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Deriving ozone dose–response of photosynthesis in adult forest trees from branch-level cuvette gas exchange assessment
Available online at www.sciencedirect.com
Environmental Pollution 153 (2008) 526e528 www.elsevier.com/locate/envpol
New initiative
Deriving ozone doseeresponse of photosynthesis in adult forest trees from branch-level cuvette gas exchange assessment C. Then a,b,*, M. Lo¨w b, R. Matyssek b, G. Wieser a a
Unit of Alpine Timberline Ecophysiology, Federal Office and Research Centre for Forests, Rennweg 1, A-6020 Innsbruck, Austria b Ecophysiology of Plants, Department of Ecology, Technische Universita¨t Mu¨nchen, Life Science Center Weihenstephan, Am Hochanger 13, D-85354 Freising, Germany Received 31 October 2007; received in revised form 4 February 2008; accepted 15 February 2008
Branch-level O3 dose dependence of photosynthesis derived from cuvette assessment yields sun-crown foliage sensitivity under whole-tree free-air O3 fumigation. Abstract Branch-level gas exchange provided the basis for assessing ozone flux in order to derive the doseeresponse relationship between cumulative O3 uptake (COU) and carbon gain in the upper sun crown of adult Fagus sylvatica. Fluxes of ozone, CO2 and water vapour were monitored simultaneously by climatized branch cuvettes. The cuvettes allowed branch exposure to an ambient or twice-ambient O3 regime, while tree crowns were exposed to the same O3 regimes (twice-ambient generated by a free-air canopy O3 exposure system). COU levels higher than 20 mmol m2 led to a pronounced decline in carbon gain under elevated O3. The limiting COU range is consistent with findings on neighbouring branches exposed to twice-ambient O3 through free-air fumigation. The cuvette approach allows to estimate O3 flux at peripheral crown positions, where boundary layers are low, yielding a meso-scale within-crown resolution of photosynthetic foliage sensitivity under wholetree free-air O3 fumigation. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Fagus sylvatica; Ozone flux; Gas exchange; Branch cuvette fumigation; Doseeresponse
1. Introduction Tropospheric ozone (O3), as prevailing at chronically enhanced exposure regimes, is regarded to be one of the most detrimental air pollutants to forest trees (Matyssek et al., 2007). Adverse effects depend on both the amount of O3 entering the leaves and the defence capacity of the impacted tissues (Musselman et al., 2006). Trees may counteract O3 impact through stomatal closure restricting O3 uptake (stress avoidance) and detoxification through biochemical * Corresponding author. Ecophysiology of Plants, Department of Ecology, Technische Universita¨t Mu¨nchen, Life Science Center Weihenstephan, Am Hochanger 13, D-85354 Freising, Germany. Tel.: þ49 8161 714787; fax: þ49 8161 714576. E-mail address: [email protected] (C. Then). 0269-7491/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2008.02.015
reactions in the leaves (defence, stress tolerance). The extent of O3-induced injury was suggested to depend on the amount of available antioxidants per unit of stomatal O3 uptake (Musselman et al., 2006). Hence, assessment of stomatal O3 uptake into the leaf mesophyll is crucial in view of O3 risk assessment, as it is the actual O3 uptake which represents the metabolically active and, hence, the ‘‘phytomedically’’ relevant O3 dose (Matyssek et al., 2007). Methods for quantifying O3 uptake include eddy covariance, sap-flow, and cuvette techniques (see Wieser et al., 2008 for an overview). Eddy covariance techniques assess the total O3 deposition to tree stands, whereas sap-flow measurement can be used to quantify stomatal O3 flux into the foliage of whole trees (Ko¨stner et al., 2008). Branch-level approaches for assessing O3 uptake and photosynthesis are a crucial scaling step in linking O3 flux and carbon gain
C. Then et al. / Environmental Pollution 153 (2008) 526e528
between the leaf and the whole-tree level (Wieser et al., 2008). The latter can be derived under canopy free-air O3 fumigation in combination with the sap-flow approach of whole-tree O3 flux (Ko¨stner et al., 2008). Fluxes of O3, CO2, and water vapour at the leaf level can concurrently be measured by means of cuvette techniques as shown recently in a chamber experiment under controlled conditions (Grulke et al., 2007). However, cuvette techniques also have the potential of quantifying doseeresponse relationships at the branch level. Such latter approaches allow continuous monitoring of both O3 uptake and net carbon gain of entire branches under canopy conditions, especially in cases of open crown structure, when boundary layer effects on O3 uptake are low. Such conditions prevail in peripheral sun-lit branches of the upper canopy. Therefore, in this study we made use of a fully automated, climate-controlled cuvette system for branch-level gas exchange assessment, which continuously (day and night) provided apparent O3, CO2, and water vapour fluxes to derive a flux-based doseeresponse relationship between O3 uptake and carbon gain. To our knowledge previous branch-level O3 fumigation studies with cuvettes or bags have missed a continuous concurrent assessment of O3 and CO2 uptake although the relationship between these two parameters is crucial for phytomedically relevant O3 risk assessment of trees (Matyssek et al., 2007). The focus was on peripheral sun branches of Fagus sylvatica in the uppermost canopy of Kranzberg Forest (Germany). Boundary layers of such canopy positions were shown to only marginally affect O3 flux under the prevailing wind conditions at the site, where in parallel to the branchlevel approach a free-air O3 fumigation system was operated within the stand canopy (Werner and Fabian, 2002). 2. Materials and methods The study was carried out at Kranzberg Forest near Munich, Germany (485 m a.s.l., 48 250 N, 11 390 E). During the growing season of 2004, 60-year-old beech trees (F. sylvatica) were exposed to either ambient or twice-ambient O3 regimes (1 O3 and 2 O3, respectively), the latter regime being generated by a free-air canopy O3 exposure system (Werner and Fabian, 2002). Six study trees were selected, three trees were exposed to 1 O3 (control), and the other three to 2 O3. To prevent the risk of acute O3 injury, maximum O3 levels in the latter regime were restricted to 150 nl O3 l1. Scaffolding allowed access to the upper sun crown 25 m above ground, where attached terminal sections of peripheral sun-lit branches of each study tree were sealed into transparent, climate-controlled cuvettes (32 cm long, 12 cm in diameter). The cuvettes allowed O3 fumigation during gas exchange assessment (see Wieser et al., 2001 for a detailed description of the system), while tracking their diurnal (24 h) and seasonal fluctuations of O3, CO2, and water vapour fluxes. The system was in operation on the same branches continuously between June 29 and September 5, 2004. Air from each cuvette was analyzed alternately by means of a solenoidbased, gas-switching system by 70-s intervals. Water vapour and CO2 concentrations were compared each between the air entering and leaving the cuvettes with a LI 6200 CO2/H2O gas analyzer (Li-Cor, Lincoln, USA) operating in differential mode. Air flow rates through cuvettes were monitored with electronic mass flow-meters (Tylan, Eching, Germany). In parallel, O3 concentrations of the 1 O3 and 2 O3 regimes were sampled through Teflon tubing at the outlet of each cuvette and assessed with an O3 analyzer (Model 8810, Monitor Labs, San Diego, USA). O3 concentrations of the external 1 O3 and 2 O3 regimes (the latter released through free-air fumigation, cf. Werner
527
and Fabian, 2002) were analyzed by a second O3 analyzer. Signals of both O3 analyzers were transmitted to a data acquisition and feedback control system (Campbell Scientific, Ltd., Shepshed, U.K.) to generate the 2 O3 regime relative to 1 O3 through motor-operated mixing valves (Walz, Effeltrich, Germany). Environmental and gas exchange data were transmitted to an AM 416 multiplexer (Campbell Scientific, Ltd., Shepshed, U.K.) and recorded with a Campbell CR10 data logger, programmed to record 20 min means. The data logger also controlled the switching of the solenoids. O3 flux into the leaves (FO3 ) was calculated according to the flux equation: FO3 ¼ ½O3 GH2 O 0:613 where [O3] is the ambient air O3 concentration, GH2 O is the stomatal conductance for water vapour and 0.613 is a conversion factor to account for the lower diffusivity of O3 relative to water vapour. The equation is based on evidence by Laisk et al. (1989) that O3 concentration approaches nil in the intercellular leaf air space. Cumulative O3 uptake (COU) was calculated from FO3 as integrated over the exposure period. On a daily basis, mean carbon gain and COU were calculated for the individual branch sections sealed into the 1 O3 and 2 O3 cuvettes. As whole-tree free-air fumigation started on 25 April 2004, COU before branch enclosure into the entire cuvettes was 5 mmol m2, irrespective of O3 exposure (Lo¨w et al., 2007). Apparent net photosynthesis before enclosure did not differ significantly between regimes and averaged 5.1 0.9 mmol m2 s1 (Then et al., 2007); mean daily carbon gain varied between 2.8 and 6.5 mmol m2 s1 (averaging at 4.71 1.6 g C m2 d1), irrespective of the O3 regime. To account for the natural within-tree variation in mean daily carbon gain, responses to COU under 2 O3 were expressed in proportion of those under 1 O3 as occurring during same days (Lo¨w et al., 2007).
3. Results and discussion After 69 days of exposure, COU was 17.6 1.2 mmol m2 of projected leaf surface area in branches under 1 O3 and 29.4 1.1 mmol m2 under 2 O3 (Fig. 1). During the first 45 days of the experiment (day of year 181 through 225), the mean daily carbon gain of 2 O3 branches did not differ significantly from that of branches under 1 O3 (Fig. 2). However, after 45 days of exposure, when COU exceeded 20.3 mmol m2, the mean daily carbon gain under 2 O3 regime showed a pronounced decline. This decline persisted
Fig. 1. Time course of cumulative ozone uptake of adult Fagus sylvatica leaves exposed to ambient (1 O3) (open symbols) and twice-ambient (2 O3) (closed symbols) O3 concentration in branch cuvettes from June 29 (day 181) throughout September 5 (day 249) of 2004. For ease of comparison only mean values of three branches per treatment are shown.
528
C. Then et al. / Environmental Pollution 153 (2008) 526e528
for whole-tree free-air O3 fumigation and related flux assessment, as boundary layer width varies across the range of different crown positions, and as branches just represent one stage in scaling O3 flux between the single-leaf and wholetree level (Wieser et al., 2008). For reasons of validation and associated whole-tree O3 risk assessment (Matyssek et al., 2007), the cuvette approach can be linked, however, with independent trunk sap-flow assessment techniques (Ko¨stner et al., 2008) that offer derivation of O3 uptake at the whole-tree level. Acknowledgements
Fig. 2. Relative reduction in daily mean carbon gain of fully developed Fagus sylvatica leaves continuously exposed to 2 O3 for 69 days (solid symbols and solid line) and light-saturated net photosynthesis (Amax) of neighbouring leaves exposed to 2 O3 under free-air conditions (open symbols and dotted line; from Lo¨w et al., 2007). Carbon gain and Amax of the 2 O3 twigs at each date is expressed relative to the respective value of the 1 O3 control (¼1). The curve was fit by the polynomial regression: y ¼ 0.0009x2 þ 0.0081x þ 1.0314; r2 ¼ 0.498. Branch-cuvette leaves were monitored continuously from June 29 throughout September 5, of 2004, while Amax was measured 5 times between May 24 and September 14, of 2004. For ease of comparison only mean values of three cuvette branches per treatment are shown, while Amax values are the mean SD of five trees.
throughout the remainder of the experiment, as the mean daily carbon gain of 2 O3 branches was reduced by 10e30% relative to that of 1 O3 branches (Fig. 2). Similar O3-induced reductions of photosynthesis were found in single leaves exposed to 2 O3 outside of the cuvettes, i.e. under the employed free-air fumigation (Lo¨w et al., 2007; Then et al., 2007). COU above 20 mmol m2 had also incited a 10e30% reduction in light-saturated net photosynthesis (Fig. 2) and electron transport rate of 2 O3 leaves (Lo¨w et al., 2007). In addition, comparative analysis of foliar gas exchange and biochemical parameters also failed to detect significant differences between branches enclosed in cuvettes and branches outside cuvettes exposed to either 1 O3 or 2 O3 (Then et al., 2007). We conclude, therefore, that the branch-level cuvette approach allows for estimating O3 flux at peripheral crown positions, where boundary layers are low, yielding meso-scale within-crown resolution of photosynthetic foliage sensitivity under whole-tree free-air O3 fumigation. Nevertheless, the branch-level approach that intrinsically minimizes boundary layer effects on O3 flux during O3 fumigation is not a substitute
The present study was supported by the EU project CASIROZ ‘‘The Carbon Sink Strength of Beech in a Changing Environment: Experimental Risk Assessment by Mitigation of Chronic Ozone Impact’’ (EVK2-2002-00165). Special thanks to T. Gigele and H. Lohner for excellent technical assistance. References Grulke, N.E., Paoletti, E., Heath, R.L., 2007. Comparison of calculated and measured foliar O3 flux in crop and forest species. Environmental Pollution 146, 640e647. Ko¨stner, B., Matyssek, R., Heilmeier, H., Clausnitzer, F., Nunn, A.J., Wieser, G., 2008. Sap flow measurements as a basis for assessing tracegas exchange of trees. Flora 203, 14e33. Laisk, A., Kull, O., Moldau, H., 1989. Ozone concentration in leaf intercellular air spaces is close to zero. Plant Physiology 90, 1163e1167. Lo¨w, M., Ha¨berle, K.-H., Warren, C.R., Matyssek, R., 2007. O3 flux-related responsiveness of photosynthesis, respiration, and stomatal conductance of adult Fagus sylvatica to experimentally enhanced free-air O3 exposure. Plant Biology 9, 197e206. Matyssek, R., Bytnerowicz, A., Karlsson, P.-E., Paoletti, E., Sanz, M., Schaub, M., Wieser, G., 2007. Promoting the O3 flux concept for forest trees. Environmental Pollution 146, 587e607. Musselman, R.C., Lefohn, A.S., Massman, J., Heath, R.L., 2006. A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects. Atmospheric Environment 40, 1869e1888. Then, C., Herbinger, K., Blumenro¨ther, M., Haberer, K., Heerdt, C., Oßwald, W., Rennenberg, H., Grill, D., Tausz, M., Wieser, G., 2007. Evidence that branch cuvettes are reasonable surrogates for estimating O3 effects in entire tree crowns. Plant Biology 9, 309e319. Werner, H., Fabian, P., 2002. Free-air fumigation on mature trees: a novel system for controlled ozone enrichment in grown-up beech and spruce canopies. Environmental Science and Pollution Research 9, 117e121. Wieser, G., Tausz, M., Wonisch, A., Havranek, W.M., 2001. Free radical scavengers and photosynthetic pigments in Pinus cembra L. needles as affected by ozone exposure. Biologia Plantarum 44, 225e232. Wieser, G., Matyssek, R., Then, C., Cieslik, S., Paoletti, E., Ceulemans, R. Upscaling ozone flux in forests from leaf to landscape. Italian Journal of Agronomy, in press.
Environmental Pollution 153 (2008) 526e528 www.elsevier.com/locate/envpol
New initiative
Deriving ozone doseeresponse of photosynthesis in adult forest trees from branch-level cuvette gas exchange assessment C. Then a,b,*, M. Lo¨w b, R. Matyssek b, G. Wieser a a
Unit of Alpine Timberline Ecophysiology, Federal Office and Research Centre for Forests, Rennweg 1, A-6020 Innsbruck, Austria b Ecophysiology of Plants, Department of Ecology, Technische Universita¨t Mu¨nchen, Life Science Center Weihenstephan, Am Hochanger 13, D-85354 Freising, Germany Received 31 October 2007; received in revised form 4 February 2008; accepted 15 February 2008
Branch-level O3 dose dependence of photosynthesis derived from cuvette assessment yields sun-crown foliage sensitivity under whole-tree free-air O3 fumigation. Abstract Branch-level gas exchange provided the basis for assessing ozone flux in order to derive the doseeresponse relationship between cumulative O3 uptake (COU) and carbon gain in the upper sun crown of adult Fagus sylvatica. Fluxes of ozone, CO2 and water vapour were monitored simultaneously by climatized branch cuvettes. The cuvettes allowed branch exposure to an ambient or twice-ambient O3 regime, while tree crowns were exposed to the same O3 regimes (twice-ambient generated by a free-air canopy O3 exposure system). COU levels higher than 20 mmol m2 led to a pronounced decline in carbon gain under elevated O3. The limiting COU range is consistent with findings on neighbouring branches exposed to twice-ambient O3 through free-air fumigation. The cuvette approach allows to estimate O3 flux at peripheral crown positions, where boundary layers are low, yielding a meso-scale within-crown resolution of photosynthetic foliage sensitivity under wholetree free-air O3 fumigation. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Fagus sylvatica; Ozone flux; Gas exchange; Branch cuvette fumigation; Doseeresponse
1. Introduction Tropospheric ozone (O3), as prevailing at chronically enhanced exposure regimes, is regarded to be one of the most detrimental air pollutants to forest trees (Matyssek et al., 2007). Adverse effects depend on both the amount of O3 entering the leaves and the defence capacity of the impacted tissues (Musselman et al., 2006). Trees may counteract O3 impact through stomatal closure restricting O3 uptake (stress avoidance) and detoxification through biochemical * Corresponding author. Ecophysiology of Plants, Department of Ecology, Technische Universita¨t Mu¨nchen, Life Science Center Weihenstephan, Am Hochanger 13, D-85354 Freising, Germany. Tel.: þ49 8161 714787; fax: þ49 8161 714576. E-mail address: [email protected] (C. Then). 0269-7491/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2008.02.015
reactions in the leaves (defence, stress tolerance). The extent of O3-induced injury was suggested to depend on the amount of available antioxidants per unit of stomatal O3 uptake (Musselman et al., 2006). Hence, assessment of stomatal O3 uptake into the leaf mesophyll is crucial in view of O3 risk assessment, as it is the actual O3 uptake which represents the metabolically active and, hence, the ‘‘phytomedically’’ relevant O3 dose (Matyssek et al., 2007). Methods for quantifying O3 uptake include eddy covariance, sap-flow, and cuvette techniques (see Wieser et al., 2008 for an overview). Eddy covariance techniques assess the total O3 deposition to tree stands, whereas sap-flow measurement can be used to quantify stomatal O3 flux into the foliage of whole trees (Ko¨stner et al., 2008). Branch-level approaches for assessing O3 uptake and photosynthesis are a crucial scaling step in linking O3 flux and carbon gain
C. Then et al. / Environmental Pollution 153 (2008) 526e528
between the leaf and the whole-tree level (Wieser et al., 2008). The latter can be derived under canopy free-air O3 fumigation in combination with the sap-flow approach of whole-tree O3 flux (Ko¨stner et al., 2008). Fluxes of O3, CO2, and water vapour at the leaf level can concurrently be measured by means of cuvette techniques as shown recently in a chamber experiment under controlled conditions (Grulke et al., 2007). However, cuvette techniques also have the potential of quantifying doseeresponse relationships at the branch level. Such latter approaches allow continuous monitoring of both O3 uptake and net carbon gain of entire branches under canopy conditions, especially in cases of open crown structure, when boundary layer effects on O3 uptake are low. Such conditions prevail in peripheral sun-lit branches of the upper canopy. Therefore, in this study we made use of a fully automated, climate-controlled cuvette system for branch-level gas exchange assessment, which continuously (day and night) provided apparent O3, CO2, and water vapour fluxes to derive a flux-based doseeresponse relationship between O3 uptake and carbon gain. To our knowledge previous branch-level O3 fumigation studies with cuvettes or bags have missed a continuous concurrent assessment of O3 and CO2 uptake although the relationship between these two parameters is crucial for phytomedically relevant O3 risk assessment of trees (Matyssek et al., 2007). The focus was on peripheral sun branches of Fagus sylvatica in the uppermost canopy of Kranzberg Forest (Germany). Boundary layers of such canopy positions were shown to only marginally affect O3 flux under the prevailing wind conditions at the site, where in parallel to the branchlevel approach a free-air O3 fumigation system was operated within the stand canopy (Werner and Fabian, 2002). 2. Materials and methods The study was carried out at Kranzberg Forest near Munich, Germany (485 m a.s.l., 48 250 N, 11 390 E). During the growing season of 2004, 60-year-old beech trees (F. sylvatica) were exposed to either ambient or twice-ambient O3 regimes (1 O3 and 2 O3, respectively), the latter regime being generated by a free-air canopy O3 exposure system (Werner and Fabian, 2002). Six study trees were selected, three trees were exposed to 1 O3 (control), and the other three to 2 O3. To prevent the risk of acute O3 injury, maximum O3 levels in the latter regime were restricted to 150 nl O3 l1. Scaffolding allowed access to the upper sun crown 25 m above ground, where attached terminal sections of peripheral sun-lit branches of each study tree were sealed into transparent, climate-controlled cuvettes (32 cm long, 12 cm in diameter). The cuvettes allowed O3 fumigation during gas exchange assessment (see Wieser et al., 2001 for a detailed description of the system), while tracking their diurnal (24 h) and seasonal fluctuations of O3, CO2, and water vapour fluxes. The system was in operation on the same branches continuously between June 29 and September 5, 2004. Air from each cuvette was analyzed alternately by means of a solenoidbased, gas-switching system by 70-s intervals. Water vapour and CO2 concentrations were compared each between the air entering and leaving the cuvettes with a LI 6200 CO2/H2O gas analyzer (Li-Cor, Lincoln, USA) operating in differential mode. Air flow rates through cuvettes were monitored with electronic mass flow-meters (Tylan, Eching, Germany). In parallel, O3 concentrations of the 1 O3 and 2 O3 regimes were sampled through Teflon tubing at the outlet of each cuvette and assessed with an O3 analyzer (Model 8810, Monitor Labs, San Diego, USA). O3 concentrations of the external 1 O3 and 2 O3 regimes (the latter released through free-air fumigation, cf. Werner
527
and Fabian, 2002) were analyzed by a second O3 analyzer. Signals of both O3 analyzers were transmitted to a data acquisition and feedback control system (Campbell Scientific, Ltd., Shepshed, U.K.) to generate the 2 O3 regime relative to 1 O3 through motor-operated mixing valves (Walz, Effeltrich, Germany). Environmental and gas exchange data were transmitted to an AM 416 multiplexer (Campbell Scientific, Ltd., Shepshed, U.K.) and recorded with a Campbell CR10 data logger, programmed to record 20 min means. The data logger also controlled the switching of the solenoids. O3 flux into the leaves (FO3 ) was calculated according to the flux equation: FO3 ¼ ½O3 GH2 O 0:613 where [O3] is the ambient air O3 concentration, GH2 O is the stomatal conductance for water vapour and 0.613 is a conversion factor to account for the lower diffusivity of O3 relative to water vapour. The equation is based on evidence by Laisk et al. (1989) that O3 concentration approaches nil in the intercellular leaf air space. Cumulative O3 uptake (COU) was calculated from FO3 as integrated over the exposure period. On a daily basis, mean carbon gain and COU were calculated for the individual branch sections sealed into the 1 O3 and 2 O3 cuvettes. As whole-tree free-air fumigation started on 25 April 2004, COU before branch enclosure into the entire cuvettes was 5 mmol m2, irrespective of O3 exposure (Lo¨w et al., 2007). Apparent net photosynthesis before enclosure did not differ significantly between regimes and averaged 5.1 0.9 mmol m2 s1 (Then et al., 2007); mean daily carbon gain varied between 2.8 and 6.5 mmol m2 s1 (averaging at 4.71 1.6 g C m2 d1), irrespective of the O3 regime. To account for the natural within-tree variation in mean daily carbon gain, responses to COU under 2 O3 were expressed in proportion of those under 1 O3 as occurring during same days (Lo¨w et al., 2007).
3. Results and discussion After 69 days of exposure, COU was 17.6 1.2 mmol m2 of projected leaf surface area in branches under 1 O3 and 29.4 1.1 mmol m2 under 2 O3 (Fig. 1). During the first 45 days of the experiment (day of year 181 through 225), the mean daily carbon gain of 2 O3 branches did not differ significantly from that of branches under 1 O3 (Fig. 2). However, after 45 days of exposure, when COU exceeded 20.3 mmol m2, the mean daily carbon gain under 2 O3 regime showed a pronounced decline. This decline persisted
Fig. 1. Time course of cumulative ozone uptake of adult Fagus sylvatica leaves exposed to ambient (1 O3) (open symbols) and twice-ambient (2 O3) (closed symbols) O3 concentration in branch cuvettes from June 29 (day 181) throughout September 5 (day 249) of 2004. For ease of comparison only mean values of three branches per treatment are shown.
528
C. Then et al. / Environmental Pollution 153 (2008) 526e528
for whole-tree free-air O3 fumigation and related flux assessment, as boundary layer width varies across the range of different crown positions, and as branches just represent one stage in scaling O3 flux between the single-leaf and wholetree level (Wieser et al., 2008). For reasons of validation and associated whole-tree O3 risk assessment (Matyssek et al., 2007), the cuvette approach can be linked, however, with independent trunk sap-flow assessment techniques (Ko¨stner et al., 2008) that offer derivation of O3 uptake at the whole-tree level. Acknowledgements
Fig. 2. Relative reduction in daily mean carbon gain of fully developed Fagus sylvatica leaves continuously exposed to 2 O3 for 69 days (solid symbols and solid line) and light-saturated net photosynthesis (Amax) of neighbouring leaves exposed to 2 O3 under free-air conditions (open symbols and dotted line; from Lo¨w et al., 2007). Carbon gain and Amax of the 2 O3 twigs at each date is expressed relative to the respective value of the 1 O3 control (¼1). The curve was fit by the polynomial regression: y ¼ 0.0009x2 þ 0.0081x þ 1.0314; r2 ¼ 0.498. Branch-cuvette leaves were monitored continuously from June 29 throughout September 5, of 2004, while Amax was measured 5 times between May 24 and September 14, of 2004. For ease of comparison only mean values of three cuvette branches per treatment are shown, while Amax values are the mean SD of five trees.
throughout the remainder of the experiment, as the mean daily carbon gain of 2 O3 branches was reduced by 10e30% relative to that of 1 O3 branches (Fig. 2). Similar O3-induced reductions of photosynthesis were found in single leaves exposed to 2 O3 outside of the cuvettes, i.e. under the employed free-air fumigation (Lo¨w et al., 2007; Then et al., 2007). COU above 20 mmol m2 had also incited a 10e30% reduction in light-saturated net photosynthesis (Fig. 2) and electron transport rate of 2 O3 leaves (Lo¨w et al., 2007). In addition, comparative analysis of foliar gas exchange and biochemical parameters also failed to detect significant differences between branches enclosed in cuvettes and branches outside cuvettes exposed to either 1 O3 or 2 O3 (Then et al., 2007). We conclude, therefore, that the branch-level cuvette approach allows for estimating O3 flux at peripheral crown positions, where boundary layers are low, yielding meso-scale within-crown resolution of photosynthetic foliage sensitivity under whole-tree free-air O3 fumigation. Nevertheless, the branch-level approach that intrinsically minimizes boundary layer effects on O3 flux during O3 fumigation is not a substitute
The present study was supported by the EU project CASIROZ ‘‘The Carbon Sink Strength of Beech in a Changing Environment: Experimental Risk Assessment by Mitigation of Chronic Ozone Impact’’ (EVK2-2002-00165). Special thanks to T. Gigele and H. Lohner for excellent technical assistance. References Grulke, N.E., Paoletti, E., Heath, R.L., 2007. Comparison of calculated and measured foliar O3 flux in crop and forest species. Environmental Pollution 146, 640e647. Ko¨stner, B., Matyssek, R., Heilmeier, H., Clausnitzer, F., Nunn, A.J., Wieser, G., 2008. Sap flow measurements as a basis for assessing tracegas exchange of trees. Flora 203, 14e33. Laisk, A., Kull, O., Moldau, H., 1989. Ozone concentration in leaf intercellular air spaces is close to zero. Plant Physiology 90, 1163e1167. Lo¨w, M., Ha¨berle, K.-H., Warren, C.R., Matyssek, R., 2007. O3 flux-related responsiveness of photosynthesis, respiration, and stomatal conductance of adult Fagus sylvatica to experimentally enhanced free-air O3 exposure. Plant Biology 9, 197e206. Matyssek, R., Bytnerowicz, A., Karlsson, P.-E., Paoletti, E., Sanz, M., Schaub, M., Wieser, G., 2007. Promoting the O3 flux concept for forest trees. Environmental Pollution 146, 587e607. Musselman, R.C., Lefohn, A.S., Massman, J., Heath, R.L., 2006. A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects. Atmospheric Environment 40, 1869e1888. Then, C., Herbinger, K., Blumenro¨ther, M., Haberer, K., Heerdt, C., Oßwald, W., Rennenberg, H., Grill, D., Tausz, M., Wieser, G., 2007. Evidence that branch cuvettes are reasonable surrogates for estimating O3 effects in entire tree crowns. Plant Biology 9, 309e319. Werner, H., Fabian, P., 2002. Free-air fumigation on mature trees: a novel system for controlled ozone enrichment in grown-up beech and spruce canopies. Environmental Science and Pollution Research 9, 117e121. Wieser, G., Tausz, M., Wonisch, A., Havranek, W.M., 2001. Free radical scavengers and photosynthetic pigments in Pinus cembra L. needles as affected by ozone exposure. Biologia Plantarum 44, 225e232. Wieser, G., Matyssek, R., Then, C., Cieslik, S., Paoletti, E., Ceulemans, R. Upscaling ozone flux in forests from leaf to landscape. Italian Journal of Agronomy, in press.