813
The influence of floral bud phenology on the potential mating system
of a wind-pollinated Douglas-fir orchard
DONALD L. COPES
Forestry Sciences Laboratory, Pacific Northwest Research Station, USDA Forest Service,
3200 Jefferson Way, Corvallis, OR 97330 U.S.A.
AND
RICHARD A. SNIEZKO
Department of Forest Science, Oregon State University; Corvallis, OR, U.S.A.
Received May 30, 1990
Accepted January 3, 1991
COPES, D. L., and SNIEZKO, R. A. 1991. The influence of floral bud phenology on the potential mating systeril of
a wind-pollinated Douglas-fir orchard. Can. J. For. Res. 21: 813-820.
Reproductive bud phenology from 1983 through 1989 Iimited potential outcross efficiency to a maximum of 58 to 870Jo,
in a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) clonal orchard near Monmouth, Oregon. Potential outcross
efficiency was calculated for 20 clones from dates of male _and female bud opening and pollination mechanism infor­
mation. Cool weather before bud opening of the earliest clones delayed and compressed the breeding period and resulted
in a greater percentage of trees having synchronous periods of poIIen release and receptive seed strobili. Length of
breeding season among years averaged 20 days and ranged from 16 to 27 days. Differences in phenology significantly
impacted the breeding system because the overall breeding period of the orchard clones exceeded the 8-day receptive
period of individual clones by two or three times and often prohibited or limited potential outcrossing between the
earliest and latest clones. Outcrossing was greatest in clones with intermediate phenology and least in the earliest clones.
The breeding s ystem appears to be an almost continuous series of overlapping breeding subpopulations. Each year's
breeding subpopulations were different from those of other years because of large shifts in rank order of bud opening
by 10 to 200Jo of the clones and because of great differences in the length of breeding season. Average temperature
during March was linearly associated with time of floral bud opening. Geneticists may be able to use average tempera­
ture of the 4-week periods prior to opening of the earliest floral buds as a tool to identify seed crops formed during
years with compressed breeding seasons. Such seed crops have potential of being more diverse than do seed crops pro­
duced during years with extended breeding seasons. Use of procedures in this report will allow tree-improvement workers
to calculate the maximum outcross efficiency of each seed crop produced by their orchards.
COPES, D. L., et SNIEZKO, R. A. 1991. The influence of floral bud phenology on the potential mating system of. a
wind-pollinated Douglas-fir orchard. Can. J. For. Res. 21 : 813-820.
La phenologie des bourgeons reproductifs a limite, de 1983 a 1989, l'efficacite potentielle de l'inter-fecondation a
un maximum de 58 a 87% dans un verger a graines de sapin de Douglas (Pseudotsuga menziesii (Mirb.) Franco) pres
de Monmouth, Oregon. L'efficacite potentielle de l'inter-fecondation a ete calculee pour 20 clones ii partir des dates
d'ouverture des bourgeons miiles et femelles et d'informations au sujet des mecanismes de pollinisation. Une temperature
froide precedant l'ouverture des bourgeons des clones hiitifs reportait et comprimait la periode d'accouplement, et
resultait en un plus grand pourcentage d'arbres demontrant des periodes synchrones de liberation du pollen et de recep­
tivite des chatons femelles. La longueur moyenne de Ia saison d'accouplement au fil des ans etait de 20 jours, la longueur
annuelle variant de 16 a 27 jours. Les differences au niveau de la phenologie avaient un impact significatif sur le systeme
d'accouplement puisque la periode entiCre d'accouplement des clones du verger etait de deux a trois fois plus grande
que la periode de receptivite des clones inctividuels qui etait de 8 jours, empechant ou limitant souvent l'inter-ffcondation
entre les clones les plus hatifs et les plus tardifs. L'inter-ffcondation etait la plus grande pour les clones avec une phenologie
intermediaire, et la plus faible pour les clones les plus hiitifs. Le s ystCme d'accouplement apparait etre tel une sfrie
presque continue de sous-populations d'accouplement se juxtaposant. A chaque annee, les sous-populations d'accouple­
ment etaient differentes a cause de changements de rang considerables dans I'ordre d'ouverture des bourgeons de 10 ii
20% des clones, et a cause de differences importantes dans la longueur de la saison d'accouplement. La temperature
moyenne du mois de mars etait lineairement associee au temps d'ouverture des bourgeons floraux. Les geneticiens pour­
raient utiliser la temperature de la periode de 4 semaines precedant l'ouverture des bourgeons floraux des plus hiitifs
comme un outil permettant d'identifier les recoltes de semences produites lors d'annees oU la saison d'accouplement
est comprimee. Ces recoltes de semences ont le potentiel d'etre plus diversififes que les recoltes produites lors d'annees
oU la saison d'accouplement est allongee. L'utilisation des procedures dfcrites dans cet article permettra au personnel
implique en amelioration des arbres de calculer l'efficacite maxi.male d'inter-fecondation pour chaque rfcolte de semences
produites par leurs vergers.
[Traduit par la redaction]
Introduction
One factor lowering genetic efficiency of wind-pollinated
seed orchards is the absence or reduction of panmictic cross­
pollination among clones in the orchards. Genetic efficiency
is defined by Adams (1983) as the degree to which seed crops
reflect the high genetic quality and broad genetic variability
of orchard clones. Seed orchard crops cannot exhibit maxiPrinted in Canada I lmprime au Canada
mum genetic efficiency unless the orchard is a closed, pan­
mictic population (Fashler and El-Kassaby 1987), but in the
real world, orchards seldom achieve panmixia (Adams 1983;
El-Kassaby et al. 1988; Erickson 1987). This negatively
influences both the genetic quality and the variability of seed
crops. The proportion of outcrossed progeny is generally
814
CAN. J. FOR. RES. VOL.
high in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)
orchard clones (Omi andAdams 1986). but significant dif­
ferences in outcrossing are found among reproductive pheno­
logical classes (El-Kassaby et al. 1988) and among individ­
ual trees (Erickson 1987; Omi and Adams 1986; Shaw and
Allard 1982).
Imperfect synchronization of reproductive phenology is
a major factor that hinders panmixia in Douglas-fir
orchards. It is usually impossible for all trees in an orchard
to exchange genes because of the disparity in periods of
pollen shed and conelet receptivity (El-Kassaby et al. 1984;
Erickson 1987; Fashler and El-Kassaby 1987). In Norway
spruce (Picea abies (L.) Karst.) it was estimated that 66 of
190 possible crosses (35%) among 20 Norway spruce clones
rarely or never occur because of large differences in repro­
ductive phenology (Eriksson et al. 1973).
Previous studies of reproductive processes in Douglas-fir
have shown that the duration of pollen dispersal of indi­
vidual trees ranges from 3 to 5 days (Barner and Christiansen
1962; Ebell and Schmidt 1964). Cool months before the
flowering season have been found to generally delay and
shorten the breeding season. Cumulative heat sums from
January through April strongly influence the date of flower­
ing in British Columbia (El-Kassaby et al. 1984). The conelet
receptivity and pollen flight period of a British Columbia
orchard averaged 24 to 25 days in years with warm springs
and only 17 to 18 days in years with cool springs or in years
with warm springs if the trees are cooled with water sprays
(Fashler and El-Kassaby 1987). A shortened flowering period
in a randomized clonal orchard improves panmixia by allow­
ing more trees to cross-pollinate than if a longer pollina­
tion period exists among the same clones (Fashler and
El-Kassaby 1987). Considerable variability exists among
Douglas-fir clones during reproductive bud burst, but the
relative rank of phenological groups of clones generally is
similar over years (El-Kassaby et al. 1984).
The effect of phenological differences on the breeding sys­
tem is becoming better understood as more accurate infor­
mation is obtained on the pollination mechanism. Pollen
arriving first has a greater chance of fertilizing ovules than
does pollen reaching the receptive flowers later (Daniels
1978; Ho 1980; Owens et al. 1981; Webber and Yeh 1987).
Ovule development is complete at time of bud. burst, and
maximum pollination can take place at bud opening (Owens
et al. 1981). In one study, the receptive period of individ­
ual seed strobili (female flowers) was about 6 days (Owens
et al. 1981), but most pollination occurs within the first
4 days after bud-scale opening. Few pollen grains gain access
to the micropyle chamber if the pollen arrives after the
6th day, and those pollen grains have little chance of
pollinating the ovules (Owens and Simpson 1982). Trees that
become receptive when the greatest number of trees are
releasing pollen have the highest outcrossing rates (El-Kassaby
et al. 1988). Phenological patterns over time are important
to geneticists and to others who are interested in the genetic
quality of seed, because they are one of parameters that can
influence filled seed set, as well as the genetic quality and
genetic diversity of seed crops in seed orchards.
In this study, current knowledge on the pollination mech­
anism in Douglas-fir is integrated with 6 years of pheno­
logical observations to investigate the effects of floral
phenology on the breeding system. The role of phenology
21, 1991
on the potential outcross efficiency (percentage of theoreti­
cally possible crosses) was calculated each year, and clonal
stability and rank changes in phenology were examined on
the same 20 clones during 6 years. Study results will help
geneticists, tree-improvement workers, and seed orchardists
in understanding and quantifying the effect of phenology
on genetic efficiency. Procedures are described that will
permit calculation of maximum potential outcross efficiency
of different orchards and different seed years. The results
may also be of help in selecting seed crops that are more
genetically diverse and in identifying clones that have little
chance of crossing with other clones in the orchard.
Methods
Floral phenology records from an experimental breeding orchard
were used to illustrate the effects of phenology on the breeding
system of a wind-pollinated orchard. The orchard is a 2-ha unit
located about 7 km southwest of Monmouth, Oregon. Ramets for
the phenological study came from 20 clones that had been grafted
in 1973 or 1974 as part of a breeding program to develop com­
patible rootstocks. Ramets originated from ortets that ranged from
18 to about 100 years old in 1983, when phenological observations
began. All ortets grew west of the crest of the Cascade Range in
Oregon and Washington between 50 and 1000 m elevation. A dis­
tance of about 550 km separated the most northern and southern
trees. Each clone was represented in the orchard by 4 to 19 ramets.
Initial spacing of trees was 3.65 x 3.65 m, but irregular spacing
existed in 1983 because of mortality resulting from poor soil
drainage. Arrangement of ramets of clones differed. Seven clones
were grafted in clonal rows, with the same clone extending across
the entire width of the orchard; others with fewer ramets per clone
were grafted consecutively in rows or were arranged in irregular
groups encompassing several rows. Ramet height in the orchard
was maintained at 5 to 6 m by topping.
Reproductive phenology was recorded from 1983 through 1989
as a part of the rootstock breeding project. Not all ramets of a
clone and not all clones necessarily flowered each year. No floral
bud stimulation techniques were used. Date of bud burst of seed
strobili was noted each year for every clone that flowered and was
obtained by observing the status of all flowers on selected ramets.
The date of bud opening was generally the same for all ramets of
a clone, so it was necessary to observe only one or two ramets of
each clone. About 900Jo of the female buds of individual clones
burst within a 2-day period, thus the date recorded indicated the
first, or peak, date of bud burst for each clone. Observations on
female and male flowering were made during the breeding season
at 1- to 2-day intervals. Bud burst for a clone was said to have
occurred when the first seed strobili had bud scales opened enough
to expose the upper half of the flower. Date of first pollen shed
was determined by shaking branches in the lower crown of several
ramets of each clone and observing whether pollen was released
into the air from the catkins. Six years of flowering observations
(1983-1989, excluding 1984) were used to calculate the average date
of female bud opening for each clone. Either 5- or 6-year aver­
ages were used to rank the clones from 1 to 20, with the earliest
opening clone being ranked 1. A ranking of clones based on pollen
phenology was not done, but was very similar to the development
pattern of the seed strobili and is assumed to be the same for
purposes of this study.
The effect of reproductive phenology on the breeding system
was estimated from the nwnber of potential crossing combinations
that could possibly occur each year given synchronized seed strobili
and pollen periods. Since there were 20 clones in this experimen­
tal orchard, the maximum number of outcross combinations for
each clone would be 19, assuming that phenological synchroniza­
tion of all clones occurred and that all 20 clones flowered that year.
TABLE I. Date of bud opening, day Of breeding season, and clonal rank of 20 clones for 6 years ranked by average March temperature (year is in parentheses)
Clone
number
a
I
2
3
4
5
6
7
8
9
10 11
12
13 14
15
16
17
18
19 20
Length of
breeding
seasonb
Avg. range
Date
Day
4-7
4-7
4-7
4-8
4-7
4-9
4-8
4-8 4-8
4-9
4-8 4-11
4-11 4-11 4-11
4-14
4-11
4-14
4-15 4-11
I
I
I
I
I
I
2
5
3
2
2
2
3
2
5
5
5
5
8
5
8
9
5
10
5
5
5
10 5
12
12 12
12
18
12
18
20
12
I
Rank
I
Date
4-10
4-9
4-10
4-12
4-12
4-13
4-13
4-12
4-13
4-13
4-13
4-13
4-13 4-16
4-16
4-18
4-14
4-16
4-18 4-16
Day
2
I
2
4
4
5
5
4
5
5
5
5
5
8
8
10
6
8
10 8
16
8.6°C (1987)
8.1°C (1988)
7.1°C ( l 989)
6.1°C (1985)
Rank
2
I
2
4
4
7
7
4
7
7
7
7
7
15
15
19
14
15
19
15
17
Date
Day
3-31
4-6
4-6
4-6
4-6
4-7
4-6
4-10
4-6
4-7
4-7
4-10
4-7
4-10
4-10 4-12
4-10
4-12
4-13 4-12
I
I
7
7
7
7
8
7
11
7
8
8
11
8
11
11
13
11
13
14 13
2
2
2
2
8
2
12
2
8
8
12
8
12 12
16
12
16
20 16 Rank
20
Date
3-29
3-29
3-29
4-1
4-1
4-1
4-1
4-1
4-6
4-8 4-6
4-8
4-8 4-8
4-8
4-8
4-8
4-6
4-6 4-10 Day
9.4°C (1983)
Rank
I
I
I
I
I
I
4
4
4
4
4
9
11
9
11 11
11
11
11
11
9
9
13
4
4
4
4
4
9
13
9
13 13
13
13
13
13
9
9
20
20 10.3°C (1986)
Date
Day
Rank
Date
3-24
3-23
4-3
3-31
3-31
2
2
13
10
10
6
3
3
3-27
3-25
3-25
3-27
I
-
-
3-31
4-3
4-3
10
13
13
I
-
3
6
6
-
-
-
4-5
4-5
4-10
4-5 4-7
15
15
20
15
17
10
10
15
10
13
-
4-10
4-7
4-3 4-10
-
-
20 17
13
20
15 13 6
15
27
Day
3
I
I
3
Rank
3
I
I
3
-
-
-
3-30
3-30
3-30
3-30
4-1
4-1
3-30
4-1
4-1
4-1
4-1
4-3
4-3
4-6 4-6
6
6
6
6
8
8
6
8
8
8
8
10
10
13
13
5
5
5
5
10
10
5
10 10 10
10
10
17 18 18
Among-year
range in
date of bud
opening (days)
18 18 17 17 13 15
15 14
15
13 13
15
13
16
16 18 12 14
16
11 ()
0
.,,
"'
,.
z
0
z
iii
N
"'
0
20 15.0
NOTE: Dates are given as month-day.-, clone did not flower. °Clones ranked in order of earliest (I) to latest (20) female bud opening based on data from 5 or 6 years. Clones 5 and 6, 9 and 10, and 15 and 16 ranked the same by using average day of first bud opening. bLength of breeding season equals number of days between first opening of flowers of the earliest clone and the last receptive day of female flowers of the latest clone. 00
"'
CAN.
816
J. FOR. RES. VOL.
21, 1991
28
27
26
25
24
23
22
(!)
z
z
w
0..
0
0
:::>
<D
w
_J
<(
::;;;
w
LL
LL
0
><(
0
21
20
19
18
17
16
17
15
13
14
18
19
15 14
11
13
rn
12
12
11
9
•
10
9
'
7
5
8
7
4
'
6
5
4
3
2
1
6
7
8
9
11
10
AVERAGE MARCH TEMPERATURE (°C)
FIG. 1. Linear regression lines showing relation between day of female bud opening and average March temperature for 20 clones
(day I of female bud opening corresponds to March 23).
The assumption was made that no other factors limit potential
crossing combinations, therefore the estimate is likely to be the
upper limit of the number of crossing combinations that can occur.
Criteria used to determine the average number of days when seed
strobili could be pollinated and the average number of days when
pollen was shed were based primarily upon the most accurate infor­
mation available at this time, that being the observations of Owens
et al. (1981) and Ebell and Schmidt (1964). Thus, for purposes of
this study, seed strobili receptivity of each clone was assumed to
be .8 days (6-day receptive period of individual flowers plus 2 days
for within-clone variation in date of bud opening) and pollen release
4 days. For simplicity, the average pollen release period of each
clone was assumed to occur between day 3 and day 6 of the female
bud opening of that clone. Some deviation in time of pollen shed
occurred between years, but not enough to have a major influence
on estimates of outcross potential. Self-pollinated crosses were not
included in cross totals.
Average March temperature (average of maximum and minimum
daily extremes) of each crop year was obtained to correlate with
reproductive phenology. March at the Monmouth site encompasses
much of meiosis and practically all cone bud enlargement stages
after winter dormancy. Weather data from the Dallas 2 NE sta­
tion, as published in Climatological Data, Oregon (United States
Department of Commerce 1983-1989) were used. The Dallas 2 NE
weather station is located about 6 miles (1 mi.
1.6 km) north­
west of the orchard. Average March temperatures from 1983
through 1989 ranged from 6.1°C (43°F) to 10.3°C (50.5°F).
The relation of strobili opening and the percentage of potential
outcrossing to average March temperature was examined by fit­
ting linear regressions for each of the 20 clones. Observations from
all 6 years were available for 16 clones and observations from
5 years for four clones.
=
Results
About 90"7o of the female buds on ramets of individual
clones opened within a 2-day period. Thus the assumption
of an 8-day receptive period of individual clones should
include the most receptive period of the majority of buds
on each tree. Pollen release by individual clones was essen­
tially complete after 4 days, but was lengthened by cool,
rainy periods during the normal release period. The assump­
tion that pollen release for each clone normally began on
day 3 and ended on day 6 of the seed strobili receptive
period was verified by the field observations for this orchard,
but minor deviations occasionally occurred, especially in
years with colder than average March temperatures.
The length of the breeding season from 1983 through 1989
varied considerably among years. Average March tempera­
tures ranged from 6.1 to I0.3°C during that time. The
earliest a clone was receptive was March 23, 1983, when the
average March temperature was 9.4°C (Table I). In 1985,
that same clone did not open its buds until April 7, when
the average March temperature was only 6.1°C. A signifi­
cant (P < 0.05) linear relation between time of flowering
and temperature was found for 13 of the 20 clones. For the
remaining 7 clones, there was also support of a linear rela­
tion (P < 0.11 in all cases, and R2 ranged from 0.54 to
0.84). An examination of the linear regressions for the
20 clones showed that time of flowering was earlier in years
with warmer March temperatures and that there appeared
to be relatively little interaction between clones and temper­
ature (Fig. 1).
817
COPES AND SNIEZKO
TABLE
2. Number of possible outcross combinations and average potential outcrossing efficiency for 20 clones
by average March temperature ( year is in parentheses)
No. of possible outcross combinations by avg. Mar.
temp.
a
Clone
number
( rank)'
6.1°C
(1985)
7. 1°C
(1987)
8.1°C
(1988)
8.6°C
(1987)
9.4°C
(1983)
10.3°C
(1986)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
16
16
16
16
16
16
16
16
16
18
16
19
19
19
19
10
19
10
8
19
13
13
13
17
17
19
19
17
19
19
19
19
19
16
16
13
19
16
13
16
0
14
14
14
14
17
14
18
14
l7
17
18
18
18
l7
12
18
12
8
12
7
7
7
11
11
11
11
11
16
11
16
11
11
11
11
11
11
16
16
11
1
1
11
9
9
14
8
8
14
Flowering clones
Crosses
Mean crosses
per clone
Potential
outcrossing
efficiencyc
20
322
20
332
20
286
20
228
No. of
average
crosses
per year
7
11
11
7
16
16
16
16
16
16
16
16
16
16
16
14
14
9
9
8.5
9.8
11.5
13.5
13.4
15.8
14.2
14.8
15.3
16.2
16.3
16.2
15.0
15.7
15.0
12.4
14.7
13.2
10.8
12.3
l7
158
19
266
19.3
265.3
9
11
11
14
14
7
14
11
16.1
16.6
14.3
11.4
9.3
14.0
13.7
84.7
87.4
75.3
60.0
58.1
77.8
74.9
Avg. potential
outcrossing
efficiency (OJo )c
45.6
52.3
62.7
73.2
72. 3
84.1
76.8
80.7
83.3
86.2
89.0
88.2
80.9
85.5
81.5
66.2
79.0
71.8
59.3
66.5
NOTE:-, clone did not flower. aoutcross combinations including reciprocal crosses. b
clone number (phenotypic rank) is an average from 1983 to 1989 (1984 was not included because the five latest clones failed to flower that year).
'Potential outcross efficiency
=
[mean outcrosses per clone
+
In general, cool weather in March seemed to delay the
date of bud opening and considerably shortened the overall
breeding period compared with years with warmer March
temperatures. For the 20 clones studied, the actual length
of the breeding season ranged from 16 days at 6.1°C (1985)
to 27 days at 9.4°C, (1983) (Table !).
Clonal variation for time of strobili opening occurred
every year (Table I). Yearly rankings of clones were gener­
ally similar, but flowering was out of the anticipated
sequence (based on 6-year average) for approximately 16%
of the clones (range 10 to 20%) (Table I). Major departures
of 4 or more days for at least one clone were detected in
5 of the 6 years. Large changes in rank order for flower
receptivity and pollen shed resulted in the affected clones
breeding with some different subpopulations of clones
(Table 1). Larger rank changes occurred more often in the
early- and late-flowering group of clones than in the inter­
mediate clones.
The potential breeding system in this experimental orchard
consisted of a fairly continuous series of subpopulations over
time if it was assumed that females of any particular clone
were receptive for 8 days and pollen was shed for 4 days
(day 3 through 6 of the female receptive period). Assuming
this type of female-male synchronization, a clone could
(the maximum possible flowering clones
I)] x JOO.
pollinate all clones whose female receptive periods ranged
from 5 days earlier to 5 days after the female flowers of the
pollen donor first opened (Fig. 2). Figure 2 shows the
makeup of three hypothetical subpopulations. In this figure
the clones that first opened their female flowers on the !st,
10th, or 14th day of the breeding season are depicted. During
1983-1989, in the orchard observed in this study, the mean
number of potential outcrossing combinations (out of a
maximum of 19) differed from 9.3 (1983) to 16.6 (1989)
(Table 2). Over all years, an average of 13.7 unique outcross­
ing combinations per parent was phenologically possible.
The most complete synchronization among clones in floral
bud opening occurred during the 2 years with coldest aver­
age March temperature. It was possible in those years for
some clones to cross with all clones. A significant linear rela­
tion between percentage of possible outcrosses and average
March temperature was found for only 2 of the 20 clones.
In general, fewer crosses per clone were possible as temper­
ature increased.
Later and shorter breeding periods occurred in years with
cooler than average temperatures during the 3- or 4-week
period preceding strobili opening. Short breeding periods
increased potential outcrossing opportunities (Tables I
and 2). The maximum outcross efficiency that could
CAN. J.
818
2
3
z
0
4
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DAYS SEED STROBILI ARE RECEPTIVE
(pollen shed dates are underlined)
FIG. 2. Diagram depicting three breeding subpopulations for clones first opening their female buds on the lst, 10th, or 14th day,
respectively , of a 29-day breeding season. (Length of seed strobili receptivity was estimated as 8 days and pollen release period as 4 days.)
possibly occur during the 6 years averaged 75% (range from
58% at 9.3°C to 87% at 6.1'C). Ramets of at least 17 of
the 20 clones flowered all 6 years. Data from 1984 were not
used because they did not include the full phenological range
of clones (7 clones, including the 5 latest clones, failed to
flower).
The average potential outcrossing during the years with
the three coolest March temperatures (1985, 1988, and 1989)
averaged 82.4% compared with a 65.3% average for the
3 years with the warmest March temperatures (1983, 1986,
and 1987). The warmest year, 1986, was atypical, however.
Outcross efficiency for that year was much higher (78%)
than for the other two years with warm March temperatures
(58% in 1983 and 60% in 1987). In 1986, the early group
of clones opened their flowers unusually late and, as a result,
the breeding season was shorter than expected. Potential out­
cross efficiency over the 6 years of this study was highest
among clones that were intermediate in time of female bud
opening and was lowest in clones that were earliest in bud
opening time (Fig. 3; Table 2).
The earliest and latest groups of clones were not as effec­
tive in outcrossing as were the more intermediate clones.
Fewer clones were shedding pollen when the early clones
were receptive, while the late clones had shed their pollen
when there were few clones with receptive female flowers
(Table 2). For example, opening of floral buds of the earliest
clone in 1988 was not synchronized with any other clone in
the orchard, and the two earliest clones in 1983 could only
pollinate each other. Also, much of the receptive period for
the latest clones occurred after most orchard clones had
already completed pollen shed (Table I).
Discussion
Reproductive phenology data from six pollination seasons
were used to estimate limits to the breeding system of an
experimental orchard consisting of 20 clones. A maximum
potential outcross efficiency of about 87% was phenologi­
cally possible during the year with the second shortest
breeding season (1989), while just 58% potential outcross
efficiency was possible for the year with the .longest breeding
season (1983). Cool March temperatures tended to compress
the length of the breeding season and thereby increase the
possibility of panmixia in the orchard. However, the actual
outcrossing that can occur is always less than the phenotypic
maximum because of variables such as selfing, unequal
fecundity, unequal number of ramets, pollen contamination,
and less than random distribution of ramets within the
orchard. The reported 75% average outcross efficiency over
the 6 years is the biological upper limit that could be
obtained with the 20 clones when no other factors limit
panmixia. In a simulation study, Fashler and Sziklai (1980)
estimated that 60% of the potential outcrossing combina­
tions in Douglas-fir had little or no chance of occurring when
a long (37-day) breeding season occurs. Phenological dif­
ferences in Norway spruce prohibited the occurrence of
about 35"7o of all possible combinations among 20 clones
(Eriksson et al. 1973). For Douglas-fir, greater genetic effi­
ciency will occur in seed orchards when breeding seasons
are preceded by 3 or 4 weeks of cooler than normal weather.
The same result occurs in orchards where water sprays are
used to artificially lower temperatures of floral buds (Fashler
and EI-Kassaby 1987). Potential outcross efficiency could
be 30% less in the orchards when warmer than normal tem­
peratures occur 3 to 4 weeks preceding floral bud opening.
The "first-on, first-in" condition in Douglas-fir indicates
that pollen grains that land first on receptive seed strobili
have a greater chance of fertilizing ovules (Daniels 1978;
Owens and Simpson 1982; Webber and Yeh 1987). No
attempt in this study was made to adjust potential outcross­
ing results for differences in stage of development of recep­
tive seed strobili when the first pollen arrived from each
clone. Clones that release pollen during the !st through the
3rd day of the 8-day receptive period of female strobili prob­
ably are more effective in promoting fertilization of those
flowers than are clones that release pollen that arrives later.
Unfortunately, information to make adjustments for such
819
COPES AND SNIEZKO
differences is not yet available. The same lack of informa­
tion exists for quantifying rate of pollen release. Clones in
the orchard dispersed most of their pollen during days 2
and 3 of the pollen-shed period of each clone. Significant
amounts of pollen did not occur on day 1 when some of
the pollen catkins became dry enough to begin releasing
pollen grain to the atmosphere, and little pollen remained
in the catkins for dispersal on day 4. The quantity and rate
of pollen shed is dependent on relative humidity and rain­
fall patterns (Ebell and Schmidt 1964). Proper weighting of
male gamete contribution by each clone according to time
and quantity of pollen shed would give more accurate esti­
mates of the male contributions. Even with the limitations
to current phenological records, this study provides a por­
trayal of the type of overlapping subpopulations that are
present in many orchards. The potential for low genetic
variability, high selfing, and pollen contamination in seed
of early- and late-flowering clones is also evident. If some
of the best clones are early or late flowering, then potential
gain from use of orchard seed can be reduced. In this case,
use of supplemental mass pollination could be especially
valuable.
Phenological difference during seed strobili opening prob­
ably generated an almost continuous series of overlapping
breeding subpopulations. A surprising range in date of
receptivity was observed in the population of just 20 clones.
One possible explanation for this variability is that the clones
in the rootstock breeding orchard came from broader geo­
graphic area than occurs in breeding units of production seed
orchards. The observed 16- to 27-day breeding seasons are
similar, however, to results from other orchards contain­
ing many more clones. Fashler and El-Kassaby (1987)
reported breeding seasons of 17 and 24 days for cool years
and warm years, respectively, in a British Columbia orchard
composed of 100 clones and seedling families. Griffith (1968)
reported a long 38-day breeding season for a population of
154 Douglas-fir trees.
The early and late clones had lower potential outcross effi­
ciency than the clones with more intermediate floral phe­
nology. It was impossible in some years for many clones to
interbreed because the effective length of the breeding period
for the group of clones exceeded the estimated receptive
period of individual clones by two to three times. The highest
and lowest levels of potential outcrossing by clones averaged
over six breeding seasons were 89 and 46%, respectively.
These numbers are probably overestimates since constraints
on panmixia other than floral phenology are not considered
here. Isozyme analysis of seed . from another orchard also
indicated greater outcrossing by clones of intermediate
phenology classes (El-Kassaby et al. 1988). Seed from clones
with intermediate reproductive phenology probably have the
widest genetic base.
Relative rank order of reproductive bud opening for
clones has been reported to be stable among years when
classes or groups of clones are evaluated (El-Kassaby et al.
1984). That same trend for groups of clones was noted
in this study, but individual clones sometimes exhibited
unexpected changes in rank order among years. Clonal
instability among years for date of seed strobili opening
necessitates collecting and averaging phenological informa­
tion for a number of seed years if more accurate ranking
of clones is desired. The small 1- to 3-day changes in rank
order may not be significant and may simply reflect sam­
100
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FIG. 3. Average potential outcross efficiency of 20 clones from
the earliest (clone !) to the latest (clone 20).
piing differences, but departures of 4 to 11 days from
expected opening dates occurred in one or two clones in 5 of
the 6 study years. Large changes in clonal order of bud open­
ing alter the composition of the breeding subpopulations and
allow new outcrossing combinations to occur. Combining
seed lots from different years may increase genetic hetero­
geneity over that obtained in a single breeding season.
Cumulative heat sums or units have been used to account
for phenological change by many researchers (Ebel! and
Schmidt 1964; El-Kassaby et al. 1984; Fashler and El-Kassaby
1987). Average March temperature was used in the present
study, because it is a simpler temperature measurement and
is readily available from conventional weather records,
therefore it is much more likely to be used by tree­
improvement workers. Reproductive bud opening of
Douglas-fir clones growing in western Oregon was strongly
correlated with average March temperature. The tempera­
tures that occur in the 3- or 4-week period preceding bud
opening in Douglas-fir are thought to be most important
because that is the period of the year when meiosis and floral
bud expansion occurs. Little development occurs during
January and February, so it is not likely that temperature
differences at that time significantly impact date of floral
bud opening.
Weather records for average temperature during the 3 or
4 weeks before normal flower bud opening should provide
useful estimates of whether seed from past years was pro­
duced in long or short breeding seasons. Such information
is a tool for geneticists to use in selecting seed crops that
have greater genetic variability as a result higher outcross­
ing rates. It is acknowledged that changes in the number of
clones that flower, amount of pollen produced by each
clone, and proportion of ovules that are fertilized by trees
located outside the orchard all subtract from the potential
outcross efficiency values based solely on floral phenology.
Study results also suggest that wind-pollinated seed from
extremely early or late clones should be avoided in seed years
with extended breeding seasons.
820
CAN.
J.
FOR. RES. VOL. 21,
Tree-improvement workers will be able to use the outlined
procedures to calculate maximum potential outcross effi­
ciency for other Douglas-fir seed orchards. The phenological
and weather records needed to calculate outcross efficiency
by this procedure are easily obtained or already in orchard
data banks. Flowering dates and the relation between seed
strobili receptivity and pollen shed within clones may vary,
but substitution of local values and the calculation of out­
cross efficiency is simple.
ADAMS, W.T. 1983. Application of isozymes in tree breeding.
In lsozymes in plant genetics and breeding. Part A. Edited by
S.D. Tanksley, and T.J. Orton. Elsevier Science Publishers B.V.,
Amsterdam, Netherlands. pp. 381-400.
BARNER, H., and CHRISTIANSEN, H. 1962. The formation of
pollen, the pollination mechanism and the determination of the
most favorable time for controlled pollination in Pseudotsuga
menziesii. Silvae Genet. 11: 89-102.
DANIELS, J.D. 1978. Efficacy of supplemental mass-pollinations
in a Douglas-fir seed orchard. Silvae Genet. 17: 52-58.
EBELL, L.F. , and SCHMIDT, R.L. 1964. Meteorological factors
affecting conifer pollen dispersal on Vancouver Island. Can. Dep.
For. Pub!. No. 1036.
EL-KASSABY, Y.A., FASHLER, A.M., and SZIKLAI, 0. 1984.
Reproductive phenology and its impact on genetically improved
seed production in a Douglas-fir seed orchard. Silvae Genet. 33:
120-125.
EL-KASSABY, Y.A., RITLAND, K., FASHLER, A.M., and DEVITT,
W.J.B. 1988. The role of reproductive phenology upon the
mating system of a Douglas-fir seed orchard. Silvae Genet. 37:
76-82.
ERICKSON, V.J. 1987. The influence of distance and floral
phenology on.pollen gene flow and mating system patterns in
1991
a coastal Douglas-fir seed orchard. M.S. thesis, Oregon State
University, Corvallis.
ERIKSSON, G., JOHNSSON, A., and LINGREN, D. 1973. Flowering
in a clonal trial of Picea abies Karst. Stud. For. Suec. 110: 1-45.
FASHLER, A.M., and EL-KASSABY, Y.A. 1987. The effect of water
spray cooling treatment on reproductive phenology in a Douglas­
fir seed orchard. Silvae Genet. 36: 245-249.
FASHLER, A.M., and SZIKLAI, 0. 1980. The importance of flower
phenology in seed orchard designs. For. Chron. 56: 237-241.
GRIFFITH, B.G. 1968. Phenology, growth, and flower and cone
production of 154 Douglas-fir trees on the University Research
Forest as influenced by climate and fertilizer, 1957-67. Faculty
of Forestry, University of British Columbia, Vancouver. Bull.
No. 6.
Ho, R.H. 1980. Pollination mechanism and seed production
potential in Douglas-fir. For. Sci. 26: 522-528.
OM!, S.K., and ADAMS, W.T. 1986. Seed set and the proportion
of progeny due to self-fertilization in a Douglas-fir seed orchard.
Can. J. For. Res. 16: 502-507.
OWENS, J.N., and SIMPSON, S.J. 1982. Further observations on
the pollination mechanism and seed production of Douglas-fir.
Can. J. For. Res. 12: 431-434.
OWENS, J.N., SIMPSON, S.J., and MOLDER, M. 1981. The pollina­
tion mechanism and optimum time of pollination in Douglas­
fir (Pseudotsuga menziesil). Can. J. For. Res. 11: 36-50.
SHAW, D.V., and ALLARD, R.W. 1982. Estimation of outcross­
ing rates in Douglas-fir using isozyme markers. Theor. Appl.
Genet. 62: 113-120.
UNITED STATES DEPARTMENT OF COMMERCE. 1983-1989.
Climatological data, Oregon. Vols. 89-95. United States Depart­
ment of Commerce, Washington, DC.
WEBBER, J.E., and YEH, F.C.H. 1987. Test of the first-on, first­
in pollination hypothesis in coastal Douglas-fir. Can. J. For. Res.
17: 63-68.