OF FORESTRY, \lol. 60, No. 4, April 1962
Purchased by the U. S. Forest Service for Official Use.
Repritnted fro1n the JOURNAL
J
"
.
Does Rainy Weather Influence
Seed Set of Douglas-Fir?
R.oy R.. Silen and
Kenneth W. K rueger
RAINY WEATHER during the polli­
nation period has been considered
one of the reasons for low yields of
seed from Douglas-fir cones. In
Australia and Europe, periods of
rain and relatively high humidity
have been observed to impede pol­
len dispersal and flight (1, 9). In
Europe these conditions have been
found to result in poor pollination
of Betula verrucosa and B. pubes­
cens (8). Gross effect of rainy pe­
riods on size of cone crop has been
investigated in the Pacific North­
west (2), but no previous studies
of the effect of rain on set of filled
seed in Douglas-fir cones have been
reported.
A first evaluation was possible
during 1959, using data from a· de­
tailed study of Douglas-fir flower­
ing on 90 to 180 trees along several
elevational clines in Oregon and
Washington,' At each 250-foot ele­
vation interval along each cline,
the inclusive period of flowering
for 10 individual trees. had been
recorded. An inspection of U. S.
Weather Bureau rainfall records
indicated that at least one rainy
period of 8 to 11 days had occurred
on each transect during floweri g.
In each case, several clear days
adequate in number to favor abun­
dant shedding of pollen had both
preceded and followed the rainy
period. Profuse
elevational zones
of 1959 assured
trees would have
flowering at all
during the spring
that some study
been in flowering
THE AUTHORS are research foresters, Pa
cifie Northwest Forest and Range Expt.
Sta., Forest Service, U. S. Dept. Agric.,
Portland, Ore. They are stationed at
Corvallis in cooperation with the School
of Forestry, Oregon State University.
1Silen, Roy R. The progression of
Douglas-fir pollen shedding and female
flower receptivity with elevation in west­
ern Oregon and Washington. 1959. (In
preparation
for
publication.
Pacific
Northwest Forest and Range Expt. Sta.,
U. S. Forest Service, Portland, Ore.)
Seattle transect, seven stations
varying in elevation from 60 to
3,020 feet were available, and the
stage when rains occurred. In late
summer, cones were collected both
from sample trees that were recep­
tive to pollen during the spring
one corresponding most closely to
the elevation of a study tree was
used. Thus, weather data are more
reliable for· the Seattle transect
since differences in amount and
rainy period and from trees recep­
tive during clear periods before or
after. A comparison was then made
of number of filled seed in the
cones.
period of rain may occur over short
distances. The low-elevation sta­
Method
tions generally give a conservative
record of both amount and dura­
tion of rain at higher elevations.
Nevertheless, the low-elevation sta­
When weather and flowering rec­
ords were first compared, it was
apparent that no one rainy period
fully covered the 20- to 30-day in­
terval during which Douglas-fir
stands are receptive at a given ele­
vation. Hence, any gross eleva­
tional effect on seed set was un­
likely. But rainy periods more
nearly coincided with the 5- to 15­
day period during which individ­
ual trees are receptive. In order to
isolate the influence of rain on pol­
lination, only those trees receptive
during predominately rainy pe­
riods were compared with trees re­
ceptive during· prerain or post­
rain periods that were predomi­
nately clear.
Comparisons were made along
three transects: (1) from Corval­
lis, at an elevation of 250 feet, to
Marys Peak, at 3,500 feet in the
Oregon Coast Range ; ( 2) from
Seattle, at 75 feet, eastward along
U. S. Highway 10 to 2,500 feet in
the Cascade Range of Washington;
and (3) from Olympia, at 25 feet,
to 2,500 feet in the Olympic Moun­
tains northwest of Shelton, Wash.
(Fig. 1). Pollen sources were abun­
dant along these transects.
U. S. Weather Bureau records
were obtained from the station
nearest each study tree. The Cor­
vallis Station was used for the Cor­
vallis transect, and both the Olym­
pia and Shelton Stations were used
for the Olympic transect. All three
are low-elevation stations. For the
242
tions would he expected to .record
some rain on the same day it fell
at higher elevations i f more than
trace amounts occurred.
The rainy periods were charac­
terized hy 8 or more days with ap­
preciable amounts of rain during
·
an 8- to 11-day period. Clear pe­
riods had 3 to 8 clear days during
a 5- to 11-day period. The rainy pe­
riods, of course, were not entirely
rainy, nor were the clear periods
entirely clear. These periods were
considerably longer than average
for April and May, however, and
were the longest periods of rainy
or clear weather that occurred dur­
ing the 1959 study of Douglas-fir
flowering.
For each transect, equal num­
bers of trees that had been recep­
tive2 during a prerain clear pe­
riod, a rainy period, and a post­
rain clear period were included in
the analysis. Thus, the smallest
number of trees that had been re­
ceptive to pollen within any rainy
or clear period limited the sample
size for a given transect. All
Sample trees representing a weather
2Here the "receptive" period was con­
sidered the interval from the time bracts
on female strobili spread until they were
closed. This may be somewhat longer
than the period pollination can occur,
but it was used to permit eonsistent ob­
servation between trees. The period cov­
ers all flowers on a single tree.
, ,,
,APRIL 1962
243
.
'
period on a particular transect are
located within a 500-foot eleva­
tion.al range, and most sample
groups are within a 250-foot zone.
Fully developed· cones were col­
lected from several parts of the
crown of each sample tree. From
among these, 10 insect-free cones
were selected for cutting tests of
seeds in the central two-thirds of
each cone. Seeds were classified
into three categories: (1) filled,
(2) normal in size and appearance
but empty save for the dry, shriv­
eled remains of the female gameto­
phyte, and (3) flat and undevel­
oped, 'voody in appearance, and
often containing a pitchy sub­
stance. Seeds in each cOne were
WEATHER
STATIONS
Seattle
Snoqualmie Falls
ELEVATION
so'
4401
Landsburg
5301
Palmer
9001
Cedar Lake
Snoquolmie Poss
1,5601
3,0201
Olym pi a
100'
'
40
Corvallis
200'
Shelton
examined until the combined num­
ber in classes 1 and 2 totaled 25.
Seeds in class 3 were not used in
the analysis since they are consid­
-
. -- - -·
ered incapable of normal develop­
ment.3 Preliminary observations
had shown that a percentage based
on this sample size is representa­
tive of all seeds in the central part
Of a one.
Filled seeds were assumed to in­
dicate pollination, and the no'rmaf­
appearing but empty seeds to indi­
cate nonpollination. This assump­
tion may involve two small errors.
First, filled seed may possibly
arise by apomixis. The occurrence
of 22 filled seeds in 11 unpollinated
eones has been reported on a sin­
gle tree ( 6). If this proportion
holds true generally, our cutting
tests would introduce a positive
error of about 8 percent. The like­
lihood of such a high error on sev­
eral trees is believed small, how­
ever.
Second, self"pollination may pro­
duce ·aborted ovules (5) which we
would have classed as unpollinated.
Although aborted ovules may be a
source of variability in this study,
they should not bias percentage of
filled seed of samples pollinated
during rainy or clear periods. In­
dividual trees are highly variable
in both respects (5, 6) .
Percentage of filled seed was cal­
culated for each of 10 cones from
a sample tree. Differences in per­
8This procedure was adopted after con­
1:1ultation with Dr. A. L. Orr-Ewing and
Dr. T. M. Ching, tree seed authorities
for Douglas-fir. Such· seed may corre·
spond· to the "aborted" ovules described
by Lyons (4) for Pinus resi1n.osa.
FIG. 1.-Location of transects and weather stations.
centage of filled seed between trees
receptive to pollination during
rainy and clear periods were then
analyzed. Variation among indi­
vidual trees receptive during the
same period was also studied.
Rainy Weather and Pollination
Evidence from this analysis does
not lend much support to the the­
ory that rainy weather inhibits
pollination of Douglas-fir. Aver­
age percentage of filled seed for
rainy-period receptivity was sig­
nificantly less (5-percent level)
than for clear-period receptivity
only on the Corvallis transect. In
this transect, the difference be­
tween rain and prerain periods
averaged 13 percent; between rain
and postrain periods, 18 percent
(Table 1) . On the Seattle and
Olympia transects, differences be­
tween rainy and clear periods were
too small ( 0 to 11 percent) to
provide statistical significance. As
shown in Table 1, intensity and
duration of rainfall were much
greater on the Olympia and Seattle
transects during rainy-period re.
ceptivity than on the Corvallis
transect.
Thus, if rainy weather
is a major factor affecting pollina­
tion, differences in _percentage of
filled seed should have been more
pronounced on the Washington
transects.
seed
data
any
that
A study of the filled­
records and related rainfall
(Table 1) fails to bring out
clear-cut failures in seed set
can be attributed to rain.
Variation Among Individual
Trees
Striking differences in percent­
age of filled seed for both rainy­
and · clear-weather periods were
found among individual trees on
all transects. Analyses of these dif­
ferences showed they are consis­
tently highly significant (1-per­
cent level) within each weather
period on all three transects.
Two trees on the Seattle tran­
sect furnish a striking example of
between-tree variation. Located at
a 2,200-foot elevation, the trees are
growing
side
by
side
(with
244
JOURNAL OF FORESTRY
TABLE 1.-RAINFALL DURING POLLINATION PERIOD ,AND PERCENT OF FILLED SEED FOR DOUGLAS-FIR TREES ON THREE TRANSECTS Item
Corvallis transect Seat le transect
PrePost- PrePostrain Rain rain _rain Rain rain
Basis (number of trees)
Average receptive period· per
tree (days)
Average number of rainy
days during receptive
period (percent)
Average rainfall during recep-
tive period (inches)
Average.number filled seed
(percent) Olympia transect
PostPrerain Rain rain
10
10
10
6
6
6
5
5
5
10
10
11
11
11
5
8
8
10
20
80
45
36
91
40
25
100
40
1.2
0.1
73
60
1.21
78
0.6
67
6.8
56
0.6
63
0.6
57
6.8
57
1.1
64
1 Although the same rainfall occurred in the rain and postrain periods at Corvallis'
rain was concentrated in a much shorter span in the postrain period.
branches interlacing) and have
identical receptive periods. Cones
in one were consistently low yield­
ing, averaging 57.5 percent filled
seed. Cones of the other were con­
sistently high yielding, averaging
91.6 percent. The entire crop of 8
and 10 cones, respectively, was ex­
amined from each tree, eliminating
any chance for sampling error.
Occurrence of flat, undeveloped
seeds (class 3) was also highly
variable between individnal trees.
Cones from most trees averaged
from one to five class 3 seeds for
every 25 class 1 and 2 seeds sam­
pled. Comparative fignres ranged
from a low of zero to a high of 20
for
occasional
trees,
however.
When high or low numbers of
class . -3 seeds were encountered,
thiS trait was consistent for all
cones fronl 'a tree.
Discussion
Two considerations have prime
importance in a discussion of these
results. First, Douglas-fir stands
are receptive to pollen for a 20- to
30-day period at a given elevation.
Second, for an individual tree the
receptive period is reduced to only
5 to 15 days, and for a single flow­
er the receptive period may be
much shorter. Thus, while persist­
ent rain might conceivably' affect
some flowers or whole trees, rains
prolonged enough to preclude pol­
lination over a whole elevational
zone would be a very rare occur­
rence.
Although the trees nsed in this
study were carefully selected to
match receptivity with a predomi­
nately rainy or clear period, a
measurable reduction in· percent­
age of filled seed occurred only on
the Corvallis transect. Almost 7
inches of rain fell during the rainy
period on the Seattle and Olympia
transects, bli_t- trees receptive dur­
ing this period did not show a sig­
nificantly lower percentage of filled
seed. Effective pollination appar­
ently occurred on most trees with­
out relation to amount and dura­
tion of rain. PresUmably, pollina­
tion can take place during even
brief clearing. This preliminary
study thus indicates that rainy
weather does not seriously reduce
seed set of Douglas-fir.
For practical seed collection pur­
poses, any small adverse effect
from rain during pollination could
be more than offset by deliberately
collecting seed from trees that dis­
play a high count of filled seed per
coile. Such trees were found ·among
.particular tree year after year.
This study indicates that al­
though rainy periods may in some
circumstances be related to re.
duced seed set of Douglas-fir, other
unmeasured factors probably exert
an even greater influence. - One -of
these is apparently the very large
variation in seed set from tree to
tree, which may be a heritable
·
trait.
A renewed effort should be
made to uncover the causes_ of this
variation.
Summary of Results.
A preliminary study during 1959
indicates that rain during pollina'
could not cause a ma­
tion period
jor reduction in Douglas-fir seed
set over a broad elevational zone.
Rainy periods did not exceed 11
days on the three transects studied.
In contrast, trees at a give11 eleva
tion were receptive to pollen over a
20- to 30-day period. A study of
individual trees that were recep­
tive to pollen during predominate­
ly rainy periods showed little re­
bility in seed set among individual
trees may be physiological. The
capacity for fertilization in spruce
has been found to be genetically in­
fluenced ( 7) and percentage of
hollow pine seed in Switzerland has
likewise been found to be· a herit­
able trait (3); Both investigators
stressed the importance of select­
ing good seedbearers, especially
when collecting for breeding. pur­
poses.
-'·
Very low percentages of filled
seed cir very high proportions of
small, fiat, undeveloped seed, as ob­
served ill this study, may both be
heritable characteristics. Thus, in
. choosing parental stock fOr.· . see(!
orchards,
trees' with these traits
·
should usually be rejected, partfo­
ularly if the trait is observed on Jt
·
duction in filled seed per cone when
compared with trees receptive dur­
ing clear periods. Variations in
seed set among individual trees
were large, however, for . both clear
and rainy periods, indicating that
unmeasured factors were mainly
responsible.
all pollination periods sampled
without regard to weather during
pollination. Many professional tree
seed collectors have followed this
practice. for years.
At least part of the great varia­
-i
Literature Cited
1. FIELDING, J. M. 1957. Notes on the
dispersal of pollen by Monterey pine.
Austral. Forestry 21(1) :17-22.
2. GARMAN, E. R. 1955. Regeneration
problems and their silvicultural sig.
nificance in the coastal forests ot
British Columbia. Brit. Columbia For·
est Serv. Tech. Pub. 41. 67 pp. Illus.
3. KARSCHON, R. 1950. Uber den Hohl·
kornanteil bei FOhrensaatgut autoch
thoner Populationen. (A b s t r a c t}
Schweiz. Z. Forstw. 101(6) :295-296.
4. LYONS, L. A. 1956. The seed capac­
ity and efficiency of red pine cones.
(Pinus resinosa). Canad. Jour. Bot.
34(1) '27·36. Illus.
5. ORR-EWING, A. L. 1954. Inbreeding­
eiperiments with the Douglas :fir.
Forestry Chron. 30(1) :7-16. Illus.
6. . 1957. Possible occur­
rence of viable unfertilized seed 'in
Douglas-fir.
Forest Sci. 3(3) :243248. Ilfus.
7. ROHMEDER, E. 1954, Un\velt unct - Erbanlagen bei der Fichtensamenaus· beute. Z. Forstgenet. 3(6) :113-118.
8. SARVAS, RISTO.
1956. Investigations into the :flowering and seed quality of forest trees. Commun. Inst. for Fenn. 45(7
, ) ,1.37. Illus.
9. ScAMONI, A. 1955. Boebachtungen
iiber den Pollenfiug der· Wiildbaum&
.J.J-: in Eberswalde . .Z. Forstgenet. 4(4/5)
113-122..