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Hydraulic limitations and water-use efficiency in Pinus pinaster along a chronosequence Federico Magnani, Abdelkader Bensada, Sergio Cinnirella, Francesco Ripullone, and Marco Borghetti
Abstract: Hydraulic constraints to water transport and water-use efficiency were studied in a Pinus pinaster Ait. chronosequence in Italy, consisting of four even-aged stands ranging from young (10 years old) to mature (75 years old), to explore the mechanisms involved in the decline of stand productivity as tree grow taller. Leaf-specific transpiration was estimated from sapflow rates measured by the heat dissipation technique, leaf-specific hydraulic conductance was computed from the slope of the relationship between transpiration and leaf water potential, long-term water-use efficiency was estimated from carbon isotope discrimination (13C) in xylem cores, and photosynthetic capacity was assessed from CO2 assimilation/CO2 intercellular concentration curves. Leaf-specific transpiration decreased with stand development, suggesting a reduction in stomatal conductance, and a negative relationship was found between leaf-specific hydraulic conductance and tree height, suggesting a role of hydraulic constraints in the decline of current annual increment. Minimum daily leaf water potential did not change with stand height, suggesting that homeostasis in leaf water potential is achieved through a reduction in leaf transpiration. The 13C values increased with stand development, indicating a decline of water-use efficiency. Leaf level stomatal conductance was higher in the younger stand; no significant difference in maximum carboxylation rate was found among stands. Re´sume´ : Les contraintes hydrauliques au transport de l’eau et l’efficacite´ d’utilisation de l’eau ont e´te´ e´tudie´es le long d’une chronose´quence de Pinus pinaster Ait. dans le but d’explorer les me´canismes responsables du de´clin de la productivite´ des peuplements a` mesure que la hauteur des arbres augmente. La chronose´quence est situe´e en Italie et est constitue´e de quatre peuplements de structure e´quienne couvrant des peuplements jeune (10 ans) a` mature (75 ans). La transpiration foliaire spe´cifique a e´te´ estime´e a` partir de mesures du taux d’e´coulement de la se`ve a` l’aide de la technique base´e sur la dissipation de chaleur; la conductance hydraulique foliaire spe´cifique a e´te´ calcule´e a` partir de la pente de la relation entre la transpiration et le potentiel hydrique foliaire; l’efficacite´ d’utilisation de l’eau a` long terme a e´te´ estime´e a` l’aide de la discrimination isotopique du carbone (13C) dans des barrettes de xyle`me et la capacite´ photosynthe´tique a e´te´ estime´e a` partir de courbes reliant l’assimilation du CO2 a` la concentration de CO2 intercellulaire. La transpiration foliaire spe´cifique diminuait avec le de´veloppement du peuplement, ce qui indique une re´duction de la conductance stomatique. Une relation ne´gative a e´te´ e´tablie entre la conductance hydraulique foliaire spe´cifique et la hauteur des arbres, ce qui indique que les contraintes hydrauliques jouent un roˆle dans le de´clin de l’accroissement annuel courant. La valeur minimale journalie`re du potentiel hydrique foliaire ne changeait pas avec la hauteur du peuplement, ce qui indique qu’un e´quilibre home´ostatique du potentiel hydrique foliaire est atteint par une re´duction de la transpiration foliaire. La discrimination isotopique du carbone augmentait avec le de´veloppement du peuplement, ce qui indique une diminution de l’efficacite´ d’utilisation de l’eau. La conductance stomatique foliaire e´tait plus e´leve´e dans le plus jeune peuplement. Aucune diffe´rence significative n’a e´te´ de´tecte´e entre les peuplements dans le cas du taux maximal de carboxylation. [Traduit par la Re´daction]
Introduction The increase of hydraulic limitations as trees grow taller has been assessed in a number of field studies (e.g., McDowell et al. 2002a; Delzon et al. 2004). According to the hydraulic limitation hypothesis (Ryan and Yoder 1997), the increase of hydraulic constraints with tree height reduces stomatal conductance, and this could account for the sizerelated decline in photosynthetic capacity (Bond 2000; Hubbard et al. 2001), which can eventually translate in the
decline of aboveground forest productivity (Ryan et al. 1997). As trees grow taller, they can compensate for the increase of hydraulic constraints by increasing the fine root/foliage ratio and through an altered balance between the area of conductive sapwood and transpiring foliage (Albrektson 1984; McDowell et al. 2002b); this partly counteracts the effects of height on hydraulic resistance (Mencuccini and Grace 1996) and suggests the hydraulic resistance per unit leaf area might be constant with age (Magnani et al. 2000).
Received 7 November 2006. Accepted 6 June 2007. Published on the NRC Research Press Web site at cjfr.nrc.ca on 22 January 2008. F. Magnani. Dipartimento di Colture Arboree, Universita` di Bologna, via Fanin 46, 40127 Bologna, Italy. A. Bensada, F. Ripullone, and M. Borghetti.1 Dipartimento di Scienze dei Sistemi Colturali, Forestali e dell’Ambiente, Universita` della Basilicata, viale dell’Ateneo Lucano 10, 85100 Potenza, Italy. S. Cinnirella. Istituto sull’Inquinamento Atmosferico, CNR, c/o UNICAL-Polifunzionale, 87036 Rende, Cosenza, Italy. 1Corresponding
author (e-mail:
[email protected]).
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doi:10.1139/X07-120
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On the other hand, it has also been shown that the increase of hydraulic constraints with tree size are not always accompanied by structural compensation of reduced leaf/ sapwood ratio in large trees (Phillips et al. 2003), and that the resulting decline of stomatal conductance cannot always be claimed to fully account for the reduction in photosynthetic rates; according to Niinemets (2002), biochemical constraints may compound the hydraulic constraint on photosynthetic capacity during stand development, because maximum carboxylation capacity was found to decrease significantly with increasing tree size at common intercellular CO2 and leaf nitrogen concentrations. Similarly, Katul et al. (2003), by the application of a coupled water and carbon balance model, predicted a decline in carboxylation capacity as the hydraulic capacity decreases with tree size. Summarizing the results of more than a decade of research, Ryan et al. (2006) suggest that there is no universal mechanisms to explain the decline of productivity as tree grow taller, but more components can be involved together. Further, it remains unclear how tree size impacts plant water-use efficiency (WUE; ratio of carbon gain to water loss), that is, an important trait for coupling plant productivity and efficient exploitation of water resources (Dewar 1997). In a number of studies, it has been found that WUE increases as trees grow taller. Such an increase was often interpreted as an effect of increased hydraulic and stomatal limitations, constraining water losses by transpiration more then carbon gain by photosynthesis (McDowell et al. 2002a; Delzon et al. 2004). In other cases, WUE has been found to decline with stand height, suggesting that other functional constraints, apart from hydraulic limitations, may play a role in reducing carbon gain in taller trees (Ko¨stner et al. 2002). For example, differences in light interception efficiency between small and large trees has also accounted for reduction of CO2 assimilation in the latter (Hikosaka et al. 1999). We focussed on a chronosequence of maritime pine (Pinus pinaster Ait.), a common natural and plantation species in the Mediterranean region, to further evaluate stand developmentrelated processes, by assessing transpiration, leaf water potential, hydraulic conductance, carbon isotope discrimination, and photosynthetic capacity in four even-aged stands ranging from young (10 years old) to mature (75 years old). Our aim was to test the following hypotheses: (i) that leaf water potential is maintained constant irrespective of plant age and height, so as to prevent diffuse xylem embolism and tissue damage; (ii) that such an homeostasis in leaf water potential is achieved through a reduction in transpiration rate per unit leaf area and, consequently, gas exchange in tall trees, which could account for the reduction in above ground stand productivity; (iii) that, alternatively, homeostasis in water potential is the result of a constant plant hydraulic resistance per unit leaf area, despite the increase in the length of the hydraulic pathway in tall trees; and (iv) that long-term WUE is modified during stand development and that metabolic constraints, other than hydraulic limitations, may play a role in reducing assimilation in older trees.
Materials and methods Study area and stand characterization The study took place in a P. pinaster forest growing along
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the Tyrrenian coast in central Italy (San Rossore, Pisa; 43843’N, 10820’E, sea level), where P. pinaster has been planted in homogeneous monospecific stands, traditionally managed with a rotation length of about 80 years. The climate is Mediterranean, with a mean annual rainfall of 932 mm and a mean annual temperature of 14.8 8C; the dry or semidry period starts in June and lasts until the end of September, with summer rainfall less than 100 mm. Soil is sandy (sand >98%), with a neutral to slightly alkaline pH. In spring 2000, four circular experimental areas, 20 m in diameter, were defined in four forest stands: a 10-year-old (S10), a 35-year-old (S35), a 55-year-old (S55), and a 75year-old stand (S75). Stands were of comparable topography, soil characteristics, and yield class, based on a preliminary comparison of stand parameters with local growth and yield tables (Cantiani 1975, 1985). In S10, tree crowns were easily accessible; in S35, S55, and S75, a scaffolding tower provided access to the top of the canopy. Standard dendrometric measurements were performed in each stand. In autumn 2001, eight trees of average diameter were sampled in each stand, and two xylem cores were extracted from each of these trees at breast height from the northern and western side of the stem. In the laboratory, annual growth rings were measured with a precision of 1 mm using a Aniol tree-ring measuring device, and the accuracy of cross-dating was assessed using the Catras software (Aniol 1983). Sapwood depth was determined on each core by visual analysis in front of a light source as the transition point between the translucent water conducting sapwood and the more opaque heartwood. The mean value of the two measurements per tree was used for data analysis. Stand sapwood area (As) was estimated from stand basal area, relying on the strong linear correlation between basal area (Ab) and sapwood area found at the tree level (As = 0.87Ab, R2 = 0.99). Stand volume current annual increment (CAI, m3ha–1year–1) was reconstructed in each stand from measured tree radial increments and ancillary data from local growth and yield tables (Cantiani 1975; Cantiani 1985). Leaf area index (LAI) was assessed during the 2001 season by the hemispherical photographic technique and successive analysis of gap fraction distribution according to the procedure described by Welles and Cohen (1996). In S35, S55, and S75, five hemispherical photographs were taken at points located 50 m apart each other along the two arms of a cross; measurements were made under conditions of high contrast between the crowns and the sky. To obtain a continuous estimate of LAI throughout the season, an empirical sinusoidal segmented relationship was fitted to the LAI experimental data (Fig. 1). Environmental variables A continuous record of meteorological variables was available from sensors placed on an eddy-covariance tower nearby, which was operated by JRC-IES (Joint Resarch Centre, Institute for Environmental Suistainability, European Commission, Ispra, Italy) as part of the European ‘‘CarboEuroflux’’ project. Soil water content was measured by the time domain reflectometry (TDR) technique (Topp et al. 1980). From March 2001 to November 2002, every 2 or 3 weeks, the ap#
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Magnani et al. Fig. 1. Leaf area index measured in 2001 (*) and 2002 (*) in the studied stands. Each point in the graph shows the mean of LAI values computed in the three stands (S35, S55, and S75) at each measurement date, and error bars are SEs. The broken line is the seasonal LAI course as captured by a segmented sinusoidal fitting, which explains 79% of the data variability.
Leaf area index (m2· m–2)
4.5 4.0 3.5 3.0 2.5 2.0 1.5 Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Jan.
parent dielectric constant of soil was measured at 20 randomly distributed points in each stand using a manual device sensor (Hydrosense CS620, Campbell Scientific, Logan, Utah). This device was connected to the end of two 20 cm long stainless steel cylindrical probes, 5 mm in diameter, that were fully inserted vertically into the soil beneath the litter layer at a distance of 5 cm from each other. The apparent dielectric constant was translated into an estimate of soil water content using calibration curves for sandy soils. In one stand (S75), a TDR probe connected to a datalogger (CR21X; Campbell Scientific) allowed continuous measurements of soil water content. Carbon isotope analysis From the two xylem cores extracted in autumn 2001 from each of the eight trees sampled in all experimental stands, the last five annual increments of each xylem core (woody increments of 2001, 2000, 1999, 1998, and 1997) were separated and further processed for carbon isotope analysis; woody rings produced before 1997 were sampled in groups of 5 years. All samples were individually ground to a fine powder and dried for >10 h in an oven at 80 8C. The carbon isotope fraction (13C/12C) of each sample was measured using a continuous flow triple collector isotope ratio mass spectrometer (CF-IRMS, ISOCHROM II VG; Isotech, Middlewich, UK), connected to a Dumas-combustion elemental analyzer (NA-1500; Carlo Erba, Milano, Italy). Carbon isotope ratio (13C) and carbon isotope discrimination (13C) were calculated using standard procedures and formulas (Farquhar and Richards 1984), assuming a fixed value of –8% for 13C in air. For data analysis, the mean value of the two measurements per tree was used. Leaf water potential Leaf water potential was measured from dawn to sunset in all stands on 30 and 31 August and 1 September 2000 and 29, 30, and 31 May and 24, 25, and 26 July 2001 using two intercalibrated pressure chambers (Model 1000; PMS Instruments, Corvallis, Ore.). In addition to the four experimental stands, small pine seedlings were occasionally measured in
75
2001. At each measurement time in each stand, eight pairs of 1-year-old needles were sampled in the upper part of the canopy, as far apart as possible; samples were wrapped in aluminium foil and immediately excised. Following excision, needles were put in a plastic bag moistened with a damp towel and stored in the dark until they were measured, always within 5–10 min of excision. Sapflow measurements Sapflow was continuously measured in the S35, S55, and S75 stands from May 2000 to December 2002 using the Granier heat dissipation technique (Granier 1985, 1987). Eight tree (others than those sampled for tree-ring analysis) of mean diameter were sampled in each stand, and sapflow was continuously recorded in the stem of each tree at a height of about 5 m. Each probe consisted of a pair of identical fine-wire copper–constantan thermocouples, installed in the centre of a 1.5 mm diameter, 20 mm long, hollow steel needle. A constantan heating wire was coiled around one of the needles, covering its whole length and supplied with a constant power of 200 mW. Both needles were inserted in a 2 mm diameter, 22 mm long aluminium tube forced into a smaller hole drilled into the xylem, to ensure optimal thermal contact with the xylem. Sensors were inserted horizontally at the same height on the northern size of the stem with a horizontal spacing of 8–10 cm; this horizontal arrangement was preferred over the original vertical alignment (Granier 1985), because it helps to minimize the effect of vertical temperature gradients in the absence of any heating (Andre´ Granier, Institut National de la Recherche Agronomique, Nancy, France, personal communication). Both sensors were placed on the northern side of the tree to avoid the thermal effects of direct solar radiation. Silicone grease was applied all around the drilled holes and over all the sensor housing to avoid xylem desiccation and to improve thermal insulation. Finally, the sensors were shielded from the rain and the sun with a polythene cover, and the stem was thermally insulated with thick padded aluminium foil extending approximately 50 cm above and below the sensor. Because the two thermocouples were connected in series, the voltage difference measured across them was proportional to the temperature difference (T) between the heated and reference probes. At each site, the readings from eight probes were collected by a datalogger (CR10; Campbell Scientific) through a multiplexer (AM 32; Campbell Scientific). Measurements of T were taken every 30 s and averaged by the datalogger every 15 min. Maximum temperature difference (Tmax) on each day was then computed for each probe and interpolated between contiguous days, so as to avoid any discontinuities at midnight, when different reference values would otherwise be applied all of a sudden. The rate of water flow per unit sapwood area (Df, gm–2s–1) was computed for each sensor using the empirical formula proposed by Granier (1985). Based on the results of a calibration experiment performed in the laboratory for calibrating sapflow, it was assumed that there was no change in sapflow rate with stem radial depth Leaf-specific transpiration and hydraulic conductance Leaf-specific transpiration (Eleaf, gm–2s–1) was computed #
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Photosynthetic measurements In April 2005 under conditions of soil water abundance, photosynthetic capacity was assessed from CO2 assimilation/CO2 intercellular concentration (A/Ci) curves in the S10, S35, and S75 stands. Three trees were sampled in each stand, and three or four pairs of 1-year-old needles detached from the upper crown were measured in each tree. Photosynthesis was measured with a portable infrared gas analyzer (CIRAS 1; PP Systems, Hitchin, UK) supplying a photosynthetic photon flux density of 800 mmolm–2s–1. Starting from a CO2 concentration of 370 mmolmol–1, chamber CO2 concentration was lowered to 20 mmolmol–1 and increased successively to 1600–2000 mmolmol–1 in four or five steps. Data were recorded after steady-state conditions were attained for at least 5 min at each CO2 concentration. Temperature inside the cuvette was close to ambient (20 ± 2 8C). The equations proposed by Farquhar et al. (1980) were fitted on A/Ci curves by nonlinear least squares regression procedures for estimating maximum carboxylation (Vcmax) and electron transport rate (Jmax) on the lower (Ci < 250 mmolmol–1) and higher (Ci ‡ 700 mmolmol–1) portions of A/Ci curve. Light-saturated photosynthesis at Ci ~ 370 mmolmol–1 was considered to be the maximum assimilation rate (Amax). Estimations of Jmax and Vcmax were made using the kinetic constants reported by Medlyn et al. (2002), and both parameters were referenced to 25 8C using temperature-dependence equations (Walcroft et al. 1997). Statistical analyses All statistical analyses (analysis of variance, multiple comparisons, and linear and nonlinear fittings) were performed using SAS (SAS Institute Inc. 1985) statistical software.
Results and discussion Homeostasis in leaf water potential and transpiration Leaf water potential displayed its daily minimum in early afternoon, reaching values around –2.3 MPa, which are somewhat lower than those reported for P. pinaster stands growing under the oceanic climate of southwestern France (Loustau et al. 1990; Delzon et al. 2004). No significant change in minimum leaf water potential was observed among stands (Fig. 2). Higher minimum values (up to –1.3 MPa) were measured in young seedlings on a few occasions (not shown). Figure 2 suggests homeostasis in leaf water potential and a strategy of the plant for limiting the risk of runaway xylem embolism and foliage dieback (Tyree and Sperry 1988; Magnani et al. 2000). In P. pinaster from Iberian populations, a water potential of –3.0 MPa was found to cause
Fig. 2. Relationship between minimum daily leaf water potential and stand height, for the measurement campaign of year 2000 (*) and year 2001 (*). Each point in the figure shows the mean of daily minimum values of leaf water potential computed for three measurement dates in 2000 and six measurement dates in 2001; error bars are SEs. Minimum daily leaf water potential (MPa)
for each stand as Eleaf = E/LAI, where LAI is leaf area index estimated at each date by the sinusoidal empirical relationship, and E is stand transpiration, calculated as E = DfAs, where Df and As are as previously defined. Leaf-specific hydraulic conductance (Kleaf, gm–2s–1MPa–1) was computed as the inverse of the slope of the linear relationship between stand means of leaf water potential and Eleaf (Delzon et al. 2004; Hubbard et al. 1999).
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0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 0
4
8
12
16
20
Stand height (m)
50% loss of stem xylem conductivity (Martı´nez-Vilalta and Pin˜ol 2002); a cavitation threshold of –2.2 MPa has been reported for P. pinaster trees by Delzon et al. (2004), who also found remarkable homeostasis in leaf water potential. Soil water content varied markedly throughout the season, as expected under Mediterranean conditions. In parallel, Eleaf declined remarkably in response to soil drying in summer and recovered after some intense precipitation events in September (Fig. 3). Similar patterns of stand transpiration have been observed in other Mediterranean pine stands exposed to summer drought (Cinnirella et al. 2002). At similar values of soil water content, a lower transpiration rate was generally observed in the older S75 stand (Fig. 3). Indeed, plotting Eleaf in the older stands (S55 and S75) against concurrent values measured in the 35-year-old stand (S35) yielded a rather clear pattern (Fig. 4). Although the picture changed somewhat over the season, values for the oldest stand were consistently lower than values for the youngest stand, with more evident reduction in winter and summer. The transpiration of the 35- and 55-year-old stands, on the contrary, differed little from each other. This result was restated by a comparison of maximum daily transpiration rates over the whole season; based on a linear regression analysis (not shown), mean maximum Eleaf in the 75year-old stand was 68% of the value for the 35-year-old stand. Such a decline of leaf-specific transpiration with stand development agrees with previously reported patterns (e.g., Phillips et al. 2003). Hydraulic constraints and tree growth For highlighting hydraulic constraints during stand development, changes in Kleaf were explored from the relationship between leaf water potential and Eleaf. On average, Kleaf declined markedly with decreasing soil water content (not shown), likely as a result of increasing soil hydraulic resistance (Breda et al. 1993; Irvine et al. 1998). A negative relationship was observed between Kleaf and stand height in well-watered conditions, whereas such a #
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Fig. 3. Seasonal pattern of soil water content (upper panel; each point shows the mean of 20 measurement points) and maximum daily leaf-specific transpiration (lower panel,; each point shows the mean of eight trees) during the year 2001 in the S35 (*), S55 (*), and S75 (~) stands. The lines in the upper panel represent the continuous measurements of soil water content performed in the S75 stand.
Fig. 4. Comparison of maximum daily leaf specific transpiration measured in the S55 and S75 stands with concurrent measurements recorded in the S35 stand in February (upper panel), May (middle panel), and July–August (lower panel) 2001. 0.06
6-18 February 2001 0.05
16
1 :1
0.03
12 10 8 6 4 Apr.
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June
July
Aug.
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0.0030 0.0025 0.0020 0.0015 0.0010
Leaf-specific transpiration, S55 and S75 (g·m –2·s –1)
Soil water content (%)
14
Max. leaf-specific transpiration (g·m–2·s–1)
S55 S75
0.04
0.02 0.01 0.00 0.040.00
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1 :1
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28 July -10 August 2001 0.025
S55 S75
1 :1
0.020
0.0005
0.015
0.0000 Apr.
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June
July
Aug.
Sept.
Oct.
Nov. 0.010 0.005
decrease was less evident when data from droughted periods were included (Fig. 5). In the soil–plant continuum, plant height is expected to affect the hydraulic conductance of the plant component (or rather of its aboveground part), which was more precisely estimated under well-watered conditions, because the contribution of soil hydraulic resistance was negligible in this case (Campbell 1985). Such a rationale is supported by recent experimental and modelling results by McDowell et al. (2005), who showed that hydraulic limitations associated with tree size are more likely to be manifest under conditions of atmospheric and soil water abundance. As long as an increase of hydraulic limitations with stand development was observed, a similar picture was apparent for stand growth. Comparing the CAIs of the different stands over the same period (1991–1995 and 1996–2000), an evident reduction of growth rate can be observed between 10 and 35 years (Fig. 6). Further interpretation of hydraulic constraints can be attempted taking into account stand structure. According to the model of Whitehead et al. (1984), Kleaf can be assumed to be proportional to plant sapwood / leaf area ratio (As/Al) and inversely proportional to tree height (H):
0.000 0.000
0.005
0.010
0.015
0.020
0.025
0.030
Leaf-specific transpiration, S35 (g · m–2 · s–1)
½1
Kleaf /
As =Al H
An increase of As/Al with stand development has been reported and interpreted as a compensating structural adjusment (Mencuccini and Grace 1996; McDowell et al. 2002b). A different picture emerged from our data. Fitting an empirical sinusoidal segmented relationship on LAI experimental data (Fig. 1) yielded a clear seasonal course, which is in good agreement with visual evidence of foliage production and litterfall dynamics and expected seasonal patterns in conifer stands (Jarvis and Leverenz 1983): after increasing from the beginning of May onwards, LAI reaches a maximum at the end of June before declining to a constant winter value at the end of September. However, comparing mean annual LAI values of different stands showed no significant differences (Table 1). Stand sapwood area increased from age 10 to 35 years then remained almost constant thereafter. #
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Fig. 5. Relationship between leaf-specific hydraulic conductance (Kleaf) and stand height: (*) mean Kleaf computed from all measurements (9 days throughout 2000 and 2001); (*) mean Kleaf computed from measurements performed over 3 days in May 2001 (well-watered conditions). Error bars are SEs.
K leaf (g·m–2·s–1·MPa–1)
0.035
0.030
Table 1. Characteristics of the four experimental stands. Stand Characteristic Age (years) Stand sapwood area (m2) Height (m) Stem diameter (cm) Leaf area index (m2m–2)
S10 10 25.2 5.9a 9.7a 2.2
S35 35 35.3 15.2b 26.4b 2.7a
S55 55 35.0 16.5c 29.1c 3.0a
S75 75 37.9 18.5c 32.6d 3.3a
Note: Mean values with the different letters among stands are significantly different according to the Student–Newman–Keul test (P < 0.05). Comparison of the leaf area index was restricted to stands S35, S55, and S75. LAI value for stand S10 stand provided by L. Bernasconi (University of Firenze, Firenze, Italy).
0.025
0.020
0.015
0.010 14
15
16
17
18
19
Table 2. Maximum assimilation rate at saturating light (Amax), maximum carboxylation activity of ribulose-1,5-biphosphate carboxylase/oxygenase (Vcmax), maximum electron transport rate (Jmax), ratio of intercellular to ambient CO2 concentration ratio (Ci/Ca) and stomatal conductance (gs) in stands of different ages.
Stand height (m)
Stand Fig. 6. Relationship between stand volume current annual increment (CAI) and stand age. The percentage reduction of CAI was calculated from its peak value, based on radial increments recorded for two 5 year periods (1991–1995, *; 1996–2001, *). Each point in the graph represents the mean of the eight trees sampled in each stand, and error bars are SEs. 100
(mmolm–2s–1)
Amax Vcmax (mmolm–2s–1) Jmax (mmolm–2s–1) Ci/Ca gs (mmolm–2s–1)
S10 7.7a 60.0a 130.0a 0.77a 161.2a
S35 6.5a 53.2a 105.4a 0.71a 114.2b
S75 6.1a 56.5a 102.2a 0.75a 128.5b
Note: Mean values with the different letters among stands are significantly different according to the Student–Newman–Keul test (P < 0.05).
CAI reduction (%)
80
60
40
20
0 0
20
40
60
80
Stand age (years)
Thus, in our case, no evidence can be claimed for an increase of sapwood / leaf area ratio with stand development, and the reduction in Kleaf appears to be mainly the result of an increase in tree height and is not accompanied by structural adjustments that are able to compensate the effect of the increase in the hydraulic path length. That is to say, the reduction in Kleaf appears to be linked directly to the reduction in Eleaf and mean conductance of the tree’s foliage, maintaining the minimum leaf water potential almost unchanged and the soil to leaf water potential difference constant. Leaf-level measurements of gas exchange showed that stomatal conductance was higher in the younger stand (Table 2), and recent experiments in our laboratory demonstrated the high sensitivity of P. pinaster stomata to changes in the hydraulic conductivity (Ripullone et al. 2007).
Water-use efficiency and photosynthetic capacity After an accurate cross-dating of tree rings, the carbon isotope fraction (13C/12C) of wood material was analysed from a ‘‘synchronic’’ perspective, that is, comparing xylem tissues produced by different trees in the same year under the same environmental conditions. Results suggest a relationship between carbon isotope discrimination (13C) and age or height of stands (Fig. 7) that could be interpreted (Farquhar et al. 1989) as a decrease of long-term plant WUE (roughly the ratio of carbon gain to water losses). In turn, this translates to a positive relationship between 13C and CAI reduction (Fig. 8). Although not a proof of a causal link, the relationships depicted in Figs. 6–8 suggest a role of decreasing hydraulic conductance, mean conductance of tree’s foliage, and WUE in the decline of stand productivity. The slight increase of 13C with stand development observed in our chronosequence points in an opposite direction with respect to a number of previous studies. Indeed, a decrease of 13C (or an increase of long-term WUE) with age (or height) of trees was observed in several cases and often interpreted as an effect of increasing hydraulic and stomatal limitations with tree height, constraining water losses by transpiration more than carbon gain by photosynthesis (Panek 1996; Irvine et al. 2004). The hypothesis that metabolic limitations may compound hydraulic constraints during stand development has been advanced; for instance, Niinemets (2002) observed a significant decrease of Vcmax with tree height, and the application #
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Fig. 7. Relationship between stand age (upper panel) or stand height (lower panel) and carbon isotope discrimination of xylem (13C) concurrently formed in the four stands. Stand ages are shown for two 5 year intervals (*, 1991–1995; *, 1996–2000), whereas stand height is shown only for 1996–2000. Each point in the graph shows the mean of the eight trees sampled in each stand, and error bars are SEs. For the 1996–2000 series, the mean for each tree was obtained by averaging 13C values measured for individual tree rings in the period; for the 1991–1995 series, the entire growth period was represented by one bulk wood sample from each tree.
Carbon isotope discrimination (‰)
Carbon isotope discrimination (‰)
18.6
Fig. 8. Relationship between the reduction of current annual increment from its peak value (CAI reduction) and carbon isotope discrimination of xylem (13C) for two 5 year periods (*, 1991– 1995, left axis; *, 1996–2000, right axis); different scales were used on the left and right axes to make the similar trend of the relationship in the two periods clearer. Each point in the graph is the mean of eight trees sampled in each stand, and error bars are SEs. For the 1996–2000 series, the mean for each tree was obtained by averaging 13C values measured for individual tree rings in the period; in case of the 1991–1995 series, the entire growth period was represented by one bulk wood sample from each tree.
18.4 18.2 18.0 17.8 17.6 17.4
18.9
18.4
18.8
18.3
18.7
18.2
18.6
18.1
18.5
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60
70
80
20
40
60
80
17.5 100
CAI reduction (%)
Carbon isotope discrimination (‰)
Stand age (years) 18.4 18.2 18.0 17.8 17.6 17.4 17.2 4
6
8
10
12
14
16
18
20
Stand height (m)
of a coupled water and carbon transport model predicted a decline in maximum carboxylation capacity as hydraulic conductivity decreases with tree height (Katul et al. 2003). For testing whether a decline of photosynthetic capacity might account for the decline of WUE, A/Ci curves were successively measured across the pine chronosequence in S10, S35 and S75 stands. Although a slight decrease with stand age was apparent in some cases, no significant difference was found in photosynthetic capacity among stands (Table 2); this is in agreement with recent results by Delzon et al. (2004) in a P. pinaster chronosequence. Similarly, measurements of leaf nitrogen and chlorophyll content across the same chronosequence (S. Raddi, University of Firenze, Firenze, Italy, personal communication, 2006) showed no significant differences among stands of different ages.
Thus, measurements of photosynthetic capacity did not actually provide an explanation for the increase of 13C (decline of long-term WUE) with stand development. However, leaf-level measurements of photosynthetic capacity concerned only shoots exposed to full-light conditions for a limited time, whereas 13C in tree rings represents an integrated signal of photosynthetic activity throughout the whole crown, including both shaded and sunlit portions. Because shoots in young trees are often more horizontal and less densely leafed than in old trees and can be also more efficient at light interception (Hikosaka et al. 1999; Niinemets et al. 2005), the hypothesis cannot be excluded that the apparent decline of long-term WUE during stand development might be significantly affected by variations in leaf area distribution and efficiency in capturing light. Conclusions We can draw from this study the following conclusions: (i) we demonstrated an effect of hydraulic limitations on gas exchanges during stand development in maritime pine; (ii) the reduction in transpiration was found to counterbalance the observed increase in leaf-specific hydraulic resistance, which runs contrary to the hypothesis of a constant plant resistance through changes in the allocation pattern; (iii) the observed changes in plant water relations managed to maintain minimum leaf water potential remarkably constant, thus confirming the hypothesis of water-potential homeostasis; (iv) the decreased WUE, as estimated from carbon isotope discrimination, was not explained by a decline of photosynthetic capacity, and we suggest that other factors, such as changes in leaf area distribution and lightuse efficiency, might play a role. #
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Acknowledgements This research was granted by the EU CARBO-AGE project (ENV4-CT97-0577) and by the PRIN-2000 project (‘‘Carbon balance of forest ecosystems: physiological determinants, environmental constraints and age-related effects’’) funded by the Italian Ministero dell’Universita` e della Ricerca. We thank Luca Bernasconi, Lidia Consiglio, Roberto Geloni, Gianna Giovannoni, Marco Lauteri, Sabrina Raddi, and Maurizio Teobaldelli for assistance in field and laboratory work; Piero Piussi for constructive comments and valuable criticism; and the San Rossore Regional Park for allowing us to work in the forest.
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