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Forest Sci., Vol.32, No.3, 1986, pp. 643-652
Copyright 1986, by the Society of American Foresters
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Effect of Family and Nitrogen Fertilizer on
Growth and Foliar Nutrients of
Douglas-fir Saplings
treatment was applied when trees were 12 years of age, and response was measured as cumulative
volume growth during the next 4 years. Volume growth varied among families and was increased
by an average of 7 percent by application of 224 kg ha -1 N as urea. Foliar concentrations of some
x
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0.05) among families and between fertilizer treatments.
fertilizer interaction was not significant for any growth or foliar nutrient variable.
SCI. 32:643-652.
ADDITIONAL KEY WORDS.
Pseudotsllga me nziesii, genetics, tree improvement, nutrition, family
I
.
ABSTRACT. Family x fertilizer-interactions were evaluated in a split-plot experiment involving
12 open-pollinated families of Douglas-fir (Pse udotsuga menziesii (Mirb.) Franco). The fertilizer
The family
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nutrients also differed significantly (P
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fertilizer interaction.
TREE IMPROVEMENT and fertilizer application programs are important components
of intensive .forest management. Genotypes of some forest tree species may re­
spond differentially to fertilizer application, especially in the seedling stage and
in the early years of plantation establishment (Curlin 1967, Pritchett and Goddard
1967, Jahromi and others 1976, Roberds and others 1976). Because maximum
gains in production of agricultural crops have been obtained by selecting genotypes
that are most responsive to fertilizer and other cultural or environmental com­
ponents of anticipated management regimes (Jennings 1976), a common as­
sumption is that improvement in productivity of tree crops will proceed along
similar lines. Unfortunately, information regarding genotype x fertilizer inter­
actions in older plantations of tree species is scant. One study conducted in
plantations of 8-year-old, control-pollinated loblolly pine (Pinus taeda L.) indi­
cated no genotype x fertilizer interactions of practical importance (Matziris and
Zobel 1976). Similarly, four tests involving P, NP, and NPK fertilizers applied
to 168 open-pollinated slash pine (Pinus elliottU var. elliottU Engelm.) progenies
revealed a significant progeny x fertilizer interaction for only one trait in one test
(Rockwood and others 1985).
In the Pacific Northwest, open-pollinated families of Douglas-fir (Pseudotsuga
The authors are, respectively, Principal Silviculturist, Principal Geneticist, Principal Plant Phys­
iologist, and Mathematical Statistician, USDA Forest Service. DeBell and Radwan are located at the
Forestry Sciences Laboratory, 3625 93rd Ave., SW, Olympia, WA 98502; Silen and Mandel are at
the Forestry Sciences Laboratory, 3200 Jefferson Way, Corvallis, OR 97331. They thank J. M. Kraft,
D. L. Olson, and D. W. Johnson for assistance with various phases of the field and laboratory work.
Manuscript received 22 May 1985.
VOLUME 32, NUMBER 3, 1986 / 643
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menziesii (Mirb. ) Franco) responded differentially to fertilization with Vigoro[
(Rediske and others 1968) and urea (Wilson and Anderson 1974). Another study
assessed response of control-pollinated Douglas-fir families to three levels of
nitrogen; significant family x fertilizer interactions were found in all the growth
traits examined but not in foliar nitrogen concentrations (Bell and others 1979).
As with most genotype x fertilizer interaction studies, however, these investi­
gations with Douglas-fir were limited to potted young seedlings. Therefore, the
applicability of results to actual field conditions is not known.
This paper reports results of a study designed to assess effects of families and
nitrogen fertilizer on growth and foliar concentrations of nitrogen (N), phosphorus
(P), potassium (K), calcium (Ca), and magnesium (Mg) of saplings from 12 open­
pollinated Douglas-fir families.
MATERIALS AND METHODS
Experimental Area. -The study was installed at the Genetics Test Site of the
Pacific Northwest Forest and Range Experiment Station, about 25 km north of
Corvallis in western Oregon. This site is located in low, rolling foothills at the
western edge of the Willamette Valley. Elevation is about 100 m, and the soil is
a well-drained, silty clay loam of the Bellpine series which developed from sed­
imentary parent material. The surface soil (0-15 cm) contains about 4 ppm bi­
carbonate-extractable P, 200 ppm exchangeable K, and 0.13 percent total N, as
analyzed at the Oregon State University Soil Testing Laboratory. Climate is mild;
mean annual precipitation is approximately 1,000 mm, most of which falls as
rain between October 1 and March 31. Average annual temperature is about 12°C;
the mean January minimum temperature is 1°C and the mean July maximum
temperature is 28°C. Productivity class is estimated as high site III (McArdle and
others 1961). Ground vegetation within the area was controlled by surface tillage
and spot-spraying with appropriate herbicides.
Test Trees. -The 12 open-pollinated families used in this experiment were out­
planted as 2-0 seedlings in spring 1969. The trees averaged more than 6 m in
height and were about 11 cm in dbh in spring 1979. Parents of these trees were
well-spaced throughout the 2,000-acre Wilson Wildlife Refuge, about 15 km
southeast of the test site. The progeny trees were spaced at about 3. 7 m x 3.7 m
in a randomized block design; six trees from each family were planted in a row,
and rows were randomly placed in each of six replications. Because past growth
performance differed among replications, similar replications were paired to form
three new blocks, each of which was split for fertilizer treatment to produce a
split-plot experiment. Growth measurements and collections of foliage were made
on the four interior saplings of each six-tree family row, and thereby minimized
the likelihood of adjacent families or fertilizer treatments affecting family per­
formance.
Fertilizer Treatments. -One former replication (hereafter referred to as a plot) in
each new block was randomly selected to receive an application of agricultural
grade urea (46 percent N). The fertilizer was broadcast by hand at a dosage
equivalent to 224 kg N ha-1 in February 1979. The remaining plots (one in each
new block) served as unfertilized controls.
Growth Measurements. -Height and diameter at breast height were measured
before fertilization (winter 1978-79) and 4 growing seasons later. Measurements
I
Mention of commercial products is for the information of the reader and does not constitute an
endorsement by the United States Department of Agriculture.
644 I FOREST SCIENCE
were taken by height pole to the nearest 0.01 m and by diameter tape to the
nearest 0. 1 cm. Tree volumes were estimated by equations developed for young­
growth Douglas-fir (Bruce and DeMars 1974). Data for the four trees were averaged
to provide a family observation for each fertilizer treatment in each block.
Foliage Collection, Processing, and Analysis. -Foliage was sampled in the winters
of 1979-80 and 1980-81. Samples consisted of 5 -cm tips of current year's growth
of secondary lateral branches in the upper third of the crowns cut from all sides
of the trees. Tissue collected from the four sample trees of each family in each
plot was composited and thus provided three replicate samples for each family­
fertilizer combination. The composite samples, about 5 00 g (fresh weight) each,
were individually placed in pre-cooled glass jars and transported to the laboratory
in a portable cooler.
Needles were separated from fresh twigs, ovendried to constant weight at 65°C,
ground to 40 mesh in a Wiley mill, and stored at -15°C until analyzed. Subsamples
were used for chemical determinations as follows: total N (including nitrate) by
the micro-Kjeldahl procedure (Bremner 1965 ); P by the molybdenum blue tech­
nique (Chapman and Pratt 1961); and K, Ca, and Mg by atomic absorption
(Perkin-Elmer Corporation 1976). Foliar nutrients were calculated as percent in
dry foliage.
Statistical Analyses. -Foliar nutrient concentration data were subjected to anal­
ysis of variance and growth variables were examined via covariance analysis. The
covariate was predicted size (height, diameter, or volume) determined from the
following model fitting pretreatment size to row and column position:
Y
=
bo
+
blxl
+
b2x2
+
b3xl2
+
b4x}
+
b5XlX2
where
Y
Xl
X2
bo-b5
=
=
=
pretreatment size (height, diameter, or volume)
row position column position coefficients.
This covariate was selected to remove some of the variation related to microsite
(i.e. , row-column position) without removing variation associated with family.
In addition, simple correlation coefficients were calculated to assess relation­
ships between growth response to fertilizer and growth of unfertilized trees of the
same family, and between growth and foliar nutrient concentrations.
RESULTS
Tree Growth. -At the time of treatment, the saplings were 12 years old from seed
and averaged 6.5 m in height, 11.0 cm in dbh, and 27. 6 dm3 in volume (Table
1). Substantial size differences occurred among families prior to treatment; height
ranged from 6.1 to 7.1 m, diameter from 9.8 to 11.9 cm, and volume from 22.1
to 34.2 dm3• All trees appeared healthy and showed no visible signs of any nutrient
deficiency.
During the 4-year period of our study, height and diameter of the trees increased
by more than 60 percent and volume nearly quadrupled. Families varied in mean
height growth from 3. 4 to 4.5 m, in mean diameter growth from 5.7 to 7.6 cm,
and in mean volume growth from 61.6 to 101.2 dm3 (Table 1), and family dif­
ferences were highly significant (Table 2). Although height and diameter growth
were not significantly affected by nitrogen fertilizer, together they produced a
volume growth of fertilized trees that was significantly greater than that of un­
fertilized trees. Average volume growth response to fertilizer was 7 percent.
VOLUME 32, NUMBER 3, 1986 / 645
TABLE 1. Pretreatment size and post-treatment (1978-82) growth characteris­
tics of open-pollinated Douglas-fir saplings. a
Family
number
Size at age 12 years
Height
dbh
Volume
m
cm
dm3
Height growth
N224
No
··············
m
···············
Diameter
growth
No
Volume growth
N224
No
............. c m ........ ......
N224
Response
............. dm3 ..............
Percent
6.75
10.9
28.2
4.08
4.30
6.76
7.58
79.6
93.8
17.8
2
6.69
10.8
27.2
4.22
4.23
6.74
6.42
79.3
76.6
-3.4
3
6.31
11.0
25.9
3.93
4.20
6.50
6.93
74.9
79.2
5.7
4
6.67
11.0
28.6
4.21
4.07
6.73
6.42
76.4
80.7
4.3
5
7.08
11.9
34.2
4.51
3.93
7.11
7.09
101.2
90.4
-10.7
6
6.13
9.8
22.4
4.14
4.07
6.61
7.56
68.0
81.6
20.0
7
6.50
11.4
29.0
3.98
3.90
6.75
7.61
85.1
88.9
4.5
8
6.59
11.0
27.7
4.15
3.79
6.75
7.58
81.1
87.7
8.1
9
6.31
10.2
23.7
3.36
3.65
6.21
7.52
65.1
71.5
9.8
10
6.12
10.1
22.1
3.80
3.71
5.70
6.27
61.6
70.8
14.9
11
6.43
11.5
29.6
3.95
4.04
6.15
6.48
69.9
84.5
20.9
12
6.79
11.8
32.3
4.08
4.23
7.01
7.47
94.1
91.7
-2.6
Mean
6.53
11.0
27.6
4.03
4.02
6.73
6.93
78.0
83.1
7.4
" No = unfertilized, N224 = urea applied at 224 kg N/ha. Growth data adjusted by covariance analysis
using initial size (height, diameter, or volume) predicted by row-column position within the experi­
mental area as the covariate.
In the analysis of covariance, the family x fertilizer interaction was not sig­
nificant for volume or any other growth variable (Table 2) even though family
mean volume growth responses ranged from -10.7 to 20.9 percent (Table 1).
However, regression analysis showed that volume growth response to fertilizer
was negatively correlated with volume growth of unfertilized trees of the same
family (Fig. 1). This trend indicated that response to fertilizer was greater for
smaller, slower growing families. The two largest families responded negatively.
Foliar Nutrients. -Concentrations of most nutrients differed significantly among
families in one year or both years (P, K, and Mg in the 1979-80 foliage; N, K,
Ca, and Mg in the 1980-81 foliage, Table 3). Fertilizer significantly increased N
concentrations in the 1979-80 foliage and Ca concentrations in the 1980-81
foliage; P concentrations in foliage of fertilized trees were significantly decreased
in both years. The family x fertilizer interaction was not significant for any
nutrient in foliage of either year (Table 3).
TABLE 2. Sources of variation and results of analyses of covariance of post­
treatment growth characteristics.
Source of variation
Degrees
of freedom
Significance of F-test
Height
Diameter
Volume
Position covariate
1
0.376
0.000
0.070
Replication
2
.121
.712
.107
Fertilizer
1
.145
.128
.025
Error,
2
II
.000
.004
.001
11
.110
.190
.800
Family
Family
x
fertilizer
Within + residual
646 / FOREST SCIENCE
43
+30
':'
+20
® @
CD
r
t
CD
LL +10
Y = 59.1 - 0.66 X
0.77, P < 0.Q1, d.l. = 10
® = Families
®
0
III
c
0
Q.
=
0
®
.c
I
-10
®
(!J
-20
60
FIGURE 1.
70
100
90
80
Volume Growth of Unfertilized Trees (dm3)
Correlation of 4-year volume growth response to fertilizer and 4-year volume growth of
unfertilized trees of the same family.
Because our nutrient data are voluminous and the family x fertilizer interac­
tions were never significant, data were combined to illustrate for each of the two
sampling years: (1) family differences in concentrations in foliage of unfertilized
trees (Table 4), and (2) differences between fertilized and unfertilized trees in mean
of nutrient concentrations (Table 5 ). Nutrient concentrations in 1979-80 foliage
of unfertilized trees of individual families ranged from 1.23 to 1.40 percent N,
0.16 to 0.21 percent P, 0.58 to 0.70 percent K, 0.5 1 to 0.73 percent Ca, and 0.12
to 0.17 percent Mg (Table 4). Ranges in nutrient values for 1980-81 foliage were
1.21 to 1.36 N, 0.16 to 0.19 P, 0.60 to 0.80 K, 0. 5 2 to 0.67 Ca, and 0.12 to 0.15
Mg. Rankings among families were similar both years. Without fertilizer, there­
fore, differences among families in the two years varied by 12 to 14 percent for
N, 19 to 31 percent for P, 21 to 33 percent for K, 29 to 43 percent for Ca, and
25 to 42 percent for Mg.
Fertilizer application increased foliar N concentrations from 1.31 to 1.61 per­
cent in the first year, but differences between fertilized and unfertilized trees were
negligible by the end of the second year (Table 5 ). Foliar Ca concentrations
appeared to be slightly increased by fertilizer in the first year, and this difference
became greater and statistically significant in the second year. The nitrogen fer­
tilizer also led to substantial depressions (33 to 35 percent) in foliar P concentra-
TABLE 3.
Source of
variation
Results of analyses of variance offoliar nutrient concentrations.
1979-80 foliage
N
P
K
Ca
1980-81 foliage
Mg
N
P
K
Ca
Mg
............................................................................... Significance level ........· ..................···..·.. ·..·· ......................................·
Family
Fertilizer
Family
0.567
0.006
0.000
0.075
0.000
0.010
0.053
0.000
0.020
0.000
.029
.002
.824
.245
.056
.177
.002
.271
.014
.302
.395
.131
.526
.564
.294
.711
.807
.683
.566
.386
x
fertilizer
VOLUME 32, NUMBER 3, 1986/647
TABLE 4. Foliar nutrient concentrations of unfertilized, open-pollinated Douglas-fir saplings.
1979-80 foliage
Family
number
N
P
K
Ca
1980-81 foliage
Mg
N
P
K
Ca
Mg
............................................................................................... Percent ................................................................................................
1
1.32
0.18
0.70
0.64
0.12
1.21
0.18
0.77
0.63
0.12
2
1.27
.16
.60
.66
.13
1.27
.16
.60
.66
.12
3
1.30
.19
.70
.53
.15
1.33
.17
.77
.52
.14
4
1.37
.20
.61
.60
.14
1.36
.18
.65
.56
.13
5
1.30
.17
.60
.57
.13
1.23
.18
.63
.58
.13
6
1.40
.18
.62
.73
.17
1.35
.16
.69
.67
.14
7
1.29
.21
.64
.53
.15
1.29
.19
.67
.53
.14
8
1.33
.20
.58
.58
.15
1.31
.17
.64
.58
.14
9
1.29
.19
.68
.53
.14
1.33
.18
.79
.53
.14
10
1.33
.17
.69
.58
.14
1.33
.17
.80
.54
.14
11
1.33
.18
.65
.51
.15
1.26
.17
.70
.54
.15
12
1.23
.17
.63
.57
.13
1.27
.16
.72
.52
.12
Mean
1.31
.18
.64
.58
. 14
1.30
.17
.70
.57
.13
tions which were highly significant in both years. In contrast, concentrations of
K and Mg were not significantly affected by the fertilizer in either year.
Correlations of most foliar nutrient concentrations with volume growth were
not significant. Except for Ca, significant correlations were negative; also, the
correlations usually accounted for less than 10 percent of the variation in growth.
DISCUSSION
The Unfertilized Trees (General). -Averaged over all families, pretreatment size
and subsequent growth of the unfertilized trees were excellent for their age and
the quality of this well-prepared, vegetation-free site. Concentrations ofN, P, and
K in the foliage of the unfertilized trees were less than adequate for growth; and
concentrations of Ca and Mg were more than adequate according to van den
Driessche (1979). Our data on foliar nutrient concentrations, however, are com­
parable with many other values in the literature (Heilman and Gessel 1963, Beaton
and others 1964, Lavender and Carmichael 1966, Heilman 1971).
Family Differences. -Differences among family means in pretreatment size (age
12 years) are typical of Douglas-fir and are commonly found even when parents
occupy a limited geographic area. The family differences were nearly as large as
might be expected in large populations (Silen and Mandel 1983).
Statistically significant differences among families in concentrations of most
foliar nutrients suggest that elemental uptake and composition may be under
genetic control. The differences in foliar nutrient concentrations, however, do not
help explain growth differences among families. Only Ca concentration was pos­
itively correlated with growth. These results suggest that the test families may
have differed in either luxury consumption of nutrients andlor in efficiency of
nutrient utilization. Although such differences cannot be determined from foliar
concentrations alone (i. e. , weights and nutrient concentrations for all tissues are
needed), they have been documented in families of loblolly pine (Pope 1979) and
among geographic sources of eastern cottonwood (Blackmon and others 1979).
Fertilizer Effects. -Average growth response to N fertilizer in this study, though
significant, was less than expected. The low response could be attributed to one
648 I FOREST SCIENCE
TABLE 5. Foliar nutrient concentrations of fertilized and unfertilized Douglas­
fir saplings, 1 and 2 years after fertilizer application. a
Nutrients
Year
Treatment
N
p
K
Ca
Mg
................................................................. Percent ..................................................................
1979-80 1980-81 a
0.14
Control
1.31
0.18
0.64
0.58
Fertilized
1.61
.12
.64
.62
.15
Control
1.30
.17
.70
.57
.13
Fertilized
1.32
.11
.66
.72
.14
Values are averages of the 12 families. For each family, data are averages of three composite
samples each. Year 1979-80
=
first year after fertilization, year 1980-81
=
second year after fertil­
ization.
or more of the following: (1) native N supplies at the test site may have been
adequate, (2) canopy density may have limited crown expansion and consequent
acceleration of growth, (3) factors such as water or other nutrients may have been
in limited supply, or (4) fertilizer may have provided smaller amounts of available
N than expected because retention was low and/or added N was immobilized by
soil microbes.
The apparent negative growth response of some families to fertilizer is more
puzzling, but it could be normal variation in an experiment of this statistical
sensitivity.
The negative correlation between growth response to urea fertilizer and growth
of unfertilized trees of the same family, differs from that which is typically found
in nitrogen fertilizer trials in dense, closed stands (cf. Miller and Pienaar 1973).
Better response from small trees, however, may not be uncommon in younger,
developing plantations. In young plantations, in which crowns have not yet closed
or have just recently closed, fertilizer may stimulate the growth of the smaller
trees more than that of larger trees. Such an effect was documented in a young
eastern cottonwood (Populus deltoides Bartr. ) plantation in the Mississippi Delta
(DeBell and others 1975 ). Smaller trees also responded more to fertilizer than did
large trees in a thinned 25-year-old slash pine plantation growing on a good site
in Louisiana.2 Presumably, the trees growing the fastest without fertilizer are those
that, because of genetic and/or microsite differences, are best able to absorb and
utilize native nutrient supplies and therefore respond less to added nutrients.
Application of N fertilizer affected foliar nutrient contents. The N increase in
the first year after application of fertilizer reflects the uptake of fertilizer N by the
trees. The return of foliar N concentrations to levels equal to those in the unfer­
tilized trees during the second year after fertilization is a common phenomenon
in N-fertilized conifers (e. g., Radwan and others 1984); this may indicate luxury
consumption of N during the first year or, more likely, may be related to dilution
by growth.
Phosphorus concentrations were depressed in the fertilized trees for at least 2
years after nitrogen was applied to levels considered very low for Douglas-fir (van
den Driessche 1979). This phenomenon has been observed before with Douglas­
fir (Heilman and Gessel 1963) and other conifers (Radwan and others 1984).
Tamm (1956) attributed such reduction in P to a stimulation of P-consuming soil
2
Shoulders, E., and A. E. Tiarks. 1984. Unpublished office report, Southern Forest Experiment
Station, Alexandria, Louisiana.
VOLUME 32, NUMBER 3, 1986 / 649
organisms. Although depression of foliar P can also be caused by dilution effects
associated with increased growth from the N treatment, this explanation is unlikely
in our study because P concentrations were depressed equally in all blocks of all
families regardless of relative response to fertilizer.
In contrast with P, Ca concentrations in the foliage were increased by N fer­
tilization. This agrees with earlier findings by Beaton and others (1964). Most
probably, the increased levels of Ca were caused by increased uptake of Ca from
the soil (McLean 195 7, Drake and White 1961).
There were no significant changes in foliar K and Mg levels after N fertilization.
Others (Heilman and Gessel 1963, Beaton and others 1964), however, have re­
ported larger differences in concentrations of these nutrients between fertilized
and unfertilized trees.
Family x Fertilizer Interactions. Inasmuch as all previous studies with Douglas­
fir seedlings (Rediske and others 1968, Wilson and Anderson 1974, Bell and others
1979) showed striking differences among families in response to fertilizers, the
absence of a significant family x fertilizer interaction for either growth or foliar
nutrient concentrations in our study was somewhat surprising. Experimental error
as well as a low response to N may have hindered identification of a family x
fertilizer interaction. A highly significant negative correlation between growth of
unfertilized trees and growth response to fertilizer was detected (Fig. 1), however,
and suggests that families may differ in their responsiveness to fertilizer appli­
cation.
Disagreements between results with seedlings and older trees have also been
observed in other species. For example, loblolly pine, slash pine, and eastern
cottonwood showed strong family or clone x fertilizer interactions in growth
traits for seedlings and young trees (Roberds and others 1976, Jahromi and others
1976, Curlin 1967), but interactions were less pronounced or were negligible when
tests were made in sapling-sized stands3 (Matziris and Zobel 1976, Rockwood
and others 1985 ).
Experimental design differences may account at least in part for the inconsistent
findings for some species (Ballard 1980), but we do not think that such design
differences have influenced the contrasting results in studies involving Douglas­
fir.
Rather, we believe that the inconsistencies in the Douglas-fir studies are related
to differences in tree age, development, and growth environment. There are several
reasons why such factors would lead to differences in results from genotype x
fertilizer studies with other species also. Mycorrhizae, for example, probably have
greater relative influence on nutrient absorption by older tJ;ees under field con­
ditions than by seedlings in pots. Similarly, root systems of older trees occupy
the soil space more fully than do seedlings; they thus have greater ability to extract
native nutrients. Fewer factors can be controlled in the field than in pots or nursery
environments, and some of these factors, such as water and other nutrients, are
more likely to limit growth response to nitrogen and thus family differences in
response. Also, internal and external nutrient cycling have much greater influence
on tree nutrient status in older stands and under field conditions than in young,
potted seedlings. Some environmental influences and biological processes affect
the stand as an entity rather than affecting individual trees or families of trees,
and thus may thwart expression of individual tree or family differences in older
stands. Finally, at some stage, stand stocking and canopy density will limit re­
-
3
Blackmon, B. G. 1972. Differential response of eastern cottonwood clones to fertilization. Un­
published office report, Southern Forest Experiment Station, Stoneville, Mississippi.
65 0 / FOREST SCIENCE
sponse to fertilizer and probably reduce the likelihood of differential response
among families.
IMPLICATIONS AND CONCLUSIONS
This study adds to the scant information now available on effects of fertilizer
application and fertilizer x genotype interactions in sapling stands of Douglas­
fir. The growth results, however, contained several anomalies. Response to applied
'
N was lower than expected, and the family x fertilizer interaction was not sig­
nificant. Our results thus must be tempered with the following caveats: (1) Family
performance regardless of fertilizer treatment was to a large degree associated with
pretreatment size or performance. (2) Although variation in growth response
within families was sufficient to rule out a significant family x fertilizer interaction
in our analyes of covariance, the negative correlation of growth of unfertilized
trees to growth response to fertilizer hints at the possibility of differential responses
among families; and (3) the possibility remains that differential responsiveness
could be important on other sites, at younger plantation ages, for other families,
or in more advanced generations of tree improvement, with repeated nitrogen
applications, andlor under other cultural regimes (e. g. , fertilizer with other nu­
trients, wider spacings). Until additional spacing x genotype x fertilizer exper­
iments are conducted, we suggest caution in projecting results from family x
fertilizer studies with seedlings to stands of sapling size or larger.
The significant differences among families in foliar nutrient concentrations do
not help explain growth differences among families, but they have another im­
portance in that they indicate the possibility of differences in nutrient concentra­
tions and contents in other tree components. Because of the concerns about
nutrient drain with harvest and uncertainties as to the future practice of forest
fertilization, a thorough assessment of family differences in nutrient accumulation
and efficiency of nutrient utilization (wood produced per unit of nutrient used)
seems appropriate.
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65 2 / FOREST SCIENCE
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fertilizer interaction in young Douglas-fir
I