Timing of shedding seeds and
cones, and production in different
stands of Scots pines at Abernethy
Forest, Scotland
R.W. SUMMERS* and R. PROCTOR
Royal Society for the Protection of Birds, North Scotland Office, Etive House, Beechwood Park,
Inverness IV2 3BW, Scotland
* Corresponding author. E-mail: [email protected]
Summary
A study was carried out over 11 seed years on the timing of shedding of seeds and cones, and annual
seed fall and cone production in three stands of native Scots pinewood and a Scots pine plantation
in Abernethy Forest, Scotland. Peaks in seed fall took place mainly in May, and cones were shed
mainly between June and August. There were few residual seeds remaining in shed cones.
Synchronized peaks in seed fall and cone production (mast years) took place at 3-year intervals
across the different stands. The difference between cohorts of high and low cone production ranged
from factors of 5 to 20 among sites. Coefficients of variation for cone production ranged from 62 to
84 per cent among sites. There were no significant differences in cone production among sites, but
there were site-related differences in seed fall. The larger canopy cover in the plantation probably
accounted for the higher seed fall per square metre there, though variations in the amount of seed
eaten by birds and mammals may also have been important. Canopy cover needs to be considered
when converting cone densities under crowns to cone density per unit of woodland area. A similar
calculation is difficult for seeds because they are lighter than cones and many fall outside the area
under the crowns. The results are discussed in relation to the potential for tree regeneration and the
availability of food for birds and mammals prior to seed dispersal.
Introduction
A forest of Scots pine (Pinus sylvestris L.) and
birches (Betula spp.) once spread across Highland
Scotland, but a recent survey showed that this
woodland had been reduced in size to 84 scattered
woods totalling 17 882 ha (Bennett, 1984; Jones,
© Institute of Chartered Foresters, 2005. All rights reserved.
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1999). Despite the loss of so much pinewood, the
remaining fragments are still of high conservation
interest because of their high biodiversity
(Anon. 1995). Many now have legal protection
through designations such as Sites of Special
Scientific Interest and National Nature Reserves
(Warren, 2002), or have become Caledonian
Forestry, Vol. 78, No. 5, 2005. doi:10.1093/forestry/cpi047
Advance Access publication date 8 August 2005
542
FORESTRY
Forest Reserves under the management of Forestry
Commission Scotland (Hamilton, 1995).
There was concern that poor tree regeneration
in many of the ancient native pinewoods was due
to poor production of viable seeds from old trees
(McIntosh and Henman, 1981). However, this
fear was unfounded (Nixon and Cameron, 1994)
and browsing by large numbers of deer and sheep
appeared to be more important in limiting tree
regeneration (Steven and Carlisle, 1959, p. 83;
Scott et al., 2000). Nevertheless, good seed production is required to ensure the continuity of
the native pinewoods.
Conifer seeds are produced within protective
woody cones, which shed the mature seeds when
the scales of the dehiscent cones open out. The
largely empty cones are then shed later. Seed production from conifers varies seasonally and
annually (Gordon and Faulkner, 1992). Years of
high seed production (‘mast’ years) are likely to
be the ones that lead to seedling establishment
because seed-eaters are satiated by the abundance
of food and are unable to consume much of the
production (Kelly and Sork, 2002). In Britain,
seed eating takes place while the seeds are still
within the cones (pre-seed dispersal) by crossbills
(Loxia spp.), other finches (Fringillidae), tits (Parus
spp.), great spotted woodpeckers (Dendrocopos
major (L.)) and red squirrels (Sciurus vulgaris L.),
whilst seed eating after seed dispersal is carried out
by finches, microtine rodents, ants (Formicidae)
and beetles (Coleoptera) (Steven and Carlisle,
1959; Smith and Balda, 1979). The seasonality
and size of the annual fluctuations in production
will have a bearing on the abundance, time of
breeding, movements and survival of many animals, particularly seed specialists such as crossbills (Reinikainen, 1937; Summers, 1999). If we
are to understand the population dynamics of
pinewood birds and mammals, and the Scots
pine itself, it is necessary to assess cone and seed
production.
In this paper, we made assessments of seed fall
and cone production over 11 years at three sites
within an ancient native pinewood in Scotland,
and compared these with an area of Scots pine
plantation. This provided data on the frequency
of years for potential tree regeneration and estimates of the temporal (seasonal and annual) and
spatial variation in the food availability for seedeating birds and mammals.
Methods
The study was carried out over 11 seed years
(1992–2002) at Abernethy Forest (57° 15′ N,
3° 40′ W), the largest remaining fragment of
ancient native pinewood in Scotland. Four sites
were selected, three in stands of ancient native
pinewood (Ice Wood, grid reference NJ0213;
Bognacruie, NJ0414; and Memorial Wood,
NJ0113) and one area of plantation woodland
(Tore Hill, NH9917). The tree species in these
areas were almost entirely Scots pine; the native
stands were of local origin and the plantation
grown from seeds of local provenance. The stands
in Ice Wood and Bognacruie were old, open forest, characterized by trees with a median height of
13 m, diameter at breast height (d.b.h.) of 57 cm
and density of 160 trees ha- 1 . The stands in
Memorial Wood were high crown trees (median
height of 19 m, d.b.h. of 44 cm and tree density
of 320 ha- 1 ). The trees on Tore Hill were at the
pole-stage of growth (median height of 15 m,
d.b.h. of 28 cm and tree density of 570 ha- 1 ) having been planted in 1932 (Summers et al., 1997).
Fifteen trees were systematically selected at
50–100 m intervals along 1–2 km transects set
through each of the four sites. All or a segment of
the area under the crown and out to the edge of
the crown was demarcated on the ground (defined
as a plot), and cleared of shrubs, twigs, branches
and old cones. The first collection was in February 1992, so that the few cones of the 1992 cohort
that may have fallen prior to this date may have
been discarded, though an attempt was made to
identify these. The average plot size was 8.2 m2
(range 1.9–33.6 m2). At the end of each month,
the plots were visited and the cones that had
accumulated within the demarcated areas were
removed, allotted to a given cohort (defined as
the year of shedding seed) and counted. The allocation of cones to cohorts was based on comparisons with cones still on the trees, where their
positions relative to the whorls of branches and
hence years could be assessed. The fallen cones
included those that had been removed by crossbills and red squirrels to extract seeds and then
dropped from the crown, as well as those shed by
the trees. The collection of cones from a given
cohort started in July of the year prior to shedding
seed, to June of the year after shedding seed (i.e.
24 months over three calendar years) (Table 1).
TIMING OF SCOTS PINE SEED AND CONE PRODUCTION
543
Table 1: The timing of collections of cones for the 1999 cohort
Calendar year and month
1998
J A S O N D J F M A
1999
M
J
J
2000
A
S
Cone month 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Cones older than this date were ignored. Old
cones could be identified if they were grey and
had lichens growing on them.
Although cone and seed production will be
highly correlated, cone production and seed fall
need not be, because seed fall represents that part
of the seed production that is not eaten prior
to seed dispersal. Falling seeds were collected by
setting seed traps (plastic pots 35 cm high and
40.5 cm in diameter) under or by five equidistantly spaced trees of the 15 trees in each site,
from February to September each year, though
some months were missed in the early years of the
study. However, additional trapping was carried
out throughout the winters of 1996/7 and 1997/8
to determine if there was any winter seed fall. The
mouths of pots had an area of 0.13 m2 and a
muslin cone was tied inside to catch falling seeds
and other debris. The seed fall referred only to
those seeds shed from cones, so did not include
those taken by seed-eaters prior to seed dispersal.
The seeds lost to birds and mammals included
those in cones that were removed and dropped
plus those taken from between scales when the
attached cones were partially dehiscent.
In order to count the number of seeds that were
not shed from cones or not eaten, four fallen cones
from the 2003 cohort were collected in July 2003
under the crowns of 10 of the study trees in the
four sites. Cones were heated in an oven until the
scales opened and the gaps between scales were
searched for residual seeds. The small proximal
scales do not part, so some seeds may have remained
trapped between scales and not counted.
Canopy cover was assessed at the 15 study
trees in the four sites. A grid of 25 points was set
out over an area of 20 × 20 m (5 m between
points) centred on the study tree. At each point,
the presence or absence of canopy directly above
the observer (checked with a clinometer) was
noted and the percentage cover calculated.
O
N
D
J
F
M
A
M
J
16 17 18 19 20 21 22 23 24
Analysis
Means of the counts of seeds and cones were used
to describe the patterns for each cohort (by month
and the total). However, for testing the effects of
site and cohort, Poisson regression models (log
link function) were fitted to the data. In each
model, the dependent variable was the total
number of seeds or cones produced by each tree
and for each cohort, and the independent factors
were site and cohort. Because the collecting areas
under the trees varied, log area was used as an
offset variable when modelling cone numbers.
A scale parameter using Pearson’s χ2 divided by
the residual degrees of freedom accounted for
over-dispersion (Crawley, 1993). The coefficient
of variation (CV) in seed fall and cone production
was used to express variability (Kelly, 1994).
Counts of residual seeds from four fallen
cones were summed for each tree, and a Poisson
regression model fitted to the total number of
seeds collected from each tree. The site effect was
then examined. Over-dispersion was accounted
for, as above.
Results
Seed fall
The average density of seeds intercepted by the
seed traps each month between February and
September is shown in Figure 1, excluding years
with low seed fall. During the early years (1993–
1995), seed trapping was not always carried
out during February, March and September,
but it was clear from the later years that few
seeds fell then. Likewise, when trapping was continued through winters 1996/7 and 1997/8, very
few seeds shed then, confirming that spring was
the time when most seeds were shed.
544
FORESTRY
Figure 1. Variations in the density of seeds (number for a given cohort per m2 of forest floor) falling in seed
traps each month at four sites in Abernethy Forest: Ice Wood = filled diamonds; Bognacruie = open circles;
Memorial Wood = filled triangles; Tore Hill = open squares. The year of seeding (cohort) is shown for each
figure. Cohorts with poor seed fall are not shown. Note the different scales on the y-axes.
In most years, the peak in seed fall took place
in May and high seed fall often extended into
June. In 1994 and 1998, the peak was in either
May or June. In all years apart from 2000, seed
fall was greater at the plantation on Tore Hill
than at the other sites (Figure 1).
The mean density of seeds shed by different
cohorts is shown in Figure 2. Peaks in seed
fall (mast years) took place in 1994, 1997 and
2000, and this was synchronous for the four sites.
This was confirmed in six pair-wise comparisons
among the four sites. The Pearson correlation
coefficients ranged from 0.817 (P = 0.002) to
0.985 (P < 0.001). However, within the Poisson
regression model, which included the combined
effects of cohort, site and their interaction, there
were significant differences in seed fall among
sites (χ2 = 279, d.f. = 3, P < 0.001) as well as
cohorts (χ2 = 1355, d.f. = 10, P < 0.001), and
a significant interaction between site and cohort
(χ2 = 85, d.f. = 30, P < 0.001). It was notable
that the seed density was highest for Tore Hill,
the plantation wood, but in 2002, there was poor
seed fall at this site relative to the other woods,
accounting for the interaction. The CVs for seed
fall at the four sites were as follows: Ice Wood
107 per cent; Bognacruie 106 per cent; Memorial
Wood 114 per cent; Tore Hill 99 per cent.
The number of seeds retained in fallen cones
was counted in freshly fallen cones in 2003 only.
The majority of cones had no residual seeds,
but 27 were counted in one cone (Figure 3). The
median count was zero, and the mean was 1.9.
There were no significant differences among
the four sites in the numbers of residual seeds
(χ2 = 2.33, d.f. = 3, n.s.).
Cone production
The average density of ground-collected cones
that had been shed each month from the trees is
shown in Figure 4 for different cohorts. The peak
TIMING OF SCOTS PINE SEED AND CONE PRODUCTION
Figure 2. Variations in the density of seeds for
different cohorts (total number for a given cohort
m-2 of forest floor) falling in seed traps at four sites
in Abernethy Forest: Ice Wood = filled diamonds;
Bognacruie = open circles; Memorial Wood = filled
triangles; Tore Hill = open squares.
Figure 3. Counts of residual seeds retained in fallen
cones in July 2003 for all sites (n = 155).
fall in cones took place in July (cone month 13)
in most years, only 2 months after the peak in
seed fall. However, the 2000 cohort had two
peaks, in June and August. In July 2000, there
were no days when the mean wind speed was
over 10 knots. In contrast, such days were common to all other years. Therefore, the lack of
windy days perhaps accounted for the less usual
pattern of cone fall in 2000.
Counts of cones for a given cohort stopped in
the June after the calendar year of seeding (Table 1),
leaving some cones on the trees. However, very
few cones fell in the calendar year following the
year of seeding (Figure 4, cone months 19–24).
Therefore, the underestimate of total cone production is likely to be small.
The average total density of cones (shed plus
those fed on and dropped by seed-eaters) pro-
545
duced by different cohorts is shown in Figure 5.
Peaks in cone production (mast years) were in
1994, 1997 and 2000, coinciding with peaks in
seed fall (Figure 2). Thus, there were significant
correlations between seed fall and cone production for the four sites: Ice Wood, r = 0.989, P <
0.001); Bognacruie, r = 0.943, P <0.001; Memorial Wood, r = 0.938, P < 0.001; Tore Hill, r =
0.938, P < 0.001. In addition, there was synchrony among sites in the cone production. In the
six pair-wise comparisons among the four sites,
Pearson correlation coefficients ranged from 0.759
(P = 0.007) to 0.963 (P <0.001). There were significant differences among cohorts (χ2 = 318, d.f. =
10, P < 0.001), but not amongst sites (χ2 = 2.9,
d.f. = 3, n.s.). However, there was an interaction
between site and cohort (χ2 = 45.7, d.f. = 30, P <
0.05). This was primarily due to the pattern at
Tore Hill (the plantation), where the cone productivity alternated between being the most productive and least productive relative to the other sites
(Figure 5). The peaks in cone production ranged
from 27.1 to 39.2 cones m− 2 of ground under the
crowns whilst the lows ranged from 2.0 to 5.7
cones m- 2 (Table 2). The differences between the
lows and peaks ranged from a factor of 5 at Tore
Hill to 20 at Ice Wood. The factors for the differences at Bognacruie and Memorial Wood were 10
and 11. The CVs in cone production for the four
sites were: Ice Wood 84 per cent, Bognacruie 68
per cent, Memorial Wood 75 per cent and Tore
Hill 62 per cent. Therefore, the plantation showed
least variation. The CVs for cone production were
also lower than for seed fall.
The mean density of cones in Figure 5 refers
only to the area under the crowns. However, the
density of trees varied among stands leading to
differing amounts of canopy cover. Estimates
of mean canopy cover ranged from 44.8 per cent
(SD = 13.6) and 46.1 per cent (SD = 16.4) for the
two old open stands, to 52.0 per cent (SD = 10.1)
for the high crown trees and 71.2 per cent
(SD = 8.4) for the plantation at the pole stage
of growth. Therefore, to obtain values of cone
production per unit area of woodland, the mean
density of cones under the crowns needs to be
multiplied by the estimates of canopy cover to
account for the open spaces. Table 2 shows the
relevant adjustments made for years of high and
low production. Thus, although the highest cone
density under the crowns was in Ice Wood, the
546
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Figure 4. Seasonal variations in the density of cones (number for a given cohort m- 2 of ground under
the crowns) falling below trees at four sites in Abernethy Forest, but not dropped by seed-eating birds
and mammals: Ice Wood = filled diamonds; Bognacruie = open circles; Memorial Wood = filled triangles;
Tore Hill = open squares. The cone months run from July (cone month 1) in the year prior to seeding to
June (cone month 24) in the year after seeding (see Table 1). The year of seeding (cohort) is shown for each
figure. Cohorts with poor cone production are not shown. Note the different scales on the y-axes.
low density of trees in Ice Wood meant that the
overall cone density in the wood was less than in
the plantation of dense trees and larger canopy
cover at Tore Hill.
Discussion
Some studies of cone production are based on
counting cones whilst still on the tree (Hagner,
1965; Gordon and Faulkner, 1992). Therefore,
depending when the assessment is made, this
technique may not take into account those cones
that are fed upon and dropped by squirrels and
crossbills. Such assessments may underestimate
production. In this study, we counted fallen and
dropped (fed upon) cones and were, therefore,
able to account for seed-eaters, apart from any
cones hoarded by red squirrels, though this
Figure 5. Variations in the density of cones for
different cohorts (total number for a given cohort m- 2 of ground under the crowns) falling and
dropped by seed-eating birds and mammals below
trees at four sites in Abernethy Forest: Ice Wood =
filled diamonds; Bognacruie = open circles; Memorial Wood = filled triangles; Tore Hill = open
squares.
TIMING OF SCOTS PINE SEED AND CONE PRODUCTION
547
Table 2: Mean cone densities (total number for a given cohort m- 2 ) and ranges (in brackets) on the ground
under the crowns of Scots pines for three cohorts of high production (1994, 1997 and 2000 cohorts) and four
of low production (1992, 1996, 1999 and 2001 cohorts) at Abernethy Forest, plus adjusted densities allowing
for differences in canopy cover
Cone density under the crowns
Site
Cone density in the wood
Mean
Range
Mean
Range
39.2
27.9
27.1
27.4
(30.5–44.4)
(20.9–31.9)
(20.4–30.7)
(23.5–32.6)
17.6
12.9
14.1
19.5
(13.7–19.9)
(9.7–14.7)
(10.6–16.0)
(16.7–23.2)
High production
Ice Wood
Bognacruie
Memorial Wood
Tore Hill
Low production
Ice Wood
Bognacruie
Memorial Wood
Tore Hill
2.0
2.7
2.5
5.7
(1.2–2.9)
(1.6–4.0)
(1.6–3.9)
(3.7–8.8)
0.9
1.2
1.3
4.1
(0.5–1.3)
(0.7–1.8)
(0.8–2.0)
(2.6–6.3)
Ice Wood, Bognacruie and Memorial Wood comprise stands of ancient native pinewood whereas Tore Hill
is a plantation.
appears to be rare in pinewoods (Gurnell, 1991).
In addition, counts of cones on trees can achieve
only a partial count, which requires multiplication by a constant (Gordon and Faulkner, 1992).
This constant varies according to the location
of trees, so is unsatisfactory. Our method, which
involved collecting all fallen cones under the
crown provides a better assessment of the production from a tree, though it is time consuming.
A criticism of the method is that cones retained
by the tree 1 year after shedding seed are not
counted. However, this bias is likely to be
small for Scots pines. For conifers that retain
cones for many years (e.g. lodgepole pine Pinus
contorta S.Wats.), this method is not suitable.
Our results showed annual variations in cone
production and seed fall at Abernethy Forest,
with intervals of 3 years in peaks in production
over an 11-year period. Gordon and Faulkner
(1992) state that the interval between peaks in
cone production by Scots pine is 2–3 years for
France, though cautioned that these periods may
not be applicable to Britain. However, Steven and
Carlisle (1959, p. 86) indicate that it is 3–6 years
for ancient native pinewood, and McVean (1963)
reports 4 or 5 years. A long run (1933–1974)
of qualitative assessments of cone abundance
(ranging from ‘poor’ to ‘bumper’) for Scots pine
woods in Strathspey showed ‘bumper’years every
2–5 years (Nethersole-Thompson, 1975). Where
quantitative assessments of seeds and cones have
been made, the runs of data tend to be short. For
example, during 5 years at the Black Wood of
Rannoch, two large falls of seed occurred 4 years
apart (McIntosh and Henman, 1981). Thus, the
findings of our study are in accordance with
previous findings. They also showed that there
was a similarity in production during mast years.
We found that the cone production varied by factors of 5, 10, 11 and 20 between cohorts of high
and low production at the four sites. A similar result
was obtained by McNeill (1954), who found that
plantation Scots pines produced 229–315 cones per
tree in years of high production and 11–23 in years
of low production (a factor of about 16). Kelly
(1994) criticized the assessment of inter-mast periods because of the difficulty in defining mast years.
The coefficient of variation is a more objective
measure of the variability in production and values
obtained in this study (62–84 per cent for cone production) are in the mid- to lower range for 144
woody plants (Herrera et al., 1998).
In the Black Wood of Rannoch, seed densities
of 300 m− 2 were recorded during two peak years
(McIntosh and Henman, 1981). This was higher
than the peak densities recorded in the ancient
native stands at Abernethy Forest (150–250
seeds m− 2 ), but not as high as in the plantation
548
FORESTRY
(over 400 seeds m− 2 ). Differences in canopy cover
may have accounted partly for the differences in
seed density between the different forests and
among stand types at Abernethy Forest. Whilst
many of the trapped seeds will have come from
the crown above the traps, there will also have
been seeds falling diagonally from neighbouring
trees (McVean, 1963), and this effect will have
been greater in the plantation where the canopy
cover was greater. In addition, the cone sizes
from the plantation are larger than from the
older stands, and larger cones contain more
seeds (Summers and Proctor, 1999). This too may
have influenced the differences in seed fall in the
different stands. Finally, it is possible that differences in losses due to seed-eaters among sites
affected the number of seeds being shed.
Cone production and seed fall showed similar
patterns for the different cohorts. The seed fall
referred to the portion not taken by seed-eaters
prior to seed dispersal. Clearly, the level of seed
consumption prior to seed dispersal was insufficient to disrupt the pattern as seen in the production of cones (Figures 2 and 5). It was notable,
however, that the CVs for seed fall were greater
than for cone production. If seed-eaters took
proportionately more seeds in years with low
cone production, this would lead to greater interannual variability in seed fall.
The annual variations in cone production are
believed to be influenced by the weather during
the time of cone bud initiation and pollination
(Gordon and Faulkner, 1992). However, responses
to weather are believed to be proximate triggers,
synchronizing the annual variations in seed
production so that seed-eaters are swamped in
mast years, or pollination efficiency is increased
(Kelly and Sork, 2002). Other potentially ultimate
causes are reviewed by Kelly (1994).
mast years do not coincide with ground preparation. With a periodicity of 3 years between mast
years, it is likely that the timing of seed bed preparation is not critical if trying to encourage tree
regeneration, unless the site has a lot of grass.
The seeds were largely shed from the cones over
a 2-month period leaving no or a few residual
seeds in the cones, at least in 1 year (Figure 3), and
the cones were then shed mainly in July. Thus, the
food available to crossbills and red squirrels
declines rapidly during the summer. Red squirrels
can switch to other food (e.g. fungi and berries)
(Gurnell, 1991) before the next cohort of cones
becomes available towards the end of July (Summers and Proctor, 1999). Crossbills, on the other
hand, are more dependent on conifer seeds, so
would need to switch to other conifers to avoid
the summer shortage of Scots pine seeds. The
shortage may be prolonged if a year of low cone
production follows a year of high production.
Also, given that the stands at four sites had coincident levels of production, it is likely a shortage
would occur throughout the forest and maybe
beyond. Koenig and Knops (1998) report synchronicity in Pinus up to 2500 km apart, though
Scots pines in different parts of Sweden can show
lack of synchrony (Hagner, 1965). Further work
is required to find out if different Scots pine
forests in Scotland have coincident patterns of
cone production. This has implications for maintaining numbers of birds and mammals of conservation concern (Anon. 1995).
Acknowledgements
Mrs Elizabeth Kerr at the Meteorological Office,
Edinburgh provided weather data. Drafts of the paper
were kindly commented on by Alice Broome, Mark
Hancock, David Jardine, Peter Lurz, Mick Marquiss
and Jeremy Wilson.
Implications for tree regeneration and
seed-eating birds and mammals
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Received 25 October 2004