VARIATIONS OF WOOD ANATOMY AND &13C WITHIN-TREE RINGS OF COASTAL PINUS PIN ASTER SHOWING INTRA-ANNUAL DENSITY FLUCTUATIONS - Brill
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IAWA Journal, Vol. 28 (1),2007: 61-74
VARIATIONS OF WOOD ANATOMY AND &13C WITHIN-TREE RINGS
OF COASTAL PINUS PINASTER SHOWING INTRA-ANNUAL
DENSITY FLUCTUATIONS
Veronica De Micco 1, Matthias Saurer 2 , Giovanna Aronne l , Roberto Tognetti 3
and Paolo Cherubini 4
SUMMARY
We investigated the variation of wood anatomical characteristics and
carbon isotopic composition of tree rings showing intra-annual density
fluctuations (IADFs) in plants of Pinus pinaster Ait. growing at a coastal
plantation in Tuscany (Italy). IADFs are regions of the tree ring where
wood density changes abruptly due to a sudden change of environmental
conditions, particularly of water availability. Dendrochronological
analyses allowed dating of the rings and four regions were considered
in each tree ring: earlywood, IADF, late-earlywood and latewood.
Although IADF commonly has been classified as latewood-like tissue
in the literature, we found differences in anatomical characteristics and
carbon isotopic composition between tracheids of the two regions. The
lumen area of tracheids in IADF was significantly larger than in latewood,
while still smaller than in earlywood and late-earlywood. Latewood and
IADF had a greater proportion of narrow tracheids than both earlywood
and late-earlywood. Although latewood and IADF were characterized
by tracheids with lumina lengthened in the tangential direction, while
earlywood tracheids were elongated in the radial direction, some dif-
ferences were found also between latewood and IADF. Moreover, IADF
tracheids had a higher 13e f I2 e ratio than any other region and showed
isotopic values significantly different from the latewood. The quan-
tification of anatomical features of tracheids within rings was useful to
discriminate between latewood and IADFs, as well as helpful for the
identification of tree-ring boundaries. The overall interpretation of den-
drochronological, wood anatomical and carbon isotopic data seems to be a
promising approach for the dating and the ecological interpretation of tree
rings in Mediterranean ecosystems and for gaining climatic information
with intra-annual resolution.
Key words: Dendroecology, false rings, Pinus pinaster, stable carbon iso-
topes, wood anatomy.
1) Laboratorio di Botanica ed Ecologia Riproduttiva, Dip. Arboricoltura, Botanica e Patologia
Vegetale, Universita degli Studi di Napoli Federico II, Via Universita 100,1-80055 Portici (NA),
Italy [E-mail: dernicco@unina.it].
2) Paul Scherrer Institute (PSI), CH-5232 Villigen, Switzerland.
3) Scienze e Tecnologie per I' Ambiente e il Territorio (STAT), Universita degli Studi del Molise,
1-86090 Pesche (IS), Italy.
4) WSL Swiss Federal Institute for Forest, Snow and Landscape Research, CH-8903 Birrnensdorf,
Switzerland.
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INTRODUCTION
Variations in annual tree-ring width and density have been used for reconstructing
past climates, and tree-ring information is commonly applied as a proxy estimate for
seasonal integration of temperature and precipitation effects with annual resolution
(Hughes 2002). It has been suggested that global climatic change influences woody
plant phenology (Penuelas et al. 2002), and a higher temporal resolution would be
suitable to derive specific intra-seasonal climatic information from ring-width records.
Summer temperatures for the past millennium have been reconstructed using tree rings
from high-latitude forests (Briffa et al. 2004), whereas past severe droughts have been
tracked in arid and semi-arid regions (e.g., Stahle et at. 1985; D'Arrigo & Jacoby
1991). Mediterranean-type ecosystems experience erratic weather conditions and
during dry periods rainfall may boost tree growth rate, while in wet periods growth
rate variations may strongly depend on temperature fluctuations (Attolini et al. 1990),
which induce typical growth patterns with distinct variation in the appearance of annual
rings (Cherubini et al. 2003).
In Mediterranean environments, trees may form intra-annual density fluctuations
(IADFs), also called "false rings" or "double rings" (Tingley 1937; Schulman 1938).
IADFs are usually induced by sudden drought events, occurring during the vegetative
period, that cause the formation of wood cells with smaller lumina and thicker walls
in comparison with cells formed before and after the stress. For their anatomical
appearance, elements of the IADFs are usually referred to as latewood-like cells (Tingley
1937; Schulman 1938; Reed & Glock 1939; Villalba & Veblen 1996; Rigling et al.
2001; Leavitt et ai. 2002). Such rings may hamper the dating of tree-ring series, the
cross-dating and any further analyses of tree growth (for a review, see Cherubini et at.
2003). Nevertheless, allowing intra-annual resolution, IADFs may provide detailed
information at a seasonal level. IADFs have been already suggested for understanding
ecological characteristics, such as sensitivity to drought (Rigling et at. 2002), and to
reconstruct changes in seasonal precipitation that reflect a dry period between two
wet periods during early summer (Wimmer et al. 2000; Rigling et al. 2001, 2002).
However, little is known about the ecophysiological processes which induce wood-
anatomical structures to resemble false tree rings in trees from Mediterranean-type
ecosystems, although such knowledge would be of significant help for ecological and
climatological purposes.
The stable carbon isotopic composition of tree rings is used as an indicator of grow-
ing conditions. Since cellulose is not transferred between annual growth rings, intra-
and inter-annual seasonal events are recorded permanently in the tree-ring ()13C signal.
Leavitt and Long (1989) related inter-annual variations in the carbon isotope discrimi-
nation in tree rings to average growing-season temperature and precipitation. Leavitt
(1993) indicated that variations in carbon isotope discrimination across single tree rings
in pine and maple were related to changes in soil water during the growing season.
Schulze et at. (2004) showed that re-occurring seasonal 13C-patterns are observed in
conifers, with low values in earlywood, maximum values in mid-season and declining
values in the latewood. In a recent study on summer monsoon activity, "false-latewood
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bands" (i.e. IADFs) were used as midseason markers (Leavitt et al. 2002). Carbon
stable-isotope ratios of IADFs in tree rings of Pinus ponderosa P. & C. Lawson from
southern Arizona and New Mexico were used to reconstruct past droughts (Leavitt
et al. 2002). IADFs had less negative ()13C values than earlywood, and latewood had
the lowest (most negative) values. Nevertheless, no explanation was given for ()13C
ratios of IADFs being so high in comparison with ()13C values of latewood.
In this paper, we report on properties of tree rings of Pinus pinaster Ait. from a Medi-
terranean forest stand because of the frequent occurrence of IADFs. We describe the
wood-anatomical characteristics and carbon stable-isotope composition of different
tree-ring sectors, IADF and latewood included, in order to a) quantify anatomical dif-
ferences between latewood and IADFs and evaluate whether detailed anatomical
analyses can help in distinguishing latewood from IADFs, b) examine the variations
of carbon isotopic composition along the rings showing IADFs, and c) interpret all data
to hypothesize the environmental conditions which induce the formation of IADFs.
MATERIALS AND METHODS
Study site
The study site (43° 13' 41" N, and 10° 17' 04" E) is located on the flood plain formed
by the Amo and Serchio rivers close to Pisa (Tuscany, Italy), along the Italian west coast.
This region is flat, with alluvial sandy soils, prone to saltwater intrusion during droughts
due to groundwater extraction.
The forest of San Rossore is a typical coastal Mediterranean pinewood, with a periph-
eral protection belt of Pinus pinaster, a species fairly tolerant to salty winds, and inland
stands formed by Pinus pinea L. These are the most important productive plantations
of P pinea in Europe, well known mainly for the long tradition in high-quality produc-
tion of edible seeds (pine nuts) exported world-wide (Peruzzi et al. 1998). Quercus
ilex L. and other typically Mediterranean species are present in the understorey. The
trees used for this study were sampled in a homogeneous stand dominated by P pin-
aster, that were planted after a destructive fire (that occurred in the year 1944). Forest
management plans and interviews with local foresters revealed that neither harvesting
nor thinning were carried out in this stand since planting. The average stand height is
18 m, the average diameter at breast height (DBH) is 29 cm, and stand density is 565
trees ha- 1 (84% P pinaster, 12 % P pinea and 4% Q. ilex).
Meteorological data (daily minimum and maximum temperatures, and precipitation)
for the period 1950-2000 are available from the meteorological station of the National
Hydrological Service located in Pisa, approximately 10 km from our site. The climate
is Mediterranean sub-humid. The average yearly temperature is 14.8 °c and rainfall is
900 mm. A drought usually occurs in correspondence with annual maximum tempera-
tures, generally in mid-summer (July-August).
Dendrochronological measurements
In March 2003, twenty Pinus pinaster trees in the stand were cored at 1.3 m height
with an increment corer 0.5 cm in diameter. Two wood cores were taken at 120° to each
other, taking care to avoid compression wood. Cores were transported to the labora-
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tory, carefully mounted on channeled wood in order to obtain a transversal section,
seasoned in a fresh-air dry store and sanded a few weeks later. All the tree rings were
dated, although some of the cores had many IADFs, and thus were difficult to date.
Ring-width measurements were made to the nearest 0.01 mm, using a measuring table
(Lintab) coupled with the TSAP software package (Time Series Analysis and Presenta-
tion) (both provided by Frank Rinn, Heidelberg, Germany). The raw ring widths of the
single curves of each dated tree were plotted, cross-dated visually and then cross-dated
statistically by a) the GleichHiufigkeit, which is the per cent agreement in the signs of
the 1sLdifference of two time series, and b) Student's t-test, which determines the degree
of correlation between the curves. Ring-width series were cross-dated using COFECHA
(Holmes 1983), and standard methods were used to build an averaged series for each
tree and for the site (Fritts 1976; Cook & Kairiukstis 1990).
The influence of climate on tree-ring growth was assessed using meteorological
data recorded at Pisa. In a first step, any ageing effect was removed by modeling the
ring-width series (dependent data) as a Hugershofffunction of cambial age (independ-
ent data) and an indexing procedure was used (Fritts 1976). Stepwise regressions and
response functions were then performed with the Precon5 software package to assess
the influence of climate on tree-ring growth. We checked for any autocorrelation effects
considering prior growth (Fritts 1976).
Wood anatomy
The cores were observed under a reflected light microscope (BX60, Olympus, Ham-
burg, Germany). Tree rings showing density fluctuations were selected from five ran-
domly selected cores and four regions were considered for each ring for subsequent
analyses: earlywood (Fig. la, b), intra-annual density fluctuation (IADF) (Fig. la, c),
late-earlywood (Fig. la, d) and latewood (Fig. la, e). Microphotographs were obtained
with a digital camera (CAMEDIA C4040, Olympus) from each sector, avoiding transi-
tion zones between the four regions. Images were elaborated with two software programs
for image analysis: Plant Meter-Root (Aronne & Eduardo 2001) and AnalySIS® 3.2
(Olympus). The area and shape of 1200 cells (20 cells x 4 images x 3 rings x 5 cores)
were measured avoiding ray cells and resin canals.
To define the shape of tracheid lumina in cross section, the following indexes were
considered, according to the definitions reported in AnalySIS® 3.2: "elongation" (the
greater the elongation of the cell the higher the value of the index); "sphericity" (a
spherical particle has a maximum value of 1; throughout the paper, sphericity is replaced
by circularity because the latter is a more appropriate 2-D term); "max X" (maximum
distance of all boundary points in the horizontal direction projected onto the x-axis);
"max Y" (maximum distance of all boundary points in the vertical direction projected
onto the y-axis). During the measurement process, the digital images were oriented
in such a way that "max X" and "max Y" corresponded to the maximum radial and
tangential diameters respectively.
Frequency distribution of the tracheid-Iumen area was calculated for each tree-ring
region, to relate different distributions of lumen areas to hydraulic properties. Wall
thickness of tracheids was measured both in radial and tangential walls, and their ratio
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Figure 1. End-grain surface of a wood ring showing a density fluctuation. - a: Complete wood
ring. - b: Earlywood. - c: lADE - d: Late-earlywood. - e: Latewood. - Scale bar in a =100 !-tm.
was calculated. The classification of tracheids as latewood or earlywood was done using
Mork's definition (Mork 1928): all tracheids whose common double cell wall is equal
to greater than the cell lumen are considered latewood (parameters always measured
in the radial direction).
The four regions of growth rings were compared by means of ANOVA using SPSS
statistical package (SPSS Inc., Chicago, Illinois, USA). Data on circularity were trans-
formed through arcsine function before statistical analyses. Variability between rings
was measured by calculating the coefficient of variability for each region.
Stable isotope analyses
Carbon stable-isotope composition was assessed on a sub-sample of tree rings from
five trees. We analyzed the tree rings formed during 1999-2002. Cores were separated
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into one-year intervals and further subdivided. Tree rings were dated and each ring
was split into four sectors from pith to bark similarly to those for anatomical analyses:
1) earlywood, 2) IADF, 3) late-earlywood, and 4) latewood. We carefully split the cores
under a stereomicroscope (Wild M3Z Leica, Germany) at a magnification of x 6.4-x 40,
to ensure that samples used for isotopic analysis were included within a given zone. For
the determination of the 13C/12C ratio, samples were milled using a centrifugal mill
(Retsch, Germany), weighed in tin capsules and measured in their isotopic composition
using an isotope-ratio mass spectrometer (Delta S, Finnigan MAT, Bremen, Germany),
after combustion of the wood in an elemental analyzer (EA 1108, Fisons, Italy). The
isotopic composition (b13C) of samples was calculated as:
b l3 C (%0) = (RsamplelRstandard - 1) x 1000
where Rsample and Rstandard are the ratios of 13C 112C in the sample and standard, respec-
tively; variations of isotope ratios are expressed in b-notation, i.e. the relative deviation
from the international standard PDB (PeeDee Belemnite). Details of the procedure for
the carbon isotopic analyses are described elsewhere (Saurer et al. 2003). The standard
deviation of b 13 C for the repeated analysis of commercial cellulose is 0.1 %0.
RESULTS
Dendrochronology
Twenty-four cross-dated ring-width chronologies ofthe Pinus pinaster trees growing
at San Rossore are shown in Figure 2. Correlation coefficients between the single-core
chronologies and their mean chronology were significant (p s 0.05) only for 24 cores
out of 40 (from 16 trees out of 20). Many intra-annual density fluctuations hindered
the dating, but measuring and cross-dating tree rings was still possible. The synchro-
Pinus pinaster, San Rossore
1200
1000
N
a.,... 800
•E
E- 600
~
il
.~
en
400
c
cr
200
O +-----~--~----~----~----~--~----_r----~---=~~~
1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
Year
Figure 2. Single-core ring-width chronologies of sampled trees.
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via free accessDe Micco et al. - Intra-annual density fluctuations in Pinus pinaster 67
nous occurrence of pointer years (i.e. the lowest or highest peaks, such as the negative
peak in 1996), and the presence of a common pattern in ring-width growth (e.g., the
1982-1987 series, with negative peaks in 1982,1984,1986 and positive in 1983, 1985,
1987) indicated that the tree rings were successfully cross-dated, at least since 1982.
After seedling emergence, tree-ring growth varied greatly between individuals. Com-
petition processes, for water and other resources in early stages of stand development,
could explain such different growth patterns.
Response-function analyses showed that precipitation influenced ring-width growth
significantly (p ~ 0.05) not only in May-June but also in September-October.
Wood anatomy
A typical density fluctuation, probably induced by climatic variation occurring within
the vegetative period, is shown in Figure 1. The area and shape of the tracheid lumina
appeared different between the four sectors detected within each growth ring. In par-
ticular, tracheids were smaller in latewood (287 11m2 ± c.i. 17.0) than IADF (397 11m2
± c.i. 23.8) (Fig. 1c, e). The latter showed smaller tracheids than late-earlywood (952
11m2 ± c.i. 33.2) (Fig. 1d). Tracheids of earlywood showed the highest values oflumen
area (1468 11m2 ± c. i. 43.0) and grew significantly larger than those of late-earlywood
(Fig. 1b, d).
Frequency distribution oftracheids according to lumen area showed that IADF, like
latewood, has a smaller range of tracheid-Iumen area and a greater proportion of nar-
row tracheids than both earlywood and late-earlywood regions (Fig. 3). Tracheids with
lumen area smaller than 1000 11m2 constituted almost 100 % of elements in latewood,
97 % in the IADF, 44% in late-earlywood and only 3 % in earlywood.
50
45
40
/ I~~~. . . . . . . . .
35
30
25
20
15
ftn' •.•. •. · •••....
10 Latewood
;"" rJ rt~ Late-Earlywood
~ /tDooooooBtJcrUU UU~~~o~oaaaQa"""" IADF
o 0 0 OD D Bg Q g goo g Earlywood
~ 0 Dog 0 0 0 0 0
~ ro ~ ~ g ~ g g goo
g
ID 0 0
~ ~ ~ ~ ~ ~ ~ ~ ~
Lumen area
Figure 3. Frequency distribution of tracheid lumen area (100 !-1m2 classes). In order to compare
different regions of the ring, frequency values are reported as percentage.
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via free access68 IAWA Journal, Vol. 28 (1), 2007
a E longat ion
2.0
1.5
1.0
0.5
0.0
earlywood IADF late- latewood
earlywood
b Sphericity
1.0
0.8
0.6
0.4
0.2
0.0
earlywood IADF late- latewood
earlywood
C Radial/tangential diameter ratio
1.5 ...,-- - - - - - - - - - - - - - - - - - - .
1.0
earlywood IADF late - latewood
earlywood
Figure 4. Mean values and confidence intervals (PDe Micco et al. - Intra-annual density fluctuations in Pinus pinaster 69
Table 1. Variability of data between rings based on area and shape of tracheid lumina for
each region.
Coefficient of variability
Area (l-tm2) Elongation Sphericity
Earlywood 0.253 0.158 0.206
IADF 0.522 0.207 0.246
Late-earlywood 0.302 0.166 0.212
Latewood 0.495 0.256 0.276
As expected, IADF showed the highest fluctuation in terms of lumen area (Table 1).
The variability of data was higher in latewood than in earlywood, both in terms of size
and shape of cells.
Tracheids from all sectors, with the exception oflate-earlywood, were quite expanded
in a preferential direction as confirmed by high values of elongation and low values
of circularity (Fig. 4a, b). Tracheids of IADF and earlywood presented intermediate
values between latewood and late-earlywood.
The ratio between radial and tangential diameters showed that earlywood tracheids
were expanded in the radial direction whereas elements of latewood and IADF were
enlarged in the tangential direction as the result of reduced radial expansion (Fig. 4c).
Both radial and tangential walls were thicker in latewood and IADF than in early-
wood (Fig. 5). Moreover, radial walls were thicker than tangential ones in latewood,
IADF, and late-earlywood, while no significant differences were found between the
two types of cell walls in earlywood.
Wall thickness
D radial • tangential
10
9
8
7
6
E 5
:::J..
4
3
2
0
earlywood IADF late- latewood
earlywood
Figure 5. Thickness of radial and tangential walls in the four regions of the ring. Mean values
and confidence intervals are reported (P70 IAWA Journal, Vol. 28 (1), 2007
-23
~
UJ
-24
1999 2000 2001 2002
If t I
-25
c
~
-26
U
~
00
-27
-28
-29
Figure 6. Values of wood ()13C from different intra-annual sections: EW (earlywood), IADF,
LEW (late-earlywood) and LW (latewood). Data shown are for the years 1999-2002, averages
of 5 trees (standard deviation is indicated). Missing values are caused by the narrow (one to
three cell rows) LEW that did not allow the analysis of isotopes.
Stable isotopes
A marked intra-annual fluctuation in 013C was observed (Fig. 6), which was consist-
ent over several years (SD < 0.25 %0). At the beginning of ring formation (earlywood),
013C values were intermediate between IADF (and late-earlywood) and latewood.
IADF wood was isotopically the heaviest, reaching 013C values of about -25.5%0.
Subsequently, wood progressively became isotopically lighter in late-earlywood and
reached the lowest Ol3C values in latewood, almost 2 %0 lower than in IADF. The values
of the different ring regions (pooled for all years) differed significantly from each other
(pairwise t-test, pDe Micco et al. - Intra-annual density fluctuations in Pinus pinaster 71
width. However, the quantification of anatomical features in tree rings of Pinus pinaster
provided new insight into differences between wood regions within rings, especially
between IADFs and latewood. Although IADF was classified as latewood, the lumen
size and shape were significantly different between latewood and IADF. Despite the
dominance of a single cell type, namely tracheids, gymnosperm wood is not uniform
and changes in element size may result in patterns of variation in both mechanical and
hydraulic properties (Zobel & van Buijtenen 1989; Gartner 1995; Spicer & Gartner
1998). The anatomical characterization of latewood, IADF, late-earlywood and early-
wood within tree rings may allow a better understanding of the contribution of each
ring sector to the total water flow with the application ofPoiseuille's law (Zimmermann
1983; Sperry 2003). It is possible to hypothesize that earlywood has much faster water
flow rates than latewood or IADF, although tracheids are not perfect capillaries due to
the lack of circularity and to the influence of other characteristics, such as bordered pit
features that can affect flow rate. Indeed, knowing the distribution of the tracheid lumen
area allows the calculation ofthe theoretical water flow rate (Spicer & Gartner 1998).
Our results suggest that theoretical water flow rate in late-earlywood of P. pinaster is
higher than in latewood or IADF, though probably lower than in earlywood.
According to our analysis, the progressively smaller tracheid lumen area in late-
earlywood, IADF and latewood may be due either to increasingly lower radial expansion
or to a premature ceasing of radial growth due to early deposition of secondary wall.
According to the latter hypothesis, the anatomy of IADFs could be a reflection of the
structure of the cambium zone when the stress is experienced. The risk of embolism
formation would be reduced partly by the production of smaller diameter tracheids in
IADF and latewood (Zimmerman 1983).
Latewood and IADF may occupy the same tangential space of earlywood because
the lower tangential diameter is compensated by thicker walls. A significant difference
found between tangential and radial walls in all the regions, except for earlywood,
indicates that the secondary cell wall thickening is uniform throughout the perimeter of
tracheids only in earlywood. As a consequence, there could be a different organization
at cytological level depending on the region of the ring. Indeed, secondary cell wall
thickening and lignification are controlled to a significant extent by individual xylem
elements and are regulated by environmental conditions (Donaldson 1992; Gindl et al.
2000; Donaldson 2002). It could be also possible that a slower radial expansion of the
tracheid lumen would consequently privilege thickening and lignification of radial walls.
Lignification oflatewood cells formed in autumn is usually not complete until early in
the following spring, but some trees may complete lignification of latewood prior to
the onset of winter dormancy (Donaldson 1992). This indicates that lignification can
switch on and off in response to environmental conditions (probably temperature or day
length) and thus irregularly correlate with carbon isotope signature (which integrates
the whole season).
Regarding carbon isotope composition, IADF-wood b13 C was appreciably heavier
than latewood b13 C. Water can be regarded as the overwhelming factor limiting growth
in Mediterranean summer season and stomata tend to close for sagacious water saving.
If this is the case for P. pinaster growing at San Rossore, the intercellular CO 2 concen-
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tration should decrease under water stress because CO 2 supply through stomata is
restricted, thus contributing to the increase in 6 13C in IADF tracheids, according to the
Farquhar et al. (1982) model. We did not observe any strong variation in 6 13 C between
different years, although a severe drought occurred during the summer of 200 1. We sup-
pose that stomatal closure occurred every summer at this site and, therefore, no further
stomatal reaction to the very dry summer of 2001 was found. Our results show the
limit of the application of stable carbon analysis to assess the severity of drought in
those environments characterized by seasonal aridity. Many authors compared 6 l3C in
earlywood and latewood reporting contrasting results (Wilson & Grinsted 1977; Leavitt
1993; Livingston & Spitdehouse 1996; Brooks etal. 1998; Helle & Schleser 2004). We
agree with Walcroft et al. (1997) in considering that variability in the seasonal cycle
of 6 13 C between locations may not be contradictory, but rather the result of growth
regulation by local environmental variables. In addition to climatic influences, intra-
annual6 13 C patterns may also be influenced by biochemical mechanisms (Schulze et al.
2004).
Differences between environmental conditions, most likely soil water deficit, during
the formation and differentiation of earlywood and late-earlywood may account for
observed fluctuations in tracheid profile and isotope signature. Assuming that the
concentration and isotopic composition of atmospheric CO 2 are stable over a certain
period, then the isotopic composition of the wood should reflect the long-term balance
between CO 2 supply (and water loss) and assimilation in leaves (Farquhar et al. 1982).
We suggest that drought severity may be reflected in the maximum isotope enrichment
found in the IADFs.
In conclusion, latewood-like cells ofIADF in tree rings differed from latewood cells,
in terms of cell lumen size and shape, and stable isotopic composition. Quantifying
anatomical features of IADFs could be a valuable approach to use the tracheids as
indicators of the duration and severity of the climatic stress experienced by plants in
different years, thus revealing the effect of wood density variations on xylem hydraulic
efficiency. Moreover, such analyses can be used as an additional tool for identifying
tree-ring boundaries in Mediterranean trees, which is usually very difficult when using
only microscopic observations. Although each ring region appeared to have a unique
carbon isotope signature, the latter seemed to be quite independent from climatic sum-
mer conditions.
The overall interpretation of all data from dendroecology, wood anatomy and isotope
composition can help dating past extreme events (e.g., summer drought) and seem to
be a promising tool for providing information on the duration and severity of these
events.
ACKNOWLEDGEMENTS
We thank Enrica Arlotta, an undergraduate student at the Universita di Firenze, for assistance in
measuring tree-ring width and preparing samples for isotopic analyses. This study originated from a
collaboration project with Marta Chiesi and Fabio Maselli (CNR-IBIMET, Firenze, Italy), and Marco
Bindi (Universita di Firenze). We are grateful to Pieter Baas (Nationaal Herbarium Nederland) for his
critical review of an early version of the manuscript.
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