Water table salinity, rainfall and water use by umbrella pine trees

Plant Ecology 171: 23–33, 2004.
© 2004 Kluwer Academic Publishers. Printed in the Netherlands.
23
Water table salinity, rainfall and water use by umbrella pine trees (Pinus
pinea L.)
Maurizio Teobaldelli1,∗ , Maurizio Mencuccini2 & Pietro Piussi1
1 D.I.S.T.A.F.:
Dipartimento di Scienze e Tecnologie Ambientali Forestali, Università degli Studi di Firenze;
Via S. Bonaventura 13, 50145 Firenze, Italy; 2 Terrestrial Ecosystem Group, School of GeoSciences,
Edinburgh University, Darwin Building, Mayfield Road, Edinburgh EH9 3JU, UK; ∗ Author for correspondence
(e-mail: [email protected])
Key words: Heat pulse, Pinus pinea L., salinity, sap flow, water stress, water table depth
Abstract
The interactions between environmental conditions, particularly precipitation and water table salinity and tree
water use were studied at the pinewood of Alberese, a stand of umbrella pine (Pinus pinea L.) trees growing along
the Southern coastline of Tuscany and characterised by a sandy soil and a high water table level (ranging between 1
and 2 m depth). Data on sap flow, measured by heat pulse, or compensation technique, were compared between two
contrasting sites (referred to as sites A and B), characterised by clear differences in the salinity levels of the water
table. Site A, located near the karstic Uccellina hills was characterised by more favourable hydrologic conditions as
it was likely receiving lateral rainfall drainage from the hills. Water electrical conductivity (EC) values at the upper
surface of the water table of this site were lower than 12 dS/m. By contrast, the more typical site B, located further
away from the hills, did not benefit from lateral water movement in the soil and showed values of EC of about 17–
20 dS/m, half the value of seawater. This amounted to a difference in soil osmotic potential of about 0.4–0.5 MPa
across sites. Despite this difference in salinity, measurements of needle water potential during September 2000 did
not differ across sites (average of about −1.5 and −2.4 for pre-dawn and midday water potentials, respectively).
In contrast to water potentials, the dynamics of sap flow clearly differed across sites. Larger seasonal reductions
in maximum daily sapwood-related sap flux density were recorded at site B both during summer (0.005–0.015
10−3 m3 m−2 s−1 ) and spring-autumn (0.030–0.045 10−3 m3 m−2 s−1 ) than at site A (0.010–0.018 and 0.025–
0.035 10−3 m3 m−2 s−1 , for summer and spring-autumn, respectively). The different behaviour of water potentials
and transpiration rates across sites could be explained by higher values of soil-to-leaf hydraulic resistance at site B
during the dry season. Rainfall accumulated in the soil during winter formed a top layer of fresh water, which was
then used by plants during the following spring/summer. When fresh water supplies were depleted, the pines drew
from the underlying salty water with clear seasonal differences between the two sites.
Introduction
The woodlands of umbrella pine (Pinus pinea L.)
growing along the coastline of Tuscany have important
ecological, environmental, historical and economical
functions. Umbrella pine is commonly considered
the symbol of Italian coastal forests, particularly in
Tuscany. Consequently, its conservation is one of
the most important objectives of current management
strategies.
Different studies have recently reported on a reduced health status in the pinewood of Alberese
(Maremma Regional Park, Grosseto, Italy) and analysed, using different methodologies, the growth
conditions of Pinus pinea L. timber and seed productivity from this pinewood are much lower than at
other sites along the Tuscan coastline (Ciancio et al.
1986). Conese et al. (1989), using remote sensing
techniques, identified areas within the pinewood of
Alberese where signs of deterioration were present.
24
Subsequently, Tani (1991) attributed the diffuse and
anomalous withering of the canopies to the occurrence
of a marked drought period in the autumnal and spring
months during recent years. Greater levels of salinity
of the soil were also indicated as potential factors. The
presence of saline water in the soils of the pinewood
of Alberese has been mainly attributed to phenomena
related to 1. The direct seawater infiltration occurring along the open areas existing between the dunes;
2. The existence of drainage channels created during
winter storms (Maracchi et al. 1996; Ramat 1997); 3.
The presence of a salty water table in the soil, derived
from the drainage of old marshlands.
Small concentrations of Na+ and Cl− ions have
been found in the needles and the woody tissues of
the umbrella pine trees (Barbolani et al. 1997), indicating absorption of salts. Furthermore, needle length
was found to decline significantly after years of reduced precipitation (during the period March-August)
and to change from site to site depending on the local
soil water availability (Torta and De Capua 1993; Piussi and Torta 1994). Periodic measurements of xylem
relative water content (RWC, an indicator of the cumulative levels of xylem embolism, Borghetti et al.
1991) over several years have shown that xylem water content varies significantly during the seasonal
cycle, probably reflecting seasonal cycles in the development of and recovery from xylem embolism in
response to drought stress. Furthermore, measurements of xylem RWC carried out over several years
in areas where understorey shrubs had been removed
and the tree stand was thinned, to reduce competition for water, have confirmed the presence of lower
rates of embolism compared to unthinned areas (De
Capua and Mencuccini 1993; Gandolfo and Piussi
1996). Finally, dendrochronological measurements of
tree ring growth rates showed significant differences
within the Alberese pinewood between areas favoured
by the hydraulic supply from lateral runoff from the
nearby karstic hills and areas in the central body of the
pinewood (Gandolfo 1999).
It is unclear from these reports whether the reduced health of the pinewood represents a recent phenomenon or a chronic condition linked to persistent
environmental limitations present at the site. In fact,
the hydrological history of these areas is complex,
with movements of the coastline back and forth over
long time periods (Pranzini 1983; Bartolini and Pranzini 1985; Pranzini 1996). The more recent advance
of the coastline created the conditions for the development of extensive marshlands during the last several
centuries. The environmental history of the pinewood
is also very relevant in this respect. The pinewood is
the result of afforestation carried out, partly by natural means, after the completion of the drainage of the
marshlands during the 19th and 20th centuries. These
wet areas were drained by creating drainage channels
and by pumping the excess water out to the sea.
Despite these studies, systematic information on
the water relations of umbrella pine trees in the coastal
areas of Italy and around the Mediterranean in general,
is very scanty, and almost nonexistent in terms of field
responses to drought and salt stresses.
Based on the available information, we hypothesised that the observed reductions in productivity and
possibly also the observations of a reduced health
status may be linked to persistent and periodic phenomena of combined drought and salt stress during the
prolonged summer season. For instance, if significant
reductions in sap flow could be demonstrated during
the summer season at some sites but not others, concurrent measurements of soil and air environmental
conditions would allow determining the relative roles
of soil water stress, air temperature and relative humidity in determining these inter-site differences.
Materials and methods
Area of Study
The pinewood of Alberese is located inside the
Maremma Regional Park (Tuscany, Italy, 42◦39 30 N,
11◦ 04 29 E). The climate of the zone is typically
Mediterranean, with moderate variations in temperature throughout the year and limited yearly precipitation (641.5 mm) concentrated in the period from late
autumn to early spring. Using the Thornthwaite classification (cf., Arrigoni et al. 1985), this climate can
be described as mesothermic, comprised between the
classes of dry-subhumid and semiarid.
The main features of the soils and soil water reservoirs in the area around the Park are relatively well
known because of their agricultural importance. The
soils near the mouth of the Ombrone River, where
the pinewood grows, are mostly made of old sand
dunes and contain a free water table at a depth ranging between 1 and 2 meters (Piussi et al. 1993). This
groundwater, being of great importance for the pinewood and for the understorey shrubs, is hydraulically
isolated from the much deeper layers of groundwater
under pressure by thick layers of clay, and it is only fed
by rainfall. In addition, for those areas of pinewood
near the karstic Uccellina hills, there are likely additional contributions of freshwater by lateral rainfall
25
drainage and runoff from the hills (Piussi et al. 1993).
While rainfall represents the only available input of
freshwater for this superficial lens, the losses from the
water table are essentially due to evapotranspiration by
soil and trees and by the slow outflow towards the sea,
at least where the piezometric surface is higher than
sea level. An additional complication is represented by
the presence of old dunes over which the pines grow.
The elevation of these coastal dunes is less than 4 m
above sea level. Because of this topographical variation of the level of the soil surface above sea level,
the water table is situated at a different depth relative
to the ground surface.
construct a complete rainfall series for the Alberese
site.
Local meteorological parameters were also measured in situ with a third meteorological station set
up at the experimental site B. This station continually measured incoming photosynthetically active radiation (PAR Li-Cor sensor), air temperature and relative humidity (MP300-Campbell sensors) at the top
of a mast at a height of about 2 m above the tree
canopy (dominant tree height was about 13 m at this
site). Values for each parameter were recorded every
30 seconds and averaged every 30 minutes.
Measurements of water salinity
Site selection
Measurements of sap flow in Pinus pinea L. trees were
carried out at two sites located in the pinewood of
Alberese, between March 2000 and September 2001.
The two sites (hereby referred to as sites A and B)
were selected because they have a similar stand structure (diameter at breast height = 25–35 cm; tree height
10–13 m) but strong differences in the salinity of
the water table, as shown by previous studies (Gandolfo 1999). Site A, located near the karstic Uccellina
hills, was characterised by more favourable hydrologic conditions as it was likely receiving lateral rainfall
drainage from the hills. This site had values of water
conductivity at the upper surface of the water table
lower than 12.0 dS/m throughout the year. By contrast, site B, located further away from the hills and
supposedly more representative of the general situation of the pinewood, did not benefit from lateral
water movement in the soil and showed values of water conductivity of about 20 dS/m throughout the year
(Gandolfo 1999).
Meteorological measurements
Rainfall data collected at two different meteorological
stations (ARSIA - Regional Agency for Innovation
and Development in Agriculture) were used for this
research. The first one (Alberese Foce) is located
between the pinewood of Alberese and the mouth of
the Ombrone river, while the second (Rispescia) is
located 10 km away from the research sites. All meteorological data were averaged every 60 minutes. Both
series had gaps, but there was a good agreement in the
month-to-month variability in rainfall intensity across
sites for those periods when both series were collected.
We could therefore interpolate across the series and
The general trends for water salinity at site A and
site B were already known from data collected
between 1997 and 1999 (Gandolfo 1999). However,
electrical conductivity (EC) at 25 ◦ C and water table
levels were also measured at each experimental site
from water samples collected in two piezometric wells
during the period from October 2000 to September
2001. Soil water osmotic potential was estimated with
the relation proposed by Lang (1967).
Measurement of sap flow
At each site, four Pinus pinea L. trees were selected for sap flow measurements. Sap flow rates were
measured using the heat pulse (compensation) method
(Huber 1932; Huber and Schmidt 1936; Custom
1986), which uses heat pulses as markers in the sap
stream. The velocity of the heat pulse was calculated
according to Swanson and Whitfield (1981), whereas
the volumetric sap flux density per unit cross-sectional
area of sapwood was calculated following Smith and
Allen (1996). To estimate variations of sap velocities
along radial depth, probes were placed at four different
depths (5, 10, 15 and 20 mm) in the stem, so that the
radial profile of sap flux density across sapwood could
be determined. Following Green and Clothier (1988),
we calculated the total sap flow rates through the stems
as the integrals of the sap flux density profiles over
the sapwood cross-sectional areas (Smith and Allen
1996). Finally, the spatially-averaged stem sap flux
density was obtained by dividing the total stem sap
flow by the stem sapwood cross-sectional area.
A data-logger recorded the heat pulse velocity in
the sapwood of each tree every half-hour. The position
of the probes – azimuth angle and depth in the sapwood – was decided randomly. The probes, covered
with aluminium sheets in order to limit the influence
26
Figure 1. Monthly values of rainfall for the period January 2000 – September 2001 at three sites close to the Alberese pinewood.
of direct sun irradiation on the sensors, were left in
the same position for the duration of the whole experiment. Every 20 days increment cores were extracted
from several trees in the surroundings of each of the
research sites, in order to estimate xylem relative water
content. The data were consolidated using the software
“Analysis Program for ‘Aokautere’ heat pulse velocity logger” (Dr. J.W. Smith, version 1995, CSIRO,
Division Ground Water Research, Perth, Western Australia).
Sap flow data were analysed in different ways
to represent the main observed seasonal trends, the
contrasting behaviour of the two monitored areas as
well as their response to the main environmental variables. Seasonal comparisons of the two experimental
areas were carried out using only maximum daily
values rather than daily integrals, to avoid problems
associated with measurements of low sap flow rates
during night time periods and early morning and late
afternoon hours (Becker 1998). Representative daily
courses were also calculated for four randomly chosen
days throughout the spring-summer of the study year
(23/05/00, 15/08/00, 12/09/00 and 30/10/00). Finally,
one-dimensional response curves to either vapour
pressure deficit (VPD) or incoming photosynthetic
active radiation (PAR) were also calculated for each
monthly period from May to November 2000.
Measurement of needle water potential
A Scholander-type pressure chamber (Dixon 1914;
Scholander et al. 1965) was used to measure needle
water potential, f (MPa), throughout one experimental day (September 12, 2000), at each of the two
experimental sites. These measurements were taken to
check the hypothesis that needle water potential did
not differ between site A and site B despite the differences in the values of salinity of the aquifer water
and of availability of fresh water. Measurements of
needle water potential concurrent to the measurements
of sap flow also allowed the calculation of soil-plant
hydraulic resistance (MPa m2 s m−3 , i.e., per unit of
sapwood area), by estimating the slope of the regression line between sapwood-related sap flux density
and needle water potential. To avoid confusion, we
will use the symbol Qs, sapwood-related sap flux
density (M L−2 T−1 ), to refer to values of sap flux
per unit conducting sapwood area at the fixed measurement point (1.3 meter from soil) (Edwards et al.
1996).
Results
The experimental period (2000–2001) was characterized by a typical Mediterranean rainfall regime, although a lower yearly rainfall value (around 519 mm)
(Figure 1) was recorded from June 1st 2000 to May
27
Figure 2. Monthly values of electrical conductibility for water collected at the top of piezometric wells at the experimental sites A
and B. Site A has a more favourable hydrological regime compared
to site B, because of water table recharge with freshwater from lateral drainage and runoff from the nearby hills. Site A, open circles;
site B, closed triangles.
30th 2001 in comparison to the long-term average
(641.4 m) referred to the period from 1938 to 1993.
Water table level ranged between 130 cm depth
in February 2001 and 170 cm depth in August 2001
with no significant difference between sites (data not
given).
Values for electrical water conductivity ranged
from 7 to 12 dS/m for site A, for the period February to October 2001, while these same values ranged
from 16 to 19 dS/m for site B (Figure 2). These values corresponded to values of soil osmotic potential
ranging between −0.40 and −0.55 MPa for site A and
between −0.81 and −0.98 MPa for site B (calculated
assuming a salt composition equivalent to that of seawater). Minimum EC values were reached in February
and maximum values between August and September
2001.
At the single sampling date (September 12th, 2000)
when data were collected, needle f did not differ
between the two experimental areas. Estimated predawn and midday leaf water potentials were fairly
similar across sites (−1.43 MPa and −2.49 MPa for
site A, and −1.62 MPa and −2.30 MPa for site B, for
pre-dawn and midday, respectively) (Figure 3). Particularly remarkable was the conservation of the midday
minimum water potential at both sites despite the large
Figure 3. Changes in leaf water potential and sap flux density during September 12, 2000 at the two experimental sites A and B. The
slope of the regression line is a measure of the soil-to-plant hydraulic resistance. The values suggest a resistance twice as large at
the more stressed-prone site B than at site A. Neither pre-dawn nor
minimum midday water potential differed despite a large difference
in soil osmotic potential across the two sites. Site A, open circles;
site B, closed triangles.
differences in soil osmotic potential (about −0.4 MPa)
due to the different proportions of fresh- and seawater. When needle water potential was plotted against
the simultaneous measurements of sap flow, distinct
linear relationships were obtained for the two sites
(Figure 3), with a larger slope (i.e., the estimated soilto-leaf hydraulic resistance) for site B than for site A
(91.33 versus 41.14 MPa m2 s m−3 ).
Sap flow measurements recorded at site A during the period March 2000-September 2001 showed
distinct seasonal trends for the maximum daily Qs
values (Figure 4). The averaged 2000–2001 winter
maximum daily Qs values ranged between 0.015 and
0.025 10−3 m3 m−2 s−1 . During autumn and spring of
2000 and spring of 2001, maximum daily Qs values
ranged between 0.025 and 0.035 10−3 m3 m−2 s−1 .
On the contrary, maximum daily Qs values decreased
during the summer periods of 2000 and 2001, ranging
between 0.015 and 0.018 10−3 m3 m−2 s−1 , except for
some unexplained high values in July 2001 (between
0.040 and 0.046 10−3 m3 m−2 s−1 ) (Figure 4).
Site B showed seasonal trends similar to those
of site A for the period between March 2000 and
September 2001, although with clearly larger seasonal
differences between maximum daily Qs values of the
spring and summer periods (for both 2000 and 2001).
Moreover, maximum daily Qs decreased with about
one month of delay at the beginning of summer 2001
28
Figure 4. Seasonal changes in the maximum daily values of sap flux density for the two experimental sites A and B. Clear seasonal differences
are apparent for both sites, but particularly for the stressed-prone site B. Maximum springtime values however, tended to be higher for site B
than for site A. Site A, open circles; site B, closed triangles.
29
compared to the beginning of summer 2000. A similar time lag was observed in the phenology of needle
shedding for the older two-year-old needles, which
occurred mostly in July and in August for 2001 and
2000, respectively. Further differences were also observed between the values of autumn 2000 and those
of the springs of 2000 and 2001 for the two sites,
which could also be explained, at least partially, by
differences in the phenological cycles.
Interestingly, springtime values of maximum daily
Qs values were larger at the stressed site B than at
site A. Winter maximum daily Qs values were similar to those of site A, although they covered a shorter
period (from November 2000 to February 2001) (Figure 4). The two peaks recorded respectively on March
and July 2001 are at the moment unexplainable, although the same phenomenon was recorded at Site A
(Figure 4).
Finally, we observed a high sensitivity of trees at
site B to even moderate daily rainfall. In June 2000,
for instance, sap flow values varied around 0.009
10−3 m3 m−2 s−1 , and after a single rainfall event (6
and 7 of June 2000) when 22 mm of rain fell, sap flow
measurements gradually reached a maximum value of
0.016 10−3 m3 m−2 s−1 on June 19th (Figure 4).
Analysis of the daily courses of sap flow were
also carried out during four randomly chosen days
(23/05/00, 15/08/00, 12/09/00 and 30/10/00). This
more detailed analysis allowed comparing the daily
courses of Qs (from 5 am to 8 pm) at both sites for
days with different seasonal values of water table recharge by rainfall and with different maximum daily
values of VPD. Rainfall and maximum daily VPD values during the preceding 30 days of the four specific
dates were respectively 11.5 mm and 1.5 kPa, 5.60 mm
and 2.74 kPa, 2.20 mm and 1.67 kPa and 75.80 mm
and 0.39 kPa, (Figure 5).
The daily pattern observed at site A was very
similar during the four experimental days (Figure 5).
Maximum values of sap flow were reached during
the central hours of the day in conjunctions with
high VPD values. Maximum daily values decreased
throughout the summer to a minimum of about 0.018
10−3 m3 m−2 s−1 on September 12th, 2000. The
large standard errors highlight the presence of significant differences among the four study trees (Figure 5)
probably because of differences in the capacities of
different individuals to obtain freshwater through their
root systems or because of competitive interactions
within the pine stand.
On the contrary, site B showed two different daily
maxima of sap flow during the sample days in May,
August and October, i.e., a first one late in the morning and the second one late in the afternoon. However, the sample day in September showed only one
maximum of sap flow with values of about 0.0096
10−3 m3 m−2 s−1 during the first morning hours
when VPD was around 1.40 kPa. On August 15th and
September 12th , during the middle of the day, sap
flow decreased to a minimum value of around 0.005
10−3 m3 m−2 s−1 . Finally, on October 30th sap flow
values increased, with a maximum of about 0.018
10−3 m3 m−2 s−1 with a trend similar to that of site A
(Figure 5). Contrary to site A, the standard error was
small, showing little differences between the sap flow
of the four study trees (Figure 5).
The comparison of the response functions of sap
flow to VPD and PAR for the two sites suggests a
higher absolute (i.e., the unit change in Qs per unit
change in either VPD or PAR) sensitivity to these
abiotic factors at site A than at site B, where presumably the pine trees were more influenced by water
table salinity (Figure 6). Table 1 gives the logarithmic functions calculated for each month from May
to November 2000 with the respective R2 for both sites
in relation to both VPD and PAR.
Discussion
The central objective of this project was to determine the main interactions present between changes
in environmental conditions (particularly the seasonal
changes in precipitation and water table salinity) and
water use by umbrella pine trees at the Alberese
pinewood.
We constructed a simple conceptual model of the
yearly variations of sap flow in relation to abiotic
factors (i.e., availability of fresh-water in the soil, light
and air vapour pressure deficit). We can assume that,
during winter, the limit to xylem sap flow is represented by the low values of VPD and, occasionally, by
the low values of light, while in the summer sap flow is
limited by the lack of soil water above the water table
level, by the high salinity of the water table, or by both
reasons. Consequently, larger values of sap flow might
be expected to occur during spring and autumn, rather
than during summer or winter. High values could also
occur during summer, if and where local conditions
are favourable to an accumulation of fresh water in the
soil.
The collected data seems to confirm the negative
effects of salty water and of prolonged soil drought on
30
Table 1. Synthesis of the regression equations used to describe the monthly relationships between sapwood-related
sap flux density (Qs , m3 m−2 s−1 ) and air vapour pressure deficit (VPD, kPa) or incoming photosynthetic
active radiation (PAR, mmol m−2 s−1 ). Regression equations have the form: Qs = a ln(VPD)+b, or
Qs = a ln(PAR)+b, and have been calculated separately for each of the two study areas and each month during
the period May-November 2000. The correlation coefficient, R2 , is also given.
Month/Year
Site
Qs vs. VPD
R2
Qs vs. PAR
R2
May
2000
A
B
Qs = 0.008∗ ln(VPD) + 0.0252
Qs = 0.0031∗ ln(VPD)+ 0.014
0.44
0.20
Qs = 0.0074∗ ln(PAR) + 0.0228
Qs = 0.003∗ ln(PAR) + 0.0131
0.75
0.37
June
2000
A
B
Qs = 0.0063∗ ln(VPD) + 0.0228
Qs = 0.0027∗ ln(VPD) + 0.0125
0.35
0.22
Qs = 0.0066∗ ln(PAR) + 0.0217
Qs = 0.0025∗ ln(PAR) + 0.012
0.69
0.34
July
2000
A
B
Qs = 0.0038∗ ln(VPD) + 0.0189
Qs = 0.001∗ ln(VPD) + 0.0089
0.36
0.08
Qs = 0.0045∗ ln(PAR) + 0.0191
Qs = 0.0017∗ ln(PAR) + 0.0089
0.65
0.28
August
2000
A
B
Qs = 0.0054∗ ln(VPD) + 0.0172
Qs = 0.0009∗ ln(VPD) + 0.0049
0.37
0.04
Qs = 0.0038∗ ln(PAR) + 0.0192
Qs = 0.0008∗ ln(PAR) + 0.0052
0.64
0.10
September
2000
A
B
Qs = 0.0047∗ ln(VPD) + 0.0163
Qs = 0.0019∗ ln(VPD) + 0.0068
0.42
0.26
Qs = 0.0027∗ ln(PAR) + 0.0172
Qs = 0.0011∗ ln(PAR) + 0.0072
0.50
0.32
October
2000
A
B
Qs = 0.0071∗ ln(VPD) + 0.0236
Qs = 0.0041∗ ln(VPD) + 0.0151
0.44
0.28
Qs = 0.0037∗ ln(PAR) + 0.0233
Qs = 0.0025∗ ln(PAR) + 0.0155
0.49
0.43
November
2000
A
B
Qs = 0.0047∗ ln(VPD) + 0.0187
Qs = 0.0041∗ ln(VPD) + 0.0174
0.38
0.37
Qs = 0.0028∗ ln(PAR) + 0.0206
Qs = 0.0025∗ ln(PAR) + 0.0191
0.47
0.50
the water relations of umbrella pines. In fact, strong
reductions of sap flow were found at both sites A
and B during the summer despite fairly high water
table levels. However, direct measurements of water
salinity at the upper surface of the water table only
showed marginally higher values than during winter
periods, and only at site B. Presumably the pines used
the fresh water stored at the top of the water table
during the previous winter months. When fresh water
supplies were depleted, the pines had to start drawing
water from the underlying salty water table. Therefore, although the salinity at the top of the water table
changed only marginally throughout the year, clear
seasonal differences could be observed in sap flow
between the two sites. This interpretation is confirmed
by the data collected by Gandolfo (1999). In his study,
δ 18 O was used as a marker to identify the horizons
from which water was drawn by the pine trees. During
the summer a clear isotopic shift was apparent showing uptake from the saline horizons rather than from
soil water. Clear seasonal trends were also observed in
soil salinization, probably as a result of the capillary
rise of salty water from the underlying saline water
table (Gandolfo 1999).
Sap flow greatly declined in the summer at site B,
but remained higher and more regular at site A. This
is also consistent with the combined effect of reduced
soil water content above the water table and an increased salinization of the upper layers of the water
table itself.
The yearly cycle is then completed by the start
of the autumn rains, which recharges the water table
with a new layer of freshwater deposited at the top.
This cycle, despite the high levels of salinity in the
water table, and despite the reductions in productivity,
does allow the pine to maintain a minimum level of
physiological functions and vitality. We noticed that
the maximum yearly values of sap flux density were
observed during springtime at the more stressed site B,
rather than at site A. This observation has already
been reported for chronically stressed sites compared
to more favourable sites (cf., Mencuccini 2003) and
it has also been shown experimentally in the field by
Cinnirella et al. (2002) on black pine. It is possibly
caused by a structural acclimation of the trees at the
chronically stressed site decreasing their ratios of supported leaf areas to the hydraulic transport tissues.
This could happen through a reduction in leaf area
index or through an increased allocation to transport
tissues (e.g., roots) as a result of the chronic exposure
to a stress factor.
31
Figure 5. Daily courses of sap flux density, VPD and PAR values for four randomly selected days to represent the seasonal progression from
spring to summer and to autumn. Rainfall totals for the preceding 30 days are also given above each panel. Site A, open circles; site B, closed
triangles.
We observed a clear difference in the time when
sap flow started to decline at the beginning of the
summer: the decline occurred with about one month
of delay in 2001 compared to 2000. This difference
was also present in the timing of shedding of the oldest
needle cohort. Analysis of the rainfall data (Figure 1)
for the preceding winter and spring periods confirmed
that the summer 2001 had been preceded by a wetter
winter and spring, suggesting that a greater recharge
of the saline water table by more intense rainfall was
responsible for this difference.
Comparing our data with similar studies carried
out on another Mediterranean pine species, i.e., mari-
time pine, Pinus pinaster Ait., maximum daily values
of sap flux density per unit of sapwood area were
around 0.080 10−3 m3 m−2 s−1 under optimal hydraulic conditions (relative extractable water content
of soil, REWC =120%; pre-dawn water potential of
−0.4 MPa) and around 0.020 10−3 m3 m−2 s−1 under
water stress conditions (REWC= 5%; pre-dawn water
potential of −0.7 MPa) (Loustau et al. 1990). Therefore, sap flux density for umbrella pine trees appears
to vary within a range very similar to that of maritime
pine.
Overall, our results support the evidence that umbrella pine trees show a significant level of tolerance
32
Figure 6. Response functions of sap flux density to VPD and PAR for each site for three selected months, May, August and November, to
represent the seasonal differences between spring, summer and autumn. A complete set of regression equations for each month is also provided
in Table 1. Site A, open circles; site B, closed triangles.
(cf., Levitt 1980) to water stress, confirming earlier
data obtained at the leaf level by Manes et al. (1997)
at Castel Porziano (Rome).
Acknowledgements
This study was supported by ARSIA (Regional
Agency for Innovation and Development in Agriculture), by the Ministry of University Education and
Research, by the University of Padova (Ph.D. Thesis;
Teobaldelli 2002) and by MEDCORE Project (EU
contract ICA3-2002-10003, 5◦ FP, INCO-MED Programme). Our sincere thanks to Prof. P.G. Jarvis, Dr.
S. Allen, Dr. W.R.N. Edwards, Dr. M. Smith, Dr.
J. Irvine, Prof. T. Anfodillo, Prof. F. Magnani, Dr.
U. Galligani, Dr.ssa E. Gravano and Dr. G.P. Gandolfo who provided many helpful suggestions; we
would also like to thank the Azienda Regionale di
Alberese and the administrators and rangers at the
Maremma Regional Park for their collaboration and
help during fieldwork. We would also like to thank the
33
two anonymous referees and the guest editors of the
Volume for their comments on an earlier version of
the manuscript.
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