­
tor of hemlock's response. Soil N appears to be more important
than soil P for predicting response of Douglas-fir to N ferti­
lizer on the sites studied, but P may be important on coastal
sites where levels are usually lower. Soil S does not appear
to limit response to N fertilizer of either species on the
sites studied and does not seem promising for estimating poten­
tial response to N fertilization.
Additional key words: Pseudotsuga menzies11, Tsuga heterophylla,
totpl N, mineralizable N, sulfate S, mineralizable S, extractable
P, forest floor, prediction of growth response to N fertilizer.
Operational application of synthetic fertilizer to commercial forests
in the United States began almost 20 years ago. By 1979, fertilizer had
been applied to nearly 364,000 ha (900, 000 acres) in the Southeast and about
486, 000 ha (1.2 million acres) in the Pacific Northwest (Bengtson 1979).
l principal Plant Physiologist, Forestry Sciences Laboratory, Pacific
Northwest Forest and Range Experiment Station, USDA Forest Service, and Soil
Scientist, Forest Land Hanagement Center, Washington Department of Natural
Resources, Olympia, Washington. The authors thank the Pacific Northwest
Regional Forest Nutrition Research Project, University of \olashington,
Seattle, for providing the site index and some of the growth response data,
making the studied sites available for collecting soil samples, and
reviewing the manuscript.
From Earl L. Stone, ed., Forest Soils and Treatment Impacts
North American Forest Solis Conference, June
1983,
(1984),
Proceedings of the Sixth
The University of Tennessee, Knoxville.
Reproduced by USDA Forest Service
for official use.
About This File:
This file was created by scanning the printed publication.
90
Slash pine (Pinus elliottii Engelm.) and loblolly pine (Pinus taeda L. ) have
been the principal species fertilized in the Southeast, with phosphorus (P)
fertilizer applied at planting and nitrogen (N) , P, or N plus P fertilizers applied to established stands (Pritchett and Gooding 1975, Duzan et al. 1982 ) . I n the Pacific Northwest, o n the other hnnd, N fertilizer =--usua1ly as urea
has been applied mostly to young and middle-aged stands of
Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and, occasionally, to a
few stands of western hemlock (Tsuga heterophylla (Raf.) Sarg. ) .
Regardless of geographic area, growth response to fertilizer application
has not always been consistent. For example, response of established stands
of loblolly pine to N alone or to combinations of N and P fertiliers has
shown much variability (Pritchett and Smith 1972) , Also, application of N
fertilizer to western hemlock has not generally been successful, particular­
ly in the coastal hemlock zone (Webster et al. 1976) . Furthermore, although
Douglas-fir responds well to N fertilize
about 30 percent of the unthinned
stands do not respond positively to such treatment (University of Washington
1974 ).
The exact causes of the variability in growth response to fertilizer are
presently unknown, Understanding the factors affecting response to fertiliz
tion, and development of methods which can accurately predict response would
undoubtedly increase the chances of success of the fertilization treatment
and enhance the efficient use of fertilizer,
Foliar analyses, soil tests, and stand (or site) conditions have been
su g gested as predictors of response. So far, investigations in the West and
in the Southeast have focused on evaluations of foliar N and P (Leaf 1973,
van den Driessche 1979, Lea and Ballard 1982b); foliar sulfate (Turner et
al. 1977) ; various soil-N indices (Shumway and Atkinson 1978, Powers 1980,
Lea and Ballard 1982a); extractable soil P, amount of silt and clay in sur­
face soil, and available soil moisture (Ballard and Pritchett 1975, Kushla
al, 1982) .
and Fisher 1980); and stand basal area and site index (Duzan
To date, none o f the indices proposed has been universally accepted but
the work continues, In the present study, we evaluated some of the previ­
ously suggested site characteristics and some additional soil properties as
predictors of growth response to N fertilizer by the two most important com­
mercial forest tree species in the Pacific Northwest--Douglas-fir and west­
ern hemlock. As with most studies, we recognize that the scope of this
investigation is limited by the small number of sites and soil factors stud­
ied, However, we also recognize that investigation of additional stands and
site variables in the future would refine our results and should ultimately
provide better information to enable the forest manager to successfully
select stands with the greatest potential for response.
MATERIALS AND METHODS
The sites
Twenty -five Douglas -fir sites and sixteen western hemlock sites were
selected from among the fertilizer-test installations of the Regional Forest
91
Nutnition Research Project (RFNRP) of the University of. Hashington, Sites
for both species ranged in elevation from 0 rn to 900 rn, Soil parent mate­
rial was glacial, igneous, ash, or sedimentary for Douglas-fir, and glacial,
sedimentary, or volcanic for hemlock. The Douglas-fir sites are located in
both western Hashington and Oregon; there wer0 15 unthinned and 10 thinned
stands (fig. la) . The hemlock sites are located in western Washington.
Eight sites occur in the coastal hemlock ;;:one within about 40 krn. of the
PacHic coast, and the other eight are loca.ted inland o.n the \vest slopes of
the Cascade Range (fig. lb), On average, the coastal zone is considered to
be more productive than the Cascade zone, and N fertilization is believed
to· be· more successful in the Cascades (Hebstcr et al. 1976, Olson et al.
1980). All hemlock stands were unthinned.
The Douglas-fir stands were selected from among 160 unthinned apd thin
ned RFNRP' installations in Oregon and Washington to proviqe a wide range of
response, site, and stand characteristics. The hemlock stands covered 16
of the 18 undamaged, phase-1 RFNRP installations located in \olashington.
Since there were only eight installations available on the coast, we selec·­
ted, at random, eight Cascade installations.
Site index
S te indices at 50-year, breast-height age for both Douglas-fir and
western hemlock were calculated from heights and breast-height ages,
Indices of Douglas-fir, estimated according to King (1966) , were provided
by the RFNRP. Values for hemlock, based on 60 trees at each site, were
calculated from measurements obtained from the RFNRP and use of Hiley's
tables (Hiley 1 978) ,
Growth response
For both Douglas-fir and western hemlock, growth response to 224 kg N/ ha
was estimated. Eight-year, basal-area response of Douglas-fir was calculat­
ed fcorn data provided by the RFNRP. Response lvas based on periodic annual
increments (p.a.i.) , adjusted for differences in initial basal area, and
computed from the adjusted means as the percent increase of the fertilized
(p. a.i.) over the unfertilized trees (p.a.i.) . Seven-year, radial-increment
response of hemlock was calculated using a method of pairing fertilized and
unfertilized trees (Olson 1979, Olson et al. 1979),
Forest floor and soil sampling and processing
Western hemlock sites lvere sampled for forest floor and the underlying
soil to a depth of 15 ern. Douglas-fir sites were sampled for mineral soil
only because not all sites had a distinct forest floor layer. In both cases,
30 samples, each 78,5 crn2 in surface area, were collected from the individ­
ual sites. Sampling was along three 30-m transects laid out in the untreated
strips between the RFNRP plots of each site, There were 10 samples per tran­
sect, and samples from each transect were cornposited and mixed thoroughly.
Samples were air-dried and large pieces of roots, stems, and rocks were re
moved. Forest floor samples were weighed and ground. Soil wRs passed
through a 2-mrn sieve, and the resulting fractions were \veig\1ed, For deter­
mination of total N, subsarnples of the sieved soil were pulveriz<;>d to p,ass
through a 0.5-rnrn sieve. All samples were stored at -15°C until analyzed.
l:
B
A
o CASCADE
• COAST
o THINNED
• UNTHINNED
100 KM
1
•
o2 4
o 23
0109
o108
o111
o58
"'
N
•OLYMPIA
•4 6
25•
•89
1 4
•62
o14 0
0 125
o18
14 5
•
BEND
W. WASHINGTON
0
0117
•ROSEBURG
OREGON
Figure
1. Approximate
location of
(A)
Washington and Oregon and
thinned and unthinned study stands of Douglas-fir in western
(B)
western hemlock study sites in western Washington.
93
Chemical analysis
Moisture content in forest floor and soil samples was determi'ned b y
drying to constant weight a t 65°C and 105°C, respectively. Total N was
assayed by the micro-Kjeldahl procedure (Bremner 1965a}. Mineralizable N
was determine under anaerobic conditions for soil samples and aerobic
conditions for forest floor materials. Anaerobic conditions were obtained
by waterlogging according to Waring and Bremner (1964)' and incubation for 2
weeks at 30°C.
Forest floors were run under aerobic conditions because
materials tended to float if the samples were waterlogged. Aerobic condi­
tions were achieved by mixing subsamples of forest floor with acid-washed
quartz sand and adjusting the moisture of the mixture to approximate field
capacity. Ammonium produced under anaerobic conditions was determined by
steam-distillation in presence of KCl and MgO, Similarly, ammonium and
nit rate pt'oduc tion under aerobic conditions were assayed as ammonium by
steamrdistillation in presence of KCl, MgO and Devarda's alloy (Bremner
1965b). Mineralized N was calculated by subtracting initial ammonium and
nitrate contents from postincubation concentrations.
,,
Sulfate S, extracted with Ca(HzP04 ) 2 according to Fox et al. (1964) ,
was determined by the turbidimetric method of Butters and Chenery ( 1959) .
Optical density readings were corrected for presence of color in bhe ex­
0 05 at
tracts, and sulfate was considered nil at net optical densities of
490 nm.
,
Mineralizable S was determined under aerobic conditions as describe be­
fol'e. Soils and forest floors were incubated at 30°C (Williams 1967) for 2'
and 3 months, respectively. Sulfate, extracted from incubated samples with
Ca(H 2 P04 ) 2, was determined turbidmetrically as described above.
Minerali.zed S was estimated by subtractfng initial sulfate contents from
postincubation concentrations.
Phosphorus was extracted with· Bray and Kurtz soluti,on 2 (O.OJN NH4 F in
O. lN HCl), and determined colorimetrical]y (Bray and Kurtz 1945) .
Statistical analysis
Data were subjected to analysis of variance. Correlation, coefficients
(r) between the variables determined and' growth response to N fenti:lizev
were ca,lculated (Snedecor 1961) . In all analyses, results were consldered
significant at p .2_ 0.05.
RESULTS
Site index and growth response to N fertilizer
Site index at 50-year, breast-height age averaged 3'J m for Douglas-fir.
and 34 m for western hemlock. Differences between unthinned and, thinned,
Douglas-fir and between the coastal and, Cascades hemlock were sma1l (tables
1 and 2) .
Growth response to N fertilizer varied greatly among the sites af each,
of the two species. Average response for hemlock was generally low, but
good positive response occurred on some sites in both the coastal and
94
Table 1,-­Selected site and stand characteristics and chemical properties of
the mineral soil of Douglas-f ir. a
Site number/
location
Site
index
Response
to N
m
%
37
35
32
30
30
31
36
37
26
38
38
31
35
45
37
4
0
29
55
33
33
5
31
34
1
18
36
28
-3
54
Total
N
Miner­
alized
N
Extract­
able
Miner­
alized
s
- - - - kg/ha - -
-
p
-
-
-
-
Unt hinned
Stands
99,
77,
60,
76,
43,
5,
14,
33,
50,
10,
32,
62,
25,
89,
46,
Wash.
lvash,
Wash.
Wash.
Wash.
Hash.
Hash,
Wash.
Oreg.
Oreg.
Oreg.
Oreg.
Oreg.
Oreg.
Oreg.
Average
34
1260
2149
715
619
191
685
147
445
507
1722
1109
1355
754
1394
285
24
X
889
0
0
7
0
14
0
0
0
8
0
0
0
0
0
0
24
22
6
2
0
7
7
5
4
59
15
12
24
83
11
19
X
2
X
9
48
27
41
33
32
85
27
50
13
17
11
95
73
44
6
4
5
3
1
0
0
8
0
0
0
0
0
0
2
X
40
X
39
X
X
Thinned
St ands
138,
159,
150,
114,
147,
148,
142,
145,
140,
125,
Wash.
Wash.
Wash.
Oreg.
Oreg.
Oreg.
Oreg.
Oreg.
Oreg.
Oreg.
Average
0
34
22
-6
8
47
42
45
26
8
36
26
29
46
43
32
34
35
40
36
36
X
23
917
311
1698
104 9
2568
879
962
1216
1317
1893
24
1
14
54
43
7
33
37
32
58
0
0
0
28
8
3
0
0
2
0
0
0
0
0
0
14
0
0
0
0
1
X
15
42
126
32
50
42
17
16
18
34
4
X
a Sit e numbers and site index (at 50 years) are t hose of t he RFNRP Fer­
t ilizer-test inst allations. Eight -year, basal-area growth response t o
application o f 224 k g N/ha were calculated from measurement data b y t he
RFNRP. Averages in the same vertical column f ollowed by the same
let t er are not stat istically diff erent at p <0. 05,
95
Table 2.--Selected site and stand characteristics and chemical properties of
the forest floor of western hemlock. a
Site number/
location
Response
to N
Site
index
Total
N
Miner­
alized
N
Miner­
alized
s
- - kg/ha -
m
%
34
36
37
30
35
38
33
36
1
7
17
10
3
3
-20
16
Extract­
able
p
-
Coastal
Zone
100
84
15
80
9
4
3
42
35
345
23 7
391
4 21
3 16
23 2
215
616
5
X
34 7
9
8
1
3
9
4
2
12
X
6
0
0
0
0
0
0
6
0
0
X
0.6
1.2
3.8
2. 6
1.0
1. 8
0. 4
1.8
5
1
1
3
1
1
2
5
X
2
X
1. 6
Cascade
Zone
24
23
109
108
111
58
18
117
20
35
33
36
38
32
36
34
20
9
t2
-2
2
-12
5
38
33
a
X
6
1
9
24
11
6
6
6
-2
513
3 26
559
286
299
246
360
212
}50 x
8 x
1
2
4
1
1
1
3
1
1
0
1
0
0
0
0
0
0
X
2
5.0
1.4
1. 6
2.2
1. 6
2.4
1.2
8. 2
X
Site rtumbers are those of the RFNRP fertilizer-test installations.
Site index based on heights and ages of 60 trees at each site measured
by RFNRP, Seven-year, radial-irtctement gro th rebponse to 224 kg N/ha
Averages in
determined by Olson (1979) using a tree-paitirtg method;
the same vertical column followed by the same letter are not
statistically different at p .2_ 0. 0),
X
96
Cascade zones (Olson 1979) . Average response of Douglas -fir \ as much high­
er than that of hemlock (24 vs. 6%) , and responses of the unthinned and
thinned Douglas -fir stands \ ere similar, Site index was significan tly
0,02) but
correlated with growth response of Douglas-fir (r
-0.60, p
not with growth response of hemlock,
m
m
Total N
Total N in the mineral soil of Douglas-fir averaged 889 kg/ha (0. 15%)
and 1281 kg/ha (0.16%) for the unthinned and thinned stands, respectively
In the hemlock sites, average amounts of total N were 348 kg/ ha
( table 1) .
(0.84%) and 1830 kg/ha (0, 33%) for the forest floor and mineral soil, res­
pectively; total N was also much higher in the coastal than in the Cascades
mineral soil (average, 2198 kg/ha and 0, 40% vs. 1463 and 0.26%) (tables 2
and 3) .
In addition, total N of hemlock mineral soil was much higher than
for Douglas-fir ( tables 1 and 3) .
---
Total N of hemlock forest floor was not significantly related to growth
response ( table 4) . In contrast, total N of mineral soil was negatively
correlated with growth response to N fertilizer of both Douglas-fir and
wes tern hemlock.
Mineralizable N
As with total N, the average mineralizable N values for the Douglas-fir
mineral soils \·7ere much lower than those for hemlock (23 vs. 38 kg/ h a) ; min­
eralizable N was also higher in the thinned than in the unthinned s tands
(30 vs. 19 kg/ha) (table 1) . Similarly, the average mineralizable N was
much higher in the coastal than in the Cascades mineral soils of western
hemlock (47 vs. 28 kg/ha) ( table 3) . For both Douglas-fir and western hem­
lock mineral soils, mineralizable N was significantly correlated with total
N. N mineralized in the hemlock forest floor was small; it averaged 7
kg/ha, with no significant differences between zones ( table 2) .
Mineralizable N of either the forest floor or mineral soil was not sig­
nificantly correlated with growth response of hemlock. In contrast, there
was a s trong negative relationship between mineralizable N and Douglas-fir
response (table 4, fig. 2a) .
Sulfate sulfur
Mineral soils of the Douglas-fir sites and the forest floors of the
hemlock s tands were very low in available S as measured by levels of sul­
fate S ( tables 1 and 3) , Sulfate was detec ted in only four samples of
Douglas -fir soils and in two of the hemlock forest floor samples. Mineral
soil of the hemlock s tands, on the other hand, contained much higher amounts
of sulfate (up to 50 kg/ha) , and significantly greater amounts occurred in
the coastal soils than in soils of the Cascades ( table 3) . Amount of sul­
fate S, however, was not correlated with growth response of western hemlock
or Douglas -fir.
Mineralized sulfur
Mineralization of S was much slower than that of N, and mineralized S
was lower than that of mineralized N for both Douglas-fir and western
97
Table 3.--Selected chemical properties of the mineral soils of the western
hemlock study sites.a
Site number/
location
Miner­
alized
N
Total
N
Miner­
alized
s
- - - - - - - - - - - - kg /ha - - - -
-
Extract­
able
p
- -
-
-
- -
-
Costal zone
100
84
15
80
9
4
3
42
AverAge
42
88
15
21
43
56
72
41
2096
2683
1735
1057
2224
24 22
2977
2390
2198
X
47
30
4
4
0
15
5
26
so
X
17
2
4
30
7
2
9
3
13
15
14
13
6
10
l3
20
17
X
14
9
X
X
Cascade zone
24 23
109
108
111
58
18
117
Average
a
337
1104
1597
1469
2187
1442
2962
606
4
43
28
30
32
23
54
11
0
0
38
l3
10
2
17
0
1
16
l3
13
15
11
21
0
ll
14
43
2
27
7
18
3
30
X
Site numbers are those of the RFNRP fertilizer-test installations.
Averages in the same vertical column followed by the same le'tter are ne>t
statistically different at p .2_ 0. 05.
98
Table 4,--Correlation coefficients (r) among soil nutrients and between
nutrients and growth response of Douglas-fir and western hemlock
to N fertilizer,&
Mineral soil
Douglas-fir
w. hemlock
p
r
r
p
Correlation
Forest floor w. hemlock r
P
0. 066
N vs. mN
0. 61
0,001
0. 82
0. 001
0.47
N vs. eP
-0.01
0,999
-0.56
0,023
-0,01 N vs.mS
0.83
0,001
0,62
0,011
mN vs. mS
0.68
0. 004
0.52
0,041
-0. 42
0.261
-0. 51
0,043
-0.22
0,401
-0.35
0,181
-0.01
mN vs. eP
0,836
mS vs. eP
l).
971
N vs. growth response
-0,50
0,011
-0,50
0,050
0.15
0,581
mN vs. growth response
-0.60
0.001
-0.43
0,093
-0.47
0,067
so -s vs. growth response
4
-0.34
0,203
mS vs. growth response
-0.60
0,014
-0.13
0,639
eP vs. growth response
eP/N vs. growth response
eP/mN vs. growth response
0,08
o. 715
0.44
0,084
0.77
0,001
-0.01
0. 991
0.66
0,006
0,68
0,004
0.6 7
0,005
-0,06 0,825
0, 70
0. 002
eP/ mS vs. growth response
a mN
mineralized N, eP
extractable P, mS = mineralized S.
tions considered significant at p .:<:_ 0,OS
=
=
Correla­
99
1,
hemlock
(tables
hemlock
mineral soils,
2,
and
3).
Most of the S mineralization occurred in the and amounts were negatively correlated with growth response to N fertilizer (table
4). E xtractable P
Extractable P in the Douglas-fir soils ranged
(13 to
194
ppm),
from 9 kg/ha to 126 kg/ha
with similar averages of about 40 kg/ha
the unthinned and thinned stands (table
(63 ppm) for both
For hemlock,
1).
amounts of extractable P in the forest flo()r and
the average
in mineral soil ·were sLg­
nif'icantly higher i n the Cascades than in the coastal sites (forest floor,
3. 0 vs.
1.6 kg/ha;
mineral soil,
18
9
vs.
kg/ha) (tables 2 and
3).
Hemlock
mtneTal soil also contained much less extractbble P than the Douglas-fir
soils (average
14
v&.
·Growth response of
with extractable
·p
40 kg/ha) {tables
hemloc'k to
N
1 and 3).
fertilizer was sigt1ificantly correlated
and the ratio ext.ractable P /tolal N · of
the forest floor,
and · wit:h the ratio extractable P/total N and .the ratio extractable ·P/mineral
fig.
izable N of mineral soil (table 4,
however,
2b).
Extractable P and
did not correlate with gro th response of Douglas-fir.
Its ratios,
DISCUSSION AND ·CONOilUSIONS
Growth response to N fertilizer \vas muc'h g.reater and •more consistent in
.Doug!las-fir t han in ·hemlock.
species in many respec:ts,
This reflec·ts ·i:li'fferences ·between the t\vo
including loca1tion and •properties
and stand5 ·selected for study,
as \ ellil <JS .g.rowth :ha·bii·ts.,
nut•ri't·i·onal requirements ··of t' he 'trees.
sidered in this study,
therefore,
of
the sites
physiology,
and
Si<te 'index and soil factors ·con­
difFered
in
'their value as indices to
prelliclt growth Tesponse ·tO N cfertil·izeT '•by ·eac'h
·Of
· the two spec.ies.
S·i· e i.ndex was no·t correla.ted with ,grolvt:'h -response to N fertilization
of 1hemilock.
This has been reported ·earli-er i(W
> ebst
· er et al.
In contrast
' .,
ductivi:ty was not lo\v N.
af f·eclted
1976,
Olson et
i'hemlock' s
and indicates that the pr'imary >fac<tor lt.mi'tin
al. 1980),
by N,
with 64% of
pro- ­
p·roduct
' ivity .of :Douglas-fir was
the sites 'respond·img mo1re 'th
' an
The data
20%.
also suggest that site index may be a ·poten'tiially 'use'f•u'l lind•icator <for
pred·ic't·ing response of Douglas-fir,
there.fo 're,
•Doug as-'fi·r st
• qnds ·on .poor sl tes,
would be expected to <be more respt'msive to 'N fertilizer th3n
thGse •occupying ·the more productive si-tes.
'However,
recent results from the
RFNRP ins,tallations show t•hat volume response i
· n
·
unthlnned
Dol1g as-fit· t-•as negatively
related
bo sllt·e
stands of
1mdex ove.r •the fh·st 1,
ye;;rs
after ,fertilization and .not beyond; voll•ume <response 'in •thinned stands \vas
not
significantly related 'to site index
i(:Pet•ewson
and 'Gesse<l
11983).
Still,
some if·orest managers in 'the Pacific 'Nort'hlvest
'
are now usli·ng site .index as a
guide in the selection of Douglas-fir stands for applying N
Total
sites.
N was much higher in ·the hem!loc·k si't·e'S
T·h is may expll;lin 't·he ovet
. ·allll ·llti'f.ference
between t
' he
two species.
'However,
Num
fertllizer.
'i.·n t'he 1Doag1as-f.J.t·
ln r
· esponse to N 'f·e·n i a 'l·ze'r
our data suggest 't'hat to•ta'l
'N
of ·the -sur­
face ml nel"81 soil seems promising as a predictor ·o' f response 'for both
100
100
......
0
-w
(/)
z
0
0..
(/)
w
IX
J:
.....
A DOUGLAS-FIR
80
THINNED
UNTHINNED
r = -0.60
0.01
p<
II
•
60
•
•
II
40
II
Ill
•
3: 20
0
a:
"
0
0
40
e...
20
30
40
50
60
70
MINERALIZABLE N IN SOIL (kg/ha)
10
0
B WESTERN HEMLOCK
30
w
en
z
0
0..
20
a:
J:
10
•
ffi
0
a:
"
90
80
COAST
oCASCADES
r = 0.77
p< 0.01
•
0
-10
0
0
4
2
0
10
8
6
EXTRACTABLE PIN FOREST FLOOR (kg/ha)
Figure
2.
Relationships between growth response to
mineralizable
N
N
fertilizer and
in mineral soil of Douglas-fir
(A), and
between response and extractable P in forest floor of western
hemlock
(B).
IOl
Douglas-fir and we stern hemlock.
Tu1·ner e·t ;}1. (1'979·) ha•ve
. · suggested a
combination of total N and soil paren
ma!levTG-t to predict response of
Douglas fir to N fertilizer.
There is no ]iterature on the Felationship
between soil N and fertilizer response in 1\emJ:oclt.
Shumway
and· Atkinson ( 1978) suggested' use o.f mineralizable N as a
general guide in se}ecting unthinned• stands. of Dougl'a•s-fir for N fertiliza.­
• izable N for Dougl!as-fir and western hemlocl
tVon.
Our results with mineral
.
for p.r,;,­
also· suggest that mineralizable N appears. puomising as a soil test
dicting response of Douglas-fir,
interest
illLzable N index.
Presently,, there is mud
but no.t hem.1o<ek.
among forest managers in the Pacific
Northwest in using the miner­
It should be pointed· outc, however, that in this stud'y
mineraTizable N is hilrdly better than· site .t'ndeX! i:n pl.'edkhng: response o•f
Douglas-fir to N fertilizer.
avai1ab.ility·,
which affect
N
}may ind:i:cate potential'
Hi
· nera1iza·b•lie
but it does note provtd·e any. information about other factors
growth.
8onsequently, the mineuaJizable N indeX!. like other
:tnd·ices based on limited soil or foliar ana.ly,ses,
w:tll not be useful when
other nu rients or some other growth fac v.or a·re limiting.
The very low levels of sulfate S found i:n. minera·l soi-ls of
fir sites and
For
\l o t h
Doug·las-fir and' 1vesterrn· hemlock, su],fa•te S
gro1vtl'
correJ:<Jted w.ith
other
however,
lizab1e S as an index
best,
response to N
fertilizer.
in
the sod:Ji was not
N'ineralizable S, on
hand, was re ated (negatively) tto· rresponse, only
J'el.1tiionship,
he Douglasr
the forest floors of heml'ock \vere probab1y caused: by reaching
in hemlock.
the
This
is not presently und'erstood', and the value of miner­
fDl'
pFedicting gro\v.tl\' uesponse
l!:o N l'ertilizer is,
at
questionable.
As· w:1th mineralizabJie N,
the ex.tractable P resu:lts.
there \Vas mucl\ d'iffleven<ee between species in
. ·l e P
The lack of corrre·]at:ton bet1veen· extractab
and growth response in Douglas-fir and the relationships obtained with
hemlock indicate
move important
a stt·ong
that extrac able P,
implicated as a· posstble factor
hemlock: stands to N fertiHzation befove
N'eurisse
( 1976)' reported'
producttvity
Feiationsl,ip between soil P and! sfte
has also been
in the forest floor, is
especta1}y
for hemlock than for Douglas-fir.
of hemlock.
Radwan, and• EleBell
1980, (';.ill 198'1'
Radwan and Shum\vay l983a·), and· hem·lock has vesponded' positively to
tions
i'n greenhou,se tests (Heilman andi
Rad1van
and'
Shutmvay 1983b).
Th·is,
Ekuan
P
in· the erratic response of
]9801,. Anderson· et aL
P
add:i­
1982,
togetther lv·Hh• tihe significantcorrela·tion
het\veen P/N ratios and• nesponse in hem:lcock,. sugges.t 11' and' P'/N· ratios as pro
mising indicators of response to N fertrilizer i' n hem'l'ock.
Such P· indices
also d serve study in Douglas-fir, especiali]y in coastal regions where P
may be limiting (Strand' and'. Austin• 1:966.;
at US1iJA
Radwan and' Shum�<�ay,
data on file
Forest Service Forestry Sciences !1a·boratory., @clymp.fia', \<lash.),
'Phis study invo1ved• a few sites and: a sma<ll number of soil varL·abJies.
Much more work is needed before nesponse to
N
ferti izati'on
by
the importan
forest tL"Pe species of the Pacific Noutrh<ves·t can· b" accul7atre1·y· pvedi!e ed',
ln such wot·k emphasis must be on devel10p.i•ng dragnosuh: technS'ques basedi
two· (>r more fac ors
firom among the soiL,
istic_s whlch can influence response.
lt.eve,
foliage,
s·uelv
a,n approacd
to asstHe correct predictrions when. motoe
on•
stand•, and1 s te chnracter
ts· necessaFy,
than one factor
we be_j
ts limiting.
102
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