• δ13C;
  • δ18O;
  • carbohydrates;
  • stomatal conductance;
  • water availability


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At eight different dates during the 2000 growing season, δ13C and δ18O were determined in the phloem of adult beech trees growing in natural beech stands in south-west Germany differing in stand density and local climate. In addition, stand transpiration, precipitation, photosynthetic active radiation, relative air humidity, water pressure deficit of the air, air and soil temperature, soil water potential, and sugar concentration of the phloem sap were determined directly and evaporation and canopy stomatal conductance were modelled. All parameters were related to δ13C. The study aimed to identify the time integral within which the δ13C of organic compounds transported in the phloem is an indicative measure of these environmental influences. δ13C of soluble carbon transported in the phloem was well correlated with mean stomatal conductance in a two-day integral prior to phloem sampling but did not depend on either light intensity or soil water availability. A strong positive relationship between δ13C and δ18O pointed to observed variation in δ13C of phloem sap being a result of variation in stomatal conductance. Bulk leaf δ13C was a poor indicator of changes in environmental conditions during the growing season. From these results we conclude that the analysis of δ13C in soluble carbon transported in the phloem is a reliable indicator of short-term changes in Ci/Ca. In contrast, the δ13C of structural carbon in beech foliage represents an integration of a range of factors that mask short-term influences responsible for Ci/Ca.


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The determination of carbon isotope composition in plant tissues is now widely used by plant eco-physiologists to integrate the influence of a range of environmental factors on plant performance. Due to their effects on internal concentration of CO2 (Ci), intercepted radiation as well as atmospheric and soil water deficits modify the ratio of 13C to 12C (expressed as deviation from PDB standard (δ13C)) in plant carbon (e.g. Leavitt & Long 1986; Livingston & Spittlehouse 1996; Korol et al. 1999). Stomatal closure due to water deficits reduces Ci, leading to an increase in δ13C (e.g. Guehl, Fort & Ferhi 1995; Lauteri et al. 1997). Plant water potential has hence been related to δ13C in leaves and wood and in turn to the availability of water (Damesin, Rambal & Joffre 1998; Warren, McGrath & Adams 2001). Alternatively, under light-limiting (but not water-limiting) conditions, photosynthesis and, consequently, Ci depends on radiation. As a consequence, organic carbon in leaves from the shaded part of the crown of trees is less depleted in 13C as compared to the sun-exposed crown (Leavitt & Long 1986).

The range of other influences on δ13C in plant organic matter includes external factors such as altitude and nutrition and plant internal factors such as hydraulic architecture of the water transport system (e.g. Högberg, Johannisson & Hällgren 1993; Walcroft et al. 1996; Korol et al. 1999; Hultine & Marshall 2000; Warren & Adams 2000).

The main difficulty in correlating δ13C signature in leaves of trees or tree rings with meteorological or edaphic parameters is the effect of carbon storage and re-translocation and related fractionation of carbon isotopes. The general consequences of re-use of carbon assimilated during previous growing seasons for the δ13C signatures of current leaves (and wood), remain poorly understood and of obvious significance, especially for deciduous trees. Although different authors (e.g. Dupouey et al. 1993; Livingston & Spittlehouse 1996; Macfarlane & Adams 1998; Barbour, Walcroft & Farquhar 2002) observed moderate to strong relationship between δ13C in the wood of annual growth rings and different parameters of water balance or water availability, Geßler et al. (2001) showed that both, radial growth and δ13C in tree rings of beech exhibited a variable time-lag with environmental conditions (rainfall, radiation) that precluded a significant correlation. In deciduous trees such as European beech, foliage that develops in spring is formed mainly from stored carbon and nutrients (Millard 1996; Kozlowski & Pallardy 1997) – newly assimilated carbon is mixed with the previously stored carbon to form the new leaves. Clearly, efforts to interpret δ13C signatures of current leaves in relation to prevailing conditions of light and moisture without understanding the influence of stored and re-used carbon are likely to fail (Brendel 2001).

A number of recent studies suggest that the δ13C of the phloem sap provides a strong guide to Ci/Ca during the actual growing season (Adams & Grierson 2001). Yoneyama et al. (1997) first reported carbon isotope ratios in phloem sap of wheat and Pate & Arthur (1998) and Geßler et al. (2001) developed and applied suitable methodologies for studying carbon isotopes in phloem sap in trees including eucalypts and European beech.

Development of the theoretical basis for the discrimination of stable isotopes of oxygen within plants, coupled with empirical studies demonstrating its practical application, has been another recent and significant development in ecophysiology. The δ18O signature shares dependence on stomatal conductance with δ13C signature but is not dependent on RubisCo activity (Farquhar, Barbour & Henry 1998; Barbour et al. 2000a). Hence, combined analysis of δ18O and δ13C in plant organic matter may help separate the effects of stomatal conductance and carbon fixation on δ13C (Scheidegger et al. 2000; Xu et al. 2000; Adams & Grierson 2001).

In the present study we assessed the effects of water availability [soil water potential (Ψs); precipitation (P)], radiation [photosynthetic active radiation (PAR)] and related environmental [air (Ta) and soil (Ts) temperature; relative air humidity (RH); water pressure deficit of the air (VPD); evaporation (E)] and physiological factors [stand transpiration (ST); canopy stomatal conductance (GS)] on the δ13C signature in the phloem. The study aimed to identify the time integral within which δ13C is an indicative measure for these environmental factors. A field experiment in a beech stand in southern Germany (Geßler et al. 2001; Fotelli et al. 2002) includes paired sites that differ mainly in aspect (south-west versus north-east) on either side of a small valley and, within each site, replicated plots of differing stand density. A previous study at these sites (Geßler et al. 2001) showed that water availability was least in the warm-dry south-west facing site as compared to the north-east facing site. However, analysis of phloem sap δ13C suggested this interpretation may have been confounded by differences in radiation. In the present study, both δ13C and δ18O signatures of phloem sap carbon were used to attempt to overcome this problem.


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Site description

General description

The experimental sites used for the present study are located in southern Germany (longitude: 8°40′ E; latitude: 48°00′ N), about 100 km south-south-west from Stuttgart in a low mountain range (Schwäbische Alb, 740–760 m a.s.l). Mean annual regional air temperature measured at a climate station of the DWD (Deutscher Wetterdienst, Offenbach, Germany), about 4 km from the experimental sites, is about 6.6 °C, and mean temperature during the growing season (May to October) about 11.5 °C. Mean annual precipitation is 856 mm with monthly maxima in June and July The sum of precipitation during the growing season (May to October) amounts to 410 mm.

The experimental sites are located on the two opposing sides (not more than 1000 m apart) of a single, narrow valley. One experimental site faces to the north-east (NE) and the other to the south-west (SW). Rainfall does not vary significantly across the valley (Geßler et al. 2001). The slope at both sites is moderately steep (NE: 58–100%; SW: 36–58%). Soil profiles are characterized as Terra fusca – Rendzina derived from limestone (Weissjura beta and gamma series) and are shallow on both sites, averaging less than 0.20 m depth of topsoil before becoming dominated by parent rock interspersed with pockets of organic matter and mineral soil. The soil profile at the SW site is especially rocky, containing more than 40% (volumetric basis) rocks and stones (> 63 mm diameter) in the top 0.20 m of the soil and increasing to 80% below 0.50 m. The soil at the NE site contains 15% rocks and stones in the uppermost 0.20 m of the soil and approximately 30% below 0.50 m. Soil pH (H2O) is 5.7 in the surface organic layer and 7.5 at 0.60 m depth.

On both sites, European beech (Fagus sylvatica L.) is the dominant species making up more than 90% of the total basal area of adult trees. The average age of the adult beech trees is 70–80 years with a mean height of between 25 and 27 m. The difference in aspects (NE, SW) produces a difference in radiation interception per m2 of inclined surface area with higher energy available on the SW site (Geßler et al. 2001). According to retrospective analyses of meteorological data, as well as the growth and water status of adult beech trees (Geßler et al. 2001) and beech seedlings (Fotelli et al. 2002), the SW-exposed site has permanently lower availability of water and higher air temperatures, than the NE-exposed site. Thus, the understorey vegetation differs between the two sites and the classification of the stand on the NE site is a Hordelymo-Fagetum and on the SW site, a Carici-Fagetum (Oberdorfer 1992).

On each site, two silvicultural (thinning) treatments plus controls (unthinned) were established in March 1999. The experimental design consisted of two blocks, each containing a single plot (each approx. 0.53 ha in size) of the two silvicultural treatments plus a control plot. The total basal area (BA) of trees on the untreated control plots varied between sites – on the NE site the mean BA in control plots was close to 27 m2 ha−1, while on the SW site the control plot BA was about 20 m2 ha−1. The two thinning treatments on both sites were BA = 15 and 10 m2 ha−1. Thinning decreased the leaf area index (LAI) from 5.16 (control) to 3.15 (BA = 15 m2 ha−1) and 1.68 (BA = 10 m2 ha−1) on the NE site, and from 5.12 to 3.24 and 2.12 on the SW site. One year after thinning, the density of understorey vegetation, other than natural regeneration of beech, increased in the thinned stands, in comparison with the controls (by approximately 25% on the NE site, and approximately 8% on the SW site; T. Paul & A. Reif, personal comm.).

Radiation, humidity and air temperature

Data from all microclimatic sensors were recorded every 30 s by CR23X-dataloggers (Campbell Scientific, Logan, UT, USA) and means calculated and stored every 30 min. Ta and RH were measured by HMP45D-Sensors (Vaisala, Helsinki, Finland) 1.5 m above ground (Holst et al. 2001). PAR was determined by Li-190SZ-Sensors (LiCor Inc., Lincoln, NE, USA). From the beginning of the growing season 2000 until October 2000 PAR measurements were only performed at 1.5 m above ground. Since October 2000, PAR has been available from measurements taken 6 m above the canopy for the NE and SW aspects. For the time before October, PAR above canopy level was calculated from PAR measurements below the canopy.

For a homogeneous stand the extinction of radiation in a canopy can be described by the Bouguer–Lambert law (Eqn 1):

  • PARbc = PARac exp(−LAI · k)(1)

where PARac is the PAR measured above the canopy, k is the extinction coefficient, and PARbc is the PAR reaching the ground (Baldocchi et al. 1984; Yang, Miller & Montgomery 1993; Kull et al. 1999).

The value of k was determined from measurements of PAR below and above canopy level during the 2001 growing season. Assuming comparable daily and seasonal courses of k between the two years, PAR was calculated by inverting Eqn 1.

PAR was measured via horizontally mounted radiation sensors that were influenced by the different aspect of the plots. Whereas in the morning the SW plot was shaded by the slope, in the afternoon the NE plot was influenced by a limiting horizon. On a monthly basis, the NE slope has only about 80–85% of PAR of the SW slope in winter when the elevation of the sun is less than in summer and the limited horizon has a strong influence. Throughout the growing season, and when the sun is close to its maximum elevation at this latitude, there is almost no difference in monthly PAR for both sites. The major difference between the sides of the valley that brings about the drier and warmer conditions on the SW slope is the differences in aspect: the SW slope is exposed to longer periods of radiation whereas the NE slope is more sheltered in the afternoon.

Soil water potential (Ψs) and soil temperature (Ts)

The value of Ψs was determined in four replicates at soil depths of 20, 40 and 60 cm using pressure transducer tensiometers (T4; UMS, Munich, Germany). Soil water potential was measured on the SW and NE control plots in large canopy gaps and under closed canopy. The Ψs measured in the large canopy gaps was regarded as representative of the water availability in the thinning treatments with a BA of 10 m2 ha−1, whereas the Ψs under the canopy was used for the control plots (S. Augustin, personal comm.). The Ψs data were collected every 30 min and calculated as daily means. The value of Ts was measured with Pt100 sensors at a soil depth of 5 cm at the control and the 10 m2 ha−1 treatment and were also calculated as daily means.

Transpiration, evaporation and canopy stomatal conductance

Prevailing conditions of water availability and climate were analysed by measuring xylem (water) flow densities using Granier-style probes and by scaling up flow densities from a single tree to the stand level (e.g. Granier et al. 1996). Flow densities in the water-conducting sapwood of beech were determined using the constant-heating method according to Granier (1985) and Köstner et al. (1996). Flux densities (FD; ml cm−2 sapwood area s−1) were determined every 5 min in seven to nine beech trees per plot and calculated as means of 30 min. Flux densities were converted to estimates of water use/stand transpiration (ST) using the stand sapwood area (SA; cm2 m−2) assuming that sap flux densities are uniform through the cross-sectional sap wood area according to the following equation (Granier et al. 1996):

  • ST = SA · FD(2)

Half hourly values of stand transpiration were summed for 24 h (l m−2 d−1 or mm d−1). The SA value of 25 representative trees of different breast height diameters (BHD) on each site was determined according to Glavac et al. (1989) and extrapolated to the stand level according to Granier et al. (1996).

Evaporation (E) was calculated using the water balance model WBS3, a forest-hydrological model that requires daily mean value of air temperature and daily total precipitation as meteorological inputs (Matzarakis et al. 2000). Time-independent input variables of the WBS3-simulations are: basal area of the stand, mixing ratio of deciduous trees, mixing ratio of coniferous trees, type of soil, maximum useable storage capacity of the soil, slope angle, slope direction and geographical latitude. For evapotranspiration, E, transpiration and interception of forests, validations of WBS3 showed a good agreement between results from model calculations and measurements for different areas and slopes (Fritsch 1998; Matzarakis et al. 2000).

As stomatal conductance is one of the factors determining δ13C of organic carbon, mean daily canopy stomatal conductance (Gs, mmol m−2 s−1; Fig. 2) was calculated using a simplified Penman–Monteith equation (according to Pataki, Oren & Phillips 1998):


Figure 2. Mean daily stand transpiration (ST; grey bars) and canopy stomatal conductance (Gs; black line) of adult beech trees on the SW and NE aspect in the control (C) and the thinning treatments (15 m2 ha−1; 10 m2 ha−1). ΣSΤ refers to the sum of transpiration during the growing season (15 April 2000–30 October 2000).

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  • image(3)

where Gc is canopy conductance, γ is the psychrometric constant (kPa K−1), λ is the latent heat of vaporization (J kg−1), ρ is the density of moist air (kg m−3), Js is mean daily sap flux density (g m−2 s−1), cp is the heat capacity of moist air (J kg−1 K−1), VPD is the water vapour pressure deficit (kPa), P is the atmospheric pressure (Pa), R is the gas constant (8.31 m3 Pa mol−1 K−1), T is air temperature (K), Js is mean daily sap flux density (g m−2 s−1) and LA is the leaf area (cm2 m−2). This calculation assumes, that: (1) sap flux densities are uniform throughout the cross-sectional sapwood area; (2) that stem capacitance can be neglected; and (3) that canopy aerodynamic conductance is much larger than Gs as reported by Whitehead & Jarvis (1981), which means that aerodynamic resistance can be neglected in comparison with stomatal resistance.

Plant material

Phloem sap was collected at about 0900 h MEZ at breast height of four to six adult beech trees per aspect and treatment according to the method described by Pate et al. (1998) for Eucalyptus globulus and Geßler et al. (2001) for European beech. The bark was cut with a scalpel to the depths of the wood at about 15% to horizontal. The ‘bleeding’ phloem sap was collected immediately with a Pasteur pipette. Since no phloem sap could be obtained in early spring (Geßler et al. 2001) the first samples were collected at the end of May. In total, phloem sap was collected on eight sampling days during the growing season 2000 (29 May, 28 June, 18 July, 1 August, 15 August, 29 August, 12 September, 25 September). Different trees were used at each sampling date in order to avoid the cumulative effects of bark damage.

On selected sampling dates [28 April (bud burst), 18 July, 25 September) four trees were climbed in each plot and foliage (expanding leaves on the first date, fully expanded leaves on the other dates) was sampled from two branches excised from the sun-exposed part of the tree crowns. Since δ13C signatures of leaves increased with increasing branch length (data not shown), samples were taken at a fixed position (approximately 3 m branch length) in the sun-crown.

Different trees were used for phloem sap and foliage sampling in order to avoid artefacts in the composition of phloem sap as a consequence of the harvest of foliage. Material from neighbouring trees of the same social class (co-dominant or dominant) was collected where required to prevent cumulative damage to sample trees.

Determination of sugars in the phloem sap

For the determination of soluble carbohydrates, 5–10 µL of phloem sap were diluted to 500 µL with demineralized water. Aliquots of 100 µL were injected into a high-performance liquid chromatography system (Dionex DX 500; Dionex, Idstein, Germany). Separation of sugars was achieved on a CarboPac 1 separation column (250 × 4.1 mm; Dionex) with 36 mM NaOH as an eluent at a flow rate of 1 mL min−1. Carbohydrates were measured by means of a pulsed amperometric detector equipped with an Au working electrode (Dionex DX 500; Dionex). Individual carbohydrates that eluted 8–16 min after injection were identified and quantified by internal and external standards.

Carbon and oxygen isotope composition

Leaf material was oven-dried for 3 d at 65 °C and, subsequently, ground and homogenized with a ball mill into a fine powder. Samples of 1–2 mg were transferred into tin capsules (Thermo Quest, Milan, Italy). An aliquot of undiluted phloem (10 µL) was pipetted into tin (δ13C analysis) or silver (δ18O analysis) capsules and oven-dried for 30 min at 65 °C.

Samples were injected into an elemental analyser (NA 2500; CE Instruments, Milan, Italy) for δ13C analysis and into a high temperature conversion/elemental analyser (TC/EA; Finnigan MAT GmbH, Bremen, Germany) for δ18O analysis, both coupled to an isotope ratio mass spectrometer (Delta Plus; Finnigan MAT GmbH) by a Conflo II interface (Finnigan MAT GmbH). The isotopic values are expressed in delta notation (in ‰ units), relative to VPDB (Vienna Pee Dee Belemnite) for carbon and VSMOW (Vienna Standard Mean Ocean Water) for oxygen.

Statistical analysis

Statistical analyses were conducted using SPSS 10.05 (SPSS, Inc., Chicago, IL, USA). The effects of site (NE, SW) and treatment (thinning) on the measured parameters were assessed using univariate GLM-ANOVA procedures. Correlations between δ13C and environmental parameters were calculated using the bivariate correlation procedure. Regression lines between δ13C and Gs and δ13C and δ18O were determined by linear regression analysis.


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Soil water potential and soil temperature

Figure 1 shows the seasonal patterns of Ψs at depths of 20 and 40 cm (60 cm not shown for clarity) during the growing season 2000. Data shown are for the closed canopy (Fig. 1a & b) and large canopy gaps (Fig. 1c & d) – supposed to be representative for the thinning treatments with a BA of 10 m2 ha−1– on the SW and NE site. In general, Ψs was more negative on the SW site. This pattern was most pronounced at both soil depths in May and August/September Reduced canopy cover increased water availability on the NE site as indicated by the increase in Ψs. On the SW site, there was only a slight increase in Ψs as a consequence of reduced canopy cover. Ts showed comparable seasonal patterns among aspects and control and thinning (BA of 10 m2 ha−1) treatments (data not shown) Mean Ts during the growing season amounted to 13.1 °C on the NE and 13.9 °C on the SW control site. Thinning to a BA of 10 m2 ha−1 increased soil temperatures on both sites: mean Ts during the growing season amounted to 14.4 °C on the NE site and to 15.1 °C on the SW site.


Figure 1. Soil water potential on the SW and NE aspect under the closed canopy of the control sites (a, b) and in large canopy gaps (c, d). Soil water potential was measured using four replicate tensiometers placed at depths of 20 cm (—) and 40 cm (- - - -).

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Transpiration, evaporation and canopy stomatal conductance

The sum of stand transpiration during the growing season 2000 (ΣST) was comparable between the NE and SW sites whereas the seasonal distribution of ST varied between the two aspects, with higher ST at the beginning of the growing season (until mid June) on the SW site (Fig. 2). Thinning reduced ST on both sites to c. 70% (15 m2 ha−1) and approximately 50% (10 m2 ha−1) of that of the control stand.

Evaporation from the soil was comparable between sites and increased with progressive thinning to approximately 130% (15 m2 ha−1) and 180% (10 m2 ha−1) of that from the control plots.

Mean Gs during the entire growing season amounted to approximately 85 mmol m−2 s−1 in the control treatments on both sites (Fig. 2). At the beginning of the growing season, Gs was higher on the SW control site compared to the respective treatment on the NE site (in May, SW: 55 mmol m−2 s−1; NE: 37 mmol m−2 s−1). This pattern changed in midsummer (in July: SW: 103 mmol m−2 s−1; NE: 170 mmol m−2 s−1) and at the end of the growing season (in September: SW: 50 mmol m−2 s−1; NE: 71 mmol m−2 s−1). Thinning reduced mean Gs slightly on the NE site (mean Gs during the growing season: 80 nmol m−2 s−1 at a BA of 15 m2 ha−1 and 67 nmol m−2 s−1 at a BA of 10 m2 ha−1) and more intensively on the SW site (61 nmol m−2 s−1 at a BA of 15 m2 ha−1 and 48 nmol m−2 s−1 at a BA of 10 m2 ha−1).

δ13C in the foliage and phloem

During the whole growing season, δ13C in foliage of beech was not significantly different between sites or among treatments (Table 1). In April, immediately after bud break, when young leaves were expanding, δ13C showed a seasonal maximum. The mean δ13C signature of leaves from both sites and all treatments amounted to −24.5‰ in April and decreased to −26.7‰ in July and further to −27.3‰ in September.

Table 1.  Effects of site (S; north-east, NE; south-west, SW) and treatment (T; control, 15 m2 ha−1, 10 m2 ha−1) on carbon isotope composition δ13C of phloem and leaves of beech
  1. The significance of the main effects from analysis of variance and the standard deviation (n = 4) for each treatment are shown (NS, not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001).

δ13C in phloem
 NE-C −29.9 ± 0.2−27.1 ± 0.7−29.8 ± 0.4−27.6 ± 0.1−27.8 ± 0.4−27.7 ± 0.1−27.3 ± 0.3−27.2 ± 0.4
 NE-15 −28.9 ± 0.5−25.7 ± 0.1−29.3 ± 0.1−27.9 ± 0.3−27.7 ± 0.3−26.9 ± 0.7−27.1 ± 0.3−27.7 ± 0.3
 NE-10 −29.4 ± 0.7−25.9 ± 0.4−29.9 ± 0.3−27.8 ± 0.1−27.7 ± 0.2−27.0 ± 0.3−26.9 ± 0.2−27.4 ± 0.2
 SW-C −30.3 ± 0.1−25.7 ± 0.7−29.4 ± 0.5−27.7 ± 0.2−27.8 ± 0.2−26.8 ± 0.3−26.8 ± 0.2−27.1 ± 0.2
 SW-15 −30.1 ± 0.3−25.7 ± 0.7−28.5 ± 0.3−27.6 ± 0.5−27.6 ± 0.2−26.8 ± 0.4−26.6 ± 0.3−27.2 ± 0.4
 SW-10 −29.8 ± 0.5−25.4 ± 0.6−28.3 ± 0.2−27.6 ± 0.1−27.2 ± 0.2−27.2 ± 0.2−26.3 ± 0.3−27.2 ± 0.3
 Main effects S: **S: ***S: ***S: NSS: NSS: NSS: *S: NS
T: *T: NST: ***T: NST: NST: NST: *T: NS
δ13C in leaves
 NE-C−24.6 ± 0.3  −26.9 ± 1.0    −27.3 ± 0.7
 NE-15−24.1 ± 0.2  −26.4 ± 0.4    −27.4 ± 1.0
 NE-10−24.9 ± 0.9  −26.7 ± 0.6    −27.3 ± 0.8
 SW-C−25.0 ± 0.9  −27.3 ± 0.5    −28.0 ± 0.6
 SW-15−24.1 ± 0.4  −26.2 ± 0.1    −26.7 ± 0.4
 SW-10−24.6 ± 0.7  −26.7 ± 0.7    −27.3 ± 0.9
 Main effectsS: NS  S: NS    S: NS
T: NS  T: NS    T: NS

In contrast, significant effects of site and treatment on δ13C on phloem sap were observed, especially at the beginning of the growing season with a tendency towards decreased 13C depletion on the SW site and in the thinning treatments (Table 1). During the seasonal course δ13C showed a maximum on all sites and treatments in June.

Correlation of phloem δ13C with physiological, climatic and pedospheric parameters

In contrast to δ13C in leaves, carbon isotope signature in phloem sap varied between sampling dates during the growing season and also between sites and treatments. Therefore we tested: (1) which environmental parameter(s) was (were) responsible for the δ13C patterns of phloem allocated carbon; and (2) quantified the time integral within which δ13C was influenced by this (these) parameter(s). Correlation analyses were performed between δ13C of phloem-transported carbon and different physiological (ST, Gs) meteorological (E, Ta, RH, VPD, PAR) and pedospheric parameters (Ψs in 20, 40 and 60 cm; Ts). These different parameters were calculated as mean values of 1–22 d prior to the time of phloem sap collection (Table 2).

Table 2.  Correlation between δ13C in the phloem and physiological, meteorological and pedospheric parameters, averaged over different time spans prior to phloem sap collection
  1. The table shows Pearson's correlation coefficients for bivariate correlation analysis between δ13C of all phloem samples collected at eight points of time during the growing season from each site and treatment (except for the correlation with (1) Ts and Ψs: only control/closed canopy and 10 m2 ha−1 treatments/canopy gaps and (2) PARC: only control treatments) and the respective stomatal conductance (Gs), transpiration (ST), evaporation (E), air temperature (Ta), relative humidity (RH), precipitation (P), water vapour pressure deficit of the air (VPD), PAR, soil water potential in 20, 40 and 60 cm (Ψs 20, 40 and 60) and temperature of the soil (Ts) as mean values of 1–22 d prior to the phloem sap collection. n= 16–48; *: P < 0.05, **: P < 0.01.

δ13C versus
Ψs 400.01−0.03−0.06−0.05−0.04−0.04−0.04−0.08−0.13−0.15−0.15
Ψs 600.−0.010.02

Canopy stomatal conductance (Gs) was clearly and closely related to δ13C (greatest R) within a time integral of 2 d prior to phloem sap collection. Figure 3 shows the relationship between these two parameters. For longer time integrals, R decreased steadily. Although δ13C was not correlated significantly with ST, E and Ψs, significant influences of T, RH and VPD were observed. The correlation coefficients, however, were distinctly smaller than observed for Gs. For RH and VPD, the correlation coefficient was greatest when the values were averaged for a period of 3 d prior to phloem sap sampling. The correlation between δ13C and temperature was more or less constant for the observed period of time. δ13C was not correlated significantly with the amount of PAR (as measured above the canopy, line PAR in Table 2). The value of Ci and hence δ13C, depends on radiation only under light-limiting conditions. As it may be supposed that these conditions prevailed only in the unthinned control treatments, the correlation between δ13C and PAR was calculated exclusively for control stands on both sites (line PARC in Table 2), additionally. However, when the canopy was closed and not influenced by selective felling, δ13C did also not depend significantly on PAR.


Figure 3. Regression between δ13C in phloem sap and the average daily mean stomatal conductance (Gs) over a period of 2 d prior to sampling. The regression analysis includes measurements from eight sampling times during the growing season 2000 (29 May, 28 June, 18 July, 1 August, 15 August, 29 August, 12 September, 25 September) made in all three treatments (control and the two thinning treatments BA 15 m2 ha−1 and BA 10 m2 ha−1) on both aspects (NE and SW). SWC, ▪; SW 15 m2 ha−1, □; SW 10 m2 ha−1, ○; NEC, ▴; NE 15 m2 ha−1, ▵ NE, 10 m2 ha−1, +.

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δ18O in the phloem sap

In addition to correlation analysis between δ13C and physiological, meteorological and pedospheric parameters, we examined the relationship between δ13C and δ18O of phloem sap (Fig. 4). A strong and positive linear relationship (R2 = 0.63; P < 0.001) was observed between the two parameters with a slope of 0.90‰δ13C per 1‰δ18O (i.e. 1.11‰δ18O per 1‰δ13C)


Figure 4. Regression between δ13C and δ18O of the organic matter of phloem sap. The figure includes measurements from eight different sampling times during the growing season 2000 (29 May, 28 June, 18 July, 01 August, 15 August, 29 August, 12 September, 25 September) made in all three treatments (control and the two thinning treatments BA 15 m2 ha−1 and BA 10 m2 ha−1) on both aspects (NE and SW).

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Correlation analyses between δ18O in the phloem sap and environmental parameters produced – as observed for δ13C – highest correlation coefficients for a 2–3 d integral prior to sampling (Table 3). The value of δ18O was influenced less intensively by Gs (R = − 0.69) as compared to δ13C, whereas correlation to the atmospheric parameters RH (R = 0.68) and VPD (R = 0.58) produced higher correlation coefficients. No correlation to Ψs and Ts was observed.

Table 3.  Correlation between δ18O in the phloem and physiological, meteorological and pedospheric parameters, averaged over different time spans prior to phloem sap collection
δ18O versusDays
  1. The table shows Pearson's correlation coefficients for bivariate correlation analysis between δ13C of all phloem samples collected at eight points of time during the growing season from each site and treatment. All environmental variables shown in Table 2 were tested, but only the ones producing significant correlation are displayed. Highest correlation coefficients were obtained for integrals 2–4 d prior to the phloem sap collection, hence only these integrals are shown. n= 48; *: P < 0.05, **: P < 0.01.


δ13C and carbohydrates in the phloem sap

Sucrose, the dominant sugar in the collected phloem sap, made up between 95 and 99.8% of total sugar between June and September In May however, when sugar concentrations were at a minimum (170–240 mM), fructose and glucose together made up approximately 40% of total sugar. In the different treatments on the SW site, seasonal patterns of both δ13C and sugars were comparable and maxima and minima of both variables were observed on the same sampling date (Fig. 5). Hence, a strong and significant regression relation was obtained between the two parameters (for all treatments on the SW site: R2 = 0.59; P < 0.001). Comparable and synchronous seasonal patterns of phloem sugars and δ13C were not observed for the different treatments on the NE site. Thus, the regression relation between the two parameters was weak and not significant (R2 = 0.13, P > 0.05).


Figure 5. Seasonal pattern of carbon isotope composition (▪) and sugar concentrations (○) in the phloem sap of beech from different sites (SW, NE) and different treatments (control, 15 m2 ha−1 and 10 m2 ha−1) during the growing season 2000. Data shown are means (± SD) from four to six trees per site and treatment.

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With the present approach, we combined the analysis of carbon isotopes in phloem sap (Yoneyama et al. 1997; Pate & Arthur 1998; Geßler et al. 2001), that putatively allows the analysis of short-term changes in Ci/Ca, with the model of variation in δ13C and δ18O proposed by Scheidegger et al. (2000). This model can, theoretically, differentiate between the effects on Ci of stomatal conductance and those of carbon fixation. In addition, we related δ13C signatures in the phloem to different environmental parameters averaged over different periods of time in order to characterize the time integral within which δ13C of phloem carbon is an indicator for particular environmental conditions.

Our study was conducted within replicated sites that differed in aspect (NE versus SW) and, hence, in radiation intercept and air temperature (Geßler et al. 2001), and in silvicultural treatment, that increased variation in water availability, temperature and light availability. Thinning resulted in a decrease in stand transpiration as previously observed for beech stands (Breda, Granier & Aussenac 1995) and a slight increase in evaporation from the soil on both sites. However, soil water availability for the remaining trees, as indicated by soil water potential (Fig. 1), increased only on the NE site under the open canopy – it remained constant or even decreased on the SW site, even though it never decreased under a value < −0.08 MPa. However, even a small depletion of soil water availability within the observed range may influence water balance of drought-sensitive beech, as indicated by decreasing plant water potential and xylem flow rates (Geßler et al. 2001) and changes in N metabolism (Fotelli et al. 2002).

Our initial hypothesis was that the differences in water availability (and radiation) between sites and treatments were reflected by δ13C in the phloem. In spite of this, no significant correlation was observed between δ13C in the phloem sap and water potential at soil depths of 20, 40 or 60 cm when the whole growing season was considered (Table 2). In contrast, Geßler et al. (2001) could attribute site-specific differences (NE versus SW) in phloem δ13C to differences in soil water availability in 1999. However, this effect was only observed during a one-month drought period with low amounts of rainfall, whereas in 2000 precipitation was evenly distributed during the growing season. Monthly rainfalls were not less than 90 mm and extreme shortages of water were unlikely.

Moreover, the amounts of water transpired by the trees (Fig. 2) also did not reflect the soil water availability (R2 = 0.09, P = 0.05 for Ψs20; R2 = − 0.07, P = 0.11 for Ψs40; R2 = − 0.04, P = 0.40 for Ψs60). On the contrary, transpiration was significantly, albeit weakly correlated with vapour pressure deficit (R2 = 0.3, P < 0.001). This weak correlation is an indicator of the number of influences on transpiration and their variation in influence throughout the growing season (Jones 1998). The weak correlations between stand transpiration and vapour pressure deficit on the one hand and between δ13C in the phloem and vapour pressure deficit on the other hand, additionally illustrated the lack of interdependence between δ13C in the phloem and transpiration (Table 2).

In contrast to the factors discussed above, carbon isotope composition was closely related to calculated stomatal conductance (Fig. 3), especially when conductance was integrated over a two-day period prior to phloem sampling. If phloem transport rates between 0.5 m h−1 and 1.0 m h−1 are assumed (Zimmermann & Braun 1971), transport of newly assimilated carbohydrates, from the foliage of the canopy to the base of the trunk, should take between 25 and 54 h in the trees examined. Hence, it is plausible that carbohydrates collected at the base of the trunk of beech carry an isotopic signal that is representative for physiological or environmental conditions of the previous two days.

There was also a significant – albeit less strong – correlation of phloem δ13C to VPD, RH and Ta. This finding indicates – as observed for transpiration – that Gs may not only be controlled by atmospheric factors but by different and potentially varying influences during the growing season. Since δ13C in the phloem did not correlate with PAR, it is concluded that Ci was only affected by stomatal constraints and not by limitations in carbon fixation under the environmental conditions prevailing at the sites studied in 2000. This finding is in contrast to results of a previous study carried out in 1999 at the same sites (Geßler et al. 2001), which showed evidence that the increase in light availability due to thinning influenced phloem δ13C when water supply was limited. Again, this observation was made in a drought period in 1999, whereas there were no comparable meteorological conditions during the 2000 growing season.

In order to test the hypothesis that δ13C was only affected by Gs and not by the activity of the RubisCo, we correlated δ13C of organic compounds in the phloem with δ18O. This procedure proposed by Scheidegger et al. (2000) and Barbour et al. (2002) based on suggestions from Farquhar, Ehleringer & Hubick (1989), and Farquhar et al. (1998) provides a qualitative distinction between the effects on δ13C of stomatal conductance from those of a change in photosynthetic capacity. Enrichment of 18O in leaf water depends on the ratio of the water pressures in the atmosphere and in the gaseous space within the leaf and is a function of back-diffusion of water from the sites of evaporation being opposed by convection of source water to these sites via transpiration (Dongman et al. 1974; Farquhar et al. 1998; Barbour et al. 2002). A range of studies show that this enrichment in leaf water is reflected in the oxygen isotope signature of organic matter synthesized in the leaves and, putatively, transported in the phloem (DeNiro & Epstein 1979, Yakir 1992; Farquhar et al. 1998; Roden & Ehleringer 1999a, b; Barbour et al. 2000a). Since greater stomatal conductance cools the leaf and reduces internal water vapour pressure, δ18O in organic matter is a potentially useful tool to characterize stomatal conductance, independent from effects of carbon fixation (Adams & Grierson 2001). The significant correlation between δ13O of the organic compounds in the phloem and Gs (Table 3; R =−0.69; P < 0.01) supports this hypothesis.

If the source of variation of δ13C in organic matter is changing photosynthetic capacity, δ18O should be unaffected. We have shown that this was not the case – there was instead a strong positive correlation between δ13C and δ18O (Fig. 4), a pattern related to stomatal and not photosynthetic control of δ13C (Farquhar et al. 1998; Scheidegger et al. 2000). The slope of the relationship between the two parameters (1.11‰ increase in δ18O per 1‰ increase in δ13C) was within the range described in literature (between 0.32‰δ18O per 1‰δ13C (Sternberg, Mulkey & Wright 1989) and 2.9‰δ18O per 1‰δ13C (Barbour et al. 2000b).

The observed differences between the coefficients obtained for correlation between δ13C or δ18O and environmental parameters (Tables 2 & 3) again confirm theoretical assumptions, that δ13C is mainly controlled by Gs under non-limiting light conditions (Farquhar, O’Leary & Berry 1982), whereas the ratio of water pressure of the air outside and inside the leaf, the latter influenced by Gs, is the parameter controlling evaporative enrichment of 18O (Farquhar et al. 1998; Adams & Grierson 2001).

A strong relationship has previously been demonstrated between sugar concentrations and δ13C in the phloem sap of Eucalyptus globulus. Both increased when water availability decreased (Pate et al. 1998; Pate & Arthur 1998). Geßler et al. (2001), however, were unable to observe any significant correlation between δ13C and sugar concentration in the phloem of European beech at the same stands examined here. Nevertheless, the previous study was of relatively short duration (sampling in August and September 1999) and used combined data from SW and NE sites. Figure 5 shows that a close correlation between sugar concentration and carbon isotope composition does exist for beech, but only on the SW site. This finding is again in agreement with the results of Pate & Arthur (1998), who described an almost exactly synchronous seasonal pattern of sugar concentrations and δ13C but only for a site with low water availability – correlations were less strong on a well-watered site. It is concluded that determination of sugar contents in the phloem of beech is not a reliable indicator for δ13C as proposed by Pate & Arthur (1998) for E. globulus.

Leaf δ13C did not vary between sites and treatments, hence reflecting poorly the temporary differences in environmental conditions between the different aspects and stand densities. The strong enrichment of bursting buds with 13C in April is likely to be a result of remobilization of carbon originating from stored starch that carries a δ13C signature that is about 4‰ greater than triose-P originating from the pentose phosphate cycle (Schmidt & Gleixner 1998). The reasons for this greater δ13C signature of stored and remobilized carbon remain a subject for further research.

We conclude that the analysis of δ13C in soluble carbon transported in the phloem is a reliable indicator of short-term changes of Ci/Ca. In leaves, δ13C is masked by the long-term integrating δ13C of structural carbon. If additional analysis of δ18O is performed, this approach can be used to differentiate between the effects of stomatal conductance and carbon fixation within a time integral of a few days.


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This study was part of the SFB 433 and was financially supported by the Deutsche Forschungsgemeinschaft under contract number DFG Re515/13–1. C.K. acknowledges the support of a fellowship from Landesgraduiertenförderung Baden-Württemberg. The authors thank Sabine Augustin and Ernst Hildebrand (Institute for Soil Science and Forest Nutrition, University of Freiburg) for providing the soil water potential data and Peter Escher for isotope analysis.


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