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The net carbon balance of an ecosystem is the balance of carbon gain by assimilation and carbon loss by respiration. While assimilation can be clearly assigned to green plant tissues aboveground, respiration originates from both aboveground and belowground plant tissues, as well as soil organisms, and their dependences on physical and biochemical factors or their interactions are not well yet understood (Trumbore, 2006). Recent studies indicate that CO2 release measured in a specific tissue does not always reflect local CO2 respiration and that stem CO2 efflux (EFStem), measured usually by chambers attached to the stem, does not necessarily reflect true stem respiration resulting from translocation of CO2 dissolved in xylem water (Teskey et al., 2008). Furthermore, Aubrey & Teskey (2009) stated that much of root-respired CO2 enters the xylem stream, challenging the paradigm of root-respired CO2 diffusing completely into the soil atmosphere and thus questioning the ability to derive root respiration rates by measuring soil CO2 efflux (EFSoil) only. Consequently, if part of root-respired CO2 remains inside the tree, it can be refixed by chloroplast-containing tissues (Gansert & Burgdorf, 2005; Dima et al., 2006; McGuire et al., 2009) and might contribute to tree assimilation. Thus, common approaches to partition net ecosystem CO2 fluxes into gross primary productivity and total respiration by response functions probably have to be revised.
Trees are an important link between the soil and the atmosphere, both in terms of water and carbon translocation. Stem internal CO2 (stem [CO2]) consists of CO2 respired locally by different stem tissues, such as bark, phloem, cambium and living cells within the sapwood, and of CO2 respired by roots or lower sections of the stem, dissolved in xylem water and being dislocated via sap flow (Moore et al., 2008; Aubrey & Teskey, 2009). Hence, understanding tree internal CO2 fluxes is essential when aiming at a better understanding of stem respiration and measured EFStem, but also of processes involved in soil respiration and links between forest ecosystem assimilation and respiration.
The contribution of respired CO2 to either the CO2 remaining inside the tree or to the CO2 diffusing to the atmosphere is determined by respiration rates of individual tissues/organisms and by diffusion barriers (Sevanto et al., 2011; Steppe et al., 2012), all of which are a function of the actual phenological phase. For example, respired CO2 in the bark travels through air-filled spaces when released to the atmosphere, whereas CO2 respired by sapwood also has to cross water-saturated tissue and the nearly gas-impermeable cambium (Steppe et al., 2007). Thus, translocation of CO2 via sap flow is supposed to affect EFStem especially during day-time, while at night, when transpiration but also corticular and woody photosynthesis are close to/at zero, measured EFStem is assumed to better represent the actual stem respiration rate (McGuire & Teskey, 2004).
Tree internal CO2 fluxes, such as import, export and recycling, could serve as one explanation for high spatial and temporal variability of previously reported EFStem rates (Ceschia et al., 2002; Damesin et al., 2002; Teskey & McGuire, 2005; Steppe et al., 2007). Moreover, they might explain the documented temperature hysteresis or even decoupling of EFStem from temperature (Saveyn et al., 2008a), which is in conflict with the commonly accepted theory that respiration should be related exponentially to temperature, as can be expected for purely biologically driven processes. Further factors have been identified that may also alter internal stem [CO2] and EFStem, such as substrate limitations (Stockfors & Linder, 1998; Pruyn et al., 2005), diurnal growth patterns (Daudet et al., 2005) and changes in stem water status (Saveyn et al., 2007b, 2008b). Together, these aspects make interpretation of respiration even more difficult.
Recently, Teskey et al. (2008) proposed that a correct estimate of stem respiration needs to account for internal CO2 fluxes, which can be assessed by measuring CO2 concentrations in situ in stems or branches of trees. Inserting a microelectrode (McGuire & Teskey, 2002) or a nondispersive infrared CO2 sensor (Teskey & McGuire, 2007) into the stem allows for continuous and high-resolution measurements of stem [CO2]. However, there are few studies that report continuously measured stem [CO2] in trees and most of these were conducted in the glasshouse and/or were restricted to short periods of several days or weeks (Teskey & McGuire, 2002; Saveyn et al., 2007b; Cerasoli et al., 2009). The results of these studies vary widely in terms of flux magnitude and their interpretation (Bowman et al., 2005; Teskey & McGuire, 2007; Saveyn et al., 2008a). Furthermore, different relationships for internal CO2 fluxes and EFStem have been reported. While Teskey & McGuire (2005, 2007) found EFStem to be directly proportional to stem [CO2] in sweet-gum (Liquidambar styraciflua) and sycamore (Platanus occidentalis), EFStem of poplar trees was either uncoupled from stem [CO2] or the two were inversely related to each other (Saveyn et al., 2008b); no significant correlation could be found for rimu (Dacrydium cupressinum; Bowman et al., 2005). Manipulations of xylem sap CO2 concentrations in poplar and oak trees were reflected in EFStem (Teskey & McGuire, 2002) but crown removal and hence breakdown of the transpiration stream had no effect on EFStem of loblolly pine trees, leading to the conclusion that diffusion of CO2 from the xylem to the stem surface is restricted in pine trees (Maier & Clinton, 2006). In agreement with this conclusion, Ubierna et al. (2009) could not detect any changes in the isotopic signal of EFStem of large conifer trees (Abies grandis, Thuja plicata and Larix occidentalis), neither after labelling their water source nor after crown removal. Hence, the abiotic and biotic drivers that affect the dynamics of stem [CO2] and EFStem are not yet identified.
Thus, in this study, we combine ecophysiological measurements with continuous stem [CO2] measurements that were conducted in a mature subalpine Norway spruce forest over 19 months (May 2009 to December 2010). We address the following objectives: to quantify the degree of daily and seasonal variations of stem [CO2] explicable by pure physical equilibrium processes between CO2 in gas and CO2 dissolved in water, as described by Henry's law; to test interdependences of stem [CO2] and stem, soil and air temperatures, sap flow rates and stem radius changes, as well as EFStem and EFSoil; and to address the temporal consistency of such interdependences at seasonal and daily scales over the 19 months of study. To our knowledge, this is the longest time-series of continuous in situ measurements of stem [CO2] and provides a deeper insight into the dynamic interdependences of the plant–soil system.
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Continuously measured stem CO2 concentrations (stem [CO2]) in Norway spruce at a subalpine site varied between 1 vol% and 10 vol% within the study period of 19 months (Fig. 1), which is consistent with previous reports on other tree species (Teskey et al., 2008). These concentrations are amazingly high compared with the CO2 concentration in air (0.039 vol%), indicating a significant accumulation of CO2 inside tree stems owing to high diffusion barriers (Sorz & Hietz, 2006; Sevanto et al., 2011; Steppe et al., 2012). Stem [CO2] exhibited a strong seasonality (Fig. 1, Table 1), similar to the only other long-term study we are aware of (Eklund, 1990; Eklund & Klintborg, 2000). Based on our long-term data set, we had the unique opportunity not only to quantify variations in stem [CO2] at different time scales (see later), but also to show the high consistency of relationships between stem [CO2] and its potential drivers over almost 2 yrs (Tables 1, 2). In particular, TStem and TSoil were strong driving factors for stem [CO2] in both years. Furthermore, we found tight relationships between stem [CO2] and EFSoil during both vegetation periods, while other tree physiological measures (e.g. DR, sap flow) affected stem [CO2] differently for different seasons in both years, helping us to identify possible cause–effect mechanisms.
Stem [CO2] and its dependency on Henry's law
Henry's law describes the equilibrium process between all forms of CO2 dissolved in water and CO2 in air in relation to temperature, pH and partial pressure (Teskey & McGuire, 2002; Saveyn et al., 2007a; Teskey et al., 2008). The equilibrium dynamics driven by temperature changes have been found to play an important role when interpreting stem [CO2] on seasonal (Fig. 1a) and diurnal scales (Fig. 5a, b). We found that 31% (2009) and 68% (2010) of the seasonal variability of stem [CO2] (Fig. 1a), and 49% of the diurnal variability during summer (Fig. 5a), were attributable to physical effects of temperature changes (assuming constant CO2 concentration and sap pH). Thus, equilibrium processes according to Henry's law need to be accounted for before CO2 sink and source processes within a stem segment can be identified and interpreted in terms of tree physiology.
Although sap pH is known to have a substantial effect on the dissolution of CO2 in water (Teskey et al., 2008), we did not consider potential temporal variations of pH when calculating a model stem [CO2]. However, seasonal and diurnal changes of sap pH in poplar trees were reported to be rather small, probably indicating a minor impact on changes of solubility of CO2 in water over time (Aubrey et al., 2011).
Stem [CO2] on a seasonal time-scale
In spring and during both wood growth phases, measured changes in stem [CO2] were distinctly larger than expected from Henry's law, that is, 80% larger during the 2009 wood growth period and 35% larger than the modelled stem [CO2]sap=3.5 in 2010. This is a clear indication that during these periods considerable amounts of CO2 were produced, either because of local cambial respiratory activity or it was imported from belowground (Teskey et al., 2008). Throughout most of the physiological phases during the 19 months of study, we found stem [CO2] to be most closely related to TStem (Tables 1, 2). However, during the 2009 wood growth phase and late summer 2009 and 2010, TSoil had a higher explanatory power than TStem (Table 1), indicating a potentially large influence of root-respired CO2 transported up the tree. This coincides with root growth of Norway spruce that usually peaks in late spring and again during autumn when stem growth has already ceased (Puhe, 2003; Davidson et al., 2006).
An additional indirect indication for a coupling of belowground and aboveground CO2 is the close relationship between stem [CO2] and the efflux of CO2 from the soil (EFSoil; Figs 1, 4). This relationship indicates either a coincidence of independent processes driven by closely related courses of stem and soil temperatures or a possible causality between them. EFSoil was found to be exponentially related to TSoil (Etzold et al., 2011), whereas stem [CO2] was linearly related to TStem (Fig. 3). Assuming stem [CO2] is purely a function of local stem respiration, we would expect an exponential relationship to TStem and not a linear one. Furthermore, we would not expect a close relationship to EFSoil. However, assuming a 50% or higher fraction of root-respired CO2 transported via the xylem, we would expect a high correlation between EFSoil and stem [CO2], which was actually the case (adj. r2 = 0.83). Thus, it seems to be most likely that a large amount of CO2 produced in the roots is transported up the tree. Low sap flow rates seemed to increase stem [CO2] (Fig. 5) and EFStem (Fig. 7), whereas high sap flow rates seemed to induce a dilution effect and therefore a decrease in stem [CO2] (Fig. 7). Belowground growth phenology might not only affect changing explanatory powers of TStem and TSoil on stem [CO2] during the year but also pinpoint to belowground sources of stem CO2 during wood growth and late summer phases.
By contrast, during dormancy (> 0°C), sap flow was low (Fig. 1) and little CO2 transport from belowground can be expected. Thus, local stem respiration should be the dominant source for stem [CO2] during this period. However, during these dormant periods, TSoil still played an important role in explaining stem [CO2], either as single factor (Table 1) or in combination with TStem (only 2010; Table 2). At these times, TStem and TSoil were less coupled than during the rest of the year as soils were insulated by a layer of snow, keeping TSoil almost constant at c. 0°C, whereas TStem followed TAir (Fig. 2a). Thus, in addition to local respiration, we hypothesize that CO2 respired from the roots under the snow might diffuse up the stem along gas-filled spaces. With a diffusion coefficient of CO2 in air of 1.6 × 10−5 m2 s−1 (Nobel, 1991), changes in [CO2] in the roots could theoretically affect the stem section measured via air-filled spaces in the wood within 2–3 d. According to Sachs (1887), gas makes up to 17% of the xylem volume of fir trees (and we assume a similar value for spruce) and could build the necessary pathway for CO2 diffusion. Thus, the origin of stem [CO2] during dormancy phases with temperatures > 0°C could be either root- or local stem-respired CO2.
Furthermore, during dormancy with freezing conditions, large deviations between stem [CO2] of the two trees were observed (Fig. 1a). During these phases, temperature dropped below the threshold for frost-induced shrinkage of bark. Such conditions led to a (reversible) decrease in bark thickness of 1 mm or more (Fig. 1b), whereas xylem cells are reported to remain largely unaffected in size (Zweifel & Häsler, 2000). Such rapid and large frost-/freeze-induced variations in DR are known to induce pressure changes inside the tissues (Robson & Petty, 1993; Sevanto et al., 2012), which might be responsible for changes in stem [CO2]. These freeze/thaw processes may also explain the observed deviations in stem [CO2] between the two trees (Fig. 1a) and the differences in stem [CO2] levels, as it is well known that such processes do not occur simultaneously in all stems (Zweifel & Häsler, 2000). Overall, during winter stem [CO2] seems to be dominated by local respiration, but also affected by physically induced CO2 concentration changes, such as pressure changes and/or diffusion processes.
Nonrecurring stem [CO2] peak during wood growth phase 2009
In the year 2009, TStem and DR could explain most of the variability of stem [CO2], during the wood growth phase, while in the year 2010, TStem and sap flow were the most important predictors (Table 2). Although this is partly explicable with different growth dynamics and thus different respiration rates in the 2 yrs, it does not satisfactorily explain the exceptional peak of stem [CO2] from April to June 2009. This period appears as an anomaly in all our analyses (Tables 1, 2, Figs 1, 3, 4) and could be induced by a physiological response to sensor installation, as the hole drilled into the stem to place the sensor was 2 cm diameter and 6 cm deep. Increased stem CO2 efflux as wound response is a well-described phenomenon and usually interpreted as increased respiration owing to healing processes, such as cell repair, callus formation, lignification and suberization (Bloch, 1941; Uritani & Asahi, 1980; Schmitt & Liese, 1993; Levy et al., 1999). The fact that the stem [CO2] peak occurred 3 months after installation might indicate a delayed wound response after tree dormancy ceased and radial stem growth started at the end of May (Dunn et al., 1990; Schmitt & Liese, 1993; Dujesiefken et al., 2005).
Furthermore, annual amplitudes of stem [CO2] signals seem to have decreased over the two study years (mean amplitude ± standard deviation of stem [CO2] within a 7-d window during vegetation period: 2009, 0.51 ± 0.50; 2010, 0.45 ± 0.26). However, data sets even longer than in this study (19 months) would be needed to reliably test potential sensor signal degradation, which might occur because of wound closure and thus increased diffusion resistances between sensor and measurement tissue of interest.
Stem CO2 on a diurnal time-scale
On a diurnal scale, stem [CO2] in the morning (8:00–10:00 h) was found to increase more slowly than expected from Henry's law (Fig. 5a, only sunny summer days were analysed). One explanation could be that sap flow transports water with lower [CO2] into the measured stem section (Teskey & McGuire, 2007), thus acting as a dilution factor in the first hours after dawn (Fig. 5). Furthermore, water was withdrawn from the bark (as shown by shrinking stems, Fig. 5c,f), in parallel with the onset of sap flow. As bark water is reported to contain much lower [CO2] than xylem water (Cernusak & Marshall, 2000; Wittmann et al., 2006), this could be a second dilution factor.
During the second half of the day, the increase in stem [CO2] was also slightly lower than expected according to Henry's law (Fig. 5a), meaning that stem CO2 could have been diluted by sap flow, as also indicated by the analyses of sap flow in combination with EFStem (Fig. 7) and the findings of Teskey & McGuire (2007). With decreasing sap flow in the early evening, stem [CO2] remained constant, while modelled stem [CO2]sap=3.5 decreased, suggesting an accumulation of locally respired CO2 under low sap flow rates during night. The different effects of sap flow rates on EFStem in growth- and non-growth periods (Fig. 7) might have to do with the generally higher stem [CO2] during the growth period (Fig. 1). During this time, the stem (growth) respiration is assumed to be highest (Ryan, 1990) and the difference between locally produced CO2 and imported CO2 from belowground might be biggest.
Stem [CO2] was strongly related to stem and soil temperatures, which in turn affect physical equilibrium processes within the stem according to Henry's law but also affect different respiration processes, making disentangling of single factors difficult. As the relationships of stem [CO2] to tree physiological measures changed over time, long-term data sets are critical for the interpretation of stem [CO2] dynamics. Clear indications were found for CO2 translocation within the tree. However, whether these observations resulted from sap flow or axial diffusion, or a combination of both, remains unclear. Nevertheless, although the fate of stem [CO2] is not yet known, it has huge implications on the partitioning of net ecosystem CO2 exchange fluxes. If a large fraction of sap CO2 is recycled internally, then gross primary productivity estimated based on light response curves as well as ecosystem respiration estimated based on temperatures will both be underestimated. Furthermore, as the potential for refixation of root-respired and translocated CO2 changes strongly with phenophases, the underestimates might change during the year.