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RESEARCH NOTE
Effects of stand structure,
composition and treatment on barkstripping of beech by grey squirrels
E.P. MOUNTFORD1 AND G.F. PETERKEN2
1
2
Ecoscope Applied Ecologists, c/o 17 Butler Road, Wem, Shropshire, SY4 5YP, England
Beechwood House, St Briavels Common, Lydney, Gloucestershire, GL15 6SL, England
Introduction
Beech (Fagus sylvatica L.) is particularly vulnerable to bark-stripping by grey squirrels (Sciurus
carolinensis Gmelin), especially when it is
medium-sized and growing rapidly with a high
average phloem volume. Patterns of bark-stripping
are best explained as a consequence of agonistic
behaviour, food-seeking or learning, which is
amplified by tree-growth factors, including phloem
volume and bark-structure (Shorten, 1954; Taylor,
1966, 1969; Mackinnon, 1976; Kenward, 1982,
1983; Rowe, 1984; Rowe and Gill, 1985;
Kenward and Parish, 1986; Gurnell, 1987;
Kenward et al., 1988a, b, 1996; Kenward, 1989;
Gill, 1992; Kenward and Dutton, 1996; Mountford, 1997). Bark-stripping seems more intense in
stands that have been silviculturally thinned, i.e.
where growth rate and phloem volume have
increased, and accordingly modified patterns of
stand treatment have been proposed (Kenward et
al., 1988a; Kenward and Dutton, 1996).
This note records supporting evidence for the
influence of thinning on the intensity of barkstripping. It relates to Lady Park Wood, a mixed
deciduous woodland growing on and above steep
slopes on Carboniferous limestone overlooking
the river Wye (UK), where stand dynamics and
the growth rate of individual trees have been
closely observed for over 50 years (Peterken and
Jones, 1987, 1989; Mountford, 1994, 1997;
© Institute of Chartered Foresters, 1999
Peterken and Mountford, 1995, 1996, 1998).
The reserve comprises a non-intervention area
containing both old-growth and young-growth
stands, and an adjacent compartment which had
a similar composition and treatment history until
a part was thinned in 1982.
Stand treatment
Both the non-intervention area and the adjacent
compartment were treated as coppice until 1902,
when a programme of thinning was initiated
which was designed to convert the stands to
beech high forest. In 1940–45 most of the stands
were felled, leaving a shelterwood of oak
(Quercus petraea (Matt.) Liebl.) and beech, but
parts of what is now the non-intervention reserve
were retained and now constitute the old-growth
stands of the reserve. All stands were allowed to
re-grow naturally, until a decision was taken to
manage the adjacent compartment as a source of
timber and small wood, regenerating only by
stump regrowth or natural seeding. In 1982, part
of this ‘managed’ compartment was accordingly
thinned. In 1992–3, a larger part was thinned,
including some portions which had also been
thinned in 1982: post-1982 stump regrowth was
cut away to leave an open canopy of mainly 50year old stems with about 50 per cent crown
cover. Only mature stands overlooking the Wye
Forestry, Vol. 72, No. 4, 1999
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F O R E S T RY
and stands on steep narrow slopes higher up
remained unthinned, i.e. their stand history was
similar to that of the old-growth stands within the
non-intervention reserve. The stands available
were therefore:
To compare damage levels, a quantitative index
of damage was generated by treating the damage
score for each level as a measure, and averaging
the total for all individuals in a particular sizeclass:
• Old-growth, non-intervention, canopy age
about 80–190 years. These grow (1) above a
central cliff line on dry, freely drained soils, and
(2) below the cliff line on a mosaic of rock outcrops and deep, moist, alkaline clay loam.
• Young-growth, non-intervention, canopy age
about 50 years, but with older standards.
• Thinned stands in the managed compartment,
canopy age about 50 years, with a scatter of
older standards.
• Unthinned stands in the managed compartment, age intermediate between old- and
young-growth in the non-intervention reserve.
3 s)
Damage Index = S(n
_______
N
where:
n = number of stems in each damage category
s = maximum damage score
N = total number of stems in each size-class
Chi-square tests of association were used to
compare the numbers of stems in particular size
or damage categories. Analysis of variance
(ANOVA) was used to compare mean growth
rates in different stands.
Results
Methodology
The changes in the stands of the non-intervention
reserve were studied by means of nine permanent
transects (Peterken and Backmeroff, 1988). A
tenth transect was established in the managed
compartment by Tim Barfield of the Nature Conservancy Council in 1984. All trees on transects
I–IX were recorded in April/May 1992 or June
1993, and on transect X in April 1997. The condition of each beech stem standing alive was
assessed as part of this record. Bark-stripping
damage to the main trunk and crown forks was
assessed against a five-point scale:
0 = no damage (no bark removed)
1 = limited damage (< 10 per cent bark removed)
2 = moderate damage (10–50 per cent bark removed)
3 = severe damage (> 50 per cent bark removed)
4 = very severe damage (ring-barked)
Bark-stripping damage was recorded first for the
lower (< 2 m height) and then for the upper trunk
(> 2 m height), and categorized according to the
maximum damage score. Thus a stem that scored
0/2 (i.e. no lower stem damage and moderate
upper stem damage) was classified into the moderate damage category, whereas a stem that
scored 3/1 was classified into the severe damage
category.
Table 1 shows that the intensity of bark-stripping
in the thinned stand was greatest for stems of
10–35 cm d.b.h. This compares with Mountford’s (1997) finding in the non-intervention
reserve that the vulnerable size range was 10–25
cm d.b.h. Since the high index value for the
30–34.9 cm class was due to single stem of 31
cm d.b.h., the vulnerable size range is regarded
to be 10–30 cm d.b.h. Within this range 34 of 49
stems were at least moderately damaged, a significantly higher level than for the smaller or
larger size ranges (x2 = 53.1, d.f. = 2, P < 0.001
(Table 1).
Tables 2 and 3 give damage indices for five
types of stand and three size classes of beech.
These suggest that three further factors influence
the intensity of damage in addition to stem size.
Density of beech
No damage was found in the old-growth stands
below the cliff, where beech was sparse (< 100
stems ha–1), but some damage was found in the
old-growth above the cliff, albeit at a low density,
where beech formed the major component of the
stand (> 250 stems ha–1). Damage was most substantial in all parts of transect X and in the younggrowth non-intervention stands where beech
density was high and vulnerable 10–30 cm d.b.h.
stems were abundant.
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EFFECTS ON GREY SQUIRREL BARK-STRIPPING
381
Table 1: Severity of bark-stripping to beech stems in the thinned sections of transect X in the managed
compartment. Damage is shown in relation to stem size and five bark-stripping categories (see text for details).
For each size-class an overall damage index is given (see text for details)
Stem d.b.h.
class (cm)
Bark-stripping category
——————–———————————————————————————
None
Limited
Moderate
Severe
Very severe
(n)
(n)
(n)
(n)
(n)
Damage
index
<5
5–9.9
10–14.9
15–19.9
20–24.9
25–29.9
30–34.9
35–39.9
40+
62
9
1
1
1
0
0
0
2
11
7
3
5
4
0
0
1
0
3
0
0
9
1
1
1
0
0
4
3
5
2
2
2
1
0
0
0
1
4
5
2
1
0
0
0
0.4
1.0
2.6
2.2
2.0
3.0
2.5
1.0
0.0
All stems
76
31
15
19
13
1.1
Age of stand
Damage intensity in the stands growing above the
cliff in the non-intervention stands was far greater
in the young-growth than in the old-growth, and
the difference was disproportionate to the difference in density. Damage was substantial in the
unthinned stand on transect X, which was intermediate in age between the old- and younggrowth.
Management
The thinned stand was damaged far more than
the unthinned portions of transect X, and very
few beech escaped some damage. Most importantly, the number of damaged stems was greater
in the most vulnerable size class (x2 = 6.21, d.f. =
1, P < 0.025), including stems with severe or
greater damage (x2 = 4.12, d.f. = 1, P < 0.05)
(Table 3). The thinned stand was also damaged
more than the young-growth non-intervention
stands. However, in the most vulnerable size class
only the total number of damaged stems was
greater (x2 = 8.09, d.f. = 1, P < 0.005), with no
significant associations obtained for stems with
moderate or greater damage (x2 = 0.61, d.f. = 1,
P > 0.1), or severe or greater damage (x2 = 0.66,
d.f. = 1, P > 0.1) (Table 3).
The recent rates of increment growth in relation
to stem size are given for the thinned and
unthinned portions of transect X (Figure 1) and
the young-growth non-intervention stands (Figure
2). Average growth rate of vulnerable-sized stems
was greatest in the thinned stands (0.438 cm a–1)
(ANOVA F = 36.88young-growth, 19.73unthinned; Pmin
< 0.0001): the unthinned (0.275 cm a–1) and
young-growth stands (0.291 cm a–1) had similar
lower average rates (ANOVA F = 0.35; P = 0.55).
In all stands, most rapidly-growing stems were
within the vulnerable size range, and damage to
these was especially frequent. However, slowgrowing vulnerable-sized stems were also selected,
especially in the young-growth stands.
Tables 4 and 5 compare damage levels in stands
according to the date of thinning. Within the
vulnerable size range, stands thinned in 1982 had
a slightly higher damage index than stands
thinned in 1993, principally because stems with
severe or very severe damage were more numerous in the earlier thinned stand (x2 = 3.38, d.f. =
1, P < 0.1). Stands thinned in both years had
intermediate levels of damage, but the few larger
stems had all been at least moderately damaged.
Discussion
Grey squirrel populations and the intensity of
bark-stripping vary from year to year. Furthermore, populations at Lady Park Wood were controlled by poisoned bait until 1996, when Forest
Old-growth below cliff
Old-growth above cliff
Transect X: unthinned
Young-growth
Transect X: thinned
Stand
1992
1992
1997
1993
1997
Survey date
0.48
1.02
0.29
1.99
0.60
Transect area
(ha)
81
254
497
332
272
6
57
155
106
82
30
115
91
410
100
0.00
0.03
0.09
0.62
0.49
Stems up to 10 cm
Density of
d.b.h.
10–29.9 cm ————————
d.b.h. beech Number Damage
(n/ha)
of stems index
3
58
45
208
49
0.00
0.16
1.56
2.17
2.35
Stems 10–29.9 cm
d.b.h.
————————
Number Damage
of stems index
6
77
3
16
5
0.00
0.01
0.67
0.56
1.20
Stems 30 cm d.b.h.
and above
————————
Number Damage
of stems index
382
Density of
all beech
stems
(n/ha)
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Table 2: Bark-stripping damage indices (see text for details) for five stands with different densities of beech, ages, stem sizes, and treatments
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EFFECTS ON GREY SQUIRREL BARK-STRIPPING
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Table 3: Number of beech stems in the 10–29.9 cm d.b.h. size-class with different levels of bark-stripping (see
text for details) in three stands of similar composition but different treatment
Stand
Transect X: thinned
Transect X: unthinned
Young-growth
Not
damaged
(n)
3
11
51
Damaged
(n)
Less than
moderate
damage
(n)
Moderate or
greater
damage
(n)
46
34
157
15
27
76
34
18
132
Less than Severe or
severe
greater
damage
damage
(n)
(n)
26
33
97
23
12
111
Figure 1. Relationship between stem size, growth rate, and severity of bark-stripping to beech in the thinned
(left) and unthinned (right) stands of transect X. The zone within which stems spent more than half the
period in the vulnerable size range (10–30 cm d.b.h.) is indicated by dashed lines. Stems with severe/very
severe damage (d), moderate (s), or none/limited (+) damage are identified.
Enterprise decided to suspend this operation
(Simon Wallace, personal communication). Comparisons between the non-intervention reserve and
managed compartment could have been influenced
by such variation. However, no substantial
increase in bark-stripping was noted during
routine surveillance of all stands after 1993, so the
difference in years of recording is discounted as a
significant factor. In any case, baiting appeared to
have little effect before 1993 (Mountford, 1997).
At Lady Park Wood, several factors appear to
be important in determining which stands have
suffered high levels of damage. The most vulnerable were those with beech growing as a major
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F O R E S T RY
Figure 2. Relationship between stem size, growth rate, and severity of bark-stripping to beech in the younggrowth stands. The diagram is divided into two for clarity: stems with severe/very severe damage (d) are
shown to the left, and stems with moderate (s) or none/limited (+) damage are shown to the right. The
zone within which stems spent more than half the period in the vulnerable size range (10–30 cm d.b.h.) is
indicated by dashed lines. NB: each point may represent more than one stem.
Table 4: Bark-stripping damage indices (see text for details) in relation to stem size for beech in three stands that
were thinned at different dates
Date stand thinned
Thinned 1982 only
Thinned 1982 and 1993
Thinned 1993 only
Stems up to 10 cm d.b.h. Stems 10–29.9 cm d.b.h.
——————————— ———————————
(n)
Damage
(n)
Damage
index
index
60
28
12
0.40
0.64
0.58
component, including many stems in the vulnerable size range, i.e. 10–30 cm d.b.h, some of
which were fast-growing. However, it should be
noted that growth rate measured over more than
a decade may be different from the actual growth
rate when the bark was stripped.
12
11
26
2.75
2.45
2.12
Stems 30 cm d.b.h. and
above
———————————
(n)
Damage
index
0
3
2
0.00
1.67
0.50
A correlation between growth rate and barkstripping has been reported or inferred in several
other studies (Mackinnon, 1976; Kenward, 1982,
1983; Kenwood and Parish, 1986; Kenward et
al., 1988a, b; Kenward, 1989; Kenward et al.,
1996; Kenward and Dutton, 1996). In Lady Park
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EFFECTS ON GREY SQUIRREL BARK-STRIPPING
385
Table 5: Number or beech stems in the 10–29.9 cm d.b.h. size-class with various levels of bark-stripping
damage (see text for details) in three stands that were thinned at different dates
Date stand thinned
Thinned 1982 only
Thinned 1982 and
1993
Thinned 1993 only
Damaged
(n)
Less than
moderate
damage
(n)
Moderate or
greater
damage
(n)
2
10
2
10
3
9
0
1
11
25
3
10
8
16
6
17
5
9
Not
damaged
(n)
Wood, damage has been greatest in the size-class
of stems which are moving into their period of
fastest growth, and within that size class it has
tended to be greater in the stems that were actually growing fastest. Damage within the nonintervention old-growth stands has been limited,
mainly because stems in the vulnerable size range
occupy the sub-canopy and are thus growing
slowly (Mountford, 1997), whereas growth of
such stems in the young-growth has been faster,
and in the thinned stands it has been faster still.
In thinned portions of transect X, growth rates
within the vulnerable range were greater than
those in the unthinned portion, and damage levels
were correspondingly higher. Recent release after
thinning may explain why several apparently
slow-growing stems in the thinned stands were so
severely debarked in 1997. Growth rates in the
non-intervention young-growth may have accelerated after rapid loss of birch following the 1976
drought (Peterken and Jones, 1989; Mountford,
1994), and this could have stimulated bark-stripping (Mountford, 1997).
A history of bark-stripping appears important.
Once squirrels have learnt to strip bark the habit
is likely to be repeated (Kenward et al., 1988b).
This helps explain the high levels of damage to the
non-intervention young-growth stands, where
localized damage patches of both slow and fast
growing vulnerable size stems have developed
since an outbreak in 1983 (Peterken and Jones,
1989; Mountford, 1997). Damage in the
managed compartment has noticeably accelerated
following the thinning in 1993.
Spatial factors may also be significant. The incidence of grey squirrel damage has been patchy,
even within a uniform stand (Mountford, 1997)
and this may influence comparisons between the
Less than Severe or
severe
greater
damage
damage
(n)
(n)
relatively small samples available in this study. The
very low density of beech in the old-growth below
the cliff may explain the lack of damage, even
though the few beech were generally fast growing.
The relatively high levels of damage in the
unthinned portions of transect X may partly reflect
the close proximity of these stands to the most
severely damaged thinned portions.
Implications for management
Beech is more susceptible to bark-stripping than
other native trees (Shorten, 1954; Rowe, 1984;
Rowe and Gill, 1985): at Lady Park only localized stripping of birch (Betula pendula Roth), ash
(Fraxinus excelsior L.), field maple (Acer
campestre L.), hazel (Corylus avellana L.), and
willow (Salix caprea L./S. cinerea L.) has been
observed. Many other mixed beech stands in
Britain have suffered substantial damage, and
alternative silvicultural treatments have been proposed in order to mitigate damage (Kenward et
al., 1988b; Kenward and Dutton, 1996). Based
on the present observations, three silvicultural
treatments are recommended where control of
grey squirrels is impractical:
• Growing alternative species less susceptible
than beech to bark-stripping. In the case of
Lady Park Wood, it would have been better to
retain more ash, oak, small-leaved lime (Tilia
cordata Mill.), large-leaved lime (Tilia platyphyllos Scop.) and birch.
• Growing beech as a minority component of
mixtures until final thinning. Squirrels appear
to have overlooked vulnerable beech where
they were rare in the stand.
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• Growing beech-dominated stands under continuous cover treatments rather than uniform
thinning. This will help maintain slow growth
through the vulnerable size range.
Acknowledgements
This work formed part of the RENFORS project –
REgeneration of Native FORest Stands for timber production and environmental value – funded by the European Communities Directorate-General for Agriculture
(Contract FAIR PL95-0420). Access to Lady Park
Wood was provided by the Forestry Commission,
English Nature and the Countryside Council for Wales,
who jointly manage the site as a National Nature
Reserve. Robin Gill and an anonymous referee kindly
provided useful comments on draft manuscripts.
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