Eur J Forest Res
DOI 10.1007/s10342-007-0180-8
ORIGINAL PAPER
Cork oak (Quercus suber L.) wood growth and vessel
characteristics variations in relation to climate and cork
harvesting
Sofia Leal Æ Elsa Nunes Æ Helena Pereira
Received: 25 August 2006 / Revised: 17 January 2007 / Accepted: 24 April 2007
Springer-Verlag 2007
Abstract Variations in tree ring growth of Quercus suber
L. were analysed using dendrochronological techniques on
cork oak discs from trees harvested in the cork producing
region of Alentejo, Portugal. A tree-ring chronology containing a strong common signal and covering the period
from 1970 to 1995 was build for ca. 30-year-old cork oaks
never submitted to cork harvesting using 14 trees that
crossdated satisfactorily out of 30 sampled trees. The tree
ring indices correlated positively with September temperature (r = 0.48, P < 0.05) and very strongly with precipitation totals from previous October until current February
(r = 0.82, P < 0.001) showing that the water stored in the
soil during the autumn and winter months prior to the
growing season has a primordial effect on the growth of the
given season. The effects of cork harvesting were analysed
by comparing mean ring width, mean annual vessel area,
vessel density (nvessels/mm2), and vessel coverage (percentage of transverse surface occupied by vessels) between
three mature cork oak trees and three young trees, for the
period from 1987 to 1996, corresponding to the growth
between two consecutive cork removals in the case of
mature trees. In 1988, 1989 and 1996 (corresponding to the
first and second years after cork removal, and 1996 to a
year of cork removal), the ratios between ring widths of
young versus mature trees was twice that for the rest of the
period. However, an effect of cork removal indicated by
eventual alterations in vessel size and distribution in the
Communicated by Rainer Matyssek.
S. Leal (&) E. Nunes H. Pereira
Centro de Estudos Florestais, Instituto Superior de Agronomia,
Tapada da Ajuda, 1349-017 Lisbon, Portugal
e-mail: spleal@yahoo.co.uk
wood rings corresponding to the years 1988, 1989 and
1996 in the mature cork oaks was not observed.
Keywords Quercus suber L. Wood anatomy
Wood vessels Tree rings Dendrochronology
Precipitation Cork harvesting
Introduction
Cork oaks (Quercus suber L.) are usually found in agroforestry systems that are characterized by sparse forests
exploited for cork production, in association with agricultural crops and pasture for cattle grazing. These systems
are common in south-western Europe, in Portugal and
Spain, where they are called respectively montado and
dehesa (Pereira and Tomé 2004).
The Quercus suber is a species known for producing
cork as a thick outer layer in the bark around stem and
branches. This cork layer can be removed since the tree
is capable of forming a new cork bark by adding new
layers of cork every year, therefore allowing a sustained
cork production during the cork oak lifetime. Cork removal is performed for the first time when the tree is
around 25–30 years old (with a breast height perimeter
above 70 cm) and afterwards every nine or more years in
order to obtain a cork with a thickness adequate for
industrial processing. Cork is used for several industrial
purposes, but mainly for production of cork stoppers for
wine bottling.
The research on cork oak has been so far focused on
issues related to cork production and not much has been
investigated on the tree growth and stem wood characteristics, namely on the effect of climate and cork removal on
wood growth.
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Eur J Forest Res
Cork oak growth is sensitive to climate and its seasonal
changes, mostly to precipitation (Costa et al. 2001). The
effect of precipitation has been more thoroughly studied in
relation to the cork and good correlations were reported
between fall and winter cumulated rainfall and cork ring
width in mature trees under cork production in several
regions (Caritat et al. 2000; Costa et al. 2001). For wood
growth, a similar relationship was reported albeit it referred
to only one tree (González-Adrados and Gourlay 1998).
Cork removal has an immediate effect of water loss by
the exposed surface of live tissues with the subsequent
response of stomata closure, leading to interruption of the
nutritional functions, only returning to normal after 24–
30 days following the regeneration of the phellogen and
production of the first thin layers of cork (Santos 1940;
Oliveira 1995). The activity of the vascular cambium decreases and the wood growth stops (Natividade 1938).
Wood anatomy is also affected by the cork removal
resulting in an irregular distribution of vessels and also loss
of typical earlywood/latewood transition (Lupi 1938).
The effect of cork removal on tree growth has been
studied by measuring over cork stem radial growth, i.e. with
dendrometers, and for very young cork oaks no significant
effect was reported (Fialho et al. 2001). However the major
component of the tree growth in over-bark diameter corresponds to the increments in cork (Costa et al. 2001), which
increase in the 2–3 years following cork removal (Caritat
et al. 1996; Ferreira et al. 2000; Costa et al. 2002b), while
wood growth is low and shows narrow and undefined
annual rings (Lupi 1938; Gourlay and Pereira 1998).
One of the possible reasons for the few studies on wood
growth in cork-oaks lies in the difficulty of the sampling of
trees for wood ring measurements since strict legal
restrictions apply to the harvest of cork oaks and felling
authorisations of living trees are given only in a few situations, namely when public interest issues are involved.
The number of healthy productive trees available for study
is therefore very limited.
This paper reports the results of a dendroclimatological
analysis of the effects of climate on wood annual growth in
young cork oaks. The effect of cork removal on wood
growth and anatomy was also analysed through a comparative study between mature cork oaks in full cork production and younger cork oaks before the first cork
extraction, where wood ring widths and vessel characteristics were measured during a full 9-year cork production
cycle.
Materials and methods
The cork oak trees used for this study come from authorized fellings carried out in the cork production region to
123
the south of river Tagus, in the Alentejo region of southern
Portugal. Figure 1 shows the location of the Alentejo region (administrative borders) as well as the northern climatic border given by the Tagus river (Alentejo means
beyond the Tagus river).
The climate in the region is described using the data
from nine meteorological stations (Alcácer do Sal, Alvalade, Beja, Elvas, Évora, Mértola, Pegões, Portalegre, and
Viana do Alentejo, Fig. 1), which are considered by the
National Meteorology Institute as the ones that best characterise the climate of the Alentejo region. Figure 2 shows
the mean temperature and rainfall distribution for the
region as the mean values of these stations. The climate is
typically mediterranean, with the highest mean temperatures occurring in July and August (~24C), when the
precipitation is close to zero, and a dry season extending
from May through September. The average annual temperature for the region is 16C and the average annual
precipitation is 600 mm.
The harvested trees were isolated trees without betweentree competition from low-density stands typical of the
Fig. 1 Location of the study sites and climatic stations (black
circles), with the Alentejo administrative region borders (dashed
lines) and the river Tagus that climatically defines the study region
(grey area)
45
90
40
80
35
70
30
60
25
50
20
40
15
30
10
20
5
10
0
Jan Feb Mar Apr Mai Jun
Temperature
Jul
Precipitation (mm)
Temperature (°C)
Eur J Forest Res
0
Aug Sep Oct Nov Dec
Precipitation
Fig. 2 Climatic diagram for the study region
montado system. They were harvested from three locations
(Fig. 1) as described below.
(Holmes 1994) was used to execute the standardisation and
the most conservative approach, the negative exponential
curve, was applied to our data because it works well with
trees from open-canopy stands where growth is more-orless undisturbed (Cook and Briffa 1990b), which is the case
of the present study. The obtained tree-ring indices for the
several trees were averaged together by calendar year
producing one tree-ring chronology.
The following statistical indicators (Cook and Kairiukstis 1990a) were used to assess the signal strength, or
amount of common variability present in all trees, since a
high common signal in a site indicates that common
environmental factors, such as climate, control tree growth:
1.
Effects of climate on wood growth
A total of 30 trees were harvested in VLV—Herdade da
Tapada Real, Vila Viçosa, Évora (Fig. 1). The trees were
around 30 years old, never submitted to cork harvesting,
and presented good vitality and phytosanitary conditions.
One wood disc with approximately 5 cm of thickness was
cut at 1.30 m of tree height.
After an acclimatisation period of 4 weeks in a chamber
at 25–30C, the wood discs were sanded through several
successively decreasing grades of grit in order to remove
surface irregularities that could obstruct the identification
of tree ring limits. Ring boundaries were observed at 70·
magnification under a low power microscope, and marked
in each wood disc along two radial directions at a minimum
of 90 of distance. The patterns in the sequences of tree
ring were graphically represented using skeleton plots
(Stokes and Smiley 1968). The tree rings were dated and
missing or false rings were identified by crossdating, first
the two plots from the same tree, and then between plots
representing different trees.
The ring widths from successfully crossdated trees were
measured to the nearest 0.01 mm using a Bannister increment measurer. A final crossdating quality control was
preformed on the tree-ring series with COFECHA (Holmes
1994), a computer program designed to identify possible
problems and measurement errors. Each pair of measurement series from one tree was averaged to minimise within
tree variation, creating mean-tree series of measurements.
Standardisation was used to isolate the contribution of
climate by removing the influence from other factors, such
as disturbances, tree’s individual variability or age-dependent decreasing trend in ring width. This consisted in fitting
a curve to the growth trend of each tree-ring series, and
dividing the tree-ring widths by the values of this fitted
curve creating a series of dimensionless tree-indices with
means of 1.0 (Fritts 1976). The program ARSTAN
Effective chronology signal (reff), a chronology signal
estimate based on correlation coefficients which
incorporate both within- and between-tree signals
reff ¼
rbt
rwt þ 1rc wt
where rwt is the within-tree signal calculated as the average
of the correlation coefficients between index series from
the same tree for all the trees, rbt is the between-tree signal
obtained by averaging the correlation coefficients calculated for all the possible pairs of series drawn from different trees, and c is the number of cores per tree.
2. Mean Sensitivity (msx), a measure of how sensitive a
tree-ring series is, i.e. how much do tree ring widths
vary in comparison with previous years
msx ¼
X 2ðxtþ1 xt Þ
1 t¼n1
n 1 t¼1
xtþ1 þ xt
where xt and xt + 1 are the ring widths of a series in a given
year and in the following, and n is the number of years.
Single values of the sensitivity range between zero, when
there is no difference between consecutive rings, and two,
when a zero value occurs next to a non-zero;
3. Expressed population signal (EPS) quantifies to which
degree the chronology signal is expressed when series
are averaged
EPS ¼
4.
reff
reff
þ 1rn eff
Signal-to-noise ratio (SNR), an expression of the
strength of observed common signal between trees
SNR ¼
nrbt
ð1 rbt Þ
The relationships between the tree-ring chronology and
the time-series of climate variables (average monthly
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Eur J Forest Res
temperatures and monthly precipitation totals from September, from previous year, to October, from current year)
were analysed using Pearson correlations calculated over
the period 1970–1995. A linear regression model was applied to estimate the climate variable showing the best
agreement with tree-ring indices using the chronology treering indices as independent variable.
Effects of cork harvesting on wood growth
and anatomy
Six trees, including three mature trees under full cork
production with an age above 60 years and three young
trees from which cork was never removed and with an age
around 35 years, were collected respectively from HC,
Herdade da Carvoeira, Benavente, Santarém; and PV,
Pavia, Mora, Évora (Fig. 1).
From each tree, one stem disc was taken at 1.3 m
height. Radial strips, free of damaged areas, covering the
whole wood growth on the radial direction (from pith to
bark) were cut from each disc. Per disc three radial strips
and two radial strips, respectively from the mature and
the young cork oaks, were prepared for measurements.
Each strip was split axially in two paired sub-strips, one
kept for ring measurements and the other for vessel
measurements. The measurements were performed for all
the trees on the sections of the radial strips corresponding to the years between 1987 and 1996. This 10-year
period is situated between the two last subsequent cork
removals in the mature trees (respectively in 1987 and
1996). For each calendar year, mean tree ring-width
values were calculated for each type of tree (non-debarked and debarked).
The sub-strips for ring measurements were polished on a
belt sander, through three grades of grit and the tree rings
were marked on the wood with a thin pencil by observation
with a low power microscope. Ring width measurements
were made on the images obtained from the samples, at
25· magnification, with a low power microscope (Olympus
SZH10) connected to a video camera (JVC model TKC1380E), using the image analysis software LEICA Qwin.
The transversal surfaces of the radial sub-strips for
vessel measurements were sanded manually, through three
grades of grit, and rubbed with a white wax crayon to
impregnate the vessels in order to distinguish them from
the rest of the tissues under a low power microscope. The
adopted protocol proved the best for cork oak wood after
trying out several methods of surface cutting, smoothing
and vessel enhancement.
Sequential images were collected from the wood cross
sections, using a low power microscope (Olympus SZH10)
with a video camera (JVC model TK-C1380E), and stored
in JPEG graphic format using the image analysis software
123
LEICA Qwin. With a total magnification of 30·, each
image (measuring frame) covered 2,612 lm on the radial
direction and 1,592 lm on the tangential direction. The
section of the radial strip under study was covered by the
successive images. The images were converted to binary
format and, after applying threshold and minimum size
settings, vessels were clearly identified as separate objects.
In each measuring frame the area of each vessel was
automatically recorded. The average vessel area, vessel
density (number of vessels per square millimeter), and
vessel coverage (percentage of cross sectional area covered
by vessels) were calculated for each growth ring. For each
calendar year, mean vessel variables values were calculated for each type of tree (non-debarked and debarked).
The two types of trees were compared by calculating,
for each calendar year, the ratios between non-debarked
and debarked trees of the mean values of the wood variables (ring-width, vessel area, vessel density, vessel coverage).
Results
Effects of climate on wood growth
During the period from 1967 to 1995, the ring widths from
the trees used for chronology construction averaged
2.50 mm, varying between an average minimum of
0.78 mm and an average maximum of 5.76 mm. Tree ring
widths tend to decrease with cambial age from an average
of 3.27 mm in the beginning of cambial activity to an
average of 1.06 mm at 26 years of age. Figure 3 shows an
example of one series of tree-ring measurements from
VLV site and the resulting tree-ring indices after standardization. The VLV tree-ring chronology (Fig. 4) was
built using only 14 out of the 30 sampled cork oak trees
because the remaining trees did not crossdate satisfactorily.
Basic statistical indicators of chronology signal strength
are presented in Table 1 and they indicate a strong common signal.
The correlation analysis between tree-ring indices and
climatic variables reveals a strong positive correlation
(r = 0.82, P < 0.001) between tree growth and the precipitation accumulated before the start of the growing
season, i.e. precipitation totals from previous October until
current February; and a positive correlation (0.48,
P < 0.05) between tree growth and September average
temperature (Table 2).
Precipitation estimates based on a regression model
using tree-ring indices as predictors show a very good
overall agreement with the actual precipitation totals that
occurred from previous October until current February
(Fig. 5).
Eur J Forest Res
Table 2 Correlation coefficients between the VLV tree ring chronology and series of mean temperature and precipitation for several
months
3
Season
Previous year
1
1995
1993
1991
1989
1987
1985
1983
1981
1979
1977
1975
1973
1971
1969
1967
0
Ring index
September
0.09
NS
0.18
NS
October
0.02
NS
0.36
NS
November
–0.29
NS
0.38
NS
December
0.33
NS
0.42
P < 0.05
January
February
–0.07
0.24
NS
NS
0.44
0.45
P < 0.05
P < 0.05
March
–0.10
NS
–0.11
NS
April
0.00
NS
0.02
NS
May
–0.08
NS
–0.05
NS
June
–0.37
NS
0.10
NS
July
–0.17
NS
0.26
NS
August
–0.19
NS
–0.25
NS
September
0.48
P < 0.05
–0.38
NS
October
0.08
NS
0.25
NS
December– –
February
–
0.68
P < 0.001
October–
February
–
0.82
P < 0.001
Current year
2
1
1995
1993
1991
1989
1987
1985
1983
1981
1979
1977
1975
1973
1971
1969
1967
0
Year
Fig. 3 Example of one series of cork oak tree ring measurements
before standardization, raw tree ring widths (upper graph), and after
standardization, tree ring indices (lower graph)
2
Ring index
Temperature Significance Precipitation Significance
2
–
NS not significant
1
1995
1993
1991
1989
1987
1985
1983
1981
1979
1977
1975
1973
1971
1969
1967
0
Precipitation (mm)
Tree ring width (mm)
4
700
600
500
400
300
200
100
0
1970
Year
1975
1980
1985
1990
1995
Year
Fig. 4 Cork oak tree ring chronology for the VLV study site
Measured precipitation
Table 1 Basic statistical indicators of chronology signal strength
msx
reff
EPS
SNR
0.267
0.349
0.866
5.27
Estimated precipitation
Fig. 5 Real autumn and winter precipitation, from October prior to
the growing season until February, versus the precipitation estimated
using a climate-growth model; data and calculations for the years
1970–1995
Msx Mean Sensitivity, reff Effective chronology signal, EPS Expressed
Population Signal, SNR Signal-to-Noise Ratio
Effects of cork harvesting on wood growth
and anatomy
The cork oaks showed a semi-diffuse porosity. The difference in vessel size between earlywood and latewood
was higher in the young cork oaks therefore allowing a
better ring definition. The identification of wood rings was
difficult in the mature trees under cork production, and a
careful visual observation on the several radii of each stem
disc and the superposing of the curves were necessary.
Overall, during the period from 1987 to 1996, the wood
ring width ranged between 1.34 and 6.20 mm, averaging
2.61 ± 1.12 mm, for the young trees, and between 0.68 and
3.20 mm, averaging 1.63 ± 0.68 mm, for the mature trees.
Vessels were larger and more numerous in the young trees.
In the 1987 to 1996 period, the vessel area averaged
23,758 ± 4,851 lm2 and 17,827 ± 3,390 lm2 respectively
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Eur J Forest Res
3,5
ratio Young/Mature
for young and mature trees, and the vessel density
6.00 ± 1.74 vessels/mm2 and 5.25 ± 1.04 vessels/mm2
respectively (Fig. 6). The vessel coverage, which represents the conductive area, was lower in the mature cork
oaks in comparison with the young trees (9.3 ± 2.0% vs.
13.9 ± 4.2%).
With the exception of 1988, 1989 and 1996, the values
for ring-width, mean vessel area, vessel density, and vessel
coverage were on average 1.4 times larger in the case of
young trees (Fig. 7). In these years, the growth rings of
mature trees reached the lowest increments (on average
1.10, 1.02, and 1.19 mm, respectively, in 1988, 1989, and
1996) and were much thinner when compared with the
growth of the young trees in the same years (on average,
young trees ring widths were 2.8 times larger). The years of
1988 and 1989 corresponded to the first and second years
after cork removal, and 1996 to a year of cork removal. As
regards the wood anatomy, an effect of cork removal as
indicated by eventual alterations in the vessel characteris-
3
2,5
2
1,5
1
0,5
0
1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
Year
Ring width
Vessel mean area
Vessel density
Vessel coverage
Fig. 7 Ratios between young and mature trees’ mean values for the
ring variables (ring width, vessel area, vessel density, and vessel
coverage), along a 10-year period comprised between two cork
extractions
tics in the wood rings corresponding to the years 1988,
1989 and 1996 in the mature cork oaks was not observed.
However the mature cork oaks with cork extractions
showed smaller vessels and less gradation of vessel size
within the ring, resulting in poorly defined rings.
Vessel area (um2)
27000
25000
Discussion
23000
21000
19000
17000
15000
13000
Vesseldensity(n°/mm2)
1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
8
7
6
5
4
Vessel coverage (%)
1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
18
16
14
12
10
8
6
1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
Year
Fig. 6 Variations in mean vessel area (upper graph), vessel density
(middle graph), and vessel coverage (lower graph), along a 10-year
period comprised between two cork extractions, for young (black
lines) and mature (grey lines) cork oak trees
123
The knowledge on a species’ growth characteristics and on
effects that climatic variables and silvicultural management
decisions have on tree growth is obviously a key issue for
assessing and preserving the sustainability of the forests.
This is more so when the trees are managed in a sustained
exploitation of non-wood components that may have a
detrimental effect on the tree growth, and when the forests
are in ecologically sensitive areas and balance between
several multifunctional options. This is the case with the
cork oak (Quercus suber) which is exploited in agroforestry systems in the prevalently poor regions of southern
Europe and North Africa with successive cork extractions
during the tree lifetime that may be a cause of growth
decline (Pereira and Tomé 2004).
However, very little is known on cork oak growth,
namely on the wood component of the tree growth, e.g. the
cambium activity, and nothing has been published on the
quantification of the effects of climatic factors and cork
removal on it. The existing growth and yield models for the
cork oak are based on over-bark diameter growth and on
cork growth (Tomé et al. 1998; Pereira and Tomé 2004;
Sánchez et al. 2005, 2006) and most modelling efforts were
focused on predicting cork weight production (i.e. Ferreira
and Oliveira 1991; Fonseca and Parresol 2001; Ribeiro and
Tomé 2002; Vásquez and Pereira 2005).
The wood ring identification and width measurement
made in the present study were difficult tasks due to the
fact that Quercus suber rings are hard to distinguish,
Eur J Forest Res
varying in width along the tree perimeter and with the
possible presence of false rings. The use of other techniques such as microdensitometry does not solve this
problem, as also reported by Knapic et al. (2007). Observation of wood discs seems therefore a requirement for ring
width studies in mature cork oaks, confirming the results
reported by Gourlay and Pereira (1998). This fact adds
extra difficulties to perform cork oak wood growth studies
due to the legal restrictions on tree harvesting. Therefore,
information on wood growth measured in cork oak stem
discs is valuable even if the number of samples available is
not as high as in conventional ring analysis.
Effects of climate on wood growth
Cork oak trees grow well with annual rainfalls between 500
and 1,600 mm, 400 mm being the minimum required
(Natividade 1950). There was an overall good agreement
(r = 0.82, P < 0.001) between precipitation totals from
previous October to current February and ring indices in
young cork oaks; September temperature had also a positive effect on tree growth (r = 0.48, P < 0.05) but not as
strong as precipitation (Table 2 and Fig. 5). Other authors
have reported wood growth in Quercus suber to show interannual fluctuations related with variations in cumulated
precipitation (González-Adrados and Gourlay 1998; Costa
et al. 2001). Other oak species growing in Portugal (Nabais
et al. 1999) and Spain (Garcia-González et al. 1997) were
shown to provide clear climatic signals as well. In semiarid regions in general and particularly in some geographical regions like Spain, North Africa, southeast USA
and China, tree-ring growth is highly correlated with
rainfall (e.g. Richter and Eckstein 1990; Till and Guiot
1990; Stahle and Cleaveland 1992; Hughes et al. 1994).
The influence of water supply on tree growth is well
studied (e.g. Kozlowski et al. 1991). The quantity as well
as the quality of wood formed annually is strongly affected
by water supply (Fritts 1976). In the case of the present
study, the water stored in the soil during the autumn and
winter months prior to the growing season has a primordial
effect on the growth of the trees during the given season.
Effects of cork harvesting on wood growth
and anatomy
The growth period studied here corresponded to a 10-year
period between two cork extractions (1987 and 1996)
which represents the so-called 9-year cork production cycle
extending between two consecutive cork extractions (the
year of cork extraction is counted as a half-year in cork
production). The effect of cork removal on growth was
therefore searched for on the wood rings of these years as
well as on the following years (1988 and 1989).
Indirect calculations have estimated an average radial
wood increment of 1.3 mm.year–1 in one 8-year period
following a cork extraction in mature cork oaks in full cork
exploitation (Costa et al. 2002b) and the only published
references to direct wood ring width measurements on cork
oak stem discs refer a mean annual growth of 4.2 mm year–1
in the first 30 years of tree growth (Knapic et al. 2007)
and values ranging from 1 to 4 mm.year–1 in mature cork
oaks (Gourlay and Pereira 1998; González-Adrados and
Gourlay 1998).
The comparison of the mean annual individual growth
curves for the mature trees with those for the young trees
showed that the main differences appeared in the years of
cork removal and in the following years (Figs. 6, 7). In the
mature cork oaks the ring width was very low in 1988 and
1989 (1.10 mm and 1.02 mm), the years following the cork
removal in 1987, and the same happened in 1996
(1.19 mm), corresponding to a year of cork removal.
However, the years of 1987, 1988 and 1996 had high
precipitation, leading to wide rings in the young cork oaks
(3.04, 3.56 and 3.39 mm, respectively) and 1989 had an
average precipitation with an average ring width
(2.30 mm). Obviously, cork removal decreases the wood
growth in the year of cork removal or in the following
years, which was not overcome by favourable water conditions. This confirms reports about a marked decrease in
wood ring width two to three years after cork removal
(Lupi 1938; González-Adrados and Gourlay 1998; Gourlay
and Pereira 1998) and about an increase in cork growth
during this period which is said to affect wood growth
negatively (Natividade 1950; Caritat et al. 1996; Ferreira
et al. 2000; Costa et al. 2002b).
However, a wood growth reduction was not observed in
the year of cork removal in 1987. The reason is not known,
and details on the history of the trees in that period were
not recorded, namely the timing of the cork extraction. In
fact extraction of cork is made between June and August,
and studies on the seasonality of tree growth have shown
that tree radial growth is highest in early spring (Costa
et al. 2002a). Therefore a late cork extraction, i.e. in
August, would have little influence upon the tree growth in
that year. It can be speculated that the timing of cork
removal will impact on the wood growth of that year.
The effect of cork removal on cork oak wood growth
was estimated by comparing the ratios of wood ring width
in mature and in young trees for the period 1990–1995
(free of cork removal stress) and for the years of 1988,
1989 and 1996, when this stress effect was recorded
(Fig. 7). These ratios averaged 0.84 in the stress-free years
and 0.37 in the stress years (0.31, 0.44 and 0.35 in 1988,
1989 and 1996, respectively). This comparison allows
estimating an annual reduction of wood growth (ring
width) due to extraction of cork of approximately 57% in
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Eur J Forest Res
those years. Considering that the cork removal stress effect
would be registered in 2 years of the 9 years of the cork
production cycle, this allows a rough estimate of a reduction of wood growth in that cycle of 13%. Should the effect
extend to 3 years (e.g. including the year of cork removal
and the next 2 years), the reduction in wood growth would
be 19%.
The reduction in wood radial growth has a practical
consequence of reducing the tree perimeter and therefore
the surface area that can be used for cork production
(stripped surface) and therefore the obtainable cork weight.
This is due simultaneously to the perimeter reduction as
well as the allowed stem height for cork extraction that is a
legally enforced factor (debarking coefficient) based on
tree perimeter (Pereira and Tomé 2004). This should be
taken into account on long term cork yield modelling and
the timing of the first cork extraction and the periodicity of
subsequent extractions should be economically balanced
with this evidence.
Vessel variables in young trees vary within the range
reported by Leal at al. (2006). The wood anatomy of adult
cork oaks undergoing cork extractions clearly differed
from that of young trees without any cork extraction.
Vessels of adult cork oaks were 25% smaller and the size
difference between earlywood and latewood vessels was
lost to a large extent (Fig. 6). This means that the tendency
of vessels to become larger with tree age (Gasson 1987;
Voulgaridis 1990; Leal et al. 2006) did not occur in this
case (considering that the young trees had 33–37 years and
the mature trees 63–89 years). This may be the tree’s
response to the successive removals of cork, as already
referred to by González-Adrados and Gourlay (1998) and
Gourlay and Pereira (1998). However an immediate effect
was not noticed and no differences in vessel size and
density were detected on the wood rings corresponding or
following the cork extraction (Fig. 7), although it has been
reported that latewood anatomy is affected on the year of
cork removal producing smaller cells (Lupi 1938).
Conclusions
Cork oak tree ring growth in young trees is strongly controlled by the precipitation during the months prior to the
growing season, i.e. previous October until current February. It is the water available at the beginning of the growth
season, which was stored in the soil during the autumn and
winter months, that determines the growth in a given year.
Cork oak wood growth was reduced by the effect of cork
extraction and smaller ring widths were formed. The
impact of cork removal on the ring width may extend from
the year of extraction to the two following years. The wood
anatomy, as regards vessel size and distribution, of mature
123
cork oaks undergoing successive cork extractions differed
from young trees before cork extraction by smaller vessels
and conductive area. Therefore, in long-term tree growth
and forest sustainability studies, the impact of the cork
removal on the wood growth should be considered.
Acknowledgments Part of this study was made within the European project Suberwood (QLK5-CT-2001-00701), under the 5th
Framework Programme. We thank Alexandra Ferreira for help with
the preparation and measurements of some of the samples; Sofia
Knapic for help in data processing; the Instituto de Meteorologia, in
the person of Fátima Espı́rito Santo, for providing the climatic data;
David D. Stahle and Malcolm K. Cleaveland for making available the
facilities in the Tree-Ring Laboratory, University of Arkansas, and for
the useful advices. The first author acknowledges a scholarship by
Fundação para a Ciência e Tecnologia (Portugal), the second author
acknowledges financial support from J.N.I.C.T. (Portugal), Fundação
Calouste Gulbenkian (Portugal) and Fundação Luso-Americana.
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