1109
Effect of seed weight on height growth of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) seedlings in a nursery FRANK C. SORENSEN AND ROBERT
K.
CAMPBELL
;,:':'
United States Department of Agriculture Forest Service, Pacific Northwest Forest and Range Experiment Station, Forestry Sciences Laboratory, 3200 Jefferson Way, Corvallis, OR, U.S.A. 97331 Received November 13, 1984'
Accepted July 26, 1985
SORENSEN, F. C., and R. K. CAMPBELL. 1985. Effect of seed weight on height growth of Douglas-fir (Pseudotsuga menziesii
(Mirb.) Franco var. menziesii) seedlings in a nursery. Can. J. For. Res. 15: 1109-1115.
Different mean seed weights were produced within each of 10 young Douglas-fir trees by leaving some developing cones
unbagged and enclosing others in Kraft paper bags for two different durations. On the average, 10 days in the bag increased
filled-seed weight by about 1%. Unbagged cones and cones from the 117-day bagging duration were wind pollinated. Seeds
from these cones were, therefore, of comparable genetic makeup and were used in further nursery growth tests. To eliminate
the effect of germination rate or time, samples of filled seeds from each treatment on each parent tree were sown as germinant
seedlings on one date. Cotyledon number was counted and I st-year epicotyl lengths and 2nd-year total heights were measured
on all seedlings. Seedling volumes were estimated by assuming diameters were proportional to heights. On the average,
bagging cones for 117 days increased seed weight by 10.7%, I st-year epicotyl length by 9.1%, and 2-year total height by 4.0%.
All differences were statistically significant. Results were compared with other reports of the relations between seed weight
and growth and reasons for inconsistencies were discussed. Size differences were projected to later ages with a growth model
and practical implications of long-term seed effects on plant size, of increasing seed size through cultural techniques, and of
grading seed lots by size were considered.
SORENSEN, F. C., et R. K. CAMPBELL. 1985. Effect of seed weight on height growth of Douglas-fir (Pseudotsuga menziesii
(Mirb.) Franco var. menziesii) seedlings in a nursery. Can. J. For. Res. 15: 1109-1115.
Au moyen de 10 jeunes tiges de Pseudotsuga menziesii, on obtint diverses moyennes dans le poids des graines en laissant
quelques fleurs femelles non ensachees et d'autres enfermees dans des sacs de papier Kraft durant deux periodes de temps
differentes. En moyenne, un ensachcment de 10 jours augmente de I% le poids des graines plcines. Les fleurs non ensachees
et celles qui le furent pendant 117 jours furent pollinisees librement. Les graines issues de ces cones s'avererent d'une valeur
genetique valable et furent utilisees dans des tests subsequents de croissance. Afin d'eliminer les effets dus au taux de
germination ou au temps, on sema des echantillons de graines pleines originant de chaque traitement et de chaque parent en
les considerant comme des semis germants a une date donnee. On denombra le nombre de cotyledons, la longueur de l'epicotyle
de I an et celle de Ia hauteur totale de 2 ans de tous les semis. On estima le volume des semis, admettant que les diametres
etaient proportionnels aux hauteurs. Ainsi, en moyenne, les fleurs ensachees pendant 117 jours augmenterent le poids des
graines de 10,7%, Ia longueur de l'epicotyle d'un an de 9,1% et la hauteur totale de deux ans de 4,0%. Toutes ces differences
furent statistiquement significatives. Les auteurs comparent ces resultats a d'autres essais analogues sur lcs relations entre
croissance et poids des graines et discutent des raisons entralnant certaines inconsistances. On extrapola les differences des
dimensions pour des ages avances utilisant un modele de croissance; ainsi on tint compte des implications pratiques a long
terme, des effets des graines sur les dimensions des plants, de !'augmentation des dimensions des semcnces par les techniques
de culture et du classement des lots de graines par dimensions.
[Traduit par le journal]
Introduction Conclusions about the relation of seed size to seedling size
in trees are inconsistent (e.g., Lavender 1958; Burgar 1964;
Demeritt and Hocker 1975; Griffin 1972; McKersie et al.
1981). Often the relation has been confounded by real or pos­
sible genetic differences associated with seed lots of different
average seed weight. To minimize this source of confounding,
we treated individual branchlets in this study to influence seed size differentially at the same crown position on the same trees. Seeds were then genninated and the relation of seed weight to seedling size was followed through 2 years in the nursery.
Results are reported in this paper.
Materials and methods
Ten 20- to 30-year-old Douglas-fir (Pseudotsuga memiesii (Mirb.)
Franco var. menziesii) trees in a second-growth stand at about 300 m
elevation in the western Oregon Cascades (44°35' N, 122°42' W)
served as seed parents. Seed weights on these trees were altered by
manipulating the time that branchlets with developing conelets and
1
Revised manuscript received July 22, 1985.
cones were left in Kraft paper bags. 2 Three bagging durations were
used: (i) bags installed on 17-21 branchlets per tree shortly before
floral bud flush (March 27); (ii) bags installed on 4 branchlets per tree
May 13, 26 days (average) after floral buds on these trees had been at
maximum receptivity for pollen; (iii) no bags installed. Cone buds and
cones were in the bags for 164, 117, and 0 days for treatments i, ii,
and iii, respectively.
Female conelcts in the bags installed March 27 (treatment i) were
crossed with pollen from other (random) trees in the local stand. These
crosses were part of a separate investigation. Female conelets in treat­
ments ii and iii were open pollinated. All three treatments were ge­
netically equivalent for seed coat and female gametophyte, which are
maternal traits. Treatments ii and iii were also equivalent in the aver­
age genetic makeup of the embryos.
Bagging was done in the upper one-third of the crown. As far as
possible, the two bagging treatments and three collection types were
evenly distributed over that part of the crown. Bags averaged about
2No. 1140 Pollen-tector, manufactured by Carpenter Paper Co.,
Des Moines, Iowa. The use of brand names is for the convenience of
the reader and does not constitute endorsement of the product by the
U.S. Department of Agriculture or the Canadian Journal of Forest
Research.
1110
CAN.
J.
FOR. RES. VOL 15, 1985
five cones each. All three treatments were used to investigate the
15
effect of duration of bagging on seed yield and seed weight; durations
ii and iii only (open pollination seeds) were used to determine the
relations among seed weight, seedling height, and cotyledon number.
14
The primary purpose of durations ii and iii was to produce seed lots
of different weight but with the same average genetic makeup.
After harvest in the 1st week in September, cones were dried in a
covered shed until they began opening and then in a dehumidified
J:.
Ill
room at about 30°C. Cones were tumbled and seeds were processed
in a small-lot extractory. Filled seeds were determined from X-ray
,
'ijj
:
photographs.
11,
..........
(/)
All seed weights are for filled seeds and are based on four I 00-seed
lots from each family-treatment combination.
The effect of bagging on yield of filled seeds and on seed weight
was analyzed as a two-way classification of random (seed parents) and
fixed (bagging duration) treatments with four seed-weight subsamples
0
·
per combination of seed parent and bagging duration. The pooled
subsampling error was used to test the seed parent
x
sus no bagging and early versus late bagging.
Seedling growth was evaluated in an outdoor nursery at Corvallis,
Oregon (44°34' N, 123°17' W; 70 m elevation), in a split-plot design
with three replications. Seed parents were randomly assigned to main
plots and bagged or unbagged seed lots from the same seed parent
were randomly assigned to subplots. Newly germinated seeds were
sown on I day in row plots across the nursery bed; each row plot rep­
seed parent combination. Spacing
was 7. 6 cm2• There was one border row of seedlings, for which
measurements were not recorded, around the test.
Three traits were counted or measured on the seedlings: cotyledon
40
60
80
100
160
FIG. I. Relation between the number of days developing cones
filled seeds from those cones. Line I is the mean for 10 Douglas-fir
seed parents; lines 2 and 3 represent two seed trees that had the least
and the greatest increases in seed weight in response to time in bags,
respectively.
0,
observed values for line
I; ••
observed values for
lines 2 and 3. The values in parentheses are the standard errors of the
regression coefficients.
TABLE I. Effects on seed and seedling characteristics of bagging
open-pollinated cones in Kraft paper for 117 days before cones were
collected
2nd-year total seedling height to the base of the terminal bud.
1st-year
Diameters were not measured, but seedling volumes were estimated
by assuming that diameter was proportional to height. This probably
Perry, personal communication), although high genetic correlations of
140
were in Kraft paper pollination bags and mean weight (per seed) of
number, 1st-year epicotyl length to the base of the terminal bud, and
underestimates volume differences (Perry and Roberts 1964; T. 0.
120
Time in pollination bag (days)
bagging duration
interaction. Bagging treatments were further contrasted: bagging ver­
resented one bagging treatment
20
Seed
2nd-year
epicotyl
total
Bagging
weight
Cotyledons
length
height
treatment
(mg)
(no.)
(em)
(em)
0.8 between 2-year height and diameter have been found for seed­
lings grown under comparable conditions (Campbell 1985). Certainly,
Bagged
when crown competition occurs volume differences will be under­
11.85
7.05
6.58
31.1
Not bagged
10.70
6.03
29.9
% increase
from bagging
7.15
10.7
estimated because diameter increment is more reduced by supression
than is height increment (Campbell and Wilson 1973).
Analysis of variance of measured growth traits followed the mixed
model of Steel and Torrie (1960, p. 235) with random (seed parents)
and fixed (bagging treatment) variables.
Results
Bagging was associated with increased numbers of filled
seeds per cone. Control-pollinated cones produced more filled
44.3) than wind-pollinated cones (i
seeds per cone (i
24.9; P < 0.000 1) and wind-pollinated cones bagged for 1 17
days yielded more filled seeds (i 29.0) than wind-pollinated
cones that were left unbagged (i 20.3; P < 0.05), probably
because the bags reduced insect depredation.
=
(117 days)
9.1
4.0
Significance
of difference"
p < 0.01
NS
p < 0.05
p < 0.05
"NS. not significant.
able weight, the interactions involved some rank changes.
The relation of seed weight to the number of days that the
cones were in the bags was linear (P < 0.00 1; Fig. 1, line l).
On the average, 10 days in the bag increased weight of filled
seeds by 0. 1 mg or by about 1 o/o ( Fig. 1). Although seed weight
was affected by bagging, cotyledon number was not (Table 1).
=
Seed weight and cotyledon number
Bagging increased the weight of individual filled seeds; the
difference between bagging and no bagging was highly signifi­
cant (P < 0.00 1). Cones that had been in the bags longer had
heavier seeds; the difference between early and late bagging
was significant (P < 0.05). Bagging affected seed weight more
for some seed trees than for others; the bagging x seed parent
interaction was also highly significant (P < 0.00 1). Estimated
proportions of variances for the different factors and inter­
actions were seed trees (75.9%), bagging durations (20.3%),
seed trees x bagging durations (3. 1%), and pooled subsamples
(0.7%). The coefficient of variation associated with the pooled
subsamples was 1.3%. If seed trees yielded seeds of compar­
First-year epicotyl length
Seedlings from seeds extracted from bagged cones had sig­
nificantly (P < 0.05) longer epicotyls than seedlings from
seeds extracted from unbagged cones (Table 1). A 1-mg differ­
ence in seed weight equated to an average difference in epicoty1
length of 0.48 em. Bagging cones for 1 17 days produced a
10.7% increase in seed weight, which, in tum, was associated
with a 9. 1% increase in 1st-year epicotyl length. Interaction
between seed parent and bagging treatment was not significant.
Second-year total height
Seedlings from seeds of bagged cones were still 4.0% taller
< 0.05) after 2 years in the beds than seedlings from seeds
of unbagged cones (Table 1). A 1-mg difference in seed weight
was associated with about a 1-cm difference in total 2-year
(P
SORENSEN AND CAMPBELL
height. In relative terms, a 10% difference in seed weight was
associated with a 3.7% difference in total 2-year height.
Second-year stem volume
Assuming that heights and diameters were proportional,
2-year stem volumes of seedlings from seeds of bagged cones
were estimated to be 12.5% larger than seedlings from seeds
of unbagged cones. In relative terms, a 10% difference in
seed weight was associated with an 11.6% difference in stem
volume.
!Ill
length (Wrzesniewski 1981), ratio of embryo weight to seed
weight (Passmore 1934), proportion of seed weight that is in
seed coat, frequency of polyembryonic seed, speed of germina­
tion (Perry 1976), and average inbreeding coeffic'ient (Soren­
sen and Miles 1974; Righter 1945). All factofs of this type
could conceal or mask a real relation between seed size and
seedling size, either because seed size does not reflect the size
of the embryo, the megagametophyte; or both, or because
different seed sizes are not associated' with equivalent genetic
potentials for embryo vigor.
Discussion
Seed yield
Both controlled crossing and increased time in the isolation
bags increased yield of filled seeds. We made no tallies of vari­
ous factors contributing to seed loss. Bagging probably in­
creased seed yield by reducing insect depredation, however,
and cross pollination to some extent increased seed yield by
reducing embryo loss from natural inbreeding (Sorensen 1969).
The year of pollination ( 1971) was a bumper seed year for
Douglas-fir in western Oregon, but part of the increased seed
yield after controlled crossing may have resulted from artificial
pollination, giving a better distribution of the pollen to the
conelets.
Seed weight
Time in the isolation bag and pollen parent both could con­
tribute to differences in seed weight. Because pollen for con­
trolled crossing was of local origin and because weight of
conifer seeds is predominantly in tissues of maternal origin
(Righter 1945), however, most differences in seed weight are
probably caused by the bagging treatment.
Several materials used in the construction of pollen bags
have been shown to change the microclimate around enclosed
branch ends (Rohmeder and Eisenhut 1959). In grain and for­
age crops, ear temperature can markedly influence growth and
yield of seeds (Slater and Jensen 1970; Ford et al. 1976).
Presumably, in our study, isolation in the Kraft paper bags
changed the microclimate around the strobili, which resulted in
(2) Interaction between seed weight and genetic differences
in seedling growth habit. In crop plants, for example, a variety
with larger seed may mature earlier than a small seeded form
(Passmore 1934; Clements and Latter 1973; Hatridge-Esh and
Bennett 1980). Seed maturity, in turn, may be related to con­
centration of reserve substances (Wilson and Splittstoesser
1980) or various inhibitory substances (Abu-Shakra and Maye­
enudin 1972). Photosynthetic rates of smaller seed fractions
may be relatively high (Burris et a!. 1973) or early production
of photosynthetic tissue may be emphasized by small seeded
varieties and larger seeded varieties may tend toward early
growth of the root axis (Shibles and MacDonald 1962; Evans
1970; Taylor 1972). Similar associations might occur between
seeds and seedlings from different trees (Rohmeder 1972),
which again would camouflage the relation of seed size to
seedling size.
(3) Influences of the test environment on effects of seed
weight on plant size (Aldrich-Blake 1935; Kaufman and Gut­
tard 1967; Rohmeder 1972). The reasons are unclear but, under
some conditions, maturity, dormancy, or bud formation of the
seedling may be related to its size (Hatridge-Esh and Bennett
1980). If growth cessation were even partly size related, it
would weaken the relation between seed weight and seedling
size and if this were because of the test medium, it could result
in inconsistency among tests.
(4) Finally, and perhaps most important, competitive effects
among seedlings resulting from test design and differences in
significantly heavier seeds.
initial seed weights. Seed size seems to be relatively unimpor­
tant in determining performance when all the members of a plot
Relation of seed weight and seedling size
(Harper et
We have assumed that seeds from bagged and nonbagged
cones differed only in weight. Their nutrition (or metabolic)
or population represent seedlings from near-identical seed sizes
a!. 1970; Salih 1981). But, when large and small
seeded types are alternated or intermingled in plots, competi­
tion magnifies the initial advantage of large seed size (Mont­
status could also have been altered, however (Schweizer and
Ries 1969; Ries and Everson 1973), although these changes in
gomery 1912; Black 1958;Smith andCamper 1975). In normal
themselves may only negligibly affect seedling growth in small
weight of the associates (Williams et al. 1968).
For the most part, tests in forest nurseries have not been
gains (Whalley et al. 1966) and trees (Sweet et al. 1975),
competition, yield depends both on own seed weight and seed
including Douglas-fir (R. K. Campbell, unpublished data).
designed to evaluate the effect of seed size under early com­
Wilcox ( 1983) reports results similar to ours using reciprocal
crosses of Pinus radiata. His comparison is based on "families"
petitive conditions. Either seeds of different size classes have
with equivalent embryo genotypes (the reciprocal crosses),
enough to provide early competition for light. In our test, plots
but different maternal effects (seed parents), including seed
weight.
seeds (bagging, 0 days), but, because seedling rows were ran­
We have previously observed a positive relation of seed size
domly paired in subplots, the two seed classes could compete.
been assigned to different plots or spacing has not been close
were established from either large (bagging, 117 days) or small
to seedling size (Sorensen and Miles 1974; Sorensen 1973), as
Seedling density in this test (about 170 seedlings/m2) was
well as in the current test. Other workers have reported either
a similar relation (Burgar 1964; Perry 1976) or no relation
about one-half of normal nursery density. Light competition
(Lavender 1958; Bell et at. 1979). There are several possible
reasons for the inconsistency in results.
most likely did not occur until the middle of the second year.
A 3 to 4% difference in 2-year seedling height associated
with a I 0% difference in seed weight has shown up in several
(I) Maternal or genetic factors that affect seed size differ­
of our Douglas-fir nursery tests where seedlings were grown at
ently than they do growth. Seeds from different trees differ in
this spacing, well watered, and supplied with artificial nutri­
seed weight, as in this test. But seeds from different plants can
ents. This difference apparently ret1ects a real effect of seed
also differ in such characters as ratio of embryo length to seed
size in these cultural and spacing conditions. The effect would
1112
CAN. J. FOR. RES. VOL . 15. 1985
TABLE 2. Calculated height differences at 5, 8, and 14 years resulting from hypothesized
seed weight differences of 12.5-100%
2-year
Volume difference
seedling
at age (years):
Height difference at age (years):
Seed
height
weight
difference
difference
5
8
14
5
8
14
5
8
14
(em)
(em)
(em)
(%)
(%)
(%)
(%)
(%)
(%)
6.5
6.5
3.3
2.2
0.7
9.6
6.5
2.1
12.8
12.8
6.6
4.3
1.4
18.5
12.4
4.1
25.7
25.7
13.1
8.6
2.8
34.4
23.6
8.2
49.3
49.3
25.6
16.5
5.4
58.8
42.0
15.3
(%)
em
12.5
0.9
5
3.2
25
1.8
10
6.3
50
3.6
20
12.2
100
7.2
40
24.6
%
NOTE: All plants were considered to be on the same elongation growth curve fitted by a Gompenz equation. Seedling­
height differences were considered to represent solely different "age" positions on the same curve. Volume differences
were estimated assuming that the effect on diameter was proportional to the effect on height. Further details are given
in text.
probably be greater if the seedling density were greater and
perhaps less if the density were less (Ross and Harper 1972).
Variation in seed weight among seed lots is usually much larger
than 10% for most coniferous species (e.g., see Fig. l in Olson
and Silen, 1975). Consequently, the potential for growth dif­
ferences resulting from seed size differences definitely exists.
In addition, seed size affects rate (Hulten 1974) and percentage
of emergence, large seed fractions generally exhibiting higher
emergence percentages (Burris et al. 1973), particularly under
adverse conditions of heavier soils, higher temperatures, or
greater sowing depth (Rohmeder 1972; Jacobsohn and Glober­
son 1978).
Practical implications
A correlation of seed size and seedling size has practical
implications for producing seedlings in nurseries and in genetic
evaluation by progeny test.
Whether altering seed size can affect productivity from
forest nurseries depends on the effect of seed size on long­
term growth, the possibility of changing seed size by inexpen­
sive cultural treatments, and the alternatives to seed size for
increasing growth.
Growth variation caused by seed size will have only a small
effect on time to stand closure or rotation length, unless seed­
lings from large seeds also have inherently faster growth rates.
To evaluate long-term effects of seed weight, we assumed that
a 10% difference in seed weight results in a 4% difference in
2-year seedling height. We also assumed that all seedlings were
on the same elongation growth curve (Overton and Ching 1978)
and that after two nursery seasons, seed-size effects had put
seedlings from different seed-size groups at different points on
the growth curve. Seedlings from heavier seeds were larger
because they were farther advanced on the "age" or x-axis of
the curve. To illustrate, we used the mean elongation curve
measured to age 14 years for 17 open-pollinated Douglas-fir
families growing in a plantation about 30 km north of Corval­
lis, OR. A Gompertz curve was fitted to annual measurements
through age 8 years. After age 8 years, the plants were in the
grand period of elongation and annual elongation was the same
for all plants.
Heights of seedlings from different seed-size groups were
compared at selected ages by a procedure previously described
(Sorensen and Miles 1982). The procedure can be represented
diagrammatically in two steps. First, the common growth curve
is shifted along the age axis to obtain a set of curves with dif­
ferences in height, at age 2 years, equivalent to 0, 5, 10, 20,
and 40%. Using this set of curves, differences from the 0%
curve were obtained for ages 5, 8, and 14 years (Table 2).
Seed-weight differences given in column one of Table 2 are
beyond the bounds of our treatment effects. The relation to
seedling height were calculated using the ratio estimator for
seedling height to seed weight from our data. Stem-volume
percentages are also given, again assuming proportionality
between height and diameter.
Height differences after 5 or more years in the field are quite
small relative to seed-weight differences (Table 2). Even with
a two to one difference in seed weight, a well-designed test is
required to detect seed effects and avoid a statistical error of the
second kind: concluding no seed effect exists when, in fact, it
does and may be quite important to volume production.
Although effects of seed weight may be small, over the long
term they may still be important to the extent that they influ­
ence competitive position of individual trees within a planta­
tion. Plants that differ in height by 5% at the time of stand
closure will occupy slightly different positions in competition
for light and moisture. After several years of competitive
growth, they may no longer be on the same growth curves for
height and, more particularly, for diameter.
An older test we are following appears to be going through a
cycle in which early size differences first increased, then stabi­
lized (as in Table 2), and then again started to increase. The
latter increase was especially notable in diameter increment.
Seed size or embryo vigor can apparently be increased by
cultural treatments applied to seed orchards, seed production
areas, and protected seed trees. Environment of the seed tree
appears to have considerable influence on seed size (Shen and
Lindgren 198 1). In coniferous species, seeds from grafted
ramets in seed orchards have been observed repeatedly to be
heavier than seeds from the original ortets (Simak and Gustafs­
son 1954; Hadders 1963; Nanson 1972; Anonymous 1973;
Dolgolikov 1977; Parks 1978; Bjpmstad 1981). The reason for
larger seeds was ascribed in one case to the different photo­
period of the orchard, which was far south of the natural stands
of the ortets (Bjprnstad 1981). Larger crowns of orchard trees
(Podzharova 1973), longer growing season, and more favor­
able moisture (Slater and Jensen 1970) and fertilizer (Mergen
and Voight 1960) in the orchard compared with the natural
stand are also possible explanations.
Response to cultural treatments is complex and probably
interacts with genotypes. The response is not restricted to an
SORENSEN AND CAMPBELL
effect on seed size. Chemical composition of the seeds may be
TABLE
lll3
3. Effect of seed-weight differences on culling outcome
ltered as well (Schweitzer and Ries 1969), which in turn may
mfluence seed dormancy (Wiesner and Grabe 1972) rate of
i
germination (Stearns 1960), and seedling vigor, all ndepen­
dently of seed size (Ries and Everson 1973). In addition, differ­
ent genotypes may respond differently to the cultural treatment
both in seed size (Simak and Gustafsson 1954; Bjprnstad 1981)
and physiological responses (Wiesner and Grabe 1972). With­
out considerable testing and experimentation, predicting the
Seed-weight
subpopu1ations
Seed size is considered to be one of the most stable of
phenotypic characters (Puckridge and Donald 1967; Gallagher
et al. 197 5). Increases in seed weight of much more than 50%
may
e difficult to achieve culturally. Scots pine
(Pinus syl­
vestns L.) seeds from an orchard were 56% heavier than seeds
(Pinus
pondero sa Doug!. ex Laws.) seeds from an orchard were
from the ortets (Hadders 1963) and ponderosa pine
about 35% heavier than seeds from the natural stands (Parks
?
19 8) An increase in seed weight of 55% for slash pine (Pinus
:.
ellwttl! Engelm.) (Mergen and Voight 1960) and 39% for sugar
maple
(Acer saccharum Marsh.) (Chandler 1938) was obtained
from fertilizer application. In forest trees, no response to sup­
plemental fertilization of the seed tree has also been found
(Chandler 1938; R. K. Campbell, J. W. Duffield, and E. C.
Steinbrenner, unpublished).
Weight differences of almost the same magnitudes have been
observed between seed lots of trees picked in different years:
an average over several trees of 33% in noble fir
(Abies procera
Rehd.) (Sorensen and Franklin 1977) and 2 1% in Doucrlas-fir
"'
(Silen and Osterhaus 1979).
Increasing the seed size compares favorably with other nur­
sery cul ural treatments for enhancing growth, particularly if
seed weight can be increased inexpensively, or as a byproduct
of treatments already in effect in seed orchards. Three seedling
cultural treatment, early sowing, fertilization, and top heat by
the "greenhouse" effect, produced increases in growth equiv­
alent to that projected from a 50 to I 00% increase in seed
weight (Table 2). Advancing the date of sowing from May 12
12.5
Heavy
Light
Heavy
Light
Heavy
Light
Heavy
Light
effects of specific treatments on seed size is difficult.
Seed weight
differences
between
subpopulations
(%)
25
50
100
Proportion of seedlings
removed from each
seed-weigh ,subpopulation
given seedlin g culling rates of:
10
(%)
20
(%)
8.5
11.5
7.2
12.8
4.5
15.5
1.3
18.7
17.5
22.3
15.3
24.7
11 .0
29.0
4.4
35.6
I, composite populations were made up of paired subpopulations whose
12.5, 25. 50, or 100%. Seedlings were culled at the end
gro ing season based on height. Either 10 or 20% of the seedlings of the
NOTE: In Fig.
mean seed weights differed by
of the s 7cond
compos1te populat1on were culled. The last two columns list the percentages of seedlings
removed from each seed-weight subpopulation.
If emergence rate is positively correlated with seed size vari­
:
ability may be further exaggerated (Rohmeder 1962) Pro­
cedu e f r culling or so v:ing less densely have been developed
to mmimize the proportiOn of poor seedlings resulting from
such variability.
Even if sowings are not dense, seed weight can affect results
of culling. A relation between seed-weight differences and
culling is given in Table 3. Culling was based on second-season
seedling height. Stem caliper would have been a better criterion
(T.
0. Perry, personal communication; Y. Tanaka, personal
c?mmunication), but the principle should be similar for any
Size measure. The array of all seedling heights from seeds of
unbagged cones was used to establish the parameters of the
curve for a single seed-weight subpopulation. In the examples,
subpopulation pairs with different mean seed weights but equal
variances were mixed. Culling of the shortest 10 or 20% of the
seedlings in the mixed populations was then broken down into
to April 23 (19 days), increased ! -year epicotyl length by
53% (8.7 vs. 5.7 em) and 2-year total height by 15% (42.3 vs.
its separate effects on the paired subpopulations. The results
36.8 em) (Sorensen 1978). Supplemental fertilizer applied to
half the plots the I st year and to all the plots the 2nd year for
most of the culled seedlings started out from lighter seeds.
the same two sowing dates increased 1-year epicotyl length by
36% (8.3 vs. 6.1 em) and 2-year total height by 19% (42.9 vs.
36 .I em) (Sorensen 1978). Supplemental top heat obtained by
.
. a clear plastic
placmg
tent over seedlings when the germinated
seeds were sown on May 20 increased !-year height by
8% (13.4 vs. 12.4 em) and 2-year height by 15% (58.6 vs.
51.0 em).
All the above cultural effects were measured in experimental
beds under conditions that were otherwise similar to those used
in the seed-weight and seedling-size test. Results suggested
hat growth increases were, to some extent, additive. If the goal
to pro uce larger seedlings, use of large seed can probably
IS
?
be substituted for any of these cultural practices to achieve
indicate that if seed-weight differences are about 50% or more,
When the normal mixture of seeds of different sizes is sown
in the nursery, competition leads to some natural and artificial
genetic selection. Genotypes associated with small seeds will
be culled most heavily. Size grading of seed results in reduction
!
o genetic variance (Brown and Goddard 1959; Hellum 1977;
S Ilen and Osterhaus 1979). The reduction in genetic variance
is greatest for the heavy and light seed fractions.
Taking into account the genetic and operational conse­
quences, seed sizing is probably beneficiaL If, however, ae­
etic v
a ion is greatly reduced, because of few seed pare ts
m the ongmal lot or because the sizing has eliminated the ex­
treme tails of the original distribution, then the seedling classes
themselves probably should be mixed before outplanting.
about equivalent growth increases.
The correlation of seed and seedling size implies that the
distribution of seed sizes in a seed lot will affect the distribution
Acknowledgments
We gratefully acknowledge the technical assistance of
of seedling sizes resulting from those seeds. Sowing the normal
.
mix of seeds of different sizes probably inflates the inherent
Richard S. Miles and helpful reviews by Mary L Duryea,
Thomas 0. Perry, Yasuomi Tanaka, and Timothy L. White.
variability in seedling size, because of the competitive ad­
vantage of larger seedlings when grown with small seedlings
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