528
First- and Second-season Effect on Douglas-fir Cone Initiation from a Single Shade Period RoY R. SILEN
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Forestry Sciences Laboratory, Pacific Northwest Forest and Range Experimel!f Station, Forest Service, U.S. Department of Agriculmre, Corvallis, Oregon 97331 Received January 3, 1973
SILEN, R. R. J973. First- and second-season effect on Douglas-fir cone initiation from a single shade
period. Can. J. For. Res. 3, 528-534.
Shade (12-23% sunlight) applied for monthly periods between February and September to individual
branches in upper whorls of 40-year-old Pseudotsuga menziesii caused increased pollen-cone (c)) and
decreased seed-cone (Q) buds counted in autumn of the year of treatment. A consistent decreased count
in both bud types the following year indicated treatment effects were carried over, raising the question as
to when inducive events actually take place in Douglas-fir.
SILEN, R. R. 1973. First- and second-season effect on Douglas-fir cone initiation from a single shade
period. Can. J. For. Res. 3, 528-534.
L'application d'ombre (12-23% de lumiere solaire), pour des periodes mensuelles entre fevrier et
septembre, a des rameaux individuels des verticilles superieurs de Pseudotsuga menziesii de 40 ansa accru
le nombre de bourgeons males et diminue le nombre de bourgeons femelles comptes a l'automne de
I'an nee du traitement. Une decroissance consistente dans le nombre de bourgeons des deux types, l'annee
suivantc, a indique que les eftets du traitement se poursuivaient, soulevant Ia question du moment de !'in­
[Traduit par le journal)
cidence d'induction chez le sapin Douglas.
Introduction
Enhancement and regulation of cone pro­
duction has long been a goal of Douglas-fir
(Pseudotsuga menziesii [Mirb.] Franco) re­
search. Many facets of the subject have been
probed, but progress still depends on answers
to some simple questions. "When do inducive
events leading to floral bud production occur
under natural conditions?" is such a question.
Fully developed male ( & ) and female ( 9 )
cone buds of Douglas-fir are visibly distinguish­
able from vegetative buds by September of the
year before seedfall. Earlier visual distinction
between cone and vegetative buds is reason­
ably certain by dissection in July while the
buds are still developing (Owens 1969).
Owens ( 1964, 1967, 1969), who approached
the problem at cellular level, considered the
bud types as distinguishable histochemically
when they were very small on the elongating
shoot a few weeks after bud burst. Whether
inducive events occur then or earlier is not
certain. He observed that all bud initials, in­
cluding those in distinctly male positions, were
detectable by succinic dehydrogenase activity
as early as March, well in advance of elon­
gation of new shoots on which their buds
develop.
Can. J. For. Res., 3, 528 (1973)
An approach which provides evidence that
inducive events may occur even earlier is the
study of correlations between weather pattern
and abundant cone crops. Lowry ( 1966) and
van Vredenburch and la Bastide ( 1969)
showed that some correlations extend up to
27 months ahead of seedfall, or well into the
growing season a year prior to the one in
which cone buds develop .
. Another approach is to find treatments that
influence flowering, apply these at intervals
ahead of the time floral buds can be distin­
guished, and observe the earliest time at which
differences in floral bud numbers between
treated and control material occur. Such studies
for Douglas-fir (Silen 1967a, b; and Ebell
1967, 1971 a, b) indicate influence of various
treatments through the season of bud develop­
ment (i.e. March-July).
Because all such studies observe events in­
directly associated with floral induction rather
than with inducing chemicals, it cannot be
stated with certainty that results of inducive
events are actually being observed rather than
some result involving a preconditioning for
floral induction or an inhibition of floral de­
velopment following induction.
Evidence exists for the latter in that many
529
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S!LEN: FIRST- AND SECOND-SEASON EFFECT
buds in the 6 positiOn on the twig stop de­
veloping. If the shoot above such a bud is
pinched off, these latent buds then produce a
vegetative shoot (Silen 1967a). Further evi­
dence is seen in the proliferated cone, which is
partly floral and partly vegetative. Such cones
suggest that a bud, once induced to floral de­
velopment, has reverted to vegetative develop­
ment. The occurrence of proliferated cones is
quite common some years, 1 and may indicate
that inhibition of floral development following
induction could be a major mechanism con­
trolling cone crops.
The two similar studies reported here had
the goal to produce a negative effect on floral
bud numbers by applying shade at intervals
ahead of bud differentiation and to concur­
rently produce a positive effect by girdling or
fertilizing.
Methods and Materials
Dominant 30- to 40-year-old Douglas-fir trees
growing near Corvallis, Oregon, at elevations of 200,
1500, and 3000 ft (61, 457, and 914 m) were utilized
in both studies. Use of trees at a range of elevations
helped to assure that some flowering trees would be
sampled at any elevational band where flowering
might occur. Also, the same springtime phenological
stage occnrred about a month later at each suc­
cessively higher elevation. Data from trees at low
elevation could be analyzed with that of middle and
high elevation trees by comparing data taken 1 and
2 months later, respectively. At each treatment date,
each of four branches of an upper whorl randomly
received 1 month of shade with and without foliar
fertilizer (1963 study), I month of shade with or
without girdling (1964 study), or no treatment. Bud
development was recorded over two seasons.
1963 Study
A 2 X 2 factorial design within split plots consisted
of four treatments per whorl (shade, fertilizer, shade
+ fertilizer, and control) and used seven whorls per
tree for eight monthly treatment dates (March
through September). Twenty-one trees were treated,
seven trees at each elevation. Treatments at each
date consisted of one randomly chosen limb shaded
with a double-layered white muslin bag (12-23% of
full sunlight which allowed a temperature rise up to
12 °F ( 6.6 °C) in full sunlight). Another limb was
sprayed to runoff with a balanced commercial foliar
fertilizer containing nitrogen (0.5%) as nitrate +
other nutrients. A third was sprayed and then shaded,
and a fourth served as control. The most nearly simi­
lar limbs in the whorl were used.
'i.e. in 1966 a survey near Corvallis found that 11
of 37 trees produced cones which showed formation
of a definite vegetative bud at the tip.
Effects in the season of treatment were recorded
in October 1963 and second-season effects in Febru­
ary 1965. Seed-cone buds were counted on all
growth acropetal to the 1961 node, dissecting buds
as necessary to assure identification. Fully developed
pollen-cone buds as well as latent buds were counted
on a six-twig sample taken systematically over the
same section of the limb.
Analysis of treatment effect on the 10 trees that
flowered in 1963 (5, 2, and 3 from low, middle, and
high elevation, respectively) and the 10 different
trees in 1964 (5, 4, and I, by elevations) required
four separate analyses of variance ( C? and () X 2
years). Preliminary analysis showed no effect of fer­
tilizer treatment. Each analysis of variance for shade
treatment effects was in the following form:
Source
Total
Trees (T)
Dates (D)
Dormant versus elongating
Remainder
Error (a)
Shade t>ersus none (S)
SxD
S x dormant versus elongating
S x remainder
Error
d.f.
119
9
(5)*
I
4
45
1
(5)
1
4
54
*Only the middle six of the eight dates were analyzed due
to insufficient numbers of trees in the earliest and latest
dates when adjusted to common date of vegetative bud
burst.
Significance of peak responses in the shoot elon­
gation period was tested with the S X dormant
versus elongating interaction.
Large variation in flowering between and within
trees necessitated transformation of data to provide
for homogeneous variance in analysis of variance.
Square root and cubic root transformations were
used for seed- and cone-bud numbers, respectively,
because more common transformations were not
severe enough to normalize the wide fluctuations in
bud counts.
1964 Study
Procedures followed the 1963 study except for
change to a randomized block design consisting of
four treatments (shade, fertilizer, girdling, and con­
trol) at seven treatment dates. Eight other trees were
chosen at each of the three elevations. The first
monthly treatment was applied in mid-February.
Since no effect of fertilizer was observed in the 1963
study, a partial girdling treatment was substituted for
the shade + fertilizer treatment. At each treatment
date, the third, randomly chosen branch was cinched
tightly for 1 month with two No. 20 copper wires
applied about 8 in. (20 em) from the bole.
Effects were measured by bud counts as noted
before. For the seven trees that flowered in 1964
(2, 4, and 1 from low, middle, and high elevation,
respectively) and the 17 in 1965 (2, 8, and 7, by
Can. J. For. Res. Downloaded from www.nrcresearchpress.com by USDA 2015 on 01/12/15
For personal use only.
Vt
.....,
0
TABLE
1.
Average autumn seed-cone(¥) and pollen-cone(&') bud counts per shaded and unshaded branch sample in the 1963 and 1964 studies,
each covering two seasons of observations
Second season counts
(date treatment started)
First season counts (date treatment started) Treatment
1963 Study Unshaded
Shaded
Shaded minus unshaded
3/8
2.2
1.1
-I. I
3/25
5.8
4.8
1.0
4/15
3.6
2.6
-1.0
6/7
5/16
(Basis 10 trees)
5.8 4.8
6.8
3.6 l
-2.2 l j 2.0
!
7/3
7/30
--
9/3
3/8
3/25
8.4
16.0
+7.6
9.0
15.2
+6.2
4/15
<;!
6.3
3.2
-3.1
7.8
1.7
-6.1
5.3
2.9
-2.4
6.8
9.6
2.8
5/16
6/7
(Basis 10 trees)
4.8 l
9.0
2.9 l
1.4
!.9 l -7.6
7/3
7/30
9/3
10.9
5.3
-5.6
7.8
4.4
-3.4
5.3
2.1
-3.2
(')
d'
Unshaded
Shaded
Shaded minus unshadcd
1964 Study
Unshaded
Shaded
Shaded minus unshadcd
42
29
27
22
!3
-5
37
37
0
2/18
3/I9
4/14
0.2
2.0
!.8
1.2
1.2
0.0
1.0
0.4
-0.6
30
89
+59
l 28
l 57
1+29
5/17
6/23
(Basis 7 trees)
4.0 l
1.0
1.4 l
0.5
-2.6 l -0.5
39
40
+I
8/IO
I2
10
-2
II
14
+3
9/3
80
73
-7
84
96
12
2/18
3/19
89
78
-II
0.5
0.4
-0.1
4.4
3.6
-0.8
+2.1
5.3
4.4
-0.9
115
137
122
143
123
126
146
113
3.2
5.3
d'
Unsbaded
Shaded
Shaded minus unshaded
110
115
+5
104
94
-10
139
142
+3
112
137
+25
Data for all flowering trees adjusted to same phenological stage as
(914 m). Arrows show date of vegetative bud burst. Actua.l mean time
listed us basis.
NOTE:
94
87
.. 7
84
78
--7
·I 22 at 1500 ft (457 m) by combining next earlier
between 200 and 3000 ft (61 and 914 m) is
l 86
l 60
l-26
5/I7
6/23
(Basis 17 II'PPS)
9.0 l
5.8
6.2 l
3.6
-2.8 l ·-2.2
4/14
!?
1.2
0.5
-0.7
90
63
-27
l
127 I I I6 75
74
I
70
58
12
8/10
9/3
5.3
4.4
-0.9
54
45
-9
>
z
.,
0
!-'
?"
;o;;
trl
Y'
<
0
r
1.0
::-'
l.O
;;;
0.0
-~
'-"
531
SILEN: FIRST- AND SECOND-SEASON EFFECT
TABLE
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1963
1964
2.
Statistical significance of peak increase or decrease in floral-bud numbers in the shoot
elongation period
First
season
Second
season
First
season
Second
season
significant
decrease
decrease not
significant
highly significant
decrease
highly significant
decrease
significant
increase
increase not
significant
highly significant
decrease
decrease, no test
possible
elevations), three separate analyses of variance were
made ( 9 and i3 in 1964 and 9 in 1965) using the
same procedure as in the 1963 study. No analysis
was appropriate for pollen-cone data taken in 1965­
descriptive ratings of each twig were taken rather
than time-consuming counts because of insufficient
climbing personnel.
1963 STUDY
+4
+3
1963
+2
+I
- ~ f-'---L-""-'-Il..W!Jf"¥-"-1-':>...L.--"Cl.­
-2
-3
Results
Neither the fertilizing nor girdling treat­
ments had a significant effect on flowering.
Effects of a month's shade on floral-bud num­
bers, however, were quite pronounced and
strikingly similar for both studies.
Responses to bag shading, shown in Fig. 1,
were apparent for much of the growing seasons
but were usually most pronounced when elon­
gating shoots were shaded. All eight peak
values of Fig. 1 occurred in the same positive
or negative direction. All but one of them
( 9 in 1963) coincided in time. The general
similarity of patterns in two independent
studies covering three seed crops removes any
reasonable possibility of a chance occurrence.
Depressed or enhanced bud numbers from
shade treatments during shoot elongation
(Table 1 and Fig. 1) were statistically verified
for all peaks of the 1963 study and for the
1965 ¥ data of the 1964 study (Table 2).
Shading applied during bud dormancy did not
produce significant changes in flowering.
The consistent and statistically significant
depression of both 9 and 6 buds the second
season after shading treatment is an unexpected
result because the organs involved are initi­
ated after treatment. Hence, an analysis was
made to determine whether this effect might
have been due merely to bud production by
a branch in any one year having a depressing
or otherwise predictable effect the following
year. Such an effect could only influence the
analysis if a tree were used both years. About
. I , ,.
+2
1964 STUDY
1965
I
+~~~s~
-1
-2
1
i
=~ t
I
I
·~·
II
I
"
FIG. 1. Male and female bud count deviations
over two seasons from branches shaded 1 month in
two separate studies. Perpendicular dashed lines indi­
cate the time of vegetative bud burst.
three-quarters of trees that flowered did so in
about equal numbers in only one of the two
years, eliminating possibility of any general
relationship. A correlation analysis was per­
formed using all branches that did produce
first- and second-season floral buds (56 in the
1963 study and 44 in the 1964 study). The
correlation coefficients were 0.046 (nonsig­
nificant) for the 1963 study and 0.736 (highly
significant) for the 1964 study. The positive
relationship in the 1964 study indicated that
branches that produced relatively more cones
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532
CAN. J. FOR. RES. VOL. 3. 1973
in 1964-probably because of larger size­
also produced more in 1965. This was op­
posite to the depressing effect hypothesized.
A further indication that u second-year effect
was uncomplicated by previous cone crops was
shown by trees that produced no crop the first
year but flowered the second year. There were
five of these in the 1963 study and 12 in the
1964 study. A separate analysis of both these
groups again showed the depression of female
bud counts by shading during shoot elongation
to be highly· significant.
First-season effects were less consistent and
more complex. A tree's 6 bud count was often
enhanced by shading, whereas if seed-cone
bud counts were affected, they were usually de­
pressed. Individual trees varied greatly from
this pattern. Nonetheless, a statistically signifi­
cant depressing effect on seed-cone buds in the
1963 study was apparent for shading adminis­
tered late in the growing season.
Enhanced 6 bud counts from shading in
May and June were significant in the 1963
study, and a similar, but nonsignificant, pat­
tern appears in the 1964 study. However, this
enhancement occurred only for trees at grow­
ing elevations of 1500 or 3000 ft ( 457 or 914
m); no enhancement was apparent for trees at
200-ft (61 m) elevation. Total number of
buds that began development was similar at
all elevations. In general, the years 1963 and
1964 were characterized by high 'abortion',
or a halting development, of male buds early
in the season at middle and high elevations
(Silen 1967 b), whereas at low elevation, most
buds continued normal development. Bagging
at bud burst somehow prevented the arrested
development of male buds, not formation of
more male buds on the bagged branches.
Rather than a shade effect, the sheltering or
temperature effect of the bag on the developing
bud may be more important than that of the
reduced light. For example, cloth bags reduce
wind chill and elevate temperatures in sunlight.
This enhancement of male flowering has since
been widely observed in our pollination bags.
Seed-cone and i!J bud patterns appear dif­
ferent in another respect. Shade affected i!J
buds only from treatments applied near the
beginning of shoot elongation. For <il buds,
shade has an effect over most of the growing
season.
Discussion
In this study, girdling and foliar fertilizer
treatments were ineffective, whereas bagging
produced effects on bud numbers throughout
two growing seasons. The major unexpected
finding was that shading affected the bud crop
produced a year after treatment as much as
the bud crop that developed during the season
of treatment. The possibility of this effect being
an artifact of a reciprocal or other relation
from a previous crop (Owens 1969, Ebell
197lb) seems ruled out in the present study.
The second-season effect is particularly im­
portant. It implies a possible carryover of
physiological or chemical differences for at
least an 8- to 12-month period until the par­
ticular bud cells involved come into existence.
Thus the study adds support to weather pat­
tern studies previously cited showing corre­
lations with cone crops extending to 27 months
before seedfall. Here effects of shading 29
months ahead were observed.
Whether inducive events occur so much
earlier is left even less certain because effects
of shading were observed over so much of the
growing season both years. Shading effects in
both studies (Fig. 1 ) began when applied
somewhat ahead of bud burst, peaked when
applied near bud burst, then decreased gradual­
ly in effectiveness through the growing season.
Such a wavelike response pattern, particularly
that observed for seed-cone buds in 1963
(Fig. 1), has distinct similarity to the response
observed by Ebell (1971b) from applying
girdles to Douglas-fir at weekly intervals be­
tween April and July, if a somewhat longer lag
in response is assumed for girdling. The effect
on <;> buds appeared to persist longer into
the summer than that on 6 buds. Shade
applied in February or March had no consist­
ent effect either season. The effect was usually
a decreased bud number. The aberrant first­
year enhancement of i!J bud numbers appears
to be a sheltering rather than a shading effect.
Characteristic peak response on floral buds
in each study was during the period of active
growth. This suggests a correlation with the
low point in plant reserves (Krueger and
Trappe 1967), although Ebell's (1971 b)
study of high carbohydrate reserves associated
with floral response from girdling suggests
more factors against than for a direct role of
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SILEN: FIRST- AND SECOND-SEASON EFFECT
carbohydrate reserves. Depletion of reserves
was never a factor in his studies, however, nor
would such depletion rule out possible shade
effects on inhibitory processes or lack of floral
promoters leading to bud latency.
Some of the second-season decrease in
floral-bud numbers from a month of shade
might arise simply from production of a smaller
bud with fewer leaves, hence fewer potential
sites of axillary buds. Bud size data were not
taken, but total bud production did not appear
to be reduced on shaded branches.
Male response to bagging the branch ap­
pears to peak earlier (Fig. 1 ) . This suggests
that Douglas-fir may follow the same pattern
of response to increasing and decreasing day
length as reported for western red cedar
(Owens and Pharis 1971), assuming some
time lag before the shade effect maximizes.
The uncertainty in this study regarding tim­
ing of inducive events is paralleled in the cited
literature. For example, one interpretation of
the data could be that inducive events had
already occurred before bud burst, 29 months
ahead of seedfall, and that shade treatments
had somehow been capable of reducing floral
potentials almost any subsequent month of the
two growing seasons before buds were set. The
cited studies on weather pattern correlations
imply a similar timing possibility. A second
interpretation could be that inducive events
occurred early in the season of treatment, and
shade the previous season provided a morpho­
logical or chemical preconditioning that re­
duced potential cone-bud numbers. Seed-cone
and pollen-cone buds are almost certainly de­
veloping differently from vegetative buds a few
weeks after vegetative bud bursting (Owens
1969). Cone crops have been most influenced
by fertilizer and other treatments applied near
bud burst (Steinbrenner et al. 1960; Stoate
et al. 1961; Silen 1967b; Ebell 197la, b). A
third possible interpretation is that some final
inducive events might have occurred just before
or during bud differentiation postulating an
even longer carryover of such a precondition­
ing. Even this possibility is supported by other
studies involving shade, fertilizer, girdling, and
other treatments (Steinbrenner et al. 1960;
Silen 1967b; Ebell 1971b) that appear to alter
floral-bud numbers much later. Could all three
interpretations be correct?
533
A rationalization of all these observations is
to simply assume that Douglas-fir flowering re­
quires a different set of developmental genes
and hormonal controls than does vegetative
growth. Whether shading or other treatments
were applied early or late either season might
cause varying portions of the bud sites then
developing under floral genes to switch hence­
forward to development by vegetative genes.
The end result of varying such diverse factors
as weather, fertilizer, shade, or food reserves
may be in determining which set of develop­
mental genes prevails as bud tissues finally
differentiate. A support for this possibility is
that observed bud changes are usually sharply
delineated as to floral or vegetative tissue
whether observed early as forced development
of latent buds or late as proliferated cones.
Despite its perplexing results, the study sug­
gests several generalizations. It supports the
viewpoint that events associated with floral
development are more pertinent in Douglas­
fir cone crop enhancement than specific floral­
inducive or bud-initiative events (Silen 1967 a;
Owens 1969). No one period seems all­
important in floral development, but there are
probably very sensitive periods associated with
shoot development when floral development
may be most influenced. The early concept
that floral development is somehow associated
with reserve photosynthate levels again seems
to warrant more investigation. The most cer­
tain conclusion is that future investigation
should cover all the growing period for at least
two seasons.
EBELL, L. F. 1967. Cone production induced by drought
in potted Douglas-fir. Mon. Res. Notes, Can. Dep.
For. Rural Dev. 23, 26-27.
1971a. Physiology and biochemistry of flowering of
Douglas-fir. Paper for I.U.F.R.O. Working Group
Meeting on Sexual Reproduction of Forest Trees,
Varparanta, Finland, May 28-June 6, 1970. (In
Comm. lnst. For. Fenn. 1971).
1971b. Girdling: its effect on carbohydrate status
and on reproductive bud and cone development of
Douglas fir. Can. J. Bot. 49, 453-466.
KRUEGER, K. W., and TRAPPE, J. M. 1967. Food reserves
and seasonal growth ofDouglas-fir seedlings. For. Sci.
13(2), 192-202.
LowRY, W. P. 1966. Apparent meteorological require­
ments for abundant cone crop in Douglas-fir. For. Sci.
12, 185-192.
OWENS, J. N. 1964. The initiation and early development
of the seed cone of Douglas fir. Can. J. Bot. 42,
1031-!047.
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For personal use only.
534
CAN. J. FOR. RES. VOL. 3. 1973
- - - 1967. A new look at Douglas fir cone development.
1967 West. For. Conf. Proc., West. For. Conserv.
Assoc., Portland, Oreg. In Western reforestation, pp.
10-12.
---1969. The relative importance of initiation and early
development on cone production in Douglas-fir. Can.
J. Bot. 47, 1039-1049.
OWENS, J. N., and PHARIS, R. P. 1971. Initiation and
development of western red cedar cones in response
to gibberellin induction and under natural conditions.
Can. J. Bot. 49, 1165-1175.
SILEN, R. R. 1967a. Earlier forecasting of Douglas-fir cone
crop using male buds. J. For. 65, 888-892.
---1967b. How early can Douglas fir cone crops be pre­
dieted? 1967 West. For. Conf. Proc., West. For. Con­
serv. Assoc., Portland, Oreg. In Western reforesta­
tion, pp. 12-17.
STEINBRENNER, E. C., DUFFIELD, J. W., and CAMPBELL,
R. K. !960. Increased cone production of young
Douglas-fir following nitrogen and phosphorus fertili­
zation. J. For. 58, 105-110.
STOATE, T. N., MAHOOD, I., and CROSSIN, E. C. 1961.
Cone production in Douglas-fir (Pseudotsuga men­
ziesii). Emp. For. Rev. 40, 105-110.
VAN VREDENBURCH, C. L. H., and LA BASTIDE, J. G. A.
1969. The influence of meteorological factors on the
cone crop of Douglas-fir in the Netherlands. Silvae
Genet. 18(5-6), 182-186.