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The effect of cork-stripping damage
on diameter growth of Quercus
suber L.
A. COSTA*, H. PEREIRA AND A. OLIVEIRA
Instituto Superior de Agronomia, Departamento de Engenharia Florestal, Tapada da Ajuda, 1349-017 Lisboa,
Portugal
Corresponding author. E-mail: [email protected]
Summary
The Mediterranean cork oak (Quercus suber L.) agro-forestry system is oriented towards cork
production, with cork being extracted from tree stem and branches as planks by cutting with an axe
and stripping off. The effect of damage to the tree during cork stripping was studied in cork oaks,
weakened by wounding, by following the diameter growth and its seasonality during a 9-year
production cycle, and comparing them with healthy cork oaks. Tree wounding decreased diameter
growth during the following cycle, e.g. 8.5 mm a–1 and 9.8 mm a–1 for weakened and healthy trees,
respectively, mostly in the 2 years immediately following the cork stripping. The beginning of annual
growth in spring and the occurrence of the highest increments in June–August were delayed by about
1 month in the weakened trees. The cork produced by weakened trees was reduced by 13 per cent in
thickness, with average cork ring widths of 3.3 mm a–1 vs. 3.8 mm a–1 for healthy trees.
Introduction
The cork oak (Quercus suber L.) is the dominant
species of the ‘montado’ agroforestry system
prevailing in the south-western part of the
Iberian Peninsula (726 000 ha in Portugal and
510 000 ha in Spain). Typically an open forest
(50–150 trees ha–1), this complex ecological
system is economically sustained by the production of cork.
Cork oak sylviculture is oriented towards the
periodical removal of the cork. The thick cork
layer that covers the tree stem and branches is
removed up to a certain level (2–3 m or more,
depending on tree size) for the first time when the
© Institute of Chartered Foresters, 2004
tree is about 25 years of age (and a circumference
over bark at breast height >70 cm), and then subsequently at periodical intervals, usually 9 years.
The removal of cork (stripping) is made from
May to August, when the physiologically active,
swollen and thin phellogen and the newly differentiated cork cells allow an easy rupture and
separation of the tissues.
The cork is removed as large rectangular
planks (1–1.3 m long, 0.4–0.6 m wide) with a
special axe by cutting first horizontally around
the tree stem and then vertically, and by prying
them off with the axe handle. The operation is
rather demanding of the skills of the worker: sufficient strength has to be applied to the axe to cut
Forestry, Vol. 77, No. 1, 2004
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F O R E S T RY
through the hard phloem tissues that cover externally the cork planks (the cork ‘back’) and the
underlying cork layer, but stopping at the phellogen or going only very slightly into the inner
bark. In fact, the traumatic phellogen will be
regenerated at some depth in the phloem and
deep cuts will induce wound responses and a loss
of uniformity in the suberous tissue that will be
formed subsequently (Graça and Pereira, 2003).
The consequence of a cut that penetrates down to
the cambium is severe, with loss of cork production in that area for several years until the wound
has healed.
The effect of cork stripping on tree health and
growth has always been a concern for the sustainable management of the system (Natividade,
1938; Correia et al., 1992; Wargo, 1996; Pereira
et al., 1999). In addition to the danger of
wounding, cork stripping causes a large water
loss from the stripped surface (Natividade, 1938;
Santos, 1940; Correia et al., 1992) that may
induce a decrease in diameter growth (Luque et
al., 1999) and enhance additional stresses and
susceptibilities (Natividade, 1938; Werner, 1995).
In the Mediterranean cork oak forest, susceptibility to attack by fungi (e.g. Hypoxylon
mediterraneum) increases when the tree suffers
from water stress (Macara, 1974; Luque et al.,
1999). These concerns have led to regulation of
cork production practices, e.g. in Portugal, stripping height must not exceed three times the stem
perimeter at breast height in mature trees and the
minimum period between cork extractions must
be 9 years. When trees are weak or damaged it is
also usual practice to decrease the cork stripping
level or to increase the cork production cycle.
We studied the effect of tree damage on radial
growth by following the diameter growth and its
seasonality, during a 9-year production cycle, in
cork oaks weakened by wounding during the
previous stripping and in healthy cork oaks. The
objective was to evaluate the impact of damage
caused by cork removal on subsequent tree
growth and losses of cork production in the
following production cycle.
Materials and methods
The study was conducted in a typical cork oak
stand under cork production with a 9-year cycle,
characterized by a density of 67 trees ha–1,
3.2 m2 ha–1 basal area and a mean circumference
at breast height of 1.0 m. As regards cork production parameters, maximum height of debarking is on average 1.8 m, total surface of
debarking 1.8 m2 tree–1, cork yield 10 kg m–2 of
stripped area and annual cork production 900 kg
ha–1. A detailed study of this stand is given in
Sousa (1997).
The study area is located in the south-west of
Portugal in the region of Benavente (38° 46′ N,
8° 45′ W) with a Mediterranean-type climate
with some Atlantic influence: mean annual temperature 15.5°C, annual precipitation 644 mm
and a dry period from May to September. During
the study period (1991–2000) the mean temperature was 17.6°C and the average annual precipitation 595 mm with below average rainfall in
1993 and 1995 (575 mm and 550 mm, respectively).
After cork stripping in June 1991, a total of 50
healthy cork oak trees were randomly selected in
a 7000 m2 plot and manual band dendrometers
(model D1; UMS GmbH, Munich, Germany)
were installed at breast height (1.30 m above
ground) on the debarked stem. During the 9 years
of the production cycle (June 1991 to June 2000)
monthly measurements were made. This period
includes eight complete years of tree growth
(1992–1999).
Before the cork extraction in June 2000, the
tree conditions were evaluated using as analysis
parameters crown defoliation and the presence of
wounds that could be recognized as having originated from the previous cork stripping and of
visible necrosis and deterioration of sapwood
tissues. The trees were classified as weakened
when they showed 50–75 per cent or more crown
transparency and presented wounds, scars or
necrosis larger than 15 cm 5 cm (length width) in stem or branches representing >5 per
cent of the debarking stem surface. The extent of
damage varied from cuts that went deep into the
phloem but with limited dimensions (Figure 1a)
to large wounds with destruction of the cambium
by the removal of the inner bark (Figure 1b). The
weakened condition of the trees was confirmed
during the cork stripping by the fact that the
extraction of the cork planks was more difficult.
A total of 10 trees were classified as weak or
damaged.
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EFFECTS OF CORK-STRIPPING DAMAGE ON QUERCUS SUBER
a.
b.
3
At the previous cork extraction in 1991 and in
June 2000, field measurements were made in
relation to breast height diameter, total tree
height, crown area, stripping height, total
stripped surface and cork production. The stripping coefficient was calculated as the ratio of the
maximum stripping height and the perimeter at
breast height. The cork productivity was calculated per tree as a cork yield produced by unit
area of stripped surface (k gm–2).
A cork sample with dimensions 20 20 cm2
was taken, during the cork extraction of 2000, at
breast height and used to measure cork thickness,
ring width for the eight complete years
(1992–1999) and porosity. Porosity, a quality
parameter of cork, was measured as a porosity
coefficient (area of pores as a percentage of total
area) using image analysis of the transverse section,
as previously described by Pereira et al. (1996).
The non-cork growth during the eight
complete years of the cork production cycle that
includes the wood growth, external phloemic
tissues, phelloderm and two mid-years of cork
growth, was calculated for each tree based on the
difference between the total annual growth
(measured with the band dendrometer) and the
total cork ring width, measured in the cork
sample collected at breast height.
For comparison of the diameter growth, a
sample of 10 healthy trees with similar dendrometric characteristics in June 1991 to those classified in 2000 as weak or damaged, including their
cork stripping parameters was selected (Table 1).
Initial differences between weakened trees and
healthy trees with respect to diameter over and
under cork, cork stripping coefficient and cork
stripping height were compared using a t test.
Differences of total and annual diameter
increases between weakened and healthy trees
were statistically assessed using t tests. In
addition, an analysis of variance was made for
the annual diameter increase using a KruskalWallis one-way analysis of variance on ranks.
Results
Figure 1. Some examples of wounds, scars and
necrosis caused by cork stripping in the stem and
branches of weakened trees.
Initial condition
In 1991, there were no significant differences
(P = 0.05) between the 10 trees classified in 2000
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Table 1: Characterization of the weakened and healthy cork oak trees in relation to tree and cork parameters
(1991 and June 2000)
Parameters
Weakened trees
Healthy trees
49.0 ± 12.9
43.3 ± 12.4
48.9 ± 12.9
45.1 ± 12.0
11.2 ± 2.0
2.1 ± 0.4
97.3 ± 55.4
2.7 ± 0.8
1.8 ± 0.4
4.6 ± 2.3
53.1 ± 10.1
42.6 ± 10.5
50.6 ± 11.4
44.2 ± 10.5
10.9 ± 2.8
2.9 ± 1.1
89.9 ± 45.0
3.0 ± 0.7
1.9 ± 0.4
5.2 ± 1.9
1991 over cork d.b.h. (cm)
1991 under cork d.b.h. (cm)
2000 over cork d.b.h. (cm)
2000 under cork d.b.h. (cm)
Total height (m)
Stem height (m)
Crown area (m2)
Cork stripping height (m)
Cork stripping coefficient
Total cork stripping surface (m2)
Values are mean of 10 trees and standard deviation.
Table 2: Annual diameter increase (cm) for the eight complete years of growth in the cork production cycle for
weakened and healthy cork oak trees
Years
Weakened trees
Healthy trees
1.09 ± 0.30
0.70 ± 0.27
1.26 ± 0.32
0.60 ± 0.32
1.01 ± 0.20
1.00 ± 0.23
0.78 ± 0.32
0.36 ± 0.24
6.78 ± 1.25
0.85 ± 0.16
1.52 ± 0.27
0.96 ± 0.20
1.30 ± 0.31
0.71 ± 0.22
1.09 ± 0.20
0.96 ± 0.23
0.81 ± 0.23
0.52 ± 0.19
7.86 ± 1.19
0.98 ± 0.15
1992
1993
1994
1995
1996
1997
1998
1999
1992–1999
Annual average
Values are mean of 10 trees and standard deviation.
as healthy and those classified as weak or
damaged in breast height diameter, cork production and stripping height.
Diameter growth
In the 9-year production cycle, both weakened and
healthy trees showed positive annual growth increments in diameter but the total diameter growth
was smaller for the weakened trees than for
healthy trees (6.96 1.27 cm and 8.08 1.23 cm,
respectively). However, between-tree variability
was large and the difference was not statistically
significant (P > 0.06). Considering only the eight
years with complete growth (Table 2), the diameter
increase was also smaller for the weakened trees
than the healthy trees (6.78 1.25 cm and 7.86 1.19 cm, respectively).
Along the production cycle the diameter
growth showed a decreasing trend that was very
similar in all trees (Table 2), with smaller increments in the final years. In 1993 and 1995 the
diameter increase was exceptionally low.
The average annual diameter increment in the
eight years of complete growth that are included
in the cork production cycle was 9.8 1.5 mm
a–1 for healthy trees and 8.5 1.6 mm a–1 for
weakened trees (Table 2). The annual growth of
healthy trees was higher than the growth of
weakened trees for all years with the exception of
1997 with a similar value for the two groups.
Only the difference in growth for the first two
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EFFECTS OF CORK-STRIPPING DAMAGE ON QUERCUS SUBER
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Table 3: Characterization of cork production of weakened and healthy cork oak trees
Parameters
Weakened trees
Healthy trees
48.7 ± 26.6
28.2 ± 4.4
10.4 ± 5.3
9.9 ± 1.9
59.2 ± 24.2
33.5 ± 5.2
9.9 ± 4.5
11.5 ± 2.0
Cork green weight (kg tree–1)
Thickness (mm)
Porosity (%)
Cork yield (kg m–2)
Values are mean of 10 trees and standard deviation.
years in the cycle (1992 and 1993) was larger and
statistically significant (P = 0.003 and P = 0.042,
respectively).
Cork production
Cork production was on average lower in the
weakened trees in comparison with the healthy
trees but the between-tree variation is high with
coefficients of variation of the mean of 34 per
cent and 55 per cent, respectively, for healthy and
for weakened trees, and the difference was not
statistically significant (P = 0.373). The cork
yield, measured as the cork weight per unit
debarked surface, was on average 14 per cent
lower in the weakened trees (9.9 1.9 kg m–2 as
compared with 11.5 2.0 kg m–2 for healthy
trees) but the difference was not statistically
significant. The total thickness of cork obtained
from the weakened trees was reduced in comparison to healthy trees on average by 16 per cent
and this difference was statistically significant
(P = 0.017). The quality of cork, measured as its
porosity, was similar for both groups of cork
oaks (Table 3).
Cork and ‘wood’ growth
The growth of cork is the main component in the
total diameter increase of cork oaks representing
on average 77.3 per cent and 78.9 per cent,
respectively, in weakened and healthy trees. The
inter-annual variation of cork ring width showed
a pattern similar to the radial growth, with an
overall decreasing trend (Figure 2).
On average for the 8-year cycle, the cork ring
width was smaller in the weakened trees than in
the healthy trees (3.3 0.5 mm a–1 and 3.8 ±
0.4 mm a–1, respectively) but between-tree variability was important and the difference was not
statistically significant. Annually, the growth of
cork was higher in the healthy trees during the
first 5 years of the cycle, e.g. in the first and
second years (1992 and 1993) cork increased
4.8 mm and 3.9 mm in healthy trees and 3.5 mm
and 2.9 mm in the weakened trees, and remained
practically the same in the last 3 years of the cycle
(Figure 2).
The average non-cork growth (or ‘wood’
growth) of weakened and healthy trees (Figure 2)
was similar in most of the years and the small
differences found for 1992, 1994 and 1995 were
not statistically different (P < 0.05).
Seasonal pattern of diameter growth
The diameter growth showed a seasonal pattern
for all trees in the eight years of the cycle with a
clearly marked growth period followed by a
period without diameter increments. Growth
begins in March and extends to October with the
highest increments in June–August (Figure 3).
However, the beginning of annual growth and
the occurrence of the highest increments were not
simultaneous in healthy and weakened trees. The
onset of growth was delayed by ~1 month in the
weakened trees while maintaining the same
pattern of radial increments subsequently
(Figure 3), thereby also delaying the growth
peaks by approximately the same time period. A
comparison made between the growth patterns of
the weakened trees showed that those with a
higher extent of damage more clearly expressed
the later growth onset (Figure 4).
Discussion
The cork oaks that were classified as weakened
after one cork production cycle showed a
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F O R E S T RY
8.0
7.0
Weakened (cork)
Healthy (cork)
Weakened (wood)
Healthy (wood)
Annual growth (mm)
6.0
5.0
4.0
3.0
2.0
1.0
0.0
1992
1993
1994
1995
1996
1997
1998
1999
Figure 2. Cork ring width (mm) and annual growth of ‘wood’ (mm) during eight complete years of the
cork production cycle for weakened and healthy cork oak trees. Mean of 10 trees and half of standard
deviation.
4.5
Monthly diameter increments (mm)
4.0
Healthy trees increment
3.5
Weakened trees increment
3.0
2.5
2.0
1.5
1.0
0.5
Nov-99
May-99
Nov-98
May-98
Nov-97
May-97
Nov-96
May-96
Nov-95
May-95
Nov-94
May-94
Nov-93
May-93
Nov-92
May-92
0.0
Nov-91
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Figure 3. Average monthly diameter increments (mm) for healthy and weakened trees for the period of
eight complete years of the cork production cycle.
diameter growth that followed the previously
reported pattern for mature cork oaks, namely a
decreasing trend along the cycle (Costa et al.,
2001). In addition, the below average rainfall in
the previous winter 1992/1993 and 1994/1995
reduced the cork growth in 1993 and 1995
(Caritat et al., 2000; Costa et al., 2001).
The empirical arguments that cork oak radial
growth decreases in trees weakened from a bad
stripping (Natividade, 1950; Correia et al., 1992)
were confirmed by the results obtained, the
increase in diameter after one production cycle
was smaller (by 14 per cent) than in the healthy
cork oaks used for comparison, although the
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EFFECTS OF CORK-STRIPPING DAMAGE ON QUERCUS SUBER
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Monthly diameter increments (cm)
0.50
Heavy damaged
Lightly damaged
0.40
0.30
0.20
0.10
0.00
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Month
Figure 4. Monthly diameter increments (cm) for the year 1992–1993 of six weakened cork oaks with two
different degrees of damage.
difference was in the range of between-tree variation and therefore not statistically significant. If
the annual growth increments are compared
(Table 2), it can be seen that they were reduced
in the weakened trees, mainly in the first two
years of the production cycle, which showed a
statistically significant reduction in growth of
~30 per cent. It seems therefore that the influence
of the cork stripping damage on radial growth is
immediate in the two subsequent years.
Since damage extent was highly variable, as
shown in Figure 1, it would be expected to have
different reaction intensities from individual
trees. This was the case and the between-tree
growth variability was higher in the weakened
trees than in the healthy trees, i.e. the coefficient
of variation of the mean annual growth in the
first and second years of the cycle was 28 per cent
and 39 per cent for weakened trees and 18 per
cent and 21 per cent for healthy trees.
The main component of diameter growth
refers to the increase in cork thickness, which is
known to represent ~80 per cent of total diameter
increments (Costa et al., 2001). Therefore, in
agreement with the differences in total diameter
increase between weakened and healthy trees, the
cork thickness obtained from weakened trees was
16 per cent less. However, this reduction in cork
growth was not uniform along the cork cycle,
and it could be observed that the cork rings were
smaller in the weakened trees only in the first half
of the cycle and similar in the second half
(Figure 2).
The activity of the newly formed traumatic
phellogen after the cork stripping was thereby
reduced as a tree response to wounding. It is
known that traumatic phellogens have an
enhanced meristematic activity in the years
immediately following their formation, thereby
originating wider cork rings in the first years of
the production cycle (Pereira et al., 1992; Ferreira
et al., 2000). Tree wounding seemed therefore to
reduce this increased activity of the phellogen in
its first years of age. In agreement, the increment
of the non-cork component of diameter growth
(of wood, if the formation of phloem and pheloderm is disregarded) did not show a consistent
decrease in the weakened trees, except for the
first year after the cork stripping (Figure 2).
The seasonality of radial increase was also
influenced by the cork stripping damage, with
weakened trees showing about 1 month delay in
the onset of spring growth and the occurrence of
increment peaks (Figure 3), the effect being
higher for the more extensive damage (Figure 4).
This response of cork oaks to stress was previously reported regarding the influence of adverse
environmental factors on the timing of
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F O R E S T RY
phenological events (Pereira et al., 1987; Oliveira
et al., 1994; Oliveira, 1995) as well as of cork
stripping on bud burst and radial increments
(Fialho et al., 2001).
Of the initial sample of 50 healthy cork oak
trees in 1991, 20 per cent were classified as
weakened due to stem wounding after one cork
production cycle. Although the experiment was
not designed ab initio to study the incidence of
decline diseases with time and no conclusions are
to be drawn on this matter, it is nevertheless noteworthy that a significant number of trees showed
consequences of the previous cork stripping. This
calls attention to the need of careful cork extraction procedures as frequently stressed in the past
by different authors (Natividade, 1950; Pereira et
al., 1999).
In conclusion, the operation of cork stripping
could represent a potential danger of tree
wounding that will reduce tree diameter and cork
growth, especially in the years immediately
following the injury. Although cork oaks survive
many years despite suffering continuous harm by
stripping and consequent biotic attacks (Natividade, 1950), their longevity as productive trees
will depend on the protection mechanisms they
develop. Expertise and careful cutting procedures
in cork stripping are therefore essential elements
to guarantee the sustainable management of cork
oaks in cork production.
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Received 2 July 2002