Biomass accumulation and clogging in biotrickling filters for waste gas treatment. Evaluation of a dynamic model using dichloromethane as a model pollutant

A dynamic model is developed that describes the degradation of volatile acidifying pollutants in biotrickling filters (BTFs) for waste gas purification. Dynamic modelling enables the engineer to predict the clogging rate of a filter bed and the time it takes the BTF to adapt to changes in its inlet concentration. The most important mechanisms that govern the behaviour of the BTF are incorporated in the model. The time scale of the accumulation of biomass in a filter is investigated, and an approach is presented that can be used to estimate how long a BTF can be operated before its packing has to be cleaned. A three-month experiment was carried out to validate the model, using dichloromethane (DCM) as a model acidifying pollutant. Valuable experimental data about biomass accumulation and liquid hold-up in the reactor were obtained with an experimental set-up that allows the continuous registration of the weight of the BTF. The results show that in BTFs eliminating DCM from a waste gas, clogging is not to be expected up to concentrations of several g/m3. Model calculations based on the measurements also suggest that the maximum carbon load that can safely be applied per unit void packing volume should not exceed 0.5-1.6 C mol/(m3. h), depending on the density of the biofilm formed. The model is a good predictor of the elimination of the pollutant in the system, the axial gas and liquid concentration profiles, the axial biomass distribution, and the response of the system upon a stepwise increase in the DCM inlet concentration. The influence of the buffer concentrations in the liquid phase upon the performance of the BTF is investigated.     

Performance of three pilot-scale immobilized-cell biotrickling filters for removal of hydrogen sulfide from a contaminated air steam

Hydrogen sulfide (H₂S) is a major malodorous compound emitted from wastewater treatment plants. In this study, the performance of three pilot-scale immobilized-cell biotrickling filters (BTFs) spacked with combinations of bamboo charcoal and ceramsite in different ratios was investigated in terms of H₂S removal. Extensive tests were performed to determine the removal characteristics, pressure drops, metabolic products, and removal kinetics of the BTFs. The BTFs were operated in continuous mode at low loading rates varying from 0.59 to 5.00 g H₂S m⁻³ h⁻¹ with an empty bed retention time (EBRT) of 25 s. The removal efficiency (RE) for each BTF was >99% in the steady-state period, and high standards were met for the exhaust gas. It was found that a multilayer BTF had a slight advantage over a perfectly mixed BTF for the removal of H₂S. Furthermore, an impressive amount >97% of the H₂S was eliminated by 10% of packing materials near the inlet of the BTF. The modified Michaelis–Menten equation was adopted to describe the characteristics of the BTF, and K_s and V_m values for the BTF with pure bamboo charcoal packing material were 3.68 ppmv and 4.26 g H₂S m⁻³ h⁻¹, respectively. Both bamboo charcoal and ceramsite demonstrated good performance as packing materials in BTFs for the removal of H₂S, and the results of this study could serve as a guide for further design and operation of industrial-scale systems.     

Water availability and carbon isotope discrimination in conifers

The stable C isotope composition ('¹³C) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and '¹³C. A significant component of the variation in '¹³C was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km^-1 altitude, r²=0.49, and in foliage, 1.91‰ km^-1, r²=0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO₂ partial pressure into the leaf (P_a-P_i) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in P_a-P_i of stemwood (r²=0.45) and foliage (r²=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on '¹³C probably dominated the effect of water availability. We also examined the relationship between plant drought stress (O) and '¹³C within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha^-1, O and '¹³C were less (more negative) than at 250 stems ha^-1 indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater '¹³C of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage - attributes that increase rates of photosynthesis, reduce P_i and increase '¹³C. '¹³C was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the '¹³C-O relationship varied between thinning treatments. Within treatments, the relationship between '¹³C and O was strong (0.38<r²<0.58). We conclude that '¹³C may well be a useful indicator of water availability or drought stress, but only in seasonally dry climates (P/E<1) and where variation in other environmental factors can be accounted for.     

In situ oxygen microelectrode measurements of bottom-ice algal production in McMurdo Sound, Antarctica

In-situ estimates of fast-ice algal productivity at Cape Evans, McMurdo Sound, in 1999 were lower than at the same site in previous years. Under-ice irradiance was between 0 and 8 µmol photons m^-2 s^-1; the ice was between 1.9 and 2.0 m thick and the algal biomass averaged 150 mg chl a m^-2, although values as high as 378 mg chl a m^-2 were recorded. Production on 11 and 12 November was between 0.053 and 1.474 mg C m^-2 h^-1. When the data from 11 November were fitted to a hyperbolic tangent function, a multilinear regression gave estimates for P_max of 0.571 nmol O₂ cm^-2 s^-1, an ! of 0.167 nmol O₂ cm^-2 s^-1 µmol^-1 photons m^-2 s^-1 and an E_k of 3.419 µmol photons m^-2 s^-1. A P_max of 2.674 nmol O₂ cm^-2 s^-1, an ! of 0.275 nmol O₂ cm^-2 s^-1 µmol^-1 photons m^-2 s^-1, r of 0.305 nmol O₂ cm^-2 s^-1 and an E_k of 9.724 µmol^-1 photons m^-2 s^-1 were estimated from the 12 November data. The sea-ice algal community was principally comprised of Nitzschia stellata, Entomoneis kjellmanii and Berkeleya adeliensis. Other taxa present included N. lecointei, Fragilariopsis spp., Navicula glaciei, Pleurosigma spp. and Amphora spp. Variations in the method for estimating the thickness of the diffusive boundary layer were not found to significantly affect the measurements of oxygen flux. However, the inability to accurately measure fine-scale variations in biomass is thought to contribute to the scatter of the P versus E data.     

A reconnaissance approach for hydrology of atoll lagoons

As a reconnaissance tool of the hydrology of atoll lagoons in the micro-tidal environment of the Tuamotu Archipelago, we define and compute "potential" flow rates at lagoon scale under three swell regimes (high, average, and low swell) after assessment of orientation and width of reef-flat spillways using satellite images. As a direct test, the "potential" flows were compared with field measurements of (1) measured inflows across the reef flat (for eight atolls), (2) net outgoing flow through the pass (for three atolls), and (3) lagoon-level variation rates (for four atolls). Absolute values of "potential" and field flows agreed (r²=0.94, n=42, slope ~1). Computed average water renewal times (T_RAV) were also tested against concentrations of dissolved organic matter (DOM). DOM and T_RAV were positively correlated (r²=0.54, n=26; Spearman's r_s=0.54), and this relationship should enable the detection of unusual atolls. This approach would then appear to be useful for the reconnaissance of hydrodynamics processes in comparable micro-tidal environments.     

Mycorrhizal associations between Tuber melanosporum mycelia and transformed roots of Cistus incanus

We set out to establish root cultures of a host plant with the aim of obtaining dual cultures of Tuber melanosporum mycorrhiza on transformed roots. Seedlings of Cistus incanus germinated under sterile conditions from seeds collected in the wild were treated with Agrobacterium rhizogenes. Nine hairy roots collected from different seedlings were cultured individually by repeated subculturing. The hairy root clones differed in growth rates and in morphology (branching frequency and distance between side roots). Root growth in a liquid medium exhibited a lag phase of about 2 weeks and an exponential phase lasting about 12 days before the start of the stationary phase. Hairy roots could be kept alive on medium M, a special solid minimal medium (low in Fe²⁺, BO₄^3-, Ca²⁺, Cu²⁺ and Zn²⁺, very low in PO₄^3- and lacking MoO₄^2-, NH₄⁺ and Co²⁺), for more than 7 months. T. melanosporum could be grown on the same medium for long periods only by subculturing the fungus with the roots. A mycorrhizal association developed between the roots and the T. melanosporum mycelium within 3 months. The association consisted of elongated roots with a mantle and a Hartig net surrounding two to three layers of cortical cells. Swollen, club-like root tips were discernible 5 months after inoculation. The mycorrhized roots could be subcultured and propagated on medium M and maintain the mycorrhizal association.     

13C content of ecosystem respiration is linked to precipitation and vapor pressure deficit

Variation in the carbon isotopic composition of ecosystem respiration ('¹³C_R) was studied for 3 years along a precipitation gradient in western Oregon, USA, using the Keeling plot approach. Study sites included six coniferous forests, dominated by Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Juniperus occidentalis, and ranged in location from the Pacific coast to the eastern side of the Cascade Mountains (a 250-km transect). Mean annual precipitation across these sites ranged from 227 to 2,760 mm. Overall '¹³C_R varied from -23.1 to -33.1‰, and within a single forest, it varied in magnitude by 3.5-8.5‰. Mean annual '¹³C_R differed significantly in the forests and was strongly correlated with mean annual precipitation. The carbon isotope ratio of carbon stocks (leaves, fine roots, litter, and soil organic matter) varied similarly with mean precipitation (more positive at the drier sites). There was a strong link between '¹³C_R and the vapor saturation deficit of air (vpd) 5-10 days earlier, both across and within sites. This relationship is consistent with stomatal regulation of gas exchange and associated changes in photosynthetic carbon isotope discrimination. Recent freeze events caused significant deviation from the '¹³C_R versus vpd relationship, resulting in higher than expected '¹³C_R values.     

Modelling mixing parameters in concentric-tube airlift bioreactors

The mixing behaviour of the liquid phase in concentric-tube airlift bioreactors of different scale (RIMP: V_L=0.070 m³; RIS-1: V_L=2.50 m³; RIS-2: V_L=5.20 m³) in terms of mixing time was investigated. This mixing parameter was determined from the output curves to an initial Dirac pulse, using the classical tracer response technique, and analyzed in relation to process and geometrical parameters, such as: gas superficial velocity, x_SGR; top clearance, h_S; bottom clearance, h_B, and ratio of the resistances at downcomer entrance, A_d/A_R. A correlation between the mixing time and the specified operating and geometrical parameters was developed, which was particularized for two flow regimes: bubbly and transition (x_SGRА.08 m/s) and churn turbulent flow (x_SGR> 0.08 m/s) respectively. The correlation was applied in bioreactors of different scale with a maximum error of ᆲ%.     

Photosynthesis, photoinhibition, and nitrogen use efficiency in native and invasive tree ferns in Hawaii

Photosynthetic gas exchange, chlorophyll fluorescence, nitrogen use efficiency, and related leaf traits of native Hawaiian tree ferns in the genus Cibotium were compared with those of the invasive Australian tree fern Sphaeropteris cooperi in an attempt to explain the higher growth rates of S. cooperi in Hawaii. Comparisons were made between mature sporophytes growing in the sun (gap or forest edge) and in shady understories at four sites at three different elevations. The invasive tree fern had 12-13 cm greater height increase per year and approximately 5 times larger total leaf surface area per plant compared to the native tree ferns. The maximum rates of photosynthesis of S. cooperi in the sun and shade were significantly higher than those of the native Cibotium spp (for example, 11.2 and 7.1 µmol m^-2 s^-1, and 5.8 and 3.6 µmol m^-2 s^-1 respectively for the invasive and natives at low elevation). The instantaneous photosynthetic nitrogen use efficiency of the invasive tree fern was significantly higher than that of the native tree ferns, but when integrated over the life span of the frond the differences were not significant. The fronds of the invasive tree fern species had a significantly shorter life span than the native tree ferns (approximately 6 months and 12 months, respectively), and significantly higher nitrogen content per unit leaf mass. The native tree ferns growing in both sun and shade exhibited greater photoinhibition than the invasive tree fern after being experimentally subjected to high light levels. The native tree ferns recovered only 78% of their dark-acclimated quantum yield (F_v/F_m), while the invasive tree fern recovered 90% and 86% of its dark-acclimated F_v/F_m when growing in sun and shade, respectively. Overall, the invasive tree fern appears to be more efficient at capturing and utilizing light than the native Cibotium species, particularly in high-light environments such as those associated with high levels of disturbance.     

Characteristics of a DO-controlled fed-batch culture of Escherichia coli

The DO-controlled glucose limited fed-batch technique was investigated in an E. coli process for production of a recombinant protein. The k_Lac^* value (oxygen transfer rate at zero oxygen concentration) was calculated from on-line gas analysis data during the process. In the investigated processes with induced production of recombinant protein, the k_Lac^* value decreased drastically several hours after induction. The reason for the decrease was found in increasing concentrations of DNA in the medium and increased viscosity due to cell lysis. The consequences of such a dramatic decrease in the volumetric oxygen transfer coefficient on the glucose feed and specific rates are described in computer simulations and experimental data.     

Gas phase hold up in gas-liquid cocurrent upflow packed bed reactors at low Reynold's number

Experimental measurements of gas phase hold up in two phase gas-liquid cocurrent upflow packed bed reactors have been made at very low gas ((N_Re)_g < 55) as well as liquid ((N_Re)_l < 8) velocities using air-water systems with two different types of packing materials. The gas hold up values thus obtained are correlated in terms of Re_g, Re_l and l, the voidage.     

Oxygen transfer characteristics of a centrifugal, packed-bed reactor during viscous xanthan fermentation

The oxygen transfer properties of a novel, centrifugal, packed-bed reactor (CPBR) during viscous xanthan fermentation were determined with respect to the effects of the arrangement of the centrifugal, packed bed (CPB) and the recirculation loop (RL). Characterized by the maximum volumetric transfer coefficient (k_La) in xanthan broth, the aeration efficiency of CPBR was compared to those in stirred-tank reactors (STR) equipped with disc turbines (DT) or marine propellers (MP), and to that in a water-in-oil emulsion (WIO). As expected, STR-WIO showed the highest k_La (0.038 s^-1 at 2%) among all systems studied due to reduced broth viscosity; however, practical difficulties exist in product recovery. It was found that, at 3.5% xanthan the k_La in CPBR (0.018 s^-1) was higher than that of STR (0.005 s^-1) and close to that of STR-WIO (0.020 s^-1), indicating improved oxygen transfer at such a xanthan concentration. The exterior baffles along the rotating fibrous matrix offer additional agitation in the viscous broth. A gas-continuous arrangement, in which the CPB was kept above the broth, was able to elevate k_La to 0.023 s^-1, higher than that of STR-WIO. The external RL operated by a peristaltic pump was found to play an important role in CPBR aeration by providing better gas-liquid contact. With the improved oxygen transfer efficiency in CPBR at high xanthan concentrations, the CPBR system is practically the preferred system for xanthan fermentation. The characteristic roles of CPB arrangement and the RL should be considered primarily during scale-up operation.     

Influence of climate-driven shifts in biomass allocation on water transport and storage in ponderosa pine

Conifers decrease the amount of biomass apportioned to leaves relative to sapwood in response to increasing atmospheric evaporative demand. We determined how these climate-driven shifts in allocation affect the aboveground water relations of ponderosa pine growing in contrasting arid (desert) and humid (montane) climates. To support higher transpiration rates, a low leaf:sapwood area ratio (A_L/A_S) in desert versus montane trees could increase leaf-specific hydraulic conductance (K_L). Alternatively, a high sapwood volume:leaf area ratio in the desert environment may increase the contribution of stored water to transpiration. Transpiration and hydraulic conductance were determined by measuring sap flow (J_S) and shoot water potential during the summer (June-July) and fall (August-September). The daily contribution of stored water to transpiration was determined using the lag between the beginning of transpiration from the crown at sunrise and J_S. In the summer, mean maximum J_S was 31.80LJ.74 and 24.34Dž.05 g m^-2 s^-1 for desert and montane trees (a 30.6% difference), respectively. In the fall, J_S was 25.33NJ.52 and 16.36dž.64 g m^-2 s^-1 in desert and montane trees (a 54.8% difference), respectively. J_S was significantly higher in desert relative to montane trees during summer and fall (P<0.05). Predawn and midday shoot water potential and sapwood relative water content did not differ between environments. Desert trees had a 129% higher K_L than montane trees in the summer (2.41᎒^-5 versus 1.05᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001) and a 162% higher K_L in the fall (1.97᎒^-5 versus 0.75᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001). Canopy conductance decreased with D in all trees at all measurement periods (P<0.05). Maximum g_C was 3.91 times higher in desert relative to montane trees averaged over the summer and fall. Water storage capacity accounted for 11 kg (11%) and 10.6 kg (17%) of daily transpiration in the summer and fall, respectively, and did not differ between desert and montane trees. By preventing xylem tensions from reaching levels that cause xylem cavitation, high K_L in desert ponderosa pine may facilitate its avoidance. Thus, the primary benefit of low leaf:sapwood allocation in progressively arid environments is to increase K_L and not to increase the contribution of stored water to transpiration.     

Characterization and theoretical analysis on toxicological threshold of mercuric ions to Pseudomonas aeruginosa PU21 (Rip64)

This study provides a novel attempt to put forward, in general terms and explanations, the toxicological threshold for Hg(II) detoxification. Quantitative analysis of mercuric ion toxicity to a mercury-resistant strain Pseudomonas aeruginosa PU21 and identification of the threshold phenomena are included. It is revealed that rate of cellular viability loss depends upon the ratio of Hg²⁺ concentration to cell population, (or "multiplicity of toxicity" - MOT). The threshold of unacclimated cells (or "non-growth cells" PU21_u) and hyperresistant strain (PU21_r) occurs at MOT of 1.6᎒¹⁰~3.2᎒¹⁰ and 1.7᎒¹¹~3.4᎒¹¹ molecules Hg²⁺/cfu, respectively. The threshold of PU21_r increase approx. 10-fold compared with unacclimated resistant strain PU21_u. This indicated that sequential treatment of culture with selection pressures (i.e. mercury-containing media) led to over 10-fold increase in mercury resistance. Thus, this quantitative evaluation of toxicity threshold among resistant populations can be used as a design criterion for long-term Hg²⁺ detoxification bioprocesses.     

Trends in wood density and structure are linked to prevention of xylem implosion by negative pressure

Wood density (D_t), an excellent predictor of mechanical properties, is typically viewed in relation to support against gravity, wind, snow, and other environmental forces. In contrast, we show the surprising extent to which variation in D_t and wood structure is linked to support against implosion by negative pressure in the xylem pipeline. The more drought-tolerant the plant, the more negative the xylem pressure can become without cavitation, and the greater the internal load on the xylem conduit walls. Accordingly, D_t was correlated with cavitation resistance. This trend was consistent with the maintenance of a safety factor from implosion by negative pressure: conduit wall span (b) and thickness (t) scaled so that (t/b)² was proportional to cavitation resistance as required to avoid wall collapse. Unexpectedly, trends in D_t may be as much or more related to support of the xylem pipeline as to support of the plant.     

Nitrification performance and nitrifier community composition of a chemostat and a membrane-assisted bioreactor for the nitrification of sludge reject water

Nitrification performance of a chemostat and a membrane-assisted bioreactor (MBR) was assessed at pilot scale for the treatment of sludge reject waters with NH₄⁺-N concentrations up to 600 mg/L and low organic content (COD<200 mg/L). To prevent nitrifier washout the 1-m³ chemostat was operated at 20°C with minimum hydraulic retention time of F=2 days. At the 0.71 m³ MBR, F was successively reduced to 6.2 h. Complete sludge retention was achieved by means of a 2-m² 100,000-Dalton PES ultrafiltration membrane. Operation in crossflow mode with flow velocities from v_F=2.4-3.7 m/s and transmembrane pressures (p=0.5-1.2 bar yielded a long-term permeate flux of 110 L/(m²2h). In the MBR, nitrification rates up to 2,500 g N/(m³2d) were measured with biomass concentrations between 4 and 15 g TSS/L. Despite low TSS values, about 0.2 g/L of the chemostat was able to nitrify 180 g N/(m³2d). The microbial community composition differed considerably between the two reactors as determined by fluorescent in situ hybridisation (FISH) with rRNA-targeted oligonucleotide probes. For both reactors, the relative abundance of ammonia and nitrite oxidisers measured by FISH was consistent with results from dynamic simulation of the nitrification process.     

Chlorophyll fluorescence and photosynthetic response to light in 1-year-old needles during spring and early summer in <Emphasis Type="Italic">Pinus leucodermis</Emphasis>

Ten-year-old trees from four Italian populations of Pinus leucodermis (populations A, B, C and D), which were collected from different sites at different altitudes, were grown near Florence, Italy. Needle CO₂ gas exchange and chlorophyll fluorescence response to increasing light intensities were evaluated; gas exchange and chlorophyll fluorescence variation between April and July were also monitored. Populations A, B and C showed a similar photosynthetic response to increasing photosynthetic photon flux density (PPFD) intensities, while at various light intensities population D, which originated from the highest altitude, showed the highest photosynthetic rates. In this population photosynthesis was saturated at PPFDs higher than 900 µmol m^-2s^-1 and a slow decrease of effective photosystem II quantum yield and F'_V/F'_M in response to increasing PPFDs were found. The same trees also showed a faster recovery in photosynthesis from limitations induced by winter temperatures than the other three populations. This work showed that photosynthetic response to light in population D was different from the other populations; trees from this population were probably naturally selected to prevent photoinhibition due to excess light.     

Feeding rates of the woodlouse Armadillidium vulgare on herb litters produced at two levels of atmospheric CO2

The consumption and assimilation rates of the woodlouse Armadillidium vulgare were measured on leaf litters from five herb species grown and naturally senesced at 350 and 700 µl l^-1 CO₂. Each type of litter was tested separately after 12, 30 and 45 days of decomposition at 18°C. The effects of elevated CO₂ differed depending on the plant species. In Medicago minima (Fabaceae), the CO₂ treatment had no significant effect on consumption and assimilation. In Tyrimnus leucographus (Asteraceae), the CO₂ treatment had no significant effect on consumption, but the elevated CO₂ litter was assimilated at a lower rate than the ambient CO₂ litter after 30 days of decomposition. In the three other species, Galactites tomentosa (Asteraceae), Trifolium angustifolium (Fabaceae) and Lolium rigidum (Poaceae), the elevated CO₂ litter was consumed and/or assimilated at a higher rate than the ambient CO₂ litter. Examination of the nitrogen contents in these three species of litter did not support the hypothesis of compensatory feeding, i.e. an increase in woodlouse consumption to compensate for low nitrogen content of the food. Rather, the results suggest that in herbs that were unpalatable at the start of the experiment (Galactites, Trifolium and Lolium), more of the the litter produced at 700 µl l^-1 CO₂ was consumed than of that produced at 350 µl l^-1 because inhibitory factors were eliminated faster during decomposition.     

Carbon isotope discrimination in photosynthetic bark

We developed and tested a theoretical model describing carbon isotope discrimination during photosynthesis in tree bark. Bark photosynthesis reduces losses of respired CO₂ from the underlying stem. As a consequence, the isotopic composition of source CO₂ and the CO₂ concentration around the chloroplasts are quite different from those of photosynthesizing leaves. We found three lines of evidence that bark photosynthesis discriminates against ¹³C. First, in bark of Populus tremuloides, the '¹³C of CO₂ efflux increased from -24.2‰ in darkness to -15.8‰ in the light. In Pinus monticola, the '¹³C of CO₂ efflux increased from -27.7‰ in darkness to -10.2‰ in the light. Observed increases in '¹³C were generally in good agreement with predictions from the theoretical model. Second, we found that '¹³C of dark-respired CO₂ decreased following 2-3 h of illumination (P<0.01 for Populus tremuloides, P<0.001 for Pinus monticola). These decreases suggest that refixed photosynthate rapidly mixes into the respiratory substrate pool. Third, a field experiment demonstrated that bark photosynthesis influenced whole-tissue '¹³C. Long-term light exclusion caused a localized increase in the '¹³C of whole bark and current-year wood in branches of P. monticola (P<0.001 and P<0.0001, respectively). Thus bark photosynthesis was shown to discriminate against ¹³C and create a pool of photosynthate isotopically lighter than the dark respiratory pool in all three experiments. Failure to account for discrimination during bark photosynthesis could interfere with interpretation of the '¹³C in woody tissues or in woody-tissue respiration.