Interactive comment on “ Carbon flux to woody tissues in a beech / spruce forest during summer and in response to chronic elevated O 3 exposure ”

Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract The present study compares the dynamics in carbon (C) allocation of adult deciduous beech (Fagus sylvatica) and evergreen spruce (Picea abies) during summer and in response to seven-year-long exposure with twice-ambient ozone (O 3) concentrations (2 × O 3). Focus was on the respiratory turnover and translocation of recent photosyn-5 thates at various positions along the stems, coarse roots and soils. The hypotheses tested were that (1) 2 × O 3 decreases the allocation of recent photosynthates to CO 2 efflux of stems and coarse roots of adult trees, and that (2) according to their different O 3 sensitivities this effect is stronger in beech than in spruce. Labeling of whole tree canopies was applied by releasing 13 C depleted CO 2 (δ 13 C 10 of −46.9‰) using a free-air stable carbon isotope approach. Canopy air δ 13 C was reduced for about 2.5 weeks by ca. 8‰ in beech and 6‰ in spruce while the increase in CO 2 concentration was limited to about 110 µL L −1 and 80 µL L −1 , respectively. At the end of the labeling period, δ 13 C of stem CO 2 efflux and phloem sugars was reduced to a similar extend by ca. 3–4‰ (beech) and ca. 2–3‰ (spruce). The fraction of labeled 15 C (f E,new) in stem CO 2 efflux amounted to 0.3 to 0.4, indicating slow C turnover of the respiratory supply system in both species. Elevated O 3 slightly stimulated the allocation of recently fixed photosynthates to stem and coarse root respiration in spruce (rejection of hypothesis I for spruce), but resulted in a significant reduction in C flux in beech (acceptance of hypotheses I and II). The 20 distinct decreased in C allocation to beech stems indicates the potential of chronic O 3 stress to substantially mitigate the C sink strength of trees on the long-term scale.

thates at various positions along the stems, coarse roots and soils. The hypotheses tested were that (1) 2 × O 3 decreases the allocation of recent photosynthates to CO 2 efflux of stems and coarse roots of adult trees, and that (2) according to their different O 3 sensitivities this effect is stronger in beech than in spruce.
Labeling of whole tree canopies was applied by releasing 13 C depleted CO 2 (δ 13 C 10 of −46.9‰) using a free-air stable carbon isotope approach. Canopy air δ 13 C was reduced for about 2.5 weeks by ca. 8‰ in beech and 6‰ in spruce while the increase in CO 2 concentration was limited to about 110 µL L −1 and 80 µL L −1 , respectively. At the end of the labeling period, δ 13 C of stem CO 2 efflux and phloem sugars was reduced to a similar extend by ca. 3-4‰ (beech) and ca. 2-3‰ (spruce). The fraction of labeled 15 C (f E,new ) in stem CO 2 efflux amounted to 0.3 to 0.4, indicating slow C turnover of the respiratory supply system in both species. Elevated O 3 slightly stimulated the allocation of recently fixed photosynthates to stem and coarse root respiration in spruce (rejection of hypothesis I for spruce), but resulted in a significant reduction in C flux in beech (acceptance of hypotheses I and II). The

Introduction
Tropospheric ozone (O 3 ) is a major component of global climate change (IPCC, 2007), mitigating the carbon (C) sink strength of forest trees and ecosystem productivity (Sitch Since the flux of current photosynthates is considered an important driver of woody tissue and soil respiration in forests (Ryan et al., 1996;Högberg et al., 2001), limited C availability caused by O 3 stress may affect the respiratory activity and growth of stems and total belowground C allocation (Matyssek et al., 1992;Günthardt-Goerg et al., 1993;Coleman et al., 1996). As a result, root biomass and sugar concentrations 15 may be reduced (Grulke et al., 1998(Grulke et al., , 2001. Highlighting the phototoxic potential of O 3 to Central-European forests, Pretzsch et al. (2010) reported a 40% decrease in stem growth of adult beech upon eight years of twice-ambient O 3 exposure, whereas spruce showed no significant growth response. Likewise, in phytotron experiments on juvenile beech, reduced allocation of recent photosynthates to stems was identified as 20 the mechanistic basis for reduced stem growth in responses to 2 × O 3 (Kozovits et al., 2005a,b;Ritter et al., 2011).
Dynamics in C allocation of adult trees in response to chronically elevated O 3 concentrations are investigated and clarification is particularly needed for respiratory C fluxes of woody tissues. Here, we compare the allocation of recent photosynthates to 25 the respiratory turn-over in stems, coarse roots and soils in adult beech and spruce in a naturally grown forest. We noted that CO 2 efflux sampled from stem and root positions may be affected by xylem-transported CO 2 deriving from lower stem regions and/or root respiration (Teskey et al., 2008 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | CO 2 efflux was recently concluded to be rather small (Gebhardt, 2008;Aubrey and Teskey, 2009;Ubierna et al., 2009). In accordance with their contrasting O 3 sensitivity, we hypothesized that (1) 2 × O 3 decreases allocation of recent photosynthates to stem and coarse root CO 2 efflux of adult trees and (2) that this effect is stronger in beech than in spruce. To this end, 5 we took advantage of a unique free-air O 3 fumigation experiment employed in a mixed forest with adult beech and spruce trees . Stable carbon isotope labeling was performed on these trees using the isoFACE exposure system (Grams et al., 2011). In view of hypothesis evaluation, focus was on translocation of recent photosynthates and CO 2 efflux at various positions along the stems and coarse roots.

Experimental design
The study was carried out during August/early September 2006 in a 60 to 70-year-old mixed beech/spruce stand at "Kranzberger Forst" in Southern Bavaria, near Freising, Germany (elevation 485 m a.s.l., 48 • 25 N, 11 • 39 E;Pretzsch et al., 1998 (Nunn et al., 2002;Werner and Fabian, 2002 Nunn et al., 2005a). O 3 concentrations in the 2×O 3 treatment were enhanced by a factor of 1.6 because of the maximum level of 150 µL L −1 (see above). Continuous stable carbon isotope labeling was performed from 18 August through 5 September and 26 August through 12 September in beech and spruce, respectively, using a free-air stable carbon isotope exposure system ("isoFACE", for details see Grams et al., 2011). 10 In brief, from 07:00 through 19:00 LT, 13 C-depleted CO 2 (δ 13 C of ca. −46.9‰) was homogenously released into the canopy of three study trees in each O 3 regime and species (total of 12 trees) by means of micro-porous tubes. During label exposure, O 3 concentrations (means ± SE) were 29.7 ± 6.9 (1 × O 3 ) and 49.3 ± 11.9 nL L −1 (2 × O 3 ; Fig. 1a). Photosynthetic photon flux density (PPFD) was moderate due to frequently 15 overcast sky and occasional precipitation (48 and 32 mm during beech and spruce labeling period, respectively, Fig. 1b).

Isotope-ratio mass spectrometry (IRMS)
Gas samples were analyzed for δ 13  (infra-red gas analyzer (IRGA), Binos 4b.1, Rosemount AG, Hanau) and sampled once a day (∼12:00 LT) using a 100 mL syringe. Gas samples were flushed through 12 mL exetainer vials and analyzed as detailed above. During labeling, δ 13 C of canopy air was effectively decreased. Compared to the unlabeled beech control, mean reductions in sun and shade crowns under 1 × O 3 were 10 8.1 ± 0.2 and 8.9 ± 0.3‰, respectively, and under 2 × O 3 9.2 ± 0.4 and 8.4 ± 0.5‰, respectively (Table 2b). In spruce, mean reductions under 1 × O 3 was 6.0 ± 0.6‰ and 6.3±0.8‰, respectively, and under 2×O 3 7.5±0.9‰ and 6.5±0.7‰, respectively (Table 2a). CO 2 concentration in the canopy air of beech was under both O 3 regimes increased by about 110 µL L −1 , and in spruce by about 80 µL L −1 (Table 2a). In both 15 species, [CO 2 ] and δ 13 C of canopy air were similar each before and on the last day of labeling. Release of CO 2 and thus label application in beech exceeded that of the spruce experiment. The increase in CO 2 concentration of the canopy air did not affect the sap flow of labeled trees, suggesting unchanged stomatal conductance at the leaf level (Grams et al., 2011). Increase in leaf internal to external CO 2 concentration was 20 assumed to be small (< 0.02) and therefore, changes in photosynthetic discrimination against 13 C were calculated to stay below 0.4‰ (Grams et al., 2011).

Assessment of stem and coarse root CO 2 efflux
Stem and coarse root CO 2 efflux (E ) of labeled and unlabeled control trees was assessed by means of a computer-controlled open gas exchange system (for details Introduction Germany) were attached at a lower and upper stem position and at one coarse root per tree (except for the unlabeled control spruce tree). Chambers were covered with aluminized polyester foil to avoid refixation of efflux CO 2 by corticular photosynthesis. For assessment of CO 2 efflux, chambers were connected through PVC tubing to an IRGA (Binos 4b, Emerson Process Management, Weißling, Germany). Stem CO 2 ef-5 flux was based on the volume (V in m 3 ) of the stem sector behind the chamber (i.e. living tissue of bark and sapwood) and coarse root CO 2 efflux on the totally enclosed coarse root volume, respectively (Desrochers et al., 2002;Saveyn et al., 2008).

δ 13 C of stem and coarse root CO 2 efflux
Data on δ 13 C of CO 2 efflux (δ 13 C E ) sampled from stems and coarse roots are shown 10 as 24 h-means (± SE). Coarse root δ 13 C E was assessed once per day (between 10:00 and 13:00 LT) by means of a closed respiration system (for details see Grams et al., 2011). A total of six 12 mL exetainer vials were subsequently flushed with chamber air of increasing CO 2 concentration and δ 13 C E of coarse roots was calculated according to the "Keeling Plot approach" (Keeling, 1958(Keeling, , 1961. Air from stem respiration chambers 15 was automatically sampled in 12 mL exetainer vials, which were flushed with sample gas for six minutes each, at a flow rate of 0.15 L min −1 . A total of eight samples per day and chamber were assessed. Isotopic signature of CO 2 efflux of the stem was calculated after Eq. (1) using a two end-member mixing model. 20 where, [CO 2 ] sample =CO 2 concentration of sample gas from a stem respiration chamber (µLL −1 ), [CO 2 ] reference =CO 2 concentration of reference gas from an empty chamber (µLL −1 ), δ 13 C sample =δ 13 C of sample gas from a stem respiration chamber (‰) and δ 13 C reference =δ 13 C of reference gas from an empty chamber (‰). We considered that stem CO 2 efflux may not only consist of local tissue-respired CO 2 , but may be biased by xylem-transported CO 2 deriving from lower stem parts and/or root respiration (Teskey et al., 2008). The absent correlation between xylem sap flow and stem respiration rate or δ 13 C E suggests limited interference of xylemtransported CO 2 with stem CO 2 efflux (data not shown).

Fraction of labeled C in stem respiration
The fraction of labeled carbon (f E, new ) in CO 2 efflux (E ) was calculated following Lehmeier et al. (2008) and Gamnitzer et al. (2009): where, δ 13 C old represents the δ 13 C of E before labeling and δ 13 C new the δ 13 C of E of 10 a tree grown (theoretically) continuously with labeled CO 2 . The labeling period of 18 to 19 days was too short to fully achieve new isotopic equilibrium in E and therefore δ 13 C new was derived from C isotope discrimination (∆ 13 C) before labeling, following Eqs. (3) and (4): where, δ 13 C unlabeled air and δ 13 C labeled air represent the δ 13 C of canopy air before and during the labeling, respectively. Day-to-day variation in δ 13 C E may occur from variations in label incorporation and in ∆ 13 C depending on weather conditions (Pate and Arthur, 1998;Bowling et al., 2008). 20 Thus, δ 13 C E of the labeled trees were corrected for the day-to-day variations in ∆ 13 C (being rather small, i.e. < 0.5‰) of the unlabeled control trees, which showed rather stable δ 13  and lower stem positions of beech, respectively, and 19.4 ± 0.1‰ for the lower stem position of spruce.

Assessment of phloem sugars
Phloem sap was sampled on day 0 and during the last labeling day from the lower stem position following the method of Gessler et al. (2004). Small pieces of bark with 5 adherent phloem tissue (∅ 5 mm) were cored in the vicinity of the lower stem chamber and incubated (5 h at 4 • C) in 15 mM sodium polyphosphate buffer (Sigma-Aldrich, Munich, Germany). After centrifugation (12.500 rpm, 5 min), phloem sap was analyzed for water soluble sugars (sum of sucrose, fructose, glucose, raffinose and pinitol; i.e. C PS in mg) by means of HPLC (CARBOsep CHO-820 calcium column, Transgenomic, 10 219 Glasgow, UK). Freeze-dried phloem sap was analyzed for stable carbon isotope (δ 13 C sample in ‰) and element composition (C sample in mg), and δ 13 C of phloem sugars (δ 13 C PS in ‰) was calculated according to Eq. (5): with δ 13 C NPS representing δ 13 C of non-sugar C (assuming δ 13 C NPS to correspond to 15 δ 13 C sample before labeling, cf. Grams et al., 2011) and C NPS (in mg) denoting the nonsugar C content after labeling (calculated as difference between C sample and C PS ) in the phloem sap.

Sampling of leaves and fine roots
Leaves and fine roots were sampled before and during the last labeling day. Leaves 20 were collected with different exposure to compass directions in sun and shade crowns.
Recently grown fine roots (≤ 2 mm diameter) were sampled from organic soil horizons (< 10 cm soil depth) and cleaned from soil with distilled water. Dried plant material (72 h at 65 • C) was fine-ground and weighed into tin capsules for δ 13 C analysis. Soil gas samples were collected as detailed by Andersen et al. (2010). In brief, specific soil-gas sampling wells were placed belowground prior to tree labeling (distance from bole base of about 0.2 to 0.5 m) at 8 cm and 15 cm depth. Teflon tubing was used to draw 5-8 mL of soil gas from each sampler using a gas-tight syringe. Each beech and 5 spruce tree served as its own control by following the change in δ 13 C of soil-respired CO 2 throughout 2.5 weeks of labeling. In the case of beech, a total of four soil-gas sampling wells were additionally installed at an unlabeled control plot. Gas samples were subsequently filled into 12 mL exetainer vials and analyzed for δ 13 C. Calculation of δ 13 C of soil-respired CO 2 follows Eq. (1), while CO 2 of ambient air above the soil 10 served as reference. Note that soil CO 2 efflux was not adjusted by −4.4‰ to account for the more rapid diffusion of 12 C compared to 13 C (Andersen et al., 2010). δ 13 C analysis of additional gas samples taken directly above the forest floor indicated that CO 2 label was restricted to the crown and did not reach the forest soil (Grams et al., 2011).

Statistical analyses
Statistical analysis was performed using the SPSS 16.0 software package (SPSS Inc., Chicago, USA). Individual study trees were regarded as experimental units, and beech and spruce were analyzed separately. Data were statistically analyzed using General Linear Model (GLM) approach and t-tests where appropriate. Differences at p ≤ 0.05

Stem and coarse root CO 2 efflux
In general, both species displayed 1 to 4 times higher (beech) and 1 to 2 times higher (spruce) CO 2 efflux rates at the upper compared to the lower stem position (Table 3), whereas rates of coarse roots were 10 to 60 time higher than in stems. In beech, 5 2×O 3 significantly diminished the CO 2 efflux rate of the upper stem (by ca. −60%), but caused a pronounced increase in coarse roots (by ca. +65%). In spruce, CO 2 efflux rate of the upper and lower stem position was increased by a factor of 1.9 and 1.2, respectively, under 2 × O 3 . However, 2 × O 3 reduced the coarse root CO 2 efflux rate of spruce strongly by ca. −25%. ues by about 1.1‰. In both species, δ 13 C E of coarse roots were similar to the values of the lower stems and responses to 2 × O 3 were consistent with stems. While unlabeled control trees displayed minor day-to-day variations in δ 13 C E of the various organs during labeling (SD < 0.3‰), labeled trees displayed decreasing values upon label application (Fig. 2). In beech, δ 13 C E of the stems decreased from day 2 on-20 wards under both O 3 regimes (Fig. 2a), with a significantly more pronounced decline under 1 × O 3 . Likewise, coarse root δ 13 C E decreased from day 2 onwards (Fig. 2c), although this effect was less prominent than in stems. Similar to beech, δ 13 C E of stems in spruce decreased from day 3 onwards under both O 3 regimes (Fig. 2b) Fig. 2d).

Fraction of labeled C in stem and coarse root CO 2 efflux
In beech, the fraction of labeled carbon (f E, new ) in stem CO 2 efflux started to increase during labeling day 2 and was significantly lower in 2×O 3 compared to 1×O 3 from day 5 3 onwards (Fig. 3a).  Fig. 3b). Increase of f E, new in spruce coarse roots started somewhat delayed (day 3) but reached levels similar to those of the lower stem position (Fig. 3d). Contrasting with beech, 2 × O 3 did not result 15 in a consistently reduced f E, new in stems and coarse roots.

δ 13 C in leaves, phloem sugars, fine roots and soil respired CO 2 before labeling
Before labeling, no apparent differences in δ 13 C caused by the long-term 2 × O 3 exposure were found in the foliage, phloem sap of the stem, fine roots and soil respired 20 CO 2 in either species (Table 4). In general, δ 13 C in the sun leaves was significantly increased by ca. 3‰ (beech) and 2‰ (spruce) compared with shade leaves each. 3.5 Shift in δ 13 C of CO 2 efflux and organic material by the end of labeling During the 2.5 week labeling period, the δ 13 C of stem and root CO 2 efflux, soil-respired CO 2 and organic samples (phloem sugars, leaves and fine roots) in the unlabeled control trees of both species was only marginally affected (< 0.5‰, Fig. 4). In labeled beech, the drop in δ 13 C E at the end of label application in the upper stem position 5 was unaffected by O 3 (3.5 ± 0.2‰ in both O 3 treatments), but less pronounced at the lower stem position under 2 × O 3 (3.3 ± 0.1‰ and 2.3 ± 0.5‰ under 1× and 2 × O 3 , respectively) (Fig. 4b,c). Phloem sugars sampled from the lower stem position displayed similar shifts in δ 13 C of 4.0 ± 1.4‰ and 3.5 ± 0.6‰ under 1× and 2 × O 3 , respectively.
In consistency with the reduced label strength in spruce canopy air (about 6.0‰ compared to 8.2‰ in beech), the drop in stem δ 13 C E of spruce was lower than in beech ( Fig. 4e,f). Conversely to beech, the drop was somewhat increased by 2 × O 3 : upper and lower stem position of 2.4 ± 0.2‰ and 1.8 ± 0.3‰ under 1 × O 3 , respectively, and 2.8 ± 0.2‰ and 2.1 ± 0.2‰ under 2 × O 3 , respectively. Again, a similar shift was observed in phloem sugars (3.2 ± 0.3‰ and 2.5 ± 0.2‰ under 1× and 2 × O 3 , respec-15 tively). Corresponding changes of δ 13 C in leaf bulk material were much smaller (about 1.5‰). Upon labeling, belowground allocation of recent photosynthates was not affected by the O 3 treatment and, in general, was reduced compared to stem CO 2 efflux and phloem sugars. The decline upon labeling in δ 13 C E of coarse roots was 1.8±0.1‰ and 20 1.4 ± 0.1‰ in beech and 1.7 ± 0.9‰ and 2.1 ± 0.8‰ in spruce under 1× and 2 × O 3 , respectively. Under beech, changes in δ 13 C of soil-respired CO 2 were similar to coarse roots δ 13 C E (about 1.5 to 2.5‰), whereas soil CO 2 under spruce remained unchanged. (Fig. 4e,f). Similar to leaf bulk material, δ 13 C of fine roots displayed smaller changes than sampled CO 2 efflux and was in the range of 0.5‰, irrespective of the O 3 treatment.

Discussion
Our study compares the flux of recent photosynthates to the CO 2 efflux of stems and coarse roots in adult deciduous beech and evergreen spruce during summer and in response to seven-year-long 2 × O 3 treatment. The hypothesis I that long-term exposure to elevated O 3 reduces the flux of recently fixed C to CO 2 efflux of stems and coarse 5 roots was accepted for beech but rejected in the case of spruce, which is in accordance with their contrasting O 3 sensitivities (support for hypothesis II). Long-term exposure to 2 × O 3 for seven years did not significantly affect the δ 13 C of beech and spruce leaves or sugars transported in the phloem sap during late summer (  (Nunn et al., 2006). In general, δ 13 C of leaf and fine root biomass was about 2‰ higher in spruce compared to beech, likely resulting from higher leaf-level water-use efficiency 15 in the evergreen conifer compared to deciduous trees (Matyssek, 1986;Garten and Taylor, 1992;Diefendorf et al., 2010). In both beech and spruce, labeled photosynthates were detected in the upper and lower stem CO 2 efflux from day 3 onwards (Figs. 2 and 3). The fraction of labeled C (f E, new ) in the CO 2 efflux of beech stems was significantly reduced under 2 × O 3 20 (support of hypothesis I), indicating a higher dependency on C stores of the respiratory supply under 2 × O 3 . Such a response may be caused by O 3 -inhibited assimilate transport from the leaves, restricting the respiratory activity of stem tissues (Matyssek et al., 2002) and decreasing C stores in stems and roots towards the end of the growing season (Mc Laughlin et al., 1982). Consequently, re-growth and bud development in spring may become limited (Matyssek and Sandermann, 2003). The significantly decreased flux of recent photosynthates to beech stems represents the mechanistic basis for the observed loss in stem productivity of 40% under long-term exposure of  Pretzsch et al., 2010). In consistency with model predictions (cf. Sitch et al., 2007), this indicates the potential of chronic O 3 stress to substantially mitigate the C sink strength of trees . Contrasting with beech, exposure to 2 × O 3 increased the fraction of labeled C (f E, new ) in stem CO 2 efflux of spruce, rejecting hypothesis I for spruce. Accordingly, the rate of stem CO 2 efflux was significantly 5 increased under 2 × O 3 . Such a stimulation following O 3 exposure has been reported in several studies on herbaceous plants ( Grantz and Shrestha, 2006;Reiling and Davison, 1992) and is known to sustain repair-and detoxification processes (Matyssek et al., 1995;Rennenberg et al., 1996). Reduction of δ 13 C in canopy air for 2.5 weeks by about 8 and 6‰ resulted in a drop 10 of stem δ 13 C E in beech of 3-4‰ and in spruce by 2-3‰, respectively ( Fig. 4b-f). Correspondingly, f E, new of stem CO 2 efflux amounted to about 0.3 to 0.4 in both species. In parallel, δ 13 C of phloem sugar was reduced to a similar extent by about 4 and 3‰ in beech and spruce, respectively, suggesting phloem sugars to be the main C source for stem CO 2 efflux. Unlabeled C in phloem sugars may derive from "old C" atoms in C 15 skeletons of currently synthesized sucrose as a consequence of slow turnover of precursor molecules or from remobilized C stores (Gessler et al., 2008;Tcherkez et al., 2003). This suggests xylem-transported CO 2 to contribute only to a smaller extent to stem CO 2 efflux in our study species. This conclusion is supported by the lack of correlation between sap flow and both rate of stem CO 2 efflux and stem δ 13 C E in our study 20 (cf. Grams et al., 2011;Kuptz et al., 2011a,b). However, contribution of CO 2 respired in lower parts of the stem or roots to sampled CO 2 efflux can not be ruled out completely and the extent of this putative influence remains obscure (cf. Teskey et al., 2008).
In consistency with the findings on δ 13 C E in stems, 2 × O 3 distinctly reduced f E, new of coarse root efflux of beech, supporting hypothesis I. The decrease in coarse root 25 δ 13 C E during the labeling in summer was about 1-2‰ smaller than in stems, indicating a lower dependence of root CO 2 efflux on current photosynthates (Wingate, 2008;Bathellier et al., 2009;Kuptz et al., 2011a). However, soil-respired CO 2 , which includes large contributions by root-respired CO 2 of unlabeled neighboring trees and  (Högberg et al., 2001;Andersen et al., 2005Andersen et al., , 2010, was reduced in δ 13 C by 1.5 to 3 ‰. Hence, beech fine roots and associated microbes appear to be a relatively strong sink for recently fixed C during summer (Högberg et al., 2001;Plain et al., 2009). Slightly pronounced shifts in soil-respired CO 2 under 2 × O 3 fits well with previously reported increased fine-root turn-over of beech under long-term 5 O 3 exposure (Nikolova et al., 2010). Similar to C flux in spruce stems, elevated O 3 did not reduce the allocation of recent photosynthates to coarse root CO 2 efflux (cf. Andersen et al., 2010). However, the C label was hardly detectable in the soil-respired CO 2 around the trees (Andersen et al., 2010), which may indicate favored allocation of labeled C to storage and/or structural pools in the fine roots, resulting in a drop of δ 13 C in the fine root tissue during labeling (Fig. 4e, f).