Tree Physiology 25, 101–108
© 2005 Heron Publishing—Victoria, Canada
Effects of seed origin and sowing time on timing of height growth
cessation of Betula pendula seedlings
ANNELI VIHERÄ-AARNIO,1,2 RISTO HÄKKINEN,3 JOUNI PARTANEN,4 ALPO
LUOMAJOKI1 and VEIKKO KOSKI1
1
Finnish Forest Research Institute, Vantaa Research Centre, P.O. Box 18, FIN-01301 Vantaa, Finland
2
Corresponding author ([email protected])
3
Finnish Forest Research Institute, Unioninkatu 40 A, FIN-00170 Helsinki, Finland
4
Finnish Forest Research Institute, Punkaharju Research Station, Finlandiantie 18, FIN-58450 Punkaharju, Finland
Received March 12, 2004; accepted June 4, 2004; published online November 1, 2004
Summary We studied the effects of seed origin and sowing
time on height development and timing of height growth cessation of first-year silver birch (Betula pendula Roth) seedlings
in a greenhouse experiment. Seeds of seven origins ranging in
latitudes from 58° to 67° N were sown at 1–2-week intervals
eight times from May 21 to July 30, 2001. The day/night temperature in the greenhouse was set at 20/10 °C, but lighting was
natural and day length varied accordingly. Seedling height was
measured twice a week. The interaction term between seed origin and sowing date was significant, but the pattern of height
development and timing of growth cessation depended systematically on latitude of seed origin and sowing date. As seed origin became increasingly northern, growth cessation began
earlier and resulted in shorter growth periods. Later sowing
dates delayed growth cessation but also shortened the growth
period. Final seedling height systematically decreased with
increasingly northern origins and with later sowings. Linear regression analysis predicted timing of growth cessation, night
length at growth cessation, length of growth period and final
seedling height with high precision when the latitude of seed
origin and sowing time were predictor variables. The timing of
height growth cessation was determined by the seed origin,
night length and developmental stage of the seedlings.
Keywords: annual rhythm, climatic adaptation, critical night
length, growth period, photoperiod, silver birch, stage of development.
Introduction
Trees native to northern latitudes survive seasonal changes and
unfavorable wintertime conditions through well-timed acquisition of dormancy and frost hardiness. Cessation of height
growth is the first visible component of the frost hardening
process and is necessary for its further development (Weiser
1970). Changes in the photoperiod (i.e., an increase in night
length) act as the predominant trigger for cessation of height
growth (Fuchigami et al. 1982). The influence of the photo-
period on growth and dormancy of woody plants has been
shown in a number of studies (Garner and Allard 1923,
Wareing 1956, Nitsch 1957). The photoperiodic control of
vegetative growth is particularly important in species with a
free growth pattern, such as juvenile birches, which grow indefinitely under long days but stop growing and set terminal
buds under short days (Nitsch 1957, Howe et al. 1996). The effect of photoperiod is determined by the length of the dark period and not by the length of the day (Nitsch 1957, Howe et al.
1996, Thomas and Vince-Prue 1997).
Cessation of vegetative growth in response to a decreasing
photoperiod at the end of the growing season is an indirect adaptation to the seasonal change in temperature (Vaartaja 1959,
Ekberg et al. 1979). Tree species having a wide geographic
distribution have photoperiodic ecotypes that respond to different critical night lengths; northern ecotypes react to shorter
critical night lengths compared with southern ecotypes
(Sylvén 1940, Pauley and Perry 1954, Vaartaja 1954, 1959,
Heide 1974, Ekberg et al. 1979). Håbjørg (1972a) demonstrated the existence of photoperiodic ecotypes in Betula
pubescens Ehrh. and later in Betula pendula Roth among other
Scandinavian tree and shrub species (Håbjørg 1978).
Photoperiodic regulation of growth cessation is modified by
several external and internal factors. Temperature is the most
important external factor (Dormling et al. 1968, Håbjørg
1972a, 1972b, Heide 1974, Koski and Selkäinaho 1982, Koski
and Sievänen 1985, Li et al. 2002), but soil water and water
stress (Li et al. 2002), air humidity (Håbjørg 1972a) and nutrient availability (Landis et al. 1999) also affect the growth cessation of tree seedlings. Internal factors such as seedling size
(Junttila 1976) and physiological stage of development may
also affect the photoperiodic response of seedlings (Hari et
al.1970, Hari 1972, Luoranen and Rikala 1997, Landis et al.
1999, Luoranen 2000). A different selection of plants in
growth chamber experiments may result in various photoperiodic responses and critical night lengths within a certain
population (Junttila 1976, Håbjørg 1978, Ekberg et al. 1979).
Precise characterization of birch seedling materials related to
102
VIHERÄ-AARNIO, HÄKKINEN, PARTANEN, LUOMAJOKI AND KOSKI
growth cessation timing and critical night length is important
to a variety of studies, such as gene mapping of photoperiodism and frost hardening, and nursery production of seedlings
for forest cultivation (Luoranen 2000, Luoranen and Rikala
2001).
In this study, we examined how seed origin, sowing time and
a naturally changing photoperiod affect the timing of growth
cessation, length of growth period and final height of first-year
silver birch (Betula pendula Roth) seedlings. An experimental
design in which seeds of each origin were sown at different
times during the summer yielded seedlings at different stages
of development that were subjected to lengthening night conditions at the end of summer.
Materials and methods
Seed origins
Silver birch seeds were obtained from seven different latitudes
(i.e., from locations differing in photoperiodic regime) (Table 1). The southernmost origin was from Viljandi in southern
Estonia (58°10′ N) and the northernmost from Kittilä in northern Finland (67°44′ N). Each origin consisted of one selected
stand. All stands originated from natural regeneration, except
for Pudasjärvi, which was cultivated with seeds of local origin.
At each stand, a mixture of seeds from open pollination of several mother trees was taken.
Experiment
A completely randomized factorial experiment with two fixed
factors (eight sowing times and seven origins) was carried out
in a greenhouse at Haapastensyrjä Tree Breeding Station,
Loppi, southern Finland (60°37′ N, 24°26′ E) in 2001. The experiment was conducted under natural light and photoperiod
during the growing season (Figure 1). All origins were sown
eight times at 1–2-week intervals during the summer. The sowing dates were: May 21, June 4, 18, 25, July 2, 9, 16 and July
30. Each treatment (origin and sowing time combination) was
replicated twice with 12 seedlings in each plot.
For the duration of the experiment, the greenhouse temperature was kept at 20 °C during the day (0800–1800 h) and 10 °C
at night (2000–0600 h). The temperature changed during the
intervening times at a steady rate of 5 °C h –1. The temperature
was recorded once a minute.
Seeds were sown in Kekkilä M6NCW40 peat in TAKO
1210 styrox trays (12 cavities tray –1, 880 cm3 cavity –1, 50 cavities m –2, Metsä-Serla, Tampere, Finland). The peat had been
treated with long-acting (40 days) Nutricote fertilizer and
moisture retainer. After germination, seedlings were thinned
to one per cavity. The seedlings were irrigated during the summer according to normal nursery practice.
Measurements and statistical methods
Seedling height was measured twice a week (Monday and
Thursday). Measurements of the seedlings from the first sowing (May 21) began in the first week of July, and in the later
sowings, soon after the seedlings had reached the upper surface of the seedling tray. Seedlings were measured until the
height remained constant over three successive measurements
(growth cessation). Height was measured from the upper surface of the seedling tray to the estimated growing point of the
seedling. For statistical analysis, the mean value of 12 seedlings in the plot was the observation.
The response variables analyzed were the time of growth
cessation, night length, heights of seedlings at growth cessation and length of growth period. Results were evaluated with
a two-way analysis of variance. The equality of the effects of
different sowing dates within origins and the equality of the
effects of different origins within sowing dates were assessed
with simple contrast F-tests. Regression models of response
variables on seed origin and sowing date were determined by
replacing the seed origin factor with the interval scale variable
of latitude, and by replacing the sowing time factor with the
interval scale time variable.
Results
Seedling height development
The general pattern of height development of the seed origins
at different sowing times is shown in Figure 2, where seedling
growth curves from the first (May 21), the fourth (June 25)
and the last (July 30) sowing times are illustrated. The pattern
depended clearly and systematically on the latitude of the seed
origin and the sowing time.
Table 1. Silver birch seed origins and mean annual temperature sums of the original growing sites from 1965–1995.
Origin
Latitude
Longitude
Altitude
(m)
Temperature sum
( > 5 °C, degree days)
Estonia, Viljandi
Finland, Tuusula
Finland, Ruovesi
Finland, Viitasaari
Finland, Pudasjärvi
Finland, Rovaniemi municipality
Finland, Kittilä
58°10′ N
60°27′ N
62°03′ N
63°14′ N
65°14′ N
66°52′ N
67°44′ N
25°32′ E
24°58′ E
24°15′ E
26°07′ E
27°36′ E
24°55′ E
24°51′ E
70
50
100
140
130
140
200
1430
1335
1209
1116
957
872
758
TREE PHYSIOLOGY VOLUME 25, 2005
SEED ORIGIN AND SOWING TIME EFFECTS ON GROWTH CESSATION
Figure 1. Seasonal change in night length at the source location of
each silver birch seed origin and at the experiment site, Loppi (thick
line, overlaps the line of Tuusula). Sowing times are marked with arrows.
Timing of height growth cessation and night length at growth
cessation
Timing of height growth cessation varied according to seed origin and sowing time (both P < 0.001). Despite the interaction
between seed origin and sowing time (P < 0.001), there was a
systematic linear clinal pattern in the timing of growth cessation from southern to northern origins, as well as a linear trend
from early to late sowing times (Figure 3). Generally, the more
northerly the origin, the earlier the cessation of growth occurred, and therefore, the shorter the night length at growth
cessation. However, a delay in sowing time delayed growth
cessation into autumn, and thus increased the night length at
the time of growth cessation within each origin. Growth cessation of seedlings from the first sowing (May 21) varied from
August 20 (Kittilä) to September 19 (Viljandi), compared with
September 11 (Kittilä) to September 25 (Viljandi) for seedlings from the last sowing (July 30).
Because of the interaction, differences in date of growth cessation among seed origins were greater for seedlings sown earlier than for seedlings sown later and decreased with sowing
time (P < 0.001 in all sowings). The difference in date of
growth cessation between the northernmost (Kittilä) and the
southernmost (Viljandi) origins was 30 days for the first sowing and 14 days for the last sowing. Conversely, differences in
date of growth cessation among sowing times were greatest for
seedlings of northern origin and lowest for seedlings of southern origin (P < 0.017 within Viljandi, P < 0.001 within all
other origins).
103
Figure 2. Mean height development and timing of growth cessation by
seed origin in first-year silver birch seedlings sown on May 21 (Sowing 1), June 25 (Sowing 4) and July 30 (Sowing 8). Sowing times are
marked with arrows. Each value represents the mean height of two
replicates.
period length was systematic: increasingly northerly origins
and later sowing times shortened the growth period (Figure 4).
For the seedlings sown first (May 21), the growth period of
Kittilä origin was 90 days and that of Viljandi origin was
121 days. For the seedlings sown last (July 30), the growth period of Kittilä origin was 43 days and that of Viljandi origin
was 58 days.
Because of the interaction between origin and sowing date,
differences among seed origins in length of growth period
were largest within the earliest sowings and decreased as sowing date advanced (P < 0.001 within all sowings) (Figure 4).
The difference in the length of growth period between the
northernmost (Kittilä) and southernmost (Viljandi) origin was
Length of growth period
The total length of the growth period from sowing to growth
cessation varied according to the seed origin and sowing time,
and there was a statistically significant interaction between
these variables (all P < 0.001). The observed change in growth
Figure 3. Mean date of growth cessation and associated night length in
silver birch seed origins sown on different dates, as a function of the
latitude of seed origin.
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104
VIHERÄ-AARNIO, HÄKKINEN, PARTANEN, LUOMAJOKI AND KOSKI
31 days for the first sown seedlings and 15 days for the last
sown seedlings. Conversely, differences between sowing
times in the length of growth period were the largest for seedlings of southern origin and decreased with increasing latitude
(P < 0.001 within all origins) (Figure 4).
The temperature sum in the greenhouse accumulated linearly as a function of time. Thus, the temperature sums of the
growth period (Table 2) produced the same pattern of variation
as the length of the growth period (Figure 4). Increasingly
northerly origins and later sowing times resulted in lower temperature sums accumulated during the growth period.
Final height of the seedlings
Heights of seedlings at the end of the experiment varied according to the seed origin and sowing time, and there was an
interaction between seed origin and sowing time (all P <
0.001). Final heights of the seedlings decreased in a curvilinear pattern as a function of the sowing time, and southern
origin seedlings were always taller than northern origin seedlings (Figure 5). The seed origin differences in final height decreased with sowing time: the differences were significant (P
< 0.001) for the first five sowing dates, but not for the last two
(P = 0.334 and P = 0.989, respectively). Sowing time had a significant (P < 0.001 within all origins) effect on the final seedling height of each origin.
Regression models of timing of growth cessation, night
length at growth cessation, length of growth period and final
height of seedlings
The dependence of timing of growth cessation (TGC; number
of days from the beginning of the year) on the sowing time
(ST; number of days from the beginning of the year) and the
latitude of seed origin (L; °) was evaluated by regression analysis. The second-order regression model:
TGC = 418.04 – 1.073ST – 0.05088 L2 +
(0.02066ST)L
(1)
explained the variation in timing of growth cessation with high
precision (s (residual standard deviation) = 3.11, R 2 = 0.92);
and all coefficients were highly significant (P < 0.001). According to the model (Equation 1), the timing of growth cessation increased by 0.24 days per 1-day delay in sowing date at
the mean latitude (63.39°) and decreased by 2.78 day per 1° increase (≅ 110 km) in latitude of seed origin at the mean sowing
date (177.75 days). The pattern of estimated regression surface
was slightly curved because of the interaction (Figure 6).
The relationship between night length (NL; h) and TGC was
nearly linear (r > 0.99), and thus NL and TGC contained practically the same information, but on different scales. The equation for night length at growth cessation was:
NL = 25.78 – 0.0988ST – 0.004686 L2 +
(0.00190ST)L
(2)
in which s = 0.29 and R 2 = 0.92 and all coefficients were significant (P < 0.001). Night length increased by 1.3 min for every 1-day delay in sowing date at the mean latitude and decreased by 15.4 min per every 1° increase in latitude of seed
origin at mean sowing date (Equation 2).
The length of growth period (LGP), which is related to timing of growth cessation as LGP = TGC – ST, is given by:
LGP = 418.04 – 2.073ST – 0.05088 L2 +
(0.02066ST)L
(3)
in which s = 3.11 and R 2 = 0.97 and all coefficients were significant (P < 0.001). The growth period shortened by 0.76 days
for every 1-day delay in sowing date at the mean latitude and
shortened by 2.78 days for every 1° increase in latitude of seed
origin at the mean sowing date (Equation 3).
Comparing Equations 1 and 3 shows that later sowing dates
delayed growth cessation but shortened the growth period. The
length of growth period describes the age of the seedlings at
growth cessation. Thus, seedlings were younger at growth cessation when sown later. To summarize, the growth cessation
process of first-year birch seedlings depended on the interaction between seed origin, night length and the developmental
stage of the seedlings created by different sowing times.
The regression model for seedling final height (H) at growth
cessation was:
H = 1787.40 – 10.678ST – 20.499 L +
(4)
0.0098ST 2 + (0.0990ST) L
Figure 4. Mean length of growth period from sowing to growth cessation for each silver birch seed origin and sowing date as a function of
latitude of seed origin.
in which s = 4.64 and R 2 = 0.96 and all coefficients were significant (P < 0.001). Unlike the other response variables, the
dependence of seedling final height on sowing time was
strongly curvilinear (Figure 5).
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SEED ORIGIN AND SOWING TIME EFFECTS ON GROWTH CESSATION
105
Table 2. Mean temperature sums (> 5 °C, degree days) over the growth period for silver birch for each seed origin and sowing date in the experiment and the mean (1965–1995) annual temperature sum of their original growing sites.
Origin
Viljandi
Tuusula
Ruovesi
Viitasaari
Pudasjärvi
Rovaniemi
Kittilä
Sowing date
May 21
June 4
June 18
June 25
July 2
July 9
July 16
July 30
1386
1248
1312
1221
1140
1075
1042
1292
1202
1155
1102
1043
978
967
1156
1036
1036
983
919
897
865
1092
1015
978
914
828
845
786
974
911
938
900
762
741
676
902
867
851
798
719
676
579
830
764
774
743
652
637
589
632
632
559
606
532
511
485
Mean
1965–1995
1430
1335
1209
1116
957
872
758
The pattern of height development and timing of growth cessation depended on the latitude of seed origin and sowing time.
Among origins, there was a clinal trend from south to north.
Our results are in accordance with growth chamber studies
showing differentiation between tree populations in terms of
the response to photoperiod and critical night length; northern
populations have shorter critical night lengths compared with
southern populations (Håbjørg 1972a, 1978, Heide 1974,
Ekberg et al. 1979). Because photoperiodic conditions change
gradually with latitude, the critical night length and photoperiodic response of the tree populations also follows a gradual clinal pattern rather than being sharply ecotypic (Langlet
1959, Eriksson and Ekberg 2001). Sharp differentiation between birch populations is unlikely because of long- distance
dispersion of pollen, wide overlapping flowering times of distant birch stands, wide dispersion of seeds and thus effective
gene flow (Hjelmroos 1991, Luomajoki 1999, Eriksson et al.
2003). The clinal variation of growth cessation, as well as the
negative correlation between timing of growth cessation and
latitude, has also been reported in nursery and common garden
experiments (Clausen 1968, Sharik and Barnes 1976, Velling
1979).
Critical night length for growth cessation is the minimum
night length causing cessation of extension growth (Thomas
and Vince-Prue 1997). We examined the night length in a naturally changing photoperiod at growth cessation, rather than the
critical night length inducing growth cessation, making it difficult to compare our results with earlier growth chamber studies.
Håbjørg (1978) reported a mean profile of critical day
length in relation to latitude in different tree species. From his
results and Figure 1, we estimated the critical night length and
its timing at Loppi (Figure 1) for the origins used in our study
as: Viljandi 8.5 h (August 17), Tuusula 8 h (August 12),
Ruovesi 7.5 h (August 6), Viitasaari 7 h (July 31), Pudasjärvi
6 h (July 18), Rovaniemi 5 h (June 29) and Kittilä 4 h (not
reached at Loppi). In growth chamber studies, the length of the
induction period from the beginning of short-day treatment to
the cessation of height growth of birch seedlings varies from
about 1 week to several weeks (Håbjørg 1978, Rinne et al.
1997). Faster responses to short-day treatments occur with
Figure 5. Mean final height of silver birch seedlings by seed origin as
a function of sowing date.
Figure 6. Time of growth cessation of silver birch seedlings as a function of latitude of seed origin and sowing time.
Discussion
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VIHERÄ-AARNIO, HÄKKINEN, PARTANEN, LUOMAJOKI AND KOSKI
more northerly origins and shorter day lengths (Håbjørg
1978). Differences between the date of critical night length according to Håbjørg (1978) and our observed date of growth
cessation for the first sowing (May 21) of the various origins
were: Viljandi 33 days, Tuusula 26 days, Ruovesi 38 days,
Viitasaari 35 days, Pudasjärvi 40 days and Rovaniemi 53 days.
The longer induction times of the northern origins in our study
seem to contradict the results of Håbjørg (1978); however,
they may be explained by differences in the stage of development between the origins, because northern origins reach their
critical night length earlier than southern origins.
Timing of seedling growth cessation varied within an origin
according to sowing date and no unambiguous night length at
growth cessation could be shown for any origin. This is in accordance with the findings of Luoranen and Rikala (2001),
who reported that, in a nursery experiment with one seed orchard seed origin of B. pendula, later sowing times caused
poorer height growth and later growth cessation.
We used an experimental design in which seeds were sown
at different times during the summer to obtain seedlings at
different developmental stages that were then subjected to
lengthening nights at the end of summer. The number of days
between the attainment of the critical night length according to
Håbjørg (1978) and the cessation of height growth in our study
was, for Sowings 1–8, 26, 35, 35, 40, 39, 43, 41 and 44 days for
the Tuusula origin and 53, 58, 65, 68, 67, 69, 73 and 77 days
for the Rovaniemi origin. Thus, delay in sowing delayed
growth cessation following attainment of the critical night
length. Growth and development of first-year nursery seedlings can be divided into different phases according to Landis
et al. (1999), the main three being the establishment, rapid
growth and hardening phases. The effects of environmental
factors on a seedling vary in each phase (Landis et al. 1999),
and therefore the response to the long night signal can differ
(Luoranen 2000), which is in accordance with our results.
Early sown seedlings were in the rapid growth phase and
rather tall when they surpassed the critical night length,
whereas seedlings from late sowings were small, having hardly reached the establishment phase and therefore were slower
to respond to the critical night length.
In agreement with our observations, delay in sowing time
delayed growth cessation of first-year silver birch seedlings in
the studies of Koski and Selkäinaho (1982), Koski and Sievänen (1985) and Partanen (2004). These authors concluded that
the timing of growth cessation was determined by a joint effect
of night length and accumulated temperature sum, the higher
the temperature sum, the shorter the night length required to
induce growth cessation. We observed a similar interaction in
our study (Table 2, Figure 3). However, in the simulation study
by Hänninen et al. (1990) based on long-term meteorological
data, a model based on night length predicted growth cessation
more efficiently than a model based on a factor combining
night length and temperature sum or temperature sum alone
because of the year-to-year variation in temperature sum.
Sensitivity to night length increases with seedling development (Koski and Selkäinaho 1982, Koski and Sievänen 1985
and Partanen 2004). This relationship may be interpreted in
two ways. First, the critical night length varies in relation to the
stage of seedling development, as stated by Koski and Selkäinaho (1982) and supported by the results of Ekberg et al.
(1979). Alternatively, the critical night length is unaffected by
sowing date, but the speed of the response depends on the stage
of seedling development. Junttila (1976) reported that in Salix
caprea L. and Salix pentandra L., both species with a free
growth pattern like birch, the rate of response to short day
treatment in a phytotron depended on seedling size; small
seedlings were slow to respond to short days, and the time
from the start of short-day treatment to apical growth cessation
decreased with increasing initial seedling size (Junttila 1976).
This physiological difference between small and large seedlings may be related to leaf number and the production of inhibitors under short-day regimes (Wareing 1954, Eagles and
Wareing 1963, 1964).
The final height of seedlings decreased curvilinearly in relation to sowing time, although the length of the growth period
decreased linearly (Figures 4 and 5). Thus, differences in
growth of seedlings from the different sowings are a result of
differences in light conditions and in length of the growth period. Seedlings sown on the first three dates began their development during lengthening days, whereas the later sown
seedlings faced shortening days from the beginning (Figure 1).
The rate of shoot elongation increases rapidly with increasing
photoperiod above the critical day length for shoot elongation
(Junttila and Nilsen 1993). Thus, the curvilinearity in the relationship between height and sowing date was most pronounced in the seedlings of southern origins, which were
grown during long days (Junttila and Nilsen 1993).
Our experiment was carried out in controlled conditions in a
greenhouse, but the timing of growth cessation would presumably vary according to growth conditions. Temperature, in particular, has a strong modifying effect on the photoperiodic
response. Different responses to photoperiod have been shown
between low and high temperatures (Dormling et al. 1968,
Håbjørg 1972a, Heide 1974, Li et al. 2002), between fluctuating and constant temperatures (Heide 1974) and between high
and low night temperatures (Håbjørg 1972a, Heide 1974).
Other environmental factors, like soil water and water stress
(Li et al. 2002), air humidity (Håbjørg 1972a) and nutrients
(Landis et al. 1999) may also affect growth cessation of seedlings. However, the role of seedling age in the regulation of
growth cessation is poorly understood. This study was carried
out with first-year birch seedlings that manifest free growth
and are entirely controlled by environmental conditions of the
given year (Landis et al. 1999). The growth of mature birches
is partly predetermined (Kennedy and Brown 1984), and as
trees mature, endogenous conditions are presumably increasingly important in regulating growth cessation (Junttila and
Nilsen 1993, Thomas and Vince-Prue 1997).
In conclusion, height growth cessation of silver birch seedlings in the year of sowing is controlled by the interaction between night length and stage of seedling development and
varies with seed origin. Thus, the use of a single value of a
night length parameter is insufficient to characterize the
TREE PHYSIOLOGY VOLUME 25, 2005
SEED ORIGIN AND SOWING TIME EFFECTS ON GROWTH CESSATION
growth cessation of birch seedlings of differing seed origins,
and the stage of seedling development should be integrated.
Acknowledgments
The authors thank the personnel at Haapastensyrjä Tree Breeding Station for support in carrying out the experiment. Miss Marja-Leena
Annala, Mrs. Merja Lahdenpää, Mrs. Ritva Linnamäki and Mrs. Eija
Maunula are warmly acknowledged for their efforts in raising and
measuring the experimental plants. Mr. Esko Oksa at Punkaharju Research Station is thanked for his aid in data processing. Mr. Michael
Starr is thanked for his revision of the language. Prof. Katri
Kärkkäinen is acknowledged for her valuable comments on the manuscript. This study was financed by the Finnish Academy as a part of
the Life2000 – Research programme of biological functions.
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