Plant Ecology 171: 23–33, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.
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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, Universit`a 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
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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
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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
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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.
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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
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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
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