COLD-ACCLIMATION IN SEEDLING DOUGLAS-FIR RELATED
TO PHENOLOGY AND PROVENANCE'
ROBERT K. CAMPBELL
AND FRANK C. SORENSEN
Forestry Sciences Laboratory, Pacific Northiwsest Forest and Range Experiment Station
Forest Service, U.S. Department of Agriculture, Corvallis, Oregon 97331
Abstract. In October 1969, 1-year seedlings from 10 provenances growing at Corvallis were
severely damaged by frost. Provenances came from eastern and western slopes of the Coast
Ranges in western Washington and Oregon.
In the earliest provenance, bud set preceded frost by 5 weeks; in the latest, by 2.75 weeks.
For each additional week by which bud set preceded frost, the proportion of frost-damaged
seedlings decreased by 25 %. For provenances setting buds in identical weeks, southern sources
were more sensitive, the proportion damaged increasing by 4% per degree of decreasing
latitude.
Since southern sources generally set buds later and were at the same time more sensitive,
they were much more severely damaged. The southernmost coastal source (Coos Bay) suffered 78% damage, the northernmost coastal (Soleduck) 10%.
INTRODUCTION
Genetic differentiation in cold-acclimation among
infra-specific populations of forest trees has been
tested directly in terms of the effect of frost on plant
tissues (Schdnbach 1959, Ching and Bever 1960,
Schober 1963, Bialobok and Mejnartowicz 1970Pseudotsuga menziesii; Eiche 1966--Pin us silvestris)
and indirectly in terms of indices of supposed frost
hardiness, such as percentage dry matter (Langlet
1936-Pinus
silvestris), cambial growth cessation
abies), and cessation of
(Dietrichson 1964-Picea
height growth or bud set (Irgens-Moller 1958, Sweet
menziesii; Roche 1969-Picea
1965-Pseudotsuga
sp.).
Frost tests are subject to many limitations and pitfalls (Parker 1963, Alden and Hermann 1971). Some
tests include hundreds or thousands of plants, representing collections obtained with considerable difficulty. Furthermore, tests usually rely on destructive
sampling, giving rise to sacrifices that many investigators cannot accept. Therefore, in some types of ecological study, indices of hardiness are almost a necessity. An easily scored index, such as bud set, is of
particular utility. Unfortunately, the reliability of bud
set as a predictor of cold hardiness or winter rest is
not known (Perry 1971).
For Douglas-fir, quantitative evidence on this point
is given by Bialobok and Mejnartowicz (1970), who
found statistically significant relationships between a
measure of growth termination and fall-plus-winter
frost damage. Other evidence indicates that bud set
usually precedes development of hardiness (van den
Driessche 1969) in Douglas-fir, which follows the
pattern discovered in other woody plant species
(Weiser 1970).
1 Received September 8, 1972; accepted February 5,
1973.
In mid-October 1969 a severe frost damaged seedlings in our cold frames in Corvallis, Oregon. It provided an opportunity to examine seedling populations derived from 10 seed collections in respect to
the following questions, not treated in previous reports: (1) Is frost sensitivity related to time of bud
set among individual seedlings within seed sources
and among seed sources in western Oregon and Washington? (2) What proportion of differences in frost
sensitivity among seed sources can be attributed to
differences in bud-set date? (3) What proportion of
differences in frost damage can be attributed to differences in frost sensitivity at the stage of bud set?
(4) Can differences in frost sensitivity among seedlings be related to location or climate of their origin?
MATERIALS
Plants scored for frost damage came from collections made at five points along two latitudinal transects extending from northern Washington to southern Oregon. The West Coastal transect (WC) followed the Pacific Coast along the 1240 meridian; the
East Coastal transect (EC) was about 1 (approximately 80 km) further inland and generally skirted
the east side of the Pacific Coast mountain ranges.
Along the transects a collection (source) of openpollinated seed was made from 10 trees in each of
five stands. The stands were separated by approximately
1? increments
from
430
N to 48? N. From
north to south, sources in WC transect were Soleduck, Raymond, Hebo, Mapleton, Coos Bay; in EC
transect, Quilcene, PeEll, Salem, Eugene, Roseburg.
Elevation of collections averaged 240 m, ranging
from 120 to 366. Each source was tested by 90 seedlings, nine seedlings per row plot, plots being randomly assigned.
The damaging frost occurred during the night of
Late Summer 1973
COLD-ACCLIMATION
IN SEEDLING
October 13-14, 1969. A maximum-minimum thermometer located I m above the seedling and 6 m
away registered -3.3 ?C. Most surrounding weather
stations recorded -2.20C. Corvallis weather station
records for the preceding 22 years indicate that a
low temperature of -2.20C at this date has a probability of less than .04.
September and early October temperatures had
been normal. Rainfall had occurred in all but 3 of
the 30 days preceding the frost, accumulating more
than twice the normal precipitation for that period at
Corvallis. Maximum temperatures in the days following the frost reached 150 to 18'C. Frost did not occur again until late November, so the damage reported here is a response to a single event.
Plants were examined for newly formed buds every
7 days from approximately the first of August. Terminal bud set only was recorded. A seedling was considered to have set bud when scales could be first
seen in the terminal growing point. Under this criterion. the seedlings had finished their first growing
season at the time of the frost: 98% had set bud.
some as much as 8 weeks before the frost. Symptoms
of damage began to appear the day after the frost
and were fully developed within I to 2 weeks. Each
seedling was scored for damage a month after the
frost by this scale: (0) no damage, (1) yellowed
needles. (2) dead needles, (3) terminal bud and
stem damage. We settled on four damage classes
because finer discriminations became highly subjective.
METHODS AND RESULTS
Frost-damaged seedlings appeared to be differentially frost sensitive on a continuous scale. If so,
physiological damage sustained by a plant should be
related by regression to our arbitrary damage classes.
As a test, height and diameter of five randomly selected seedlings from each damage class-source combination were measured in November 1970, 1 year
after the frost. Highly significant linear regressions
of height and diameter on score indicated that injured
seedlings had grown less than uninjured seedlings.
The loss in growth was directly related to score. For
each increase in score by one class, 2-year heights
were shorter by 1.7 and 1.3 cm, respectively, in EC
and WC sources; for diameters, corresponding decreases were .15 and .16 mm. On the average, score
3 seedlings were about 11 % smaller in diameter and
12% shorter than score 0 seedlings. We concluded
that each seedling population included an array of
internal states of sensitivity to frost, and that the
array had been subjectively divided into four classes
by our scoring system.
If we assume that individual seedlings are differentially sensitive to a given degree of frost, a priori
DOUGLAS-FIR
1149
we might expect the frequency distribution of seedling sensitivities to follow the normal curve, as do
many other biological traits. The probit transformation is often applied to discontinuous data that by
deduction are thought to be underlain by continuous
variation (Bliss 1935). This variation is supposedly
measured on a hypothetical scale that makes its distribution normal. The unit of measurement is the
standard deviation of the distribution coded to probits
for convenience. Falconer (1960) describes general
properties of the transformation.
The probit analysis we used is analogous to regression methods used for dosage mortality curves
(Bliss 1935) as described by Fisher and Yates
(1949). The analysis required that the four damage
classes be reordered into two categories. To get sufficient numbers of plants in the two classes, seedlings
with score 0 and 1 were combined into one class.
scores 2 and 3 into a second class. The first analysis
was to determine if frost sensitivity was related to
time of bud set among individual seedlings within
seed sources. For each source population, probits
generated by the above two categories in each bud-set
class (i.e., seedlings setting bud within a 1-week interval) were regressed on average date of bud set, in
weeks before frost.
Among sources, the 10 regression coefficients of
bud-set date on probit ranged from -.37 to -.59 and
all differed significantly from zero. Coefficients of
determination ranged from .49 to .96, averaging .74.
This indicates a fairly strong association of frost
sensitivity and bud set. Regression coefficients did
not differ among sources and so were averaged to
-.48. The inference is that frost sensitivity distributions of all sources had retreated along the hypothetical sensitivity scale by about one-half probit per
week for each week that bud set preceded frost. The
relationship between weeks from bud set and probits
was linear. Therefore, the process by which seedlings
become frost resistant probably proceeded at nearly
the same rate for several weeks.
Our main objective was to evaluate the extent to
which frost sensitivity of a population is related to
its date of average bud set. To this end, the arithmetic mean bud-set week of sources was regressed
against probit transformations of percentage seedlings frosted per source. In Fig. 1. source averages
are plotted with the retransformed regression line.
Ninety percent of the variation in frost damage
among sources was related to week of mean bud set,
a very close relationship indeed. The four southernmost sources, Coos Bay and Mapleton in the WC
transect and Roseburg and Eugene in the EC transect. determine the position of the upper part of the
curve.
A second objective was to determine if seedlings
Ecology, Vol. 54, No. 5
ROBERT K. CAMPBELL AND FRANK C. SORENSEN
1150
* CoosBay
.7-
.8
.8 Mapeton
se0
z
3
*Roseburg
\
.5 -
\
* Eugene
.7
o .4
z
2
.3 _
\
u~.6
Ell
0
Quilcene
.2
Raymond
Salem
CD
Hebo
Soleduck
2.75
3.00
3.25
3.50
BUD SET--WEEKS
3.75
4.00
4.25
4.50
Coos Bay
LLB
4.75
u-.4
BEFORE FROST
1. Proportion of seedling damaged by frost as
related to mean date of bud set of source. Regression
line is a retransformation to proportions from probits.
Vertical line is 95 % confidence limits at overall mean
bud set.
FIG.
from the various sources suffered equivalent damage
if frosted at an equivalent stage of bud development.
Seedlings within source populations set buds over a
period of several weeks, so some seedlings of every
source set buds within the same week. If, in any
bud-set cohort, some sources sustained more damage
than others, we may infer that they were more frost
sensitive. A visual comparison related to this point
can be made in Fig. 2. For seedlings that set buds at
approximately 3.5 weeks before frost, the Coos Bay
source suffered damage to approximately 70%, Soleduck and Raymond sources to 11-30%. The EC
transect behaved similarly. Roseburg and Eugene in
particular were considerably more sensitive than
northern sources.
Thus, normal fall frost acclimation of the sources
appears to have two components. The first can be
closely tied to bud set. Although rate of development
toward frost resistance seems to be similar for
sources, frost sensitivity at points in time measured
from a phenological checkpoint need not be. The
second component accounts for these discrepancies
in sensitivity. This latter effect appears to be a function of latitude of the seed source. To examine its
influence on frost sensitivity, average bud-set dates
and latitudes of sources were regressed on proportions of seedlings damaged. The resulting equation is
Y=1.5604-.04105X1-.25473X2
where
Y
XI
X.,
z .5
proportion of seedlings in frost scores 2 and 3
north latitude of source origin, in degrees
mean bud-set date in weeks before frost.
That is, for each week by which sources differed in
average bud set, the proportion of frost damaged
z
~
H
0
0
~
~
~
Hb
\
3
3l
Mapleton
.2
Raymond
.1
H-ebo
Soleduck
0
3
1
4
2
FROST
BEFORE
BUDSET--WEEKS
5
FIG. 2 Curves were drawn from regression equations
relating profits to bud-set dates within sources in the
WC transect, after probits had been retransformed to
proportions. Vertical lines represent 95% confidence
limits at mean bud-set dates. Mean bud sets are weighted
according to requirements of the probit transformation
and differ somewhat from means in Fig. 1.
seedlings differed by .25. Also, for sources setting
buds in identical weeks, the proportion damaged
decreased by .04 per degree of increasing latitude.
The equation accurately predicted damage, 98% of
variation in proportions of seedlings frosted being
explained by bud set and latitude. Further, the contribution of each variable to the regression sums of
squares was statistically highly significant. Mean bud
set was about twice as effective as latitude in predicting frost damage; i.e., standard partial regression
coefficients were -.33 and -.75 for latitude and bud
set, respectively.
In spite of the close relationship between bud-set
date and frost hardiness, a significant part of the
sensitivity among sources was independent of bud-set
date. For example, (Fig. 2) about 5 weeks were required after first appearance of bud scales for seedlings from northern WC sources to become 90%
resistant, compared with 7-8 weeks for the Coos
Bay source. (The latter number was obtained by
projecting the curve for Coos Bay.) This differential
Late Summer 1973
COLD-ACCLIMATION
IN SEEDLING
sensitivity complicates any attempt to correlate bud
set with frost resistance.
When grown in a common environment, seedlings
of sources from the southern Willamette ValleyPuget Sound trough were more frost sensitive than
those from further north. Southern Oregon coastal
sources were most sensitive of any tested. These observations confirm data by Ching and Bever (1960)
and Bialobok and Mejnartowicz (1970). Our frost
damage was considerably more severe than reported
by either. From least to most damaged sources, percentage damage ranged from 4 to 36 (Ching and
Bever 1960), from 0 to 43 (Bialobok and Mejnartowicz 1970) and 9 to 77 (this paper). Considering
plants at the same stage of bud development, for each
degree of latitude separating sources, percentage of
plants frosted differed by 4% in our sample and by
2% in re-analyzed Ching and Bever data.
Our data also agreed with that of Ching and Bever
(1960) by showing frost damage to have been more
related to latitude than was bud set. Simple correlation coefficients between latitude and bud set, and
between latitude and frost damage percent, were .19,
.62. and -.66, -.80, for reworked Ching and Bever
data and ours, respectively. Hagner (1970) found
similar relationships in Pinus silvestris.
Effects related to latitude of sources are usually
ascribed to photoperiodic control which has developed as an adaptational mechanism to synchronize
developmental periodicity with annual climatic cycles (Heslop-Harrison 1964). In Douglas-fir, photoperiod is one of several factors that influence bud
set (Lavender et al. 1968) as well as development
towards cold hardiness (van den Driessche 1970).
The independent effect of latitude is therefore not
surprising. To advance two of many possibilities,
this effect could result from (1) a relatively tighter
photoperiodic control of hardiness in comparison to
its control of growth cessation, or (2) different adaptational strategies among sources for hardiness and
growth cessation, both based on a photoperiod timer
but using different critical daylengths.
LITERATURE
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1151
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