Reprinted from the JOURNAL OF FORESTRY, Vol. 60, No. 11, NoYember 196!
Pureha.sed by the U.S. Forest Service for official use.
Pollen Dispersal Considerations for Douglas.fir
Roy R. Silen
WITH increasing investments in
seed orchards, more exact know!.
edge of pollen dispersal patterns is
required to estimate isolation dis­
tances necessary to assure relative
freedom from pollen originating
outside these orchards.
The first pollen dispersal study
of Douglas-fir (Pseudotsuga ,,,.,,.
ziesii '[Mirb.] Franco) was made
in Indiana by Wright (12), who
classed the species among those
with a rather short dispersal dis­
tance. This classification appeared
reasonable because Douglas-1,ir has
one of the larger pollen grains
ainong western coniferous species
(11). According to Wright's data,
about 66 percent of the pollen from
a 25-foot-tall tree fell within 60
feet. Therefore, an isolation dis.
tance five to ten times the height
of mature trees should provide a
factor of safety in a seed orchard.
It thus appeared that a simple
study in western Oregon should be
adequate to verify the reported dis­
persal distance, and perhaps allow
a more specific recommendation as
to isolation requirements.
European literature on disper­
sion of forest tree pollen is a rich
source of information (1, 3, 5, 6, 7,
8, 9), but there have been relative.
ly few American studies (2, 10, 12,
13). The literature contains two
conflicting viewpoints concerning
pollen dispersal distances.
On the one hand, the amount of
pollen dispersed from individual
trees (12) or released from a point
source (9) has been found to de­
crease rapidly with distance. Dis.
tribution of ofl'spring readily iden­
tified through a pollen-transmitted
.
THE .AUTHOR ie on the et.aft' of the Paei:fl.c
Northweet Forest and Range Erpt. Sta.,
Forest Service, U. S. Dept. Agric., Port­
land, Ore.
Jack K. Carter of the Pacific North·
West Region, U. S. Forest Service, aa­
sisted with field work and counted the
pollen.
marker gene has also indicated that
efl'ective pollination distance from
individual trees is short (4).
On the other hand, Sarvas (6)
cites studies indicating that large
amounts of pollen are transported
for long distances. The amount of
pollen measured at lightships 35 to
60 kilometers seaward from the
nearest pine stands in Sweden was
about equal to that measured be­
neath the stand itself. In Sarvas'
own study in 1954, pollen fell at a
lightship 20 kilometers from the
coast of Finland in amounts ap.
proaching or even exceeding that
beneath a stand on the shore (6).
These seemingly contradictory
findings are drawn upon to explam
the results of a study of Douglas.
fir pollen :flight made in 1956 near
Corvallis, Ore.
Location. 1 Location t
Single
tree
Pair of
trees
250
250
Elevation (feet)
Distance and di­
rection from Cor·
3 (west) 5 (north)
vallis (miles)
99 and 110
78
Tree height (ft.)
Tree diam ete r
52 and 54
39
(inchea)
Distance from neareet Dougl_as-fir
(feet)
4,100
Distance from nearest Douglas-fir
atand (miles)
I.7
3,800
1.8
Pollen from the isolated trees
was sampled with vaseline.coated
slides. These were slanted at a 45°
angle facing the tree to increase
the chance of intercepting the slant­
ing descent of the pollen.2 In order
to elevate the slides above the field
crops, each was attached to a lath
stake with a clothespin stapled to
Methods
a notch near the top of the lath
(Fig. 3). A 4-inch-square alumi­
All the essential requirements
num
shield was stapled above the
for a study of pollen dispersal­
pin
to
protect the slide from rain.
isolated trees well supplied with
Although
this proved to be ineffec­
male :flowers, long periods without
tiTe
protection
from rain, the alu.
rain, and undisturbed sampling
minum
often
protected
the slides
stations-were met during 1956.
from
bird
droppings,
a
real
prob.
Pollen production from low-eleva- .
tion Douglas-fir stands in western lem where stakes provided the only
Oregon was extremely copious, and perches in large :fields of grain.
Four Jines of stakes, radiating
practically all open-grown trees
had large numbers of male :flowers. from the tree in northwest, north­
Trees isolated from other Doug. east, southeast, and southwest di­
las-fir by as much as a mile are rections, were established well
very .rare in western Oregon. In ahead of pollen :flight. Since the
the Willamette Valley, within 18 expected distance of flight was
miles of Corvallis, four single or short, successive stake intervals
close pairs of trees were chosen from the tree were 25 feet for the
that were isolated by more than first 100 feet ; then 50 feet between
3,800 feet. Only two of these, how­ 100 and 300 feet; 100 feet between
ever, were sampled over the entire 300 and 1,000 feet; and 200 feet
pollination period (Figs. 1 and 2).1 between 1,000 and 2,000 feet. Sam ­
pling extended to 2,000 feet in all
1The other two locations were aampled
quadrants
about the study trees
only once since they were more 4iistant
•
from Corvallis. It became impossible to
reach all four areas to recover slides
ahead ot impending rain. As these single
observations were useful in our conclu­
s.iona, basic data are included in Table
9:
hn+
+
.....
,,.) ..,,...,.lo,,.
..
,.
-
""'-
-
-----
.
'Guidance in development of proce·.
durea and analysis of data was provided
'bl J'. W. Wri ght who conducted the first
die on pol1e dj pe:ssf of m any :Am r­
.
,
791
NOVEMBER 1962
2.-Pair of 100-foot Douglas-fir
trees at location 2. The large poplar tree
in cluster was leafless at the time of the
study.
FIG.
Fto. 3.-Vaseline-coated slides were used
weather cleared. During the most
important period of the study,
from April 10 to May 2, only two
short rainy periods occurred. Thus,
continuous sampling was possible
over an unusually high proportion
1.-Isolated
78-foot
Douglas-fir
used in pollen dispersion study in the
Willamette Valley of Oregon-location 1.
Fra.
except to the southwest where agri­
cultural operations prevented sam­
pling more than 500 feet at loca­
tion 1 and 1,200 feet in the same
direction at location 2. No appre­
ciable amounts of pollen were ex­
pected beyond five or six times the
height of the tree; hence distances
and isolation were thought to be
more than ample.
However. to insure a sensitive
measure of the Douglas-fir pollen
density at maximum possible isola­
tion, an additional sampling was
made near the center of a treeless
area 3 by 14.5 miles in extent south
of Corvallis (Fig. 4). This is one
of the largest areas devoid of
Douglas-fir trees in all western
Ore<rnn and western Washington.
Siid-;,s were slanted at a 45° angle
arranged on 64 stakes in a square
grid at 10-foot spacings. In all but
the first observation, two similarly
oriented slides were exposed per
stake, pro,- iding 32 slides per
quadrant for a total sample of 128.
The quadrant faced by the slides
on each stake was randomly chosen.
Slides
1Yere g at h e re d
before rain
.
of the total period. Altogether,
slides were exposed during five in­
tervals, the last three in a pro­
longed rainless period in which ex­
posed slides were immediately ex­
changed for fresh ones.
Douglas-fir pollen shedding be­
gan at location 1 on April 10 and
was completed about April 21. At
location 2, shedding began on
April 12 and was completed about
April 24. These trees were some­
what early. In the treeless area,
peak pollen frequencies were found
during the period April 19-24, the
time when the majority of trees in
the surrounding country probably
were producing pollen.
Pollen counting commenced im­
mediately after the first collections
were made. Most of this was done
with a 36X binocular microscope,
but pollen of doubtful identity was
checked with a 246X microscope.
Slides were placed on a plastic
sheet etched with 144 squares per
inch. The plastic was set into a
cardboard frame suitable for hold­
ing the slide, and backed by blue
paper for contrast. Only Douglas­
fir pollen was counted for each
slide on a %-square- inch area, or
on 72 of the small squares. Identi­
fication of Douglas-fir pollen is
usually easy as it is one of the
largest grains encountered and has
for sampling pollen dispersal. These were
held by clothespins attached to stakes.
The aluminum shield, added to provide
protection from rain, proved ineffective.
all, 238,549 pollen grains were
counted.
Weather data were available
from the U. S. Weather Bureau
Station at CorvaJlis and from two
auxiliary stations. One of these is
at the Corvallis airport in the tree­
less area where hourly observations
are made. The other station, at the
O.S.U. Experimental Farm within
2 miles of location No. 2, had a
quadruple recorder, which pro­
vided a continuous record of rain­
fall, wind, humidity, and sunlight.
Weather during the fiye sam­
pling periods covered the usual
springtime variations. The first two
periods, .April 10 and April 12-14,
had clearing weather between
frontal rains with strongest winds
The period
from the southwest.
from April 16 to .April 25 was pre­
dominantly clear with variable
winds, and midday humidities were
generally below 45 percent. Strong­
est winds occurred on April 21 and
24, reaching velocities of 18 and 22
miles per hour from the west.
Results
Cumulative pollen densities for
all distances, as shown in Figure
5, averaged 2,805 grains per square
inch.
Maximum
pollen
count,
found at 50 feet from the trees,
averaged 5,116 grains and ranged
8In a few instances where identification
was doubtful, final determination was
,
-
.......
,.,.
.
793
NovEMBEB 1962
background pollen from other
sources (Fig. 5). A first estimate
of this background density can be
VERTICAL SCALE:
" Pollen Grains per Sq. In."
HORIZONTAL SCALE:
made using pollen counts in the
treeless area (Fig. 6). But the
'Distance from Tree in Ft."
background density estimated by
this means would indicate that
about 20 percent as much pollen
per square inch falls at a 2,000foot distance as at the tree.
Higher e s t i m a t i o n s of back­
ground pollen density seem more
·
LOCATION
reasonable. Theoretical studies of
pollen distribution from point
sources (9) indicate that a plotting
of pollen distribution using log­
arithmic scales for both distance
and pollen density should not de­
part greatly from a straight line.
Such a plotting (Fig. 7) of the
cumulative
curves for isolated
trees indicates that for distances
beyond 300 feet from the tree, the
distribution
departs
from
a
straight line. A trial deduction of
1,000 pollen grains per square inch
as background density shows a
similar but less marked departure.
Deducting 2,000 grains causes a
departure in the opposite direction.
A deduction of 1,800 grains makes
LOCATION 2
practically a straight line of the
distribution and thus appears to
be the best estimate. In this study,
therefore, pollen captured at dis­
tances beyond 5-10 times tree
height is considered to be largely
from sources other than the tree
sampled.
The relationship fou;.d between
wind· direction and pollen density
supports such a conclusion. Pollen
counts were related to wind meas­
urements both for the treeless area
and for distances beyond 500 feet
from study trees. This similarity
could mean that in both samplings,
mostly background pollen was
measured. Samples nearer the trees
did not show a definite relationship
between wind direction and pollen
count. Possibly the plume of pol­
len from each tree fell between the
TREELESS AREA
FIG. 5.--Cumulative pollen densities are shown by direction from Douglas-fir trees in
locations 1 and 2, and in the treeless area. For each graph the pollen captured
from April 10-15, 16-19, and April 20-May 2 is shown at bottom, middle, and top
positions, respectively. Treeless area densities are on the same scale and show pollen
captured on slides facing the same directions as shown for locations 1 and 2.
rather short dispersal distance.
However, the same data also sup­
often
port studies that indicate large
amounts of pollen fall at consid­
erable distances from the nearest
source (1, 6), especially if this
Probably only a small fraction
source is a large continuous stand
(9).
four sampling
than on them.
lines
more
Conclusions
of the pollen dispersed by a single
Douglas-fir tree falls at a distance
further than 5-10 times tree height.
This supports the early indication
f .. '"
Before choosing seed orchard lo­
cations, pollen density should be
sampled on the sites being consid-
production is heavy. Comparative pollen counts would both influence the choice and evaluate the con­
tamination problem for a partic­
ular seed orchard site. The :findings indicate that con­
tamination of seed orchards from nearby pollen sources might be re.
duced to acceptable levels by rea­
sonably narrow isolation zones of
5-10 times average tree height when
NOVEMBER 1962
795
tured during the entire dispersal
ated a high background level of
period on vaseline-coated slides at pollen, even in one of the largest
various distances up to 2,000 f eet
radiating in four directions from
the trees. Measurements of· pollen density were also made at the cen­
ter of a a by 14.5-mi!e treeless area.
•
Total counts a v e r a g e d 5,116
Douglas-fir pollen grains per square inch for samples taken 50 feet from
. the trees and a,·eraged more than
·
treeless areas in the Pacific North­
west. On-site pollen counts in
heavy-flowerii:ig years are therefore
recommended ti> help decide be­
tween alternative locations of pros­
pective seed orchards. However, full protection by isolation does not
appear to be attainable west of the
Cascades, with the possible excep­
2,000- grains per square inch at a tion of the extensive spruce-hem­
distance of 2,000 feet from the lock stands along the coast.
trees. This comparatively high
amount of pollen far from th e
nearest source is believed to orig­
mostly from surrounding
stands, since an average of 769
grains per square inch was counted
in the treeless area. Further anal­
inate
ysis supported this conclusion and
indicated that Douglas-fir pollen
from sing-le trees is dispersed pri­
marily within a few hundred feet
of its source. Ho'''ever, the omn i­
presence of Douglas-fir in this year
of heavy flowering evid ently ere.
Literature Cited
1. ANDE&SSON, ENAJL 1955.. Pollensprid­
ning och avstandsieolering av skogs­
frOplantager.
(English
summary)
Norrlands SkogsvArds:tOrb. Tidskr.
l :35-100. Illus.
2. BUELL, M. F. 1947. Mass dissemina­
tion of pine pollen. J. Elisha Mitch­
ell Sci. Soc. 63:163-167. D!us.
3. DENGLER, A. 1955. Pollenflugbeo­
bachtungen in der Umgebung von
Waldbestiinden. Z. Forstgenet. 4
('/5) oll0-113.
4. LANGER, W. 1953. Eine Mendels­
paltnng bei Aurea·Formen von Picea
&bies (L.) Krast. als Mittel zur
Klirung der Befruehtungsverhiilt­
nisse im Waide. Z. Forstg'enet. 2:49. 51. Illus.
5. PERSSON, ARNE. 1955. Frequenzen
von Kiefernpollen in Siidscbweden
1953 and 1954. Z. Forstgenet. 4
C'/5) :129-137. Illus.
6. SA&VAB, RlsTo. 1955. EiD Beitrag
zur
Fernverbreitung
des Bliiten·
staubes einiger Waldbli.ume. z. For­
stgenet. '('/5) 0137-142 Illus.
7. ScA MONI, A. 1955. Ober den gegan­
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wirtigen
St.ard
i
Pollenflug der Waldbiiums. z. For·
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8. ScHllIT'l', B. 1955. Ober die Ver­
breitung des Pollena von Pinu11
1ilvestris L. Z. Forstgenet. 4(4/5),
H2·H5. Illus.
9, STRANJ>_, LA.as. 1957. Pollen disper­
sal Z. Porstgenet. 6(5) :129-136. IJ.
!us.
10. WA NO, Caz Wu, TnouAs 0. PERRY,
and ALBERT E. JOHNSON. 1960.
Pollen
dispersion of
slash pine (Pinus elliottii) with special refer· enee -to seed orchard management. Silvae Genet. 9 : 78-86. Illus.
11. WODEHOUSE, R. P. 1935. Pollen
grains; their structure, identification
and significance in science and
medicine. McGraw-Hill Book Co.,
New York. 574 pp. Illus.
12. WRIGHT, JONATHAN W. 1952. Pollen
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Forest Service, Northeastern Forest
Expt. Sta. Paper 46. 42 pp. Illus.
. 1953. Pollen-W.penion
13.
studies :some practical applications.
Jour. Forestry 51:114-118.