Introduction 

Tropospheric ozone (O₃), a common phytotoxic secondary air pollutant, reduces growth and carbon sequestration potential of terrestrial vegetation ([34]). Despite the limited nature of observations in the 19th century, ozone concentrations are reported to have undergone an increase from the base level of 10-15 ppbv in the pre-industrial era ([28]) to current concentrations of 35-45 ppbv in large parts of the industrialised world, with an average rate of increase of about 5 ppbv per decade in the Northern Hemisphere ([7]). It is estimated that the long-term increase in tropospheric O₃ concentration ([O₃]) has led to a substantial reduction in carbon uptake, as compared to the pre-industrial era ([7]). While daily maxima of [O₃] may now be decreasing, annual mean [O₃] has been increasing over the last decade ([23]).

Recent studies at different hierarchical levels have shown that O₃ reduces photosynthetic carbon assimilation and changes carbon allocation in trees and forest ecosystems ([41], [30]). It has been demonstrated that O₃ alters the source-sink balance in plants, resulting initially in carbon retention in shoots and decreased carbon allocation in below ground biomass (reviewed by [2]). Rising [O₃] thus has a significant potential to affect terrestrial carbon sinks and regional hydrology ([41], [35]). Most of the studies were, however, conducted on seedlings or juvenile trees and the assessment of the O₃ impact on mature trees is still at an early stage ([47], [48], [9]). Accordingly, there is an increasing demand for long-term studies conducted under natural, ecologically relevant conditions ([10]).

To quantify the detrimental dose of O₃ on vegetation, several indices have been established: AOT40 (accumulated exposure over a threshold of 40 ppbv) and POD_y (accumulated ozone flux above a given flux threshold y [nmol m^-2 s^-1]). Critical levels are assessed as an estimation above which detrimental effects may occur. The magnitude of both indices depends on species, environmental conditions and the capacity of plant defence mechanisms ([22]). Verryckt et al. ([38]) reported no impact of ambient [O₃] on carbon uptake in sub-urban mature pine forest in the vicinity of Antwerp city, Belgium, although the critical levels for both AOT40 and POD₁ indices were exceeded. Similarly, an ambient [O₃] of 50 ppbv (mean value) had no effect on leaf injury in black cherry and red maple seedlings during the period of April-August. While visible symptoms of foliar injury were observed under doubled [O₃] ([29]), no visible damages of leaves nor reduction in CO₂ uptake were observed in poplar plantation at a cumulative stomatal O₃ uptake of 25-27 mmol m^-2 ([49]).

Contrary to these findings, growth decreased by 6.6% in Swiss forests during the period 1991-2011, particularly in European beech and Norway spruce ([5]), and this decline was ascribed to rising [O₃]. Similarly, Gross Primary Production (GPP) was reduced by 4-8% on average in eastern US vegetation peaking up to 11-17% during an occurrence of high O₃ episodes ([45]). Only a limited effect of O₃ on photosynthesis was observed in the broad-leaved Harvard forest ([44]). Norway spruce (Picea abies Karst.), a wide-spread tree species of the temperate zone and especially Central Europe, has previously been described as an ozone sensitive tree species ([47]). Subramanian et al. ([34]), among others, reported a decrease in photosynthetic carbon assimilation of 4.3-15.5% due to an exposure to high [O₃]. As most studies were conducted on seedlings under controlled conditions ([32]), estimation and quantification of O₃ effects at the ecosystem scale under field conditions received only limited attention.

In the present study, we attempted to quantify the effect of O₃ on Net Ecosystem Production (NEP), defined as the difference between ecosystem-level photosynthetic uptake of CO₂ and ecosystem respiratory loss of CO₂. Our attention was focussed on a mountainous spruce forest in the Czech Republic. NEP values measured by eddy covariance were compared to those estimated by the Generalized Additive Mixed Model (GAMM). We tested the efficacy of [O₃]- and stomatal ozone flux (FsO₃)-based GAMM models to predict diurnal and seasonal changes in NEP. To determine O₃ effects, NEP was calculated in a simulation model at low, pre-industrial [O₃] (12 ppbv) and FsO₃ (0.5 nmol m^-2 s^-1) as well as at high [O₃] (80 ppbv) and FsO₃ (17 nmol m^-2 s^-1) expected at the end of the century ([21], [7]). We hypothesized that tropospheric ozone reduces NEP in Norway spruce forest. To test this hypothesis, we investigated effects of [O₃] and FsO₃ on NEP during four representative days of the main growing season differing in degree of cloudiness and thus in daily maxima of solar radiation intensity, air temperature, and vapour pressure deficit. These representative days also covered seasonal course in NEP and enabled thus to model effects of [O₃] and FsO₃ on NEP throughout the growing season (May-August).

  Material and methods 

Site description

The forest stand is located at the Bílý Kríž experimental research site within the Beskydy Mountains in the north-east of the Czech Republic (49° 30′ N, 18° 32′ E; 875-908 m a.s.l.). The experimental research site forms part of several international research networks and infrastructures: CzeCOS (Czech Carbon Observation System), ICOS (Integrated Carbon Observation System), and AnaEE (Analysis and Experimentation on Ecosystems). The forest stand (99% Picea abies and 1% Abies alba) had been established in 1981 by row planting of 4-year-old Norway spruce seedlings. Mean stand slope is 13° and exposure is to the south. In 2009, the year of investigation, the average stand height was 13 m, tree density was 1420 trees ha^-1, average stem diameter at breast height was 15.9 cm, and leaf area index based on hemispherical photographs was 9.6 m² m^-2.

The area has a moderately cool (mean air temperature 6.8 °C) and humid (mean relative air humidity 84%) climate with high annual precipitation (mean annual precipitation 1318 mm; years 1998-2009). Due to an even distribution of precipitation over the year, values of soil volumetric water content remain high during the growing season ranging between 20 and 30% irrespective of sky conditions. The region is characterized by low concentrations of nitrogen oxides (below 10 ppbv) and high [O₃], exceeding 80 ppbv during summer months ([47]). Diurnal courses of selected environmental variables, including diurnal changes in [O₃], for representative days are shown at Fig. 1.

Fig. 1 - Diurnal courses of global radiation (A), air temperature (B), vapour pressure deficit (VPD, C), and ozone concentration (D) during the representative days.

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Measurement of environmental variables

The measurements were conducted from May 28 to September 30, 2009, during the daytime (06:00-18:00 GMT+1) to cover the periods characterized by well-developed turbulent mixing. A meteorological mast (36 m tall) situated within the studied stand was equipped with a set of meteorological sensors and an eddy covariance system. Air temperature (Tair) and relative air humidity (RH) were measured by an EMS33 Rotro^® sensor (EMS, Brno, Czech Republic). These variables were used to calculate the values of vapour pressure deficit (VPD) according to the formula (eqn. 1):

The barometers PTB110^® (Vaisala, Vantaa, Finland) and SPA 511 B5UB^® (CRESSTO, Roznov pod Radhostem, Czech Republic) were used to measure air pressure. Global radiation (GR) was measured by a pyranometer CM6B^® (Kipp & Zonen, Delft, Netherlands). The 2D ultrasonic anemometer 50.5 (Met One Instruments, Grants Pass, OR, USA) was used for the measurement of horizontal wind speed. All standard meteorological measurements were made on a vertical profile at heights of 2, 7, 10, 11.5, 12.25, 13, 13.5, 14, 15, 17, 21, and 28 m above the soil surface. The signals from all sensors were recorded every 30 s and stored as half-hourly averages using a data logger (Delta-T^®, Burwell, Cambridgeshire, UK). In addition, precipitation was recorded by a precipitation gauge 386C (Met One Instruments, Grants Pass, OR, USA).

Estimation of CO₂ and O₃ fluxes

An eddy covariance system was used to measure the CO₂ and water vapour fluxes between the forest stand and the atmosphere. The system consisted of a Gill R3^® ultrasonic anemometer (Gill Instruments, Hampshire, UK) and an enclosed-path infrared gas analyser LI-7200^® (LI-COR Biosciences, Lincoln, NE, USA) placed 18 m above the soil surface. The post-processing of high frequency data (20 Hz) was performed by EddyPro^® software (LI-COR Biosciences, USA) according to recent recommendations ([3]) and expressed as half-hourly estimates. This procedure included spike removal and quality check of the raw signals, rotation of wind velocity components into the planar fit coordinate system, and spectral corrections of computed fluxes. The missing and excluded data, based on the quality checking scheme, were gap-filled by the marginal distribution sampling method according to Reichstein et al. ([25]). Sign convention was adopted as follows: positive values represent CO₂ uptake, thus enabling convenient modelling of NEP as a general nonrectangular hyperbolic function of incident GR ([37]).

[O₃] was measured at 5, 15, and 25 m above the soil surface using slow-response O₃ analysers O₃41M^® (Environment S.A., Poissy, France). The signals from all O₃ analysers were recorded as half-hourly averages.

Stomatal ozone flux (FsO₃) to the Norway spruce forest was calculated according to Cieslik ([6] - eqn. 2)

where c(z) is O₃ concentration at a height z = 2 m above the canopy, R_a is the aerodynamic resistance for the turbulent layer, R_b is the laminar layer resistance for the quasi-laminar layer, and R_c is the surface or canopy resistance of the receptor. R_c was calculated as (eqn. 3):

where R_sto is the land-cover specific needle stomatal resistance to O₃ uptake, R_ext is the resistance of the external plant parts to uptake or destruction of O₃, R_inc is the land-cover specific in-canopy aerodynamic resistance to transport of O₃ towards the soil and lower parts of the canopy, R_soil is the soil resistance to destruction or absorption of O₃ at the ground surface, LAI is leaf area index, and SAI is a surface area index set equal to LAI in the growing season.

Stomatal resistance component R_sto was calculated as described in Emberson et al. ([8] - eqn. 4):

where g_max is the average maximum stomatal conductance of Picea abies to ozone (nmol O₃ m^-2 s^-1) expressed on total needle surface area. The parameters g_phen, g_light, g_temp, g_VPD, and g_SWP are expressed in relative terms between 0 and 1, and represent the modification of g_max due to phenological changes, light intensity, air temperature, vapor pressure deficit, and soil water potential, respectively. g_min is the minimum FsO₃ that occurs during daylight hours.

The model was applied using environmental variables measured at the experimental station, although soil moisture deficit was estimated as a function of precipitation and daily mean surface temperature according to the principles of water budget. For more details and the site-specific model parameterisation see Zapletal et al. ([47]).

Modelling and statistical analyses

The Generalized Additive Mixed Model (GAMM) was implemented in the “mgcv” package of the R ver. 3.4.0 program ([42]) to assess the dependencies between NEP and its predictors GR, RH, Tair, VPD, and transpiration rate along with [O₃] and FsO₃ (Tab. 1). One-hour averages of all input data were used in GAMM. Only the data of the daytime period (06:00-18:00) when GR exceeded 10 W m^-2 were used, so as to exclude periods where turbulence was not well-developed.

All predictors were tested and fitted by a linear model, in the form of splines as smooth functions and as 1^st degree linear interactions. Predictors were centred and, in the case of GR and RH, a square root transformation was used. Day of observation and autocorrelation pattern were tested as random factors. The best model was selected based on the AIC criteria and p-values. The model was started as one component containing all explanatory variables together with their interactions. By use of REML (Restricted Maximum Likelihood Estimation) the optimal structure of the random component was found. Then, optimal fixed structure using ML (Maximum Likehood) estimation was determined. Within the fixed structure, REML estimation was applied again.

  Results 

Microclimate conditions, [O₃] and FsO₃

The mean incoming daily sum of GR was 15.8 MJ m^-2, mean Tair was 15.03 °C, RH was 81.5%, and precipitation sum was 388 mm during the investigated period from May 28 to September 30. Soil volumetric water content remained high during the whole growing season ranging between 20 and 30% irrespective of actual sky conditions. Four model days, selected on the base of cloudiness degree (Fig. 1), represent (i) a sunny warm day with high Tair and VPD values (July 22) and (ii) partly-cloudy sky conditions with mild Tair and VPD (June 17 and August 16). (iii) The onset of the growing season (May 30) was characterized by the minimum values of GR, Tair and VPD. These days were also characterized by distinct [O₃] with daily maxima ranging between 38 and 64 ppbv (Fig. 1D) as well as diurnal courses of FsO₃ (Fig. S1 in Supplementary material). The most frequent FsO₃ values ranged from 8-12 nmol m^-2 s^-1 during sunlight hours when the whole dataset is considered (Fig. 2).

Fig. 2 - Relative occurrence of stomatal ozone flux (FsO₃) during the sunlight hours modelled for the period May 28-September 30, 2009.

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GAMM outputs

The results of GAMM model for NEP prediction including [O₃] and FsO₃ are shown in Tab. 2 and Tab. 3, respectively. Both models include a linear and non-linear part, as illustrated by the Edf parameter, which is presented only in the non-linear part. Only statistically significant predictors are shown. [O₃] and VPD variables are shown only in Tab. 2, as their effect on NEP was only linear. Based on the Edf parameter, it was shown that FsO₃ had a non-linear effect on NEP (Tab. 3).

Statistically significant (p < 0.01) interactions were found between [O₃] and GR and between FsO₃ and GR (Tab. 2, Tab. 3). Both interactions had negative values of parametric estimate (Tab. 2, Tab. 3) indicating that [O₃] and FsO₃ led to a larger reduction of NEP with increasing GR. For example, natural FsO₃ led to a decrease in NEP by 1.07 µmol m^-2 s^-1, comparing to pre-industrial era, at GR <300 W m^-2, whereas this reduction amounted up to 1.59 µmol m^-2 s^-1 (23.6%) at GR >500 W m^-2. The FsO₃ seems to be more tightly linked to ozone-induced decline of NEP than [O₃] itself, based on explained variance by the models (R²) and more significant p-value for FsO₃ (p = 7.77e-13) that [O₃] (p = 3.55e-06). GR was identified as the most important driver of NEP, having the most significant p values (Tab. 2, Tab. 3).

NEP modelling

NEP was predicted using [O₃] (Fig. 3A) and FsO₃ (Fig. 3B) values in the range from zero to maximal values determined during the investigated growing season (May-September). Values of other predictors were fixed to values measured at 11:00 of the representative days. Both models predict a decrease in NEP with increasing [O₃] or increasing FsO₃. Among the representative days, the highest decrease of NEP was found on May 30^th, amounting to 21.6% (model run with [O₃]) and 30.4% (model run with FsO₃) when compared NEP at minimum (affected by 12 ppbv and 0.5 nmol m^-2 s^-1) and maximum (affected by 80 ppbv and 17 nmol m^-2 s^-1) values of [O₃] and FsO₃, respectively. In July 22, the warmest month, the corresponding decreases were 24.9% and 28.9%. Diurnal courses of measured and modelled NEP during four representative days of the vegetation season are shown in Fig. 4.

Fig. 3 - Modelled relationships between net ecosystem production (NEP) and ozone concentration (A) and between NEP and stomatal ozone flux (B) for four representative days (full line). Models were applied with fixed predictors (see Tab. 1) measured at 11:00 of these representative days. Dashed lines represent the 95% confidence interval.

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Fig. 4 - Predicted diurnal courses of net ecosystem production (NEP) using Generalized Additive Mixed Model with ozone concentration (O₃, A,B,C,D) and stomatal ozone flux (FsO₃, E,F,G,H). Three levels of O₃ and FsO₃ were used to predict NEP: Blue line - low O₃ (10 ppbv) and FsO₃ (0.5 nmol m^-2 s^-1), red line - high O₃ (80 ppbv) and FsO₃ (17 nmol m^-2 s^-1), and black line - actual, measured O₃ and FsO₃. Green line represents NEP measured by an eddy covariance system. Shaded area represents prediction bounds at 95% confidence level.

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Multi-factorial analysis (Fig. S2 in Supplementary material) revealed that high FsO₃ (17 nmol m^-2 s^-1) has the most significant effect on NEP reduction in summer, while NEP decrease is driven more by GR than FsO₃ in spring and autumn. Moreover, VPD-induced stomatal closure was a main factor limiting NEP and stomatal O₃ flux in Norway spruce trees throughout the whole growing season. Therefore, a strong relationship between NEP reduction and stomatal conductance, not only the simple [O₃] outside the leaves, can be suggested.

Taking to the account the whole dataset, the GAMM based calculated NEP was higher by up to 12.4% at low, preindustrial ozone concentration (12 ppbv) as compared to the measured NEP at the present concentration of 42.9 ± 11.2 ppbv (mean ± SD - Tab. 1). Similarly, the GAMM based on FsO₃ revealed NEP to be elevated by up to 24.8% under the condition of low FsO₃ (0.5 nmol m^-2 s^-1) as compared to actual FsO₃ of 8.3 ± 3.7 nmol m^-2 s^-1 (mean ± SD). In contrast, high [O₃] (80 ppbv) and high FsO₃ (17 nmol m^-2 s^-1) were found to lead to a decrease in NEP by 24% and 38.9%, respectively as compared to preindustrial NEP values which were unaffected by high [O₃] and/or FsO₃ (Tab. 4).

In addition, we present estimated changes in NEP under the conditions of low/high [O₃] and FsO₃ during individual months of the growing season (May-September - Tab. 4). Both models confirmed a high percentage reduction of NEP occurring in spring (May-June), comparing values of current NEP to preindustrial ones, while the O₃-induced NEP reduction in September was relatively small. The largest absolute reduction occurred, however, in June and July when NEP reaches a seasonal maximum. Contrary to those findings, the largest percentage and absolute NEP reductions predicted at high [O₃] and FsO₃ were found in September whereas the lowest ones in June.

  Discussion 

Projections of O₃-induced damage are uncertain due to numerous scenarios dealing with different future [O₃] or FsO₃. Ozone in the Northern Hemisphere is projected to increase further by 20-25% between 2015 and 2050, and by 40-60% by 2100, if current ozone-precursors (CO₂, volatile organic compounds, NOx) emission trends continue ([21]). Sitch et al. ([31]) projected a decrease in NEP by 14-23% over the period 1901-2100 owing to plant damage caused by [O₃] with regional reduction peeking up to 30%. However, recent studies suggest [O₃] decreases on average by 0-2 ppbv between 2000-2030 years, rising to 10 ppbv in 2100 in north latitudes ([43]). Those findings are in accordance with Klingberg et al. ([18]), who expected a decrease of O₃ precursors, particularly biogenic volatile organic compounds (BVOCs) and nitrogen oxides (NO_X), according to RCP4.5 scenario, which should consequently lead to decrease of European vegetation damage. However, extreme periods expected under future climate conditions may lead to episodes with high [O₃]. For example, severe drought conditions in 2003 were associated with high [O₃] in the Central Europe ([33]). In relation to regional climate, Beniston ([4]) assumed that the year 2003 could be used as an analogy of future dry summer seasons resulting in high emissions of BVOCs and NO_X ([43]), which showed to depend highly on season also at Bílý Kríž ([15]). Future impacts on ozone concentration across Europe show contradicting results: under RCP8.5 scenario, [O₃] of temperate coniferous forest could increase by 10.2 ppbv as modelled for coniferous forests at Hengduan Shan, China forests ([12]). On the other hand, a modelling study for the temperate coniferous forest of the Sierra Nevada suggests a decrease in [O₃] by 6 ppbv from 2000 to 2050 ([12]) in accordance with the [O₃] decrease by 5 ppbv per decade ([7]). In our study we have contrasted the O₃ effects on NEP at low, pre-industrial [O₃] of 12 ppbv and high [O₃] of 80 ppbv.

Numerous studies have shown that elevated [O₃] generally results in reduced photosynthesis, chlorophyll content and whole-plant growth, decreased stomatal conductance, an altered antioxidant system, accelerated senescence and changes the plant metabolism, although the extent of the effects varies by species, length of exposure, [O₃] and/or co-occurrence of other stress factors (reviewed in [39], [19], [14]). In accordance with these studies we have found here that both increasing [O₃] and FsO₃ significantly reduced NEP of mature Norway spruce forest grown under natural mountain conditions (Fig. 3). Moreover, our analyses show that the use of FsO₃ provides an improved fit to NEP experimental data as compared to the predictions of a merely [O₃]-based GAMM model, particularly under hot summer days (Tab. 4). Differences between the measured and predicted NEP by the model with [O₃] and FsO₃ expressed as a model error amounted to 0.41% and 0.17%, respectively. The theory behind this is that stomata substantially regulate both CO₂ and O₃ diffusion into the leaf interior leading thus to a modulation of photosynthetic activity and the magnitude of O₃-induced injuries, respectively. We assume that stomata remain open under the conditions of low VPD and sufficient light intensity ([37]). Accordingly, high FsO₃ can be expected though ambient [O₃] is relatively low and can lead to a substantial NEP reduction. On the contrary, conditions of high VPD and/or high temperature may result in a marked closure of stomata. While an ambient [O₃] is usually high under such conditions, O₃-induced reduction of NEP may be depressed due to a low FsO₃ into a leaf interior. Therefore, models with high FsO₃ may predict higher reduction in NEP than models with high [O₃] (Fig. 3). Such results imply that not only high [O₃], but particularly high FsO₃ induces injuries on vegetation and reduces carbon assimilation ([20], [9]).

We predicted higher NEP (by 24.6% on average) at low FsO₃ of 0.5 nmol m^-2 s^-1 as compared to NEP at current FsO₃ of 8.32 ± 3.66 nmol m^-2 s^-1 with the highest absolute reduction occurring in June and July (Tab. 4). Karlsson ([16]) stated that reduction of the living biomass carbon stock caused by the ozone problem among the Central and Northern European countries ranges from 2% (Norway, Finland) to 32% in the Czech Republic, when compared to the pre-industrial age. Satellite data over the European continent suggest a decline of photosynthetic carbon uptake ranging between 0.4-30% due to O₃ damage depending on the forest type and location. Highest negative effects of O₃ were found in coniferous forests ([24]). Moreover, these authors identified air temperature, soil water content, and relative air humidity as key predictors of NEP describing more than 81% of NEP variability. Soil water content was, however, omitted into our model, since soil water content was not a limiting factor in our case study under the wet mountainous conditions. However, air temperature and relative air humidity (ultimately VPD), play significant roles in our model together with GR. Similarly to our findings, Anav et al. ([1]) reported a reduction of photosynthetic carbon uptake per day caused by [O₃] of about 22% in boreal and temperate forests. FACE fumigation experiment of enhanced CO₂ and [O₃] on temperate forest dominated by Betula papyrifera Marsh. and Acer saccharum Marsh. in USA revealed reduction of cumulative net primary productivity of 9% ([36]). However, Finco et al. ([11]) estimated relatively low decreases in biomass production by 4-5 % in Alpine larch forest caused by current [O₃] when compared to the pre-industrial era, even if the total ozone fluxes were generally high, up to 30-40 nmol m^-2 s^-1. This was due to relatively low portion of stomatal uptake by the larch forest representing only 15-16% of total O₃ deposition flux into the forest. Similar growth reductions in stem increment of 6.6% and photosynthetic carbon uptake of 4-8% were observed in Switzerland ([5]) and eastern parts of USA ([45]), respectively, due to episodes of high tropospheric [O₃].

Norway spruce, one of the most common and important timber trees in Europe, was found to be an O₃ sensitive tree species, showing substantial reductions in photosynthetic carbon uptake under elevated [O₃] ([39], [47]). For example, Subramanian et al. ([34]) reported a decrease in carbon sequestration of spruce forest in Sweden by 4.3-15.5% under ambient ozone conditions as compared to the preindustrial [O₃]. This is consistent with our estimate of NEP reduction in mountain spruce forest of the temperate zone.

It should be noted, however, that there is a contrasting evidence of long-term O₃ effects detected after several growing seasons. Since several research groups reported no negative effect of elevated [O₃] on carbon assimilation in forest ecosystems ([46], [40]), we can hypothesize large acclimation capacity of tree species, including morphological ([26], [27]) as well as biochemical adjustments (reviewed by [13]). Recent studies have, however, shown that long-term exposure to elevated [O₃] may substantially modulate an acclimation of trees to other environmental factors like elevated temperature ([17]) and/or elevated atmospheric CO₂ concentration ([27], [46]). The carbon sink strength in old forests could be thus reduced substantially or even disappear under the conditions of high [O₃], accelerating the impact of atmospheric CO₂ on climate change. Precise estimates of FsO₃ are thus needed for future predictions of ozone impacts on NEP of terrestrial ecosystems.

  Conclusions 

We have found that stomatal ozone flux model tracks diurnal changes in NEP more precisely that ozone concentration model does. NEP simulated at low, pre-industrial FsO₃ (0.5 nmol m^-2 s^-1) was higher by 24.8% as compared to NEP assessed at current FsO₃ (8.32 nmol m^-2 s^-1). Further increase in FsO₃ (up to 17 nmol m^-2 s^-1) may lead to subsequent reduction in NEP by 14.1% in average as compared to current NEP values, but this reduction may change during the growing season. These results are in agreements with observations in other European coniferous forests. Relatively high site-specific variability in [O₃] effect of photosynthetic carbon uptake is likely caused by species-specific sensitivity of stomata to environmental drivers. Also a co-occurrence of other stress factors, particularly extreme temperatures and drought, leading to the changes in stomatal conductance and stomatal ozone flux, may influence a final response of trees to [O₃].

  Acknowledgement 

This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic within the project “SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions” (CZ.02. 1.01/0.0/0.0/16_019/0000797).

  References

  Supplementary Material

Fig. S1 - Diurnal courses of modelled stomatal ozone flux (FsO₃) in Norway spruce forest during the investigated period of May 28 - September 30, 2009.
Fig. S2 - Principal component analysis revealing dependencies of environmental variables among each other.