About This File:
This file was created by scanning the printed publication.
Misscans identified by the software have been corrected;
1222
h0l.lliever, some mistakes may remain.
Effect of forest floor on growth and nutrition of Douglas-fir and western hemlock seedlings
with and without fertilizer
M.A. RADWAN
USDA Forest Service, Forestry Sciences Laboratory, Pacific Northwest Research Station,
3625 93rd Avenue, Sw, Olympia, WA 98512, U.S.A.
Received September 9, 1991 Accepted February 27, 1992 RADWAN, M.A. 1992. Effect of forest floor on growth and nutrition of Douglas-fir and western hemlock seedlings with and
without fertilizer. Can. J. For. Res. 22: 1222-1229.
Experiments were conducted to determine the effects of four different forest soils on growth and shoot nutrients of potted
Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf. ) Sarg.) seedlings, in
absence and in presence of forest floor, and with and without N and P fertilizers. Nine-month-old seedlings from low-elevation
seed sources were used, and seedlings were grown for 2 years in a roofed lathhouse. Soils were of the KIone, Vesta, Bunker,
and Shelton series; Klone and Vesta soils, and Bunker and Shelton soils, were collected from western hemlock and Douglas-fir
stands, respectively. The fertilizers ammonium nitrate at 100 kg N/ha and triple superphosphate at 226 kg P/ha were tested.
The forest floor, at 70 g/7.6-L pot, and the N and P fertilizers were added to the top of the planting pots without mixing.
The forest floors and mineral soils differed by source in many of the chemical characteristics determined. Overall, seedling
growth of Douglas-fir and western hemlock was better in the KIone and Shelton soils than in the Bunker and Vesta soils.
Seedlings, especially those of western hemlock, grew better with than without forest floor. The N fertilizer reduced seedling
growth of both species and, in some soils, reductions were more with than without forest floor. The P fertilizer improved
seedling growth of both species in all soils and, with one exception, growth was much greater in the presence than in the
absence of the forest floor. With both species, soil, forest-floor, and fertilization treatments affected concentrations and
contents of the various shoot nutrients determined. The nutritional changes observed varied by nutrient and reflected
differences in uptake of native and fertilizer nutrients, as well as changes in shoot dry weight. The results demonstrate the
importance of the forest floor to growth and nutrition of Douglas-fir and western hemlock seedlings, especially when fertilizers
are used.
RADWAN, M.A. 1992. Effect of forest floor on growth and nutrition of Douglas-fir and western hemlock seedlings with and
without fertilizer. Can. J. For. Res. 22 : 1222-1229.
Des experiences ont ete conduites afin de determiner les effets de quatre sols forestiers differents sur la croissance et les
nutriments de la partie aerienne de semis de sapin de Douglas (Pselldotsllga menziesii (Mirb.) Franco) et de pruche de rOuest
(Tsuga heterophylla (Raf. ) Sarg.) cultives en pot, en absence et en presence de la couverture morte, et avec ou sans fertilisants
azotes et phosphates. Des semis ages de 9 mois proven ant de semences de basse altitude ont ete utilises et cultives pendant
2 ans dans un abris en lattis recouvert d'un toit. Les sols etaient les series Klone, Vesta, Bunker et Shelton. Les sols des
series Klone et Vesta, et Bunker et Shelton, ont ete recoltes respectivement de peuplements de pruche de rOuest et de sapin
de Douglas. Les fertilisants testes etaient Ie nitrate d'ammonium applique au taux de 100 kg N/ha et Ie super phosphate triple
applique au taux de 226 kg P/ha. La couverture morte, au taux 70 g par pot de 7,6 L, et les fertilisants N et P ont ete ajoutes
a la surface du sol, sans melange avec ce dernier. Les couvertures mortes et les sols mineraux different selon la source pour
plusieurs des caracteristiques chimiques qui furent determinees. Dans r ensemble, la croissance des semis de sapin de Douglas
et de pruche de rOuest etait meilleure pour les series Klone et Shelton que pour les series Bunker et Vesta. Les semis,
particulierement ceux de la pruche de rOuest, ont eu une meilleure croissance avec la couverture morte ajoutee que sans
celie couverture morte. La fertilisation azotee a reduit la croissance des deux especes et pour certains sols, les reductions
etaient plus fortes lorsque la couverture morte etait presente. La fertilisation phosphatee a ameliore la croissance des semis
des deux especes pour tous les sols et, a une exception, la croissance etait de beaucoup superieure lorsque la couverture
morte etait presente que sans cette couverture morte. Pour les deux especes, Ie sol, la couverture morte et les traitements de
fertilisation ont affecte les concentrations et les contenus des differents nutriments mesures dans la partie aerienne des semis.
Les changements nutrition nels observes ont varie selon les elements et refletent des differences de prelevement des elements
endogenes et des elements outes par les fertilisants aussi bien que des changements dans la masse anhydre des parties
aeriennes. Les resultats montrent I'importance de la couverture morte pour la croissance et la nutrition des semis de sapin
de Douglas et de pruche de l'Ouest, particulierement quand des feltilisants sont ajoutes.
[Traduit par la redaction]
Introduction
response to applied fertilizer. Different forest tree species,
The importance of the forest-floor component of the forest
ecosystem to site productivity is well known. The forest floor
however, may not utilize the forest floor to the same extent,
and their growth may not be affected by it to the same degree.
contains large amounts of nutrients, especially N, P, and S
(Pritchett 1979). The forest floor insulates soil surface from
temperature extremes, protects mineral soil against erosional
For example, more roots of western hemlock (Tsuga hetero­
phylla (Raf.) Sarg.) usually occupy the forest floor than roots
of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco).
forces, preserves soil moisture, improves water infiltration,
and affects soil biology by serving as habitat to a myriad
Some studies have been made of the amount, distribution,
composition, and physical and chemical properties of the
of organisms (Gessel and Balci 1965; Wooldridge 1970;
forest floor of the coniferous forests of the Pacific North­
Pritchett 1979). The forest floor, therefore, may influence
west of the United States and Canada (Gessel and Balci
the success of forest regeneration, tree growth, and growth
1965; Youngberg 1966; Grier and McCa11 1971; Quesnel and
Printed in Canada / lmprime au Canada
RADWAN
1223
TABLE 1. Soil descriptions
Soil 1
Soil 2
Soil 3
Soil 4
Klone
Andic
Haplumbrepts
Gravelly loam
Vesta
Andic
Haplumbrepts
Silt loam
Bunker
Andic
Haplumbrepts
Silt loam
Glacial
Basalt
Basalt
Shelton
Dystric Entic
Durachrepts
Gravelly
sandy loam
Glacial
Coast
Coast
Coast
70
Western
hemlock
270
Western
hemlock
230
Douglas-fir
Item
Soil series
Soil taxonomy
Soil texture
Soil parent
material
Site location in
western Washington
Elevation (m)
Tree species
present
Lavkulich 1981; Carter and Lowe 1986). Present information
on these forest floors, however, is not commensurate with the
significant functions attributed to this important component
of the forest ecosystem. Also, much forestry research is still
being conducted without considering the effect of forest floor.
For example, fertilization trials in the field are usually con­
ducted without much regard to the forest floor, and fertiliza­
tion tests in the greenhouse are routinely run with mineral
soil, without addition of any forest floor.
This study was designed to (i) compare the chemical prop­
erties of the forest floor and underlying mineral soil collected
from different Douglas-fir and western hemlock stands in
western Washington, United States, (ii) determine the effect
of the different forest floors and mineral soils on seedling
growth of the two species, with and without fertilizer,
(iii) assess growth response of the seedlings to N and P fer­
tilizers, and (iv) estimate the effect of the various treatments
on nutrient concentration and content of seedling shoots.
Materials and methods
Test seedlings
Douglas-fir and western hemlock seeds were collected from single
trees growing at low elevation. After stratification in spring 1984,
seeds were sown in Styrob1ock containers filled with a 1: 1 (v/v)
mixture of peat moss and vermiculite, and the containers were kept
in a roofed lathouse. Water and nutrient solution (Hoagland and
Arnon 1950) were added as needed, and seedlings were thinned
gradually to one seedling per cavity by August 1984. In early spring
1985, cavities were flushed with water to remove excess nutrient salts
from the potting mixture. Seedlings were culled to uniform height
(Douglas-fir 7.5-10.0 cm; hemlock 4.5-5.5 cm) before they were
individually planted in 7.6-L plastic pots. A three-seedling group
served as the basic experimental unit, and three replications were used
for each soil - forest floor - fertilizer treatment of each species (nine
seedlings divided into three replications). Pots were placed, as groups
of three, at random on benches in the 1athhouse.
=
=
Test soils and forest floors
Four different soils, two from western hemlock stands and two
from Douglas-fir stands, were used. These soils were selected because
they have wide distribution in western Washington; they also allow
comparison of performance of each species when grown in soil col­
lected from stands of its own and those of the other species. Pertinent
information about the soils is given in Table 1.
Each of the four sites was sampled for forest floor and underlying
mineral soil to a depth of 20 cm. At each site, samples of forest
floor and mineral soil were collected separately from a minimum of
Puget Sound
trough
120
Douglas-fir
20 spots along a 30-m transect through the site. Forest floor and
mineral soil samples from each transect at each site were composited
separately. Twigs, roots, rocks, and other debris were removed, and
each composite sample of forest floor or mineral soil was thoroughly
mixed to yield a representative sample for each site. Thus, in the end,
there were a total of eight composite samples, with each site repre­
sented by two composite samples: one of forest floor and one of
mineral soil. Samples were air dried, and pots were filled with approx­
imately 6 kg of mineral soil each. To test the effect of the forest floor,
soil in one-half of the pots was covered with about 70 g of the
appropriate forest floor, evenly distributed around the newly planted
seedlings. The two forest-floor treatments allowed comparisons of
each species in each of four soils, with and without fertilizer.
Fertilization treatments
There were three fertilization treatments, including an unfertilized
control. Treatments were assigned to the seedling groups of each
species at random. The fertilizers ammonium nitrate (34% N) and
triple superphosphate (20% P) were applied on an area basis at rates
equivalent to 100 kg Nlha (0.86 g fertilizer/pot) and 226 kg Plha
(3.41 g fertilizer/pot). Seedlings were fertilized in May 1985. Fertil­
izers were uniformly distributed around the seedlings on top of
mineral soil or forest floor without mixing to simulate field fertiliza­
tion. Seedlings were watered with deionized water immediately after
fertilizers were applied and periodically thereafter as needed. Leached
soil solution, if any, was collected in shallow plastic dishes and poured
back into the corresponding pots. Seedlings were grown in the lath­
house for 2 years. All pots were protected from the cold winter with
insulation material.
Growth measurements
Height and diameter of the seedlings were measured at the begin­
ning and at the end of the experiment. At harvest in October 1986,
roots were washed free of soil, and seedling shoots and roots were
dried to constant weight at 65°C. Height and diameter growth,
dry weight of seedlings, and percent response to fertilization were
calculated.
Chemical analyses
At the start, representative subsamples of mineral soils were passed
through a 2-mm sieve, and sieved soil was used to determine chemical
characteristics. Soil subsamples to determine N and all samples of
forest floor and ovendried seedling shoots (pooled by replication and
treatment) were ground to fine powder before analysis.
Reaction (pH) of mineral soil and forest floor was determined on
1:1 mixtures with water by glass electrode. Cation exchange capacity
of soil was estimated by the NH40Ac method according to Chapman
(1965). Total N and total S in mineral soil, forest floor, and seedling
shoots were determined by the micro-Kjeldahl procedure (Bremner
and Mulvaney 1982) and by the turbidimetric method of Butters and
Chenery (1959), respectively. Total P in forest floor and seedling
1224
CAN. J. FOR. RES. VOL. 22, 1992
TABLE 2. Selected characteristics of forest floors and mineral soils
Soil series
Forest floor
pH
Kjeldahl N (%)
S ( %)
K (% )
Ca (% )
Mg (% )
P (%)
Bray 2 extractable P (ppm)
Mineral soil
pH
Kjeldahl N (% )
S (% )
Bray 2 extractable P (ppm)
Cation exchange capacity (mequiv./lOO g)
Exchangeable (NH40Ac) K (mequiv./ l OO g)
Exchangeable (NH40Ac) Ca (mequiv.l100 g)
Exchangeable (NH40Ac) Mg (mequiv.l100 g)
Klone
Vesta
Bunker
Shelton
4.20
0.78
0.09
0.16
0.28
0.08
0.08
59.60
4.40
0.83
0.10
0.12
0.24
0.15
0.08
42.80
4.80
1.06
0.10
0.16
0.40
0.10
0.07
36040
4.70
0.90
0.09
0.20
0.68
0.06
0.08
59.60
4.60
0040
0.04
6.80
37.80
0.26
0040
0.33
4.90
0.35
0.06
3.00
43.70
0.27
0.74
0.76
4.80
0046
0.06
7.80
43.10
0.28
1.28
0047
5.30
0.09
0.01
16.80
13.80
0.20
1.68
0.30
TABLE 3. Effect of soil series on growth of Douglas-fir and western hemlock seedlings,
averaged over three fertilization and two forest-floor treatments
Soil
Diameter
growth
(mm)
Height
growth
(cm)
Shoot dry
weight
(g)
Root dry
weight
(g)
Seedling
dry weight
(g)
3.9b
3.3c
3Abc
4.7a
1O.0b
8.3c
8.8bc
11.3a
4.1b
2.3d
2.8c
5.0a
1O.7b
5.2d
6.8c
11.9a
Douglas-fir
Klone
Vesta
Bunker
Shelton
4.1b
4.0b
3.9b
4.6a
27.3a
23.4b
23.4b
29.6a
6.1ab
5.0c
5Abc
6.6a
Western hemlock
Klone
Vesta
Bunker
Shelton
(p
4.3a
2. 9c
3Ab
4.2a
27.6b
18.2c
21.4c
31.8a
6.6a
2.9c
4.0b
6.9a
NOTE: Within each species, means in the same column followed by the same letter are not significantly different
< 0.05).
shoots, and Bray 2 extractable P (Bray and Kurtz 1945) of the soil
and forest floor, were determined by the molybdenum blue method
(Chapman and Pratt 1961). Potassium, Ca, and Mg in forest floor and seedling shoots, and exchangeable (NH40Ac extraction) K, Ca, and
Mg of mineral soil, were assayed by standard atomic absorption
methods (Perkin-Elmer Corporation 1976).
In this study, we collected forest-floor and mineral-soil
samples from one site per soil series. All results, therefore, apply to the samples used in the study and not to the entire range of each soil series. Statistical analyses
Growth and nutrient data were subjected to analysis of variance
using a completely randomized design model. Whenever effects were
statistically significant, Tukey's test (Snedecor 1961) was used to
separate the means. Differences were considered significant at a p­
level of 0.05 except for percent response to fertilizer, where signifi­
cance was at a p-Ievel of 0.1.
Differences in characteristics between the forest-floor composite
samples and between the four mineral-soil composite samples col­
lected from the four sites were not statistically tested because there
was only one composite sample each of forest floor and mineral soil
per site.
Forest-floor and mineral-soil properties
Samples of the forest floors and mineral soils used in the
study differed greatly by source in many of their characteris­
tics (Table 2). In the forest floor, N and pH were highest in
the Bunker and lowest in Klone soil series; K and Ca were
highest in the Shelton and lowest in the Vesta series; and
extractable P was highest in the Shelton and Klone and lowest
in the Bunker soil series.
Mineral soils of Bunker and Vesta were derived from basalt, while Klone and Shelton soils were glacial in origin. Klone soil was lowest in Ca and high in N. Vesta soil was Results and discussion
RADWAN
1225
TABLE 4. Effect of soil series on nutrients in ovendried shoots of Douglas-fir and western hemlock seedlings,
averaged over three fertilizations and two forest-floor treatments
Concentration
Content (mg/seedling shoot)
Soil
p
(ppm)
N
(% )
K
(%)
Klone
Vesta
Bunker
Shelton
577b
547b
478b
746a
2.38a
1.86b
1.67b
1.22e
0,40a
0.36a
0.37a
0,43a
Mg
(%)
Ca
(%)
P
N
K
Ca
Mg
3.8b
3.1b
2.8b
4.9a
126.0a
88.5b
82.9b
77.7b
22.6ab
18.0b
18.8b
28.2a
l3,4b
1O.7b
l3.1b
18,4a
5.6a
5.0a
5.0a
6,4a
30.0b
14.9d
21.7e
38,4a
14.3b
7.2e
11.5b
23.6a
6.9a
3.6b
4.6b
8.3a
Douglas-fir
0.21b
0.21b
0.24b
0.28a
0.09a
O.lOa
0.09a
0.10a
Western hemlock
Klone
Vesta
Bunker
Shelton
616b
500b
596b
777a
1.66a
1.53a
1.44a
0.96b
0.62a
0.69a
0.68a
0.57a
0.22e
0.25be
0.29b
0.34a
0.11a
0.12a
0.12a
0.12a
4.1b
1.6e
2.6e
5.5a
90.0a
33.8e
45.7be
62.0b
NOTE: Within each species, means in the same column followed by the same letter are not significantly different (p
<
0.05).
lowest in extractable P and highest in Mg. Bunker soil was
particularly high in N and Ca. Shelton soil was very low in
N, S, and cation exchange capacity, but high in Ca and extract­
able P. For all soils, extractable P was much lower in the
mineral soil than in the forest floor.
of this study, nutrients and other factors required for good
growth of the two species were similar and were provided
more by the Shelton and Klone soils than by the Bunker and
Vesta soils. This may not be true in the field, however, because
of the many differences between field and lathhouse conditions.
Within each species, concentrations and contents of most
shoot nutrients determined differed significantly among the
soils (Table 4). For all fertilization and forest-floor treat­
ments, nutrient concentrations in shoots of Douglas-fir and
western hemlock ranged from 478 to 777 ppm P, 0.96 to
2.38% N, 0.36 to 0.69% K, 0.21 to 0.34% Ca, and 0.09 to
0.12% Mg. Some of these values are lower than published
foliar concentrations for Douglas-fir and western hemlock
(Lavender and Carmichael 1966; Radwan and DeBell 1980)
because of dilution with wood and bark material of the shoots.
Concentrations and contents of the various nutrients in the
shoots of both species reflected differences in uptake of native
nutrients from the various soils and uptake of nutrients from
the fertilizers applied, as well as differences in shoot dry
weights. For example, P and Ca concentrations and contents
were significantly higher in shoots of Douglas-fir and western
hemlock seedlings grown in Shelton soil than in shoots of
trees grown in the other soils. Similarly, hemlock contents of
P, N, K, Ca, and Mg were higher in shoots of seedlings grown
in the Shelton and Klone soils than in the shoots of the Vesta
and Bunker trees.
Effects of soil series
For all fertilization and forest-floor treatments, growth of
both Douglas-fir and western hemlock seedlings was signifi­
cantly better in the Shelton and Klone soils than in the Bunker
and Vesta soils (Table 3). Also, differences between soils
were more pronounced with hemlock than with Douglas-fir.
For example, dry weights of hemlock seedlings produced in
Shelton or Klone soils were more than twice as large as those
of trees grown in the Vesta soil. The reason for the superiority
of the Shelton and Klone soils for both Douglas-fir and
western hemlock is not immediately apparent from the char­
acteristics of forest floors and mineral soils determined
(Table 2). It is obvious, however, that performance of both
species was best in one Douglas-fir soil (Shelton) and one
hemlock soil (Klone). This indicates that under the conditions
Effects of fertilization
For all soil and forest-floor treatments, the ammonium
nitrate and triple superphosphate fertilizers significantly
affected all growth variables measured. For both species,
growth was negatively affected by the N fertilizer (Table 5).
Average height-growth response to ammonium nitrate was
-24.8% for Douglas-fir and -40.0% for western hemlock.
Also, seedling dry weights were more depressed by the N
fertilizer than seedling heights. Average weight response
were -34.4% and -51.8% for Douglas-fir and Western hem­
lock, respectively. Negative response to N fertilizer has been
observed before with potted seedlings of Douglas-fir (Rad­
wan and Shumway 1985) and western hemlock (Radwan and
Shumway 1983) when the soil is low in extractable P. Also
severe depression of growth has been previously observed
TABLE 5. Effect of fertilization on growth of Douglas-fir and western
hemlock seedlings, averaged over four soil and two forest-floor
treatments
Height growth
Fertilization
treatment
Seedling dry weight
Increment Response Quantity Response
(cm)
(%)
(g)
(% )
Douglas-fir
Control
Ammonium nitrate
Triple superphosphate
25.8b
19,4e
32.5a
-24.8
26.0
9.3b
6.1c
l3,4a
-34,4
44.1
8.5b
4.1c
l3.2a
-51.8
55.3
Western hemlock
Control
Ammonium nitrate
Triple superphosphate
26.0b
15.6e
32.6a
--40.0
25,4
NOTE: Within each species, means in the same column followed by the same letter are
not significantly different (p < 0.05).
CAN. J. FOR. RES. VOL. 22, 1992
1226
TABLE 6. Effect of fertilization on nutrients in ovendried shoots of Douglas-fir and western hemlock seedlings, averaged
over four soil and two forest-floor treatments
Concentration
Content (mg/seedling shoot)
Fertilization
treatments
p
(ppm)
K
(% )
N
(% )
Ca
(% )
Mg
(% )
P
N
K
Ca
Mg
O.09a
O.lOa
O.lOa
3.3b
1.6c
5.9a
95.0b
73.9c
112.4a
20.6b
14.3c
30.9a
12.5b
9.2c
20.0a
5.0b
3.5c
7.9a
3.3b
1.3c
5.6a
61.8a
36.4b
75.5a
27.lb
18.0c
33.6a
13.0b
7.6c
21.7a
5.6b
2.6c
9.5a
Douglas-fir
Control
Ammonium nitrate
Triple superphosphate
597b
449c
715a
l.74b
2.21a
1. 40c
O.37a
O.42a
O.38c
Control
Ammonium nitrate
Triple superphosphate
618b
528b
72la
1.28b
1.90a
1.01c
O.58b
O.88a
O.46c
O.23a
O.24a
O.24a
Western hemlock
O.26b
O.28a
O.29a
O.lla
O.lla
O.12a
NOTE: Within each species, means in the same column followed by the same letter are not significantly different (p
<
0.05).
TABLE 7. Effect of forest floor on growth of Douglas-fir and western hemlock seedlings,
averaged over four soil and three fertilization treatments
Forest-floor
treatment
Diameter
growth
(mm)
Without forest floor
With forest floor
4.0a
4.3b
Without forest floor
With forest floor
3.4a
4.0b
Height
growth
(cm)
Shoot dry
weight
(g)
Root dry
weight
(g)
Seedling
dry weight
(g)
3.4a
4.3b
8.la
IUb
3.0a
4.lb
7.la
IO.2b
Douglas-fir
23.5a
28.3b
4.7a
6.8b
Western hemlock
(p
22.2a
27.3b
4.la
6.lb
NOTE: Within each species, means in the same column followed by the same letter are not significantly different
< 0.05).
after N fertilization of Douglas-fir on sites with low-P soils
in coastal Washington. As with seedlings, these growth
depressions may be attributed, at least in part, to stimulation
of soil organisms by the applied N and the resultant reduction
in the already low available P by immobilization.
Unlike ammonium nitrate, the P fertilizer significantly
improved growth of both species. Average height-growth
responses to P were 26.0 and 25.4% for Douglas-fir and wes­
tern hemlock, respectively. Weight responses were greater,
at 44.1 and 55.3% for Douglas-fir and western hemlock,
respectively. These responses agree with previous results with
seedlings grown in pots (Heilman and Ekuan 1980; Ander­
son et al. 1982; Radwan and Shumway 1985) and in the field
(Zasoski and Gessel 1982; Porada and Zasoski 1986; Weet­
man et al. 1989). Douglas-fir and western hemlock pole-sized
trees in closed-canopy stands, however, have failed to respond
positively to application of P fertilizer (Steinbrenner 1981;
Radwan et al. 1991). This discrepancy may be the result of
differences between seedlings and trees in age and growth
environment.
Fertilization significantly affected concentration of some
nutrients in seedling shoots (Table 6). With both species, the
N fertilizer increased concentrations of N and K, and the P
treatment increased level of P, although some of the increases
were not statistically significant. The Nand P increases prob­
ably reflect uptake of fertilizer N and P by the seedlings. The
native K pool was probably the source of increased tissue K.
Fertilization affected content of shoot nutrients in both
species. With only one exception, the P fertilizer significantly
increased weights of all nutrients. These gains probably
resulted from parallel increases in dry matter production
(Table 5) and nutrient uptake and utilization. Fertiliza­
tion with N, however, significantly reduced amounts of all
nutrients in the shoots of both species. The reductions prob­
ably resulted mostly from reductions in seedling biomass by
the N fertilizer (Table 5).
Effect of forest floor
Presence of the forest floor significantly improved growth
of Douglas-fir and western hemlock seedlings (Table 7). For
all soils and fertilization treatments, diameter and height
growth as well as shoot and root dry weights of the trees of
both species were significantly larger with than without the
forest floor. Also, presence of the forest floor apparently
benefited growth of western hemlock more than that of
Douglas-fir. For example, forest floor improved dry weight
of Douglas-fir and western hemlock seedlings by 37 and 44%,
respectively. These results can be explained, at least in part,
by the higher nutrient content of the forest floor (Table 2) and
by the known tendency of hemlock to have more feeder roots
than Douglas-fir in the soil organic layer.
Effects of forest floor on growth of both species varied by
soil and fertilization treatments. Many differences due to the
presence of the forest floor, however, were not statistically
RADWAN
TABLE 8. Effect of forest floor on dry weight (g) of Douglas-fir and
Douglas-fir
15 0
western hemlock seedlings grown in four different soils without
fertilizer
DAN/-FF E2l TSP/-FF
IiIIAN/+FF liliiii TSP/+FF
b
125
Douglas-fir
100
0
(!)
I/)
s::
0
c..
I/)
(!)
ex:
75
50
25
0
Western hemlock
Soil
Without
forest floor
With
forest floor
Without
forest floor
With
forest floor
Klone
Vesta
Bunker
Shelton
29.0a
24.9a
26.3a
23.7a
28.9a
23.2a
28.3a
38.7b
38.1a
9.6a
18.4a
29.8a
39.1a
12.2a
21.5a
35.1a
NOTE: Values are averages of three replications of three seedlings each. Within each
species and soil, means in a horizontal sequence followed by the same letter are not
significantly different (p < 0.05).
-2 5
-5 0
a
-75
a
a
a
-100
Klone
Vesta
Bunker
Shelton
Soil series
Western hemlock
350'
b
300
250
-
eft 200
'-'
(!)
15 0
0
c..
I/) 100
(!)
ex:
50
0
a
-50
-100
1227
a
a
Klone
Vesta
Bunker
Shelton
Soil series
FIG. 1. Percent response of seedling dry weight to ammonium
nitrate (AN) and triple superphosphate (TSP) fertilizers in four soils,
with and without forest floor (FF). Within each species, soil, and
fertilization treatment, bars bearing the same letter are not signifi­
candy different (p < 0.10).
significant because of the large variation between replicates.
This was expected because of the nonunifonn nature of the
forest floor added to the pots and the relatively small number
of seedlings per treatment. Still, we believe that apparent
differences between treatments are important and deserve
consideration, especially when trends were consistent. To
save space, the following discussion is limited to seedling dry
weight as an indicator of effects of the forest floor on seedling
growth.
Without fertilizer (Table 8), the forest floor significantly
improved growth of Douglas-fir only in the Shelton soil,
where seedling dry weight was increased by 63%. Seedling
dry weight of hemlock, on the other hand, was increased by
the presence of the forest floor in all soils, although the
increases were not significant. Growth of hemlock, therefore,
appeared to have been affected more than that of Douglas-fir
by the presence of the forest floor.
Effects of the forest floor on growth response of seedling
dry weight to fertilization varied by soil, species, and fertilizer
(Fig. 1). For both species, most responses to ammonium nitrate
were negative and, in some soils, growth was reduced more
with than without forest floor, although not significantly so.
Without forest floor, growth response of Douglas-fir to ammo­
nium nitrate was negative in all soils except Shelton. The
Shelton seedlings also had the smallest negative response
when the forest floor was present. Similarly, performance of
the western hemlock seedlings fertilized with ammonium nitrate
in the Shelton soil was apparently different from that in the
other three soils, with and without forest floor. Thus, the
Shelton trees showed the smallest negative response to the N
fertilizer in absence of the forest floor and the only positive
response to the same fertilizer when the forest floor was
added, although differences were not statistically significant.
Effect of the triple superphosphate fertilizer on seedling
growth was quite different from that of ammonium nitrate.
'
Without forest floor, both species in alt soils responded pos­
itively to application of the P fertilizer. Addition of the forest
floor increased growth response to P in all soils with western
hemlock and in all soils except Shelton with Douglas-fir,
although some increases were not statistically significant. The
largest increases in response to P in seedling dry weight due
to presence of the forest floor occurred in the Klone (148%)
and Vesta (305%) soils with Douglas-fir and western hem­
lock, respectively. Such increases are very much larger
than those due to forest floor in the absence of fertilizer
(Table 8). Probably data on the overall effects of forest floor
on seedling growth (Table 6) were influenced more by the
P-treated than the untreated forest floor.
The exact reasons for the observed differences in growth
response of both species to Nand P fertilizers due to the forest
floor are not known. A likely factor, however, is the change in
availability of important nutrients caused by interactions of
the different fertilizers with the forest floor. For example, the
N fertilizer can stimulate soil microorganisms and lead to a
decrease in nutrient availability to the seedlings. Alternatively,
the P fertilizer may increase nutrient absorption and utilisation
by the trees by enhancing the activity of mycorrhizal fungi.
The forest floor affected nutrient concentration and content
in seedling shoots of both species (Table 9). For all soil and
fertilization treatments, the forest floor significantly increased
CAN. J. FOR. RES. VOL. 22, 1992
1228
TABLE 9. Effect of forest floor on nutrients in ovendried shoots of Douglas-fir and western hemlock seedlings,
averaged over four soil and three fertilization treatments
Concentration
Content (mg/seedling shoot)
Forest-floor
treatment
P
(ppm)
N
(% )
K
(% )
Without forest floor
With forest floor
563b
611a
1.89a
1.68b
0.40a
0.38a
Without forest floor
With forest floor
591b
653a
1.53a
1.26b
O.64a
0.63a
Mg
(% )
P
N
K
Ca
Mg
O. l Oa
O. l Oa
2.8b
4.5a
86.4b
l O1.2a
18.1b
25.8a
11.4b
16.4a
4.5b
6.5a
2.7b
4.2a
53.7b
62.0a
22.4b
30.1a
11.4b
16.8a
4.7b
7.0a
Ca
(% )
Douglas-fir
0.24a
0.23a
Western hemlock
0.28a
0.27a
0.12a
0.12a
NOTE: Within each species, means in the same column followed by the same letter are not significantly different (p
P and decreased N concentrations, and increased content of
all nutrients determined. Concentration of K, Ca, and Mg
were not significantly affected by application of forest floor.
Factors responsible for changes in nutrient concentration and
content included nutrient uptake and utilization, dilution by
growth, and increase in shoot dry weight.
Conclusions and recommendations
In this study, growth and shoot nutrients of Douglas-fir and
western hemlock were affected by the soil in which the seed­
lings were grown, the presence of the forest floor, and the N
and P fertilizers tested. Overall, the data suggest that (i) tests
with potted seedlings, which are routinely run with mineral
soil only, can be carried out better with forest floor added to
the top of the pots without mixing, if only to simulate field
conditions; (ii) adequate amounts of forest floor on top of
mineral soil of newly regenerated sites may be beneficial and
a prerequisite to good seedling growth, especially for western
hemlock; (iii) the presence or absence of forest floor must be
taken into consideration in the design and interpretation of
results of fertilization experiments; (iv) additional research is
needed to increase our understanding of the nutrient supplying
capacity of the forest floor and its interaction with synthetic
fertilizers; and (v) application of P and not N fertilizers may
be considered for young trees on sites high in soil N and low
in available soil P. Application of N fertilizers to such sites
can result in negative growth response, especially with
western hemlock.
Acknowledgements
The author thanks J.M. Kraft, J.E. Wilcox, and D.W. John­
son, Forestry Sciences Laboratory, Olympia, Washington,
for their valuable assistance with various phases of the study.
Anderson, S., Zasoski, R.I, and Gessel, S.P. 1982. Phosphorus and
lime responses of Sitka spruce, western hemlock seedlings, and
romaine lettuce on two coastal Washington soils. Can. J. For. Res.
12: 985-991.
Bray, R.H., and Kurtz, L.T. 1945. Determination of total, organic,
and available forms of phosphorus in soils. Soil Sci. 59: 39-45.
Bremner, J.M., and Mulvaney, C.S. 1982. Nitrogen-total. In
Methods of soil analysis, part 2. Agronomy, 9: 595-624.
Butters, B., and Chenery, E.M. 1959. A rapid method for the deter­
mination of total sulphur in soils and plants. Analyst (London), 84:
239-245.
<
0.05).
Carter, RE., and Lowe, L.E. 1986. Lateral variability of forest floor
properties under second-growth Douglas-fir stands, and the use­
fulness of composite sampling techniques. Can. J. For. Res. 16:
1128-1132.
Chapman, H.D. 1965. Cation exchange capacity. In Methods of soil
analysis, part 2. Agronomy, 9: 891-901.
Chapman, H.D., and Pratt, P.F. 1961. Methods of analysis for soils,
plants, and waters. Division of Agricultural Science, University of
California. Berkeley.
Gessel, S.P., and Balci, AN. 1965. Amount and composition of forest
floors under Washington coniferous forests. In Forest-soil relation­
ships in North America. Edited by C.T. Youngberg. Oregon State
University Press, Corvallis. pp. 11-23.
Grier, C.C., and McColl, J.G. 1971. Forest floor characteristics within
a small plot in Douglas-fir in western Washington. Soil Sci. Soc.
Am. Proc. 34: 988-991.
Heilman, P.E., and Ekuan, G. 1980. Effects of phosphorus on growth
and mycorrhizal development of Douglas-fir in greenhouse pots.
Soil Sci. Soc. Am. J. 44: 115-119.
Hoagland, D.R, and Arnon, J. 1950. The water culture method of
growing plants without soil. University of California, Berkeley.
Circ. 347.
Lavender, R.P., and Carmichael, RL. 1966. Effects of three variables
on mineral concentrations in Douglas-fir needles. For. Sci. 12:
441-446.
Perkin-Elmer Corporation. 1976. Analytical methods of atomic absorp­
tion spectrophotometry. Perkin-Elmer Corp., Norwalk, Conn.
Porada, H.I., and Zasoski, RJ. 1986. Response of Douglas-fir and
hemlock seedlings to N, P, and N+P applications. Agron. Abstr.
1986: 265.
Pritchett, W.L. 1979. Properties and management of forest soils. John
Wiley & Sons, New York.
Quesnel, H.I., and Lavkulich, L.M. 1981. Comparison of the chemical
properties of forest floors, decaying wood, and fine roots in three
ecosystems on Vancouver Island. Can. J. For. Res. 11: 215-217.
Radwan, M.A., and DeBell, D.S. 1980. Site index, growth, and foliar
chemical composition relationships in western hemlock. For. Sci.
26: 283-290.
Radwan, M.A., and Shumway, J.S. 1983. Growth response of western
hemlock seedlings to N, S, and P fertilizers. The 56th Annual
Meeting of the Northwest Scientific Association, 24-26 Mar. 1983,
The Evergreen State College, Olympia Wash. Abstr. 57. Northwest
Scientific Association, Pullman, Wash.
Radwan, M.A, and Shumway, J.S. 1985. Response of Douglas-fir
seedlings to nitrogen, sulfur, and phosphorus fertilizers. USDA For.
Servo Res. Pap. PNW-346.
Radwan, M.A, Shumway, J.S., DeBell, D.S., and Kraft, J.M. 1991.
Variance in response of pole-size trees and seedlings of Douglas-fir
and western hemlock to nitrogen and phosphorus fertilizers. Can.
J. For. Res. 21: 1431-1438.
RADWAN
Snedecor, G.W. 1961. Statistical methods applied to experiments in
agriculture and biology. Iowa State University Press, Ames.
Steinbrenner, E.C. 1981. Growth response of young Douglas-fir to
repeated application of nitrogen and phosphorus. Soil Sci. Soc. Am.
1. 45: 953-955.
Weetman, G.F., Fournier, R., Barker, 1., and Schnorbus-Panozzo, E.
1989. Foliar analysis and response of fertilized chlorotic western
hemlock and western red cedar reproduction on sal ai-dominated
cedar-hemlock cutovers on Vancouver Island. Can. J. For. Res. 19:
1512-1520.
1229
Wooldridge, D.D. 1970. Chemical and physical properties of forest
litter layers in central Washington. In Tree growth and forest soils.
Edited by C.T. Youngberg and C.B. Davey. Oregon State Univer­
sity Press, Corvallis. pp. 327-337.
Youngberg, C.T. 1966. Forest floors in Douglas-fir forests: I.
Dry weight and chemical properties. Soil Sci. Soc. Am. Proc.
30: 406- 409.
Zasoski, R.J., and Gessel, S.P. 1982. Response of western hemlock
seedlings to N, P, and S fertilization in a coastal Washington soil.
Agron. Abstr. 1982: 273.