Impact of Ozone on Trees: an Ecophysiological Perspective

Estimates of biomass reductions of ozone sensitive herbaceous plants in California

Exposure to tropospheric ozone pollution impairs photosynthesis and growth in plants and this can have consequences for ecosystems. However, exposure-response research in the United States (U.S.) has historically focused on trees and crops, and less attention has been given to non-crop herbaceous species. We combined U.S. Environmental Protection Agency ozone monitoring data from the entirety of 2016 with published exposure-response relationships from controlled exposure experiments for twenty herbaceous plant species occurring in California. The U.S. Department of Agriculture PLANTS database was used to identify county-level occurrence data of these plant species. A kriged ozone exposure surface for 2016 was generated using data from monitoring stations in California and surrounding states, using Accumulated Ozone exposure over a Threshold of 40 ppb (AOT40) as an exposure metric. County-wide ozone exposure estimations were then combined with published exposure response functions for focal plants, and maps were created to estimate ozone-induced growth losses in the counties where the plants occur. Plant species had estimated annual growth losses from <1 % to >20 % based on exposure levels and sensitivity. Of the 20 species, 17 had predicted biomass loss >5 % in at least one county, emphasizing the vulnerability of herbaceous species at recent ozone concentrations. Butte, Nevada, Plumas, San Luis Obispo, and Shasta Counties, an area of about 31,652 km2, had the highest number of species (6) with >10 % estimated biomass loss, the loss threshold for European critical levels. White clover (Trifolium repens L.) was one of the most affected species with more than an estimated 10 % annual estimated growth loss over 59 % of the state. Overall, these estimated growth losses demonstrate potential for shifts in plant communities and negative effects on ecosystems. This study addresses critical policy needs for risk assessments on herbaceous species in a single year of ozone exposure.

Ozone exposure-response relationships parametrized for sixteen tree species with varying sensitivity in the United States

It is well known that exposure to ambient O3 can decrease growth in many tree species in the United States (US). Our study reports experimental data from outdoor open-top chamber (OTC) studies that quantify total biomass response changes for seedlings of 16 species native to western and eastern North America, which were exposed to several levels of elevated O3 for one or more years. The primary objective of this study is to establish a reference set of parameters for these seedling exposure-response relationships using a 3-month (92 day) 12-hr W126 O3 metric used by US Environmental Protection Agency and other agencies to assess risk to trees from O3 exposure. We classified the 16 species according to their sensitivity, based on the biomass loss response functions to protect from a 5% biomass loss. The three-month 12-h W126 estimated to result in a 5% biomass loss was 2.5-9.2 ppm-h for sensitive species, 20.8-25.2 ppm-h for intermediate species, and > 28.7 ppm-h for insensitive species. The most sensitive tree species include black cherry, ponderosa pine, quaking aspen, red alder, American sycamore, tulip poplar and winged sumac. These species are ecologically important and widespread across US. The effects of O3 on whole-plant biomass depended on exposure duration and dynamics and on the number of successive years of exposure. These species-specific exposure-response relationships will allow US agencies and other groups to better estimate biomass losses based on ozone exposures in North America and can be used in risk assessment and scenario analyses.

Visible Foliar Injury and Ecophysiological Responses to Ozone and Drought in Oak Seedlings

To verify the responses of visible foliar injury (VFI), we exposed seedlings of three oak species for 4.5 months in an open air facility, using differing ozone (O3) and drought treatments: O3 (three levels from ambient to ×1.4 ambient), and drought (three levels of irrigation from 40% to 100% field capacity). We related the accumulated phytotoxic O3 dose (POD1) and cumulative drought index (CDI) to the O3 and drought VFI and assessed growth increment (height, diameter, leaf number), biomass (of all organs), and physiological parameters: net photosynthesis per plant (Pn), photosynthetic nitrogen (PNUE) and phosphorus use efficiency (PPUE)). The results indicated that an increase in POD1 promoted O3 VFI in Quercus robur and Quercus pubescens, while Quercus ilex was asymptomatic. The POD1-based critical level at the onset of O3 VFI was lower for Q. robur than for Q. pubescens (12.2 vs. 15.6 mmol m−2 POD1). Interestingly, drought reduced O3 VFI in Q. robur but increased it in Q. pubescens. Both O3 and drought were detrimental to the plant biomass. However, Q. robur and Q. pubescens invested more in shoots than in roots, while Q. ilex invested more in roots, which might be related to a hormetic mechanism. Pn, PNUE and PPUE decreased in all species under drought, and only in the sensitive Q. robur (PPUE) and Q. pubescens (PNUE) under O3. This study confirms that POD1 is a good indicator to explain the development of O3 VFI and helps a differential diagnosis of co-occurring drought and O3 VFI in oak forests.

Growth and photosynthetic responses to ozone of Siebold's beech seedlings grown under elevated CO2 and soil nitrogen supply

Ozone (O₃) is a phytotoxic air pollutant, the adverse effects of which on growth and photosynthesis are modified by other environmental factors. In this study, we examined the combined effects of O₃, elevated CO₂, and soil nitrogen supply on Siebold's beech seedlings. Seedlings were grown under combinations of two levels of O₃ (low and two times ambient O₃ concentration), two levels of CO₂ (ambient and 700 ppm), and three levels of soil nitrogen supply (0, 50, and 100 kg N ha^-1 year^-1) during two growing seasons (2019 and 2020), with leaf photosynthetic traits being determined during the second season. We found that elevated CO₂ ameliorated O₃-induced reductions in photosynthetic activity, whereas the negative effects of O₃ on photosynthetic traits were enhanced by soil nitrogen supply. We observed three-factor interactions in photosynthetic traits, with the ameliorative effects of elevated CO₂ on O₃-induced reductions in the maximum rate of carboxylation being more pronounced under high than under low soil nitrogen conditions in July. In contrast, elevated CO₂-induced amelioration of the effects of O₃ on stomatal function-related traits was more pronounced under low soil nitrogen conditions. Although we observed several two- or three-factor interactions of gas and soil treatments with respect to leaf photosynthetic traits, the shoot to root dry mass (S/R) ratio was the only parameter for which a significant interaction was detected among seedling growth parameters. O₃ caused a significant increase in S/R under ambient CO₂ conditions, whereas no similar effects were observed under elevated CO₂ conditions. Collectively, our findings reveal the complex interactive effects of elevated CO₂ and soil nitrogen supply on the detrimental effects of O₃ on leaf photosynthetic traits, and highlight the importance of taking into consideration differences between the responses of CO₂ uptake and growth to these three environmental factors.

Spatio-Temporal Variation of Ozone Concentrations and Ozone Uptake Conditions in Forests in Western Germany

The study analyzes the long-term trends (1998–2019) of concentrations of the air pollutants ozone (O3) and nitrogen oxides (NOx) as well as meteorological conditions at forest sites in German midrange mountains to evaluate changes in O3 uptake conditions for trees over time at a plot scale. O3 concentrations did not show significant trends over the course of 22 years, unlike NO2 and NO, whose concentrations decreased significantly since the end of the 1990s. Temporal analyses of meteorological parameters found increasing global radiation at all sites and decreasing precipitation, vapor pressure deficit (VPD), and wind speed at most sites (temperature did not show any trend). A principal component analysis revealed strong correlations between O3 concentrations and global radiation, VPD, and temperature. Examination of the atmospheric water balance, a key parameter for O3 uptake, identified some unusually hot and dry years (2003, 2011, 2018, and 2019). With the help of a soil water model, periods of plant water stress were detected. These periods were often in synchrony with periods of elevated daytime O3 concentrations and usually occurred in mid and late summer, but occasionally also in spring and early summer. This suggests that drought protects forests against O3 uptake and that, in humid years with moderate O3 concentrations, the O3 flux was higher than in dry years with higher O3 concentrations.

Effects of Elevated Ozone Concentrations on Root Characteristics and Soil Properties of Elaeocarpus sylvestris and Michelia chapensis

The increasing concentration of surface ozone (O₃) was observed during recent decades in the world, which affects tree roots and forest soils. Meanwhile, the impact of ozone on tree roots is greatly affected by soil condition. However, there is a lack of knowledge about the possible effects of ozone on tree roots and soil processes. In this study, The influences of surface ozone (O₃) stress on the root biomass, morphology, nutrients, soil properties, and soil enzyme activity of Elaeocarpus sylvestris and Michelia chapensis seedlings were examined at four O₃ concentrations (charcoal-filtered air, 1 × O₃ air, 2 × O₃ air, and 4 × O₃ air). Elevated O₃ concentrations were found to significantly increase the root C content, N content, C/P ratio, and N/P ratio, and significantly decrease the root biomass, number of root tips, and root C/N ratio of both species. The soil organic matter content, pH, total N content, and urease and catalase activities of both species tended to increase. The limitation in root growth and responses in the root structure of E. sylvestris induced by elevated O₃ concentrations led to increased bulk density and decreased soil porosity and void ratio. These profound effects of O₃ concentrations on the roots and soil characteristics of these two species underscore the importance of research in O₃ science.

Water use strategy affects avoidance of ozone stress by stomatal closure in Mediterranean trees-A modelling analysis

Both ozone (O₃ ) and drought can limit carbon fixation by forest trees. To cope with drought stress, plants have isohydric or anisohydric water use strategies. Ozone enters plant tissues through stomata. Therefore, stomatal closure can be interpreted as avoidance to O₃ stress. Here, we applied an optimization model of stomata involving water, CO₂ , and O₃ flux to test whether isohydric and anisohydric strategies may affect avoidance of O₃ stress by stomatal closure in four Mediterranean tree species during drought. The data suggest that stomatal closure represents a response to avoid damage to the photosynthetic mechanisms under elevated O₃ depending on plant water use strategy. Under high-O₃ and well-watered conditions, isohydric species limited O₃ fluxes by stomatal closure, whereas anisohydric species activated a tolerance response and did not actively close stomata. Under both O₃ and drought stress, however, anisohydric species enhanced the capacity of avoidance by closing stomata to cope with the severe oxidative stress. In the late growing season, regardless of the water use strategy, the efficiency of O₃ stress avoidance decreased with leaf ageing. As a result, carbon assimilation rate was decreased by O₃ while stomata did not close enough to limit transpirational water losses.

Using Visual Ozone Damage Scores and Spectroscopy to Quantify Soybean Responses to Background Ozone

Remotely-sensed identification of ozone stress in crops can allow for selection of ozone resistant genotypes, improving yields. This is critical as population, food demand, and background tropospheric ozone are projected to increase over the next several decades. Visual scores of common ozone damage have been used to identify ozone-stress in bio-indicator plants. This paper evaluates the use of a visual scoring metric of ozone damage applied to soybeans. The scoring of the leaves is then combined with hyperspectral data to identify spectral indices specific to ozone damage. Two genotypes of soybean, Dwight and Pana, that have shown different sensitivities to ozone, were grown and visually scored for ozone-specific damage on multiple dates throughout the growing season. Leaf reflectance, foliar biophysical properties, and yield data were collected. Additionally, ozone bio-indicator plants, snap beans, and common milkweed, were investigated with visual scores and hyperspectral leaf data for comparison. The normalized difference spectral index (NDSI) was used to identify the significant bands in the visible (VIS), near infrared (NIR), and shortwave infrared (SWIR) that best correlated with visual damage score when used in the index. Results were then compared to multiple well-established indices. Indices were also evaluated for correlation with seed and pod weight. The ozone damage scoring metric for soybeans evaluated in August had a coefficient of determination of 0.60 with end-of-season pod weight and a Pearson correlation coefficient greater than 0.6 for photosynthetic rate, stomatal conductance, and transpiration. NDSI [R558, R563] correlated best with visual scores of ozone damage in soybeans when evaluating data from all observation dates. These wavelengths were similar to those identified as most sensitive to visual damage in August when used in NDSI (560 nm, 563 nm). NDSI [R560, R563] in August had the highest coefficient of determination for individual pod weight (R2 = 0.64) and seed weight (R2 = 0.54) when compared against 21 well-established indices used for identification of pigment or photosynthetic stress in plants. When evaluating use of spectral bands in NDSI, longer wavelengths in SWIR were identified as more sensitive to ozone visual damage. Trends in the bands and biophysical properties of the soybeans combined with evaluation of ozone data indicate likely timing of significant ozone damage as after late-July for this season. This work has implications for better spectral detection of ozone stress in crops and could help with efforts to identify ozone tolerant varieties to increase future yield.

Changes of Cinnamomum camphora root characteristics and soil properties under ozone stress in South China

High O₃ exposure affects the forest growth and soil characteristics. Although there is substantial evidence that O₃ does impose a stress on forest trees, the effects of O₃ on roots and soil of evergreen broad-leaved tree species in South China remain unknown. The effects of ozone (O₃) fumigation on the root biomass, root morphology, root nutrient, soil physical, and chemical properties were examined in Cinnamomum camphora seedlings grown under four O₃ treatments (charcoal-filtered air (CF) or O₃ at 1×, 2× and 4× ambient concentration). O₃ significantly decreased root biomass and root carbon (C). Regardless of O₃ level, elevated O₃ significantly resulted in reduced root surface area, volume, number of forks, and specific root length (SRL). The percentages of fine to total root in terms of root surface area and root volume of seedlings under the CF and 1 × O₃ treatments were significantly higher than those of seedlings under the 4 × O₃ treatment, indicating that high O₃ level impaired the growth performance of fine roots. O₃ affected root growth and structures, which increased soil bulk density and reduced soil total porosity and void ratio. The soil pH under all O₃ fumigation treatments significantly increased compared with CF treatment, whereas the organic matter significantly decreased. In conclusion, although the increased O₃ level enhanced root N and P under 2 and 4 × O₃ treatments compared with 1 × O₃ treatment as compensation mechanisms to prevent O₃-induced decrease in root C gain and root functions, O₃ still decreased the root biomass and root tips, and changed the soil physical and chemical properties.

Elevated ozone reduced leaf nitrogen allocation to photosynthesis in poplar

We investigated the effects of elevated ozone (O₃) concentration on leaf nitrogen (N), a key determinant of plant photosynthesis, with two clones of poplar grown in open-top chambers. We focus on the difference between mass-based leaf N concentration (N_mass) and area-based one (N_area) in their responses to elevated O₃, and the allocation of N to different leaf components: photosynthetic apparatus, cell walls, and others under elevated O₃ level. Our results showed that elevated O₃ significantly increased N_mass, but reduced N_area and leaf mass per area (LMA). The two clones showed no difference in N_mass response to O₃, but the more sensitive clone showed greater reduction of N_area and LMA due to O₃. We also found positive relationships between N_area and photosynthetic parameters, e.g. light-saturated photosynthetic rate (A_sat). Furthermore, elevated O₃ significantly reduced photosynthetic N-use efficiency (PNUE) and leaf N allocation to photosynthetic components, while increasing N allocation to cell walls and other components. We concluded that plants invested more N in cell walls and other components to resist O₃ damages at the expense of photosynthetic N. The change of N allocation in plant leaves in response to elevated O₃ could have an impact on ecological processes, e.g. leaf litter decomposition.

The passion fruit liana (Passiflora edulis Sims, Passifloraceae) is tolerant to ozone

Passiflora edulis Sims is a liana species of high economic interest and is an interesting model plant for understanding ozone action on disturbed vegetation. In this work we hypothesized that P. edulis has adaptive responses to oxidative stress that enable it to tolerate ozone damage based on its capacity to grow under a diversity of environmental conditions and to dominate disturbed areas. We exposed seedlings to three levels of ozone in a Free-Air Controlled Exposure (FACE) system (22, 41 and 58 ppb h AOT40 and 13.52, 17.24 and 20.62 mmol m^-2 POD0, over 97 days) for identifying its tolerance mechanisms. Anatomical (leaf blade structure and fluorescence emission of chloroplast metabolites), physiological (leaf gas exchange, growth rate and biomass production) and biochemical (pigments, total sugars, starch, enzymatic and non-enzymatic antioxidant metabolites, reactive oxygen species and lipid peroxidation derivatives) responses were assessed. Ozone caused decreased total number of leaves, hyperplasia and hypertrophy of the mesophyll cells, and accelerated leaf senescence. However, O₃ did not affect carbohydrates content, net photosynthetic rate, or total biomass production, indicating that the carboxylation efficiency and associated physiological processes were not affected. In addition, P. edulis showed higher leaf contents of ascorbic acid, glutathione (as well high ratio between their reduced and total forms), carotenoids, and flavonoids located in the chloroplast outer envelope membrane. Our results indicate that P. edulis is an O₃-tolerant species due to morphological acclimation responses and an effective antioxidant defense system represented by non-enzymatic antioxidants, which maintained the cellular redox balance under ozone.

Effects of Elevated Ozone Levels on Photosynthesis, Biomass and Non-structural Carbohydrates of Phoebe bournei and Phoebe zhennan in Subtropical China

To assess the impacts of ozone (O3) on carbon metabolism of subtropical broadleaved tree species, seedlings of Phoebe bournei and Phoebe zhennan were exposed to elevated O3 levels in open-top chambers (OTCs) from June to November 2014. Three treatments were conducted in nine total OTCs, including charcoal-filter air (CF) as a control treatment, low O3 treatment 'O3-1' (∼100 nl l-1), and high O3 treatment 'O3-2' (∼150 nl l-1). Our findings demonstrated that elevated O3 levels significantly decreased the net photosynthesis rates (Pn ) and leaf, root, and total biomass of both species, while it did not significantly affect the root/shoot ratio in P. bournei and P. zhennan. O3-1 treatments significantly increased water soluble carbohydrates (WSC) in leaves of both tree species, while only increased the total non-structural carbohydrates (TNC) and starch in leaves of P. bournei; effects on P. zhennan were equivalent in comparison to the control treatment (CF). Likewise, there was no effect of treatment on the polysaccharide content of both tree species. The contents of polysaccharide, starch contents in fine roots of both species, and TNC in fine roots of P. bournei increased significantly in O3-1 compared to CF. O3-2 treatment significantly decreased starch and TNC in the fine roots of P. bournei, and significantly decreased polysaccharide, starch, WSC, and TNC in the fine roots of P. zhennan. Elevated O3 had no effects on leaf polysaccharide in both species, but O3-1 significantly increased polysaccharide in the fine roots of both species, and O3-1 significantly increased WSC in the leaves while decreased that in the fine roots of both species. These results suggested that elevated O3 levels have significant impacts on the carbon metabolism of both tree species in our study, with differential responses between tree species and among leaves and roots.

Effects of nitrogen and phosphorus imbalance on photosynthetic traits of poplar Oxford clone under ozone pollution

Ozone (O₃) pollution and the availability of nitrogen (N) and phosphorus (P) in the soil both affect plant photosynthesis and chlorophyll (Chl) content, but the interaction of O₃ and nutrition is unclear. We postulated that the nutritional condition changes plant photosynthetic responses to O₃. An O₃-sensitive poplar clone (Oxford) was subject to two N levels (N0, 0 kg N ha^- 1; N80, 80 kg N ha^- 1), two P levels (P0, 0 kg P ha^- 1; P80, 80 kg P ha^- 1) and three levels of O₃ exposure (ambient concentration, AA; 1.5 × AA; 2.0 × AA) over a growing season in an O₃ free air controlled exposure (FACE) facility. The daily change of leaf gas exchange and dark respiration (R_d) were investigated at mid-summer (August). Chl a fluorescence was measured three times in July, August and September. At the end of the growing season, Chl content was measured. It was found that Chl content, the maximum quantum yield (F_v/F_m), Chl a fluorescence performance index (PI) and gas exchange were negatively affected by elevated O₃. Phosphorus may mitigate the O₃-induced reduction of the ratio of photosynthesis to stomatal conductance, while it exacerbated the O₃-induced loss of F_v/F_m. Nitrogen alleviated negative effects of O₃ on F_v/F_m and PI in July. Ozone-induced loss of net photosynthetic rate was mitigated by N in medium O₃ exposure (1.5 × AA). However, such a mitigation effect was not observed in the higher O₃ level (2.0 × AA). Nitrogen addition exacerbated O₃-induced increase of R_d suggesting an increased respiratory carbon loss in the presence of O₃ and N. This may result in a further reduction of the net carbon gain for poplars exposed to O₃.

Mesophyll conductance to CO2 in leaves of Siebold's beech (Fagus crenata) seedlings under elevated ozone

Ozone is an air pollutant that negatively affects photosynthesis in woody plants. Previous studies suggested that ozone-induced reduction in photosynthetic rates is mainly attributable to a decrease of maximum carboxylation rate (V_cmax) and/or maximum electron transport rate (J_max) estimated from response of net photosynthetic rate (A) to intercellular CO₂ concentration (C_i) (A/C_i curve) assuming that mesophyll conductance for CO₂ diffusion (g_m) is infinite. Although it is known that C_i-based V_cmax and J_max are potentially influenced by g_m, its contribution to ozone responses in C_i-based V_cmax and J_max is still unclear. In the present study, therefore, we analysed photosynthetic processes including g_m in leaves of Siebold's beech (Fagus crenata) seedlings grown under three levels of ozone (charcoal-filtered air or ozone at 1.0- or 1.5-times ambient concentration) for two growing seasons in 2016-2017. Leaf gas exchange and chlorophyll fluorescence were simultaneously measured in July and September of the second growing season. We determined the A, stomatal conductance to water vapor and g_m, and analysed A/C_i curve and A/C_c curve (C_c: chloroplast CO₂ concentration). We also determined the Rubisco and chlorophyll contents in leaves. In September, ozone significantly decreased C_i-based V_cmax. At the same time, ozone decreased g_m, whereas there was no significant effect of ozone on C_c-based V_cmax or the contents of Rubisco and chlorophyll in leaves. These results suggest that ozone-induced reduction in C_i-based V_cmax is a result of the decrease in g_m rather than in carboxylation capacity. The decrease in g_m by elevated ozone was offset by an increase in C_i, and C_c did not differ depending on ozone treatment. Since C_c-based V_cmax was also similar, A was not changed by elevated ozone. We conclude that g_m is an important factor for reduction in C_i-based V_cmax of Siebold's beech under elevated ozone.

Photosynthetic activity in relation to a gradient of leaf nitrogen content within a canopy of Siebold's beech and Japanese oak saplings under elevated ozone

The primary objective of this study was to describe parameters related to the leaf biochemical assimilation capacity of photosynthesis, such as the maximum rates of carboxylation (V_cmax) and electron transport (J_max), as a function of the leaf nitrogen content throughout a canopy of Siebold's beech and Japanese oak grown under elevated ozone (O₃) conditions during a growing season. To this end, we investigated the relationship between photosynthetic traits and leaf nitrogen content in various canopy positions of two tree species under free-air O₃ exposure (60 nmol mol^-1, during daylight hours) in June, August, and October 2012. We observed O₃-induced reduction in V_cmax and J_max without reduction of leaf nitrogen content in both tree species. In Siebold's beech, V_cmax and J_max in leaves with higher N_area were largely decreased by O₃ from August, while little effect of O₃ was observed in leaves with lower N_area. On the other hand, there was no difference in the extent of O₃-induced reduction in V_cmax and J_max across the range of N_area in leaves of Japanese oak. Reduction of leaf nitrogen content under elevated O₃ conditions was observed only in Siebold's beech in October. These results indicated that the decrease in the efficiency of photosynthetic nitrogen use is in an earlier step in O₃-induced decline of photosynthesis in Siebold's beech and Japanese oak. Based on these results, we emphasize the importance of integration of O₃ effects into the conventional estimation of V_cmax and J_max from leaf nitrogen content for evaluating canopy photosynthesis under current and future elevated O₃ conditions.

Elevated ozone affects C, N and P ecological stoichiometry and nutrient resorption of two poplar clones

The effects of elevated ozone on C (carbon), N (nitrogen) and P (phosphorus) ecological stoichiometry and nutrient resorption in different organs including leaves, stems and roots were investigated in poplar clones 546 (P. deltoides cv. '55/56' × P. deltoides cv. 'Imperial') and 107 (P. euramericana cv. '74/76') with a different sensitivity to ozone. Plants were exposed to two ozone treatments, NF (non-filtered ambient air) and NF60 (NF with targeted ozone addition of 60 ppb), for 96 days in open top chambers (OTCs). Significant ozone effects on most variables of C, N and P ecological stoichiometry were found except for the C concentration and the N/P in different organs. Elevated ozone increased both N and P concentrations of individual organs while for C/N and C/P ratios a reduction was observed. On these variables, ozone had a greater effect for clone 546 than for clone 107. N concentrations of different leaf positions ranked in the order upper > middle > lower, showing that N was transferred from the lower senescent leaves to the upper ones. This was also indicative of N resorption processes, which increased under elevated ozone. N resorption of clone 546 was 4 times larger than that of clone 107 under ambient air (NF). However, elevated ozone (NF60) had no significant effect on P resorption for both poplar clones, suggesting that their growth was only limited by N, while available P in the soil was enough to sustain growth. Understanding ecological stoichiometric responses under ozone stress is crucial to predict future effects on ecological processes and biogeochemical cycles.

Tree-ring formation as an indicator of forest capacity to adapt to the main threats of environmental changes in Lithuania

Global changes occurring under different environmental conditions have changed stand competition, as well as nutrient and light availability, which has resulted in changes in productivity. Therefore, in the present study, the characteristics of tree-ring width formation of the prevailing Lithuanian tree species, Norway spruce, Scots pine and silver and downy birch, and key factors resulting in their differences during the last 36-year period were investigated at forest sites located on poor mineral oligotrophic and on nutrient-rich organic mesoeutrophic soils. The aim of the study was as follows: first, to separately detect the maximum possible seasonal effect of three groups of variables - meteorology, acidifying pollutants and surface ozone on the stem basal area increment (BAI) of the evaluated tree species; second, to assess the significance of each group of variables affecting the BAI of these tree species integrally with the remaining groups of variables. Norway spruce was found to be well adapted to recent environmental changes, which makes it one of the most favourable tree species for silviculture in the northeastern part of Europe. The rapid increases recorded in growth intensity since 1980 were attributed to the increase in air temperature, precipitation amount, nitrogen deposition during the vegetative stage and reductions in SO₂ concentrations and S deposition. Scots pine demonstrated the highest level of resilience and capacity to adapt to recent global changes because its reaction to both negative and favourable environmental factors was best expressed. Silver and downy birch tree reactions to the effects of air concentrations of acidifying compounds, their deposition and surface ozone concentrations were the least expressed; however, a significant decline in growth intensity indicated that these tree species experienced a reduced resistance to recent changes in environmental conditions in the mature and over-mature age groups.

How important is woody tissue photosynthesis in EuCahetus dunnii Maiden and Osmanthus fragrans (Thunb.) Lour. under O3 stress?

Numerous studies have demonstrated the negative effects of elevated O₃ on leaf photosynthesis. Within trees, a portion of respired CO₂ is assimilated by woody tissue photosynthesis, but its response to elevated O₃ remains unclear. Saplings of two evergreen tree species, EuCahetus dunnii Maiden (E. dunnii) and Osmanthus fragrans (Thunb.) Lour. (O. fragrans), were exposed to non-filtered air (NF), 100 nmol mol^-1 O₃ air (E1) and 150 nmol mol^-1 O₃ air (E2) in open-top chambers from May 5 to September 5, 2016 (8 h a day; 7 days a week) in subtropical China. In this study, O₃ fumigation significantly reduced leaf net photosynthesis rate in both two tree species on most measurements. However, compared with leaf net photosynthesis rate, woody tissue gross photosynthesis rate showed less negative response to O₃ fumigation and was even stimulated to increase. Refixation rate reflects the utilization efficiency of the respired CO₂ by woody tissue photosynthesis. During the experiment period, E1 and E2 both increased refixation rate in O. fragrans compared with NF. Whereas for E. dunnii, E1 increased refixation rate until 81 days after starting of fumigation and then decreased it, and E2 decreased it all the time. Refixation rate had a significant positive correlation with woody tissue chlorophyll contents, indicating that the response of refixation rate to elevated O₃ may relate to chlorophyll contents. All these suggested that under O₃ fumigation, when atmospheric CO₂ uptake and fixation by leaf is limited, woody tissue photosynthesis can contribute more to the total carbon assimilation in trees. The findings help to understand the significance of woody tissue photosynthesis under elevated O₃ conditions.

Ozone affects leaf physiology and causes injury to foliage of native tree species from the tropical Atlantic Forest of southern Brazil

In southern Brazil, the recent increase in tropospheric ozone (O₃) concentrations poses an additional threat to the biodiverse but endangered and fragmented remnants of the Atlantic Forest. Given the mostly unknown sensitivity of tropical species to oxidative stress, the principal objective of this study was to determine whether the current O₃ levels in the Metropolitan Region of Campinas (MRC), downwind of São Paulo, affect the native vegetation of forest remnants. Foliar responses to O₃ of three tree species typical of the MRC forests were investigated using indoor chamber exposure experiments under controlled conditions and a field survey. Exposure to 70ppb O₃ reduced assimilation and leaf conductance but increased respiration in Astronium graveolens while gas exchange in Croton floribundus was little affected. Both A. graveolens and Piptadenia gonoacantha developed characteristic O₃-induced injury in the foliage, similar to visible symptoms observed in >30% of trees assessed in the MRC, while C. floribundus remained asymptomatic. The underlying structural symptoms in both O₃-exposed and field samples were indicative of oxidative burst, hypersensitive responses, accelerated cell senescence and, primarily in field samples, interaction with photo-oxidative stress. The markers of O₃ stress were thus mostly similar to those observed in other regions of the world. Further research is needed, to estimate the proportion of sensitive forest species, the O₃ impact on tree growth and stand stability and to detect O₃ hot spots where woody species in the Atlantic Forest are mostly affected.

Woody-plant ecosystems under climate change and air pollution-response consistencies across zonobiomes?

Forests store the largest terrestrial pools of carbon (C), helping to stabilize the global climate system, yet are threatened by climate change (CC) and associated air pollution (AP, highlighting ozone (O3) and nitrogen oxides (NOx)). We adopt the perspective that CC-AP drivers and physiological impacts are universal, resulting in consistent stress responses of forest ecosystems across zonobiomes. Evidence supporting this viewpoint is presented from the literature on ecosystem gross/net primary productivity and water cycling. Responses to CC-AP are compared across evergreen/deciduous foliage types, discussing implications of nutrition and resource turnover at tree and ecosystem scales. The availability of data is extremely uneven across zonobiomes, yet unifying patterns of ecosystem response are discernable. Ecosystem warming results in trade-offs between respiration and biomass production, affecting high elevation forests more than in the lowland tropics and low-elevation temperate zone. Resilience to drought is modulated by tree size and species richness. Elevated O3 tends to counteract stimulation by elevated carbon dioxide (CO2). Biotic stress and genomic structure ultimately determine ecosystem responsiveness. Aggrading early- rather than mature late-successional communities respond to CO2 enhancement, whereas O3 affects North American and Eurasian tree species consistently under free-air fumigation. Insect herbivory is exacerbated by CC-AP in biome-specific ways. Rhizosphere responses reflect similar stand-level nutritional dynamics across zonobiomes, but are modulated by differences in tree-soil nutrient cycling between deciduous and evergreen systems, and natural versus anthropogenic nitrogen (N) oversupply. The hypothesis of consistency of forest responses to interacting CC-AP is supported by currently available data, establishing the precedent for a global network of long-term coordinated research sites across zonobiomes to simultaneously advance both bottom-up (e.g., mechanistic) and top-down (systems-level) understanding. This global, synthetic approach is needed because high biological plasticity and physiographic variation across individual ecosystems currently limit development of predictive models of forest responses to CC-AP. Integrated research on C and nutrient cycling, O3-vegetation interactions and water relations must target mechanisms' ecosystem responsiveness. Worldwide case studies must be subject to biostatistical exploration to elucidate overarching response patterns and synthesize the resulting empirical data through advanced modelling, in order to provide regionally coherent, yet globally integrated information in support of internationally coordinated decision-making and policy development.

Species characteristics and intraspecific variation in growth and photosynthesis of Cryptomeria japonica under elevated O3 and CO2

In order to predict the effects of future atmospheric conditions on forest productivity, it is necessary to clarify the physiological responses of major forest tree species to high concentrations of ozone (O3) and carbon dioxide (CO2). Furthermore, intraspecific variation of these responses should also be examined in order to predict productivity gains through tree improvements in the future. We investigated intraspecific variation in growth and photosynthesis of Cryptomeria japonica D. Don, a major silviculture species in Japan, in response to elevated concentrations of O3 (eO3) and CO2 (eCO2), separately and in combination. Cuttings of C. japonica were grown and exposed to two levels of O3 (ambient and twice-ambient levels) in combination with two levels of CO2 (ambient and 550 µmol mol-1 in the daytime) for two growing seasons in a free-air CO2 enrichment experiment. There was no obvious negative effect of eO3 on growth or photosynthetic traits of the C. japonica clones, but a positive effect was observed for annual height increments in the first growing season. Dry mass production and the photosynthetic rate increased under eCO2 conditions, while the maximum carboxylation rate decreased. Significant interaction effects of eO3 and eCO2 on growth and photosynthetic traits were not observed. Clonal effects on growth and photosynthetic traits were significant, but the interactions between clones and O3 and/or CO2 treatments were not. Spearman's rank correlation coefficients between growth traits under ambient conditions and for each treatment were significantly positive, implying that clonal ranking in growth abilities might not be affected by either eO3 or eCO2. The knowledge obtained from this study will be helpful for species selection in afforestation programs, to continue and to improve current programs involving this species, and to accurately predict the CO2 fixation capacity of Japanese forests.

Photosynthetic responses to ozone of upper and lower canopy leaves of Fagus crenata Blume seedlings grown under different soil nutrient conditions

We aimed to clarify the effects of ozone (O₃) on photosynthetic ability of upper and lower canopy leaves of Fagus crenata Blume seedlings grown under different soil nutrient conditions. To accomplish this objective, we analyzed the response of photosynthetic parameters such as maximum carboxylation rate (V_cmax) to cumulative stomatal O₃ uptake (ΣF_st) and reduction rate of V_cmax per unit ΣF_st as an index of detoxification capacity for O₃. The seedlings of Fagus crenata were grown for two growing seasons (2014-2015) in nine treatments comprised of a combination of three levels of gas treatments (charcoal-filtered air or 1.0- or 1.5-times ambient O₃ concentration) and three levels of soil nutrient treatments (non-fertilized or a supply of relatively low or high concentrations of compound fertilizer). The nutrient supply significantly increased the degree of O₃-induced reduction in V_cmax in September. However, nutrient supply did not significantly increase ΣF_st and reduce the detoxification capacity for O₃. On the other hand, the degree of O₃-induced reduction in V_cmax of upper canopy leaves was higher as compared with that of lower canopy leaves in August due to the higher ΣF_st. However, the reduction rate of V_cmax per unit ΣF_st in lower canopy leaves was higher than that in upper canopy leaves, indicating lower detoxification capacity for O₃ in lower canopy leaves. Reduction rate of V_cmax per unit ΣF_st over the threshold, which is assumed to be proportional to gross photosynthetic rate, was similar between upper and lower canopy leaves. Therefore, capacity of photosynthetic CO₂ assimilation is likely to be associated with detoxification capacity for O₃ in upper and lower canopy leaves of F. crenata seedlings grown under different soil nutrient conditions.

Global topics and novel approaches in the study of air pollution, climate change and forest ecosystems

Research directions from the 27th conference for Specialists in Air Pollution and Climate Change Effects on Forest Ecosystems (2015) reflect knowledge advancements about (i) Mechanistic bases of tree responses to multiple climate and pollution stressors, in particular the interaction of ozone (O3) with nitrogen (N) deposition and drought; (ii) Linking genetic control with physiological whole-tree activity; (iii) Epigenetic responses to climate change and air pollution; (iv) Embedding individual tree performance into the multi-factorial stand-level interaction network; (v) Interactions of biogenic and anthropogenic volatile compounds (molecular, functional and ecological bases); (vi) Estimating the potential for carbon/pollution mitigation and cost effectiveness of urban and peri-urban forests; (vii) Selection of trees adapted to the urban environment; (viii) Trophic, competitive and host/parasite relationships under changing pollution and climate; (ix) Atmosphere-biosphere-pedosphere interactions as affected by anthropospheric changes; (x) Statistical analyses for epidemiological investigations; (xi) Use of monitoring for the validation of models; (xii) Holistic view for linking the climate, carbon, N and O3 modelling; (xiii) Inclusion of multiple environmental stresses (biotic and abiotic) in critical load determinations; (xiv) Ecological impacts of N deposition in the under-investigated areas; (xv) Empirical models for mechanistic effects at the local scale; (xvi) Broad-scale N and sulphur deposition input and their effects on forest ecosystem services; (xvii) Measurements of dry deposition of N; (xviii) Assessment of evapotranspiration; (xix) Remote sensing assessment of hydrological parameters; and (xx) Forest management for maximizing water provision and overall forest ecosystem services. Ground-level O3 is still the phytotoxic air pollutant of major concern to forest health. Specific issues about O3 are: (xxi) Developing dose-response relationships and stomatal O3 flux parameterizations for risk assessment, especially, in under-investigated regions; (xxii) Defining biologically based O3 standards for protection thresholds and critical levels; (xxiii) Use of free-air exposure facilities; (xxiv) Assessing O3 impacts on forest ecosystem services.

Effects of long-term ambient ozone exposure on biomass and wood traits in poplar treated with ethylenediurea (EDU)

This is the longest continuous experiment where ethylenediurea (EDU) was used to protect plants from ozone (O3). Effects of long-term ambient O3 exposure (23 ppm h AOT40) on biomass of an O3 sensitive poplar clone (Oxford) were examined after six years from in-ground planting. Trees were irrigated with either water or 450 ppm EDU. Above (-51%) and below-ground biomass (-47%) was reduced by O3 although the effect was significant only for stem and coarse roots. Ambient O3 decreased diameter of the lower stem, and increased moisture content along the stem of not-protected plants (+16%). No other change in the physical wood structure was observed. A comparison with a previous assessment in the same experiment suggested that O3 effects on biomass partitioning to above-ground organs depend on the tree ontogenetic stage. The root/shoot ratios did not change, suggesting that previous short-term observations of reduced allocation to tree roots may be overestimated.

Growth overcompensation against O3 exposure in two Japanese oak species, Quercus mongolica var. crispula and Quercus serrata, grown under elevated CO2

To assess the effects of elevated concentrations of carbon dioxide (CO2) and ozone (O3) on the growth of two mid-successional oak species native to East Asia, Quercus mongolica var. crispula and Quercus serrata, we measured gas exchange and biomass allocation in seedlings (initially 1-year-old) grown under combinations of elevated CO2 (550 μmol mol(-1)) and O3 (twice-ambient) for two growing seasons in an open-field experiment in which root growth was not limited. Both the oak species showed a significant growth enhancement under the combination of elevated CO2 and O3 (indicated by total dry mass; over twice of ambient-grown plants, p < .05), which probably resulted from a preferable biomass partitioning into leaves induced by O3 and a predominant enhancement of photosynthesis under elevated CO2. Such an over-compensative response in the two Japanese oak species resulted in greater plant growth under the combination of elevated CO2 and O3 than elevated CO2 alone.

Chronic drought stress reduced but not protected Shantung maple (Acer truncatum Bunge) from adverse effects of ozone (O3) on growth and physiology in the suburb of Beijing, China

A two-year experiment exposing Acer truncatum Bunge seedlings to elevated ozone (O3) concentrations above ambient air (AO) and drought stress (DS) was carried out using open-top chambers (OTCs) in a suburb of Beijing in north China in 2012-2013. The results suggested that AO and DS had both significantly reduced leaf mass area (LMA), stomatal conductance (Gs), light saturated photosynthetic rate (Asat) as well as above and below ground biomass at the end of the experiment. It appeared that while drought stress mitigated the expression of foliar injury, LMA, leaf photosynthetic pigments, height growth and basal diameter, due to limited carbon fixation, the O3 - induced reductions in Asat, Gs and total biomass were enhanced 23.7%. 15.5% and 8.1% respectively. These data suggest that when the whole plant was considered that drought under the conditions of this experiment did not protect the Shantung maple seedlings from the effects of O3.

Effects of combined ozone and cadmium stresses on leaf traits in two poplar clones

Information on plant responses to combined stresses such as ozone (O3) and cadmium (Cd) is scarce in tree species. On the other hand, high O3 concentrations in the atmosphere and heavy metal contaminations in water and soil simultaneously affect forest ecosystems. Toxic metals may exacerbate the consequences of air pollutants. In this research, two poplar clones, differently sensitive to O3 ("I-214" O3-tolerant and "Eridano" O3-sensitive), were grown for 5 weeks in pots supplied with 0 and 150 mg Cd kg(-1) soil and then exposed to a 15-day O3 fumigation (60 nl l(-1), 5 h a day) or supplied with charcoal-filtered air under the same conditions (referred to as control samples). The effects of the two stressors, alone or in combination, on Cd accumulation, photosynthetic capacity, ethylene emission and oxidative state were investigated in fully expanded leaves. Cadmium accumulation in leaves caused a reduction, but not complete failure, of photosynthesis in Eridano and I-214 poplar clones. The reduction in assimilation rate was more important following O3 fumigation. Stomatal aperture after O3 treatment, instead, increased in I-214 and decreased in Eridano. Overall, Cd treatment was effective in decreasing ethylene emission, whereas O3 fumigation increased it in both clones, although interacting with the metal treatment. Again, O3 fumigation induced a significant increase in ascorbate (ASA) + dehydroascorbate (DHA) content, which was strongly oxidised by O3, thus decreasing the redox state. On the other hand, Cd treatment had a positive effect on ASA content and redox state in I-214, but not in Eridano. Although Cd and O3 are known to share some common toxicity pathways, the combined effects induced distinct clone-specific responses, underlying the complexity of plant reactions to multiple stresses.

Whole-tree seasonal nitrogen uptake and partitioning in adult Fagus sylvatica L. and Picea abies L. [Karst.] trees exposed to elevated ground-level ozone

The effect of long-term exposure of twice-ambient O(3) (2 × O(3)) on whole-tree nitrogen (N) uptake and partitioning of adult beech and spruce was studied in a mixed forest stand, SE-Germany. N uptake as (15)N tracer and N pools were calculated using N concentrations and biomass of tree compartments. Whole-tree N uptake tended to be lower under 2 × O(3) in both species compared to trees under ambient O(3) (1 × O(3)). Internal partitioning in beech showed significantly higher allocation of new N to roots, with mycorrhizal root tips and fine roots together receiving about 17% of new N (2 × O(3)) versus 7% (1 × O(3)). Conversely, in spruce, N allocation to roots was decreased under 2 × O(3). These contrasting effects on belowground N partitioning and pool sizes, being largely consistent with the pattern of N concentrations, suggest enhanced N demand and consumption of stored N with higher relevance for tree-internal N cycling in beech than in spruce.

Contrasting carbon allocation responses of juvenile European beech (Fagus sylvatica) and Norway spruce (Picea abies) to competition and ozone

Allocation of recent photoassimilates of juvenile beech and spruce in response to twice-ambient ozone (2 × O(3)) and plant competition (i.e. intra vs. inter-specific) was examined in a phytotron study. To this end, we employed continuous (13)CO(2)/(12)CO(2) labeling during late summer and pursued tracer kinetics in CO(2) released from stems. In beech, allocation of recent photoassimilates to stems was significantly lowered under 2 × O(3) and increased in spruce when grown in mixed culture. As total tree biomass was not yet affected by the treatments, C allocation reflected incipient tree responses providing the mechanistic basis for biomass partitioning as observed in longer experiments. Compartmental modeling characterized functional properties of substrate pools supplying respiratory C demand. Respiration of spruce appeared to be exclusively supplied by recent photoassimilates. In beech, older C, putatively located in stem parenchyma cells, was a major source of respiratory substrate, reflecting the fundamental anatomical disparity between angiosperm beech and gymnosperm spruce.

Ozone fumigation increases the abundance of nutrients in Brassica vegetables: broccoli (Brassica oleracea var. italica) and Chinese cabbage (Brassica pekinensis)

Brassicaceae vegetables, among them broccoli and Chinese cabbage, are well recognized due to the nutritional properties. Four-week-old Chinese cabbage and broccoli seedlings were fumigated with O3 for 3 days before being transplanted into the field. The effect of O3 treatment was determined after reaching marketable quality (ca. 10 weeks). The inflorescences of O3-treated broccoli were enriched in vitamin E (α-tocopherol and γ-tocopherol), whereas Chinese cabbage heads had an increased content of anthocyanins and β-carotene. Ozone treatment did not significantly affect the productivity of both examined vegetables.

Forest stand growth dynamics in Central Europe have accelerated since 1870

Forest ecosystems have been exposed to climate change for more than 100 years, whereas the consequences on forest growth remain elusive. Based on the oldest existing experimental forest plots in Central Europe, we show that, currently, the dominant tree species Norway spruce and European beech exhibit significantly faster tree growth (+32 to 77%), stand volume growth (+10 to 30%) and standing stock accumulation (+6 to 7%) than in 1960. Stands still follow similar general allometric rules, but proceed more rapidly through usual trajectories. As forest stands develop faster, tree numbers are currently 17–20% lower than in past same-aged stands. Self-thinning lines remain constant, while growth rates increase indicating the stock of resources have not changed, while growth velocity and turnover have altered. Statistical analyses of the experimental plots, and application of an ecophysiological model, suggest that mainly the rise in temperature and extended growing seasons contribute to increased growth acceleration, particularly on fertile sites.

The growth dynamics of forest ecosystems undergoing climatic change are not well understood. Here Pretzsch et al. show that two of the dominant tree species of Central Europe have undergone significantly accelerated growth dynamics during the past century.

Photosynthetic responses of Monarch birch seedlings to differing timings of free air ozone fumigation

To study the effects of different periods of ozone (O3) fumigation on photosynthesis in leaves of the Monarch birch (Betula maximowicziana), we undertook free air O3 fumigation to Monarch birch seedlings at a concentration of 60 nmol mol(-1) during daytime. Plants were exposed to O3 at early, late or both periods in the growing season. The light-saturated net photosynthetic rate (A(sat)) in July and August was reduced by O3 exposure through a reduction in the maximum rate of carboxylation (V(c,max)). In early September, on the other hand, despite a reduction in V(c,max), A(sat) was not reduced by O3 due to a counteracting increase in the stomatal conductance. Through the experiment, there was no difference in sensitivity to O3 between maturing and matured leaves. We analyzed the relationship between A(sat), V(c,max) and accumulated stomatal O3 flux (AF(st)). Whereas V(c,max) decreased with increasing AF(st), the correlation between A(sat) and AF(st) was weak because the response of stomatal conductance to O3 was affected by season. We conclude photosynthetic response of Monarch birch to O3 exposure changes with season. This is due to the inconstant stomatal response to O3 but not due to the respose of biochemical assimilation capacity in chloroplasts.

Canopy carbon budget of Siebold's beech (Fagus crenata) sapling under free air ozone exposure

To determine the effects of ozone (O3) on the canopy carbon budget, we investigated photosynthesis and respiration of leaves of Siebold's beech saplings under free air O3 exposure (60 nmol mol(-1), during daytime) in relation to the within-canopy light gradient; we then calculated the canopy-level photosynthetic carbon gain (PCG) and respiratory carbon loss (RCL) using a canopy photosynthesis model. Susceptibilities of photosynthesis and respiration to O3 were greater in leaves of upper canopy than in the lower canopy. The canopy net carbon gain (NCG) was reduced by O3 by 12.4% during one growing season. The increased RCL was the main factor for the O3-induced reduction in NCG in late summer, while contributions of the reduced PCG and the increased RCL to the NCG were almost the same in autumn. These results indicate contributions of changes in PCG and RCL under O3 to NCG were different between seasons.

Seasonal ozone uptake by a warm-temperate mixed deciduous and evergreen broadleaf forest in western Japan estimated by the Penman-Monteith approach combined with a photosynthesis-dependent stomatal model

Canopy-level stomatal conductance over a warm-temperate mixed deciduous and evergreen broadleaf forest in Japan was estimated by the Penman-Monteith approach, as compensated by a semi-empirical photosynthesis-dependent stomatal model, where photosynthesis, relative humidity, and CO2 concentration were assumed to regulate stomatal conductance. This approach, using eddy covariance data and routine meteorological observations at a flux tower site, permits the continuous estimation of canopy-level O3 uptake, even when the Penman-Monteith approach is unavailable (i.e. in case of direct evaporation from soil or wet leaves). Distortion was observed between the AOT40 exposure index and O3 uptake through stomata, as AOT40 peaked in April, but with O3 uptake occurring in July. Thus, leaf pre-maturation in the predominant deciduous broadleaf tree species (Quercus serrata) might suppress O3 uptake in springtime, even when the highest O3 concentrations were observed.

Ozone fumigation results in accelerated growth and persistent changes in the antioxidant system of Brassica oleracea L. var. capitata f. alba

The growth response and antioxidant capacity of Brassica oleracea var. capitata f. alba plants treated with 70ppb of ozone was examined. Four week old cabbage seedlings were fumigated with O3 for 3 days before being transplanted into the growing field. The effect of O3 treatment was determined directly after fumigation and over the course of field cultivation. Plants subjected to O3 treatment had an increased diameter of rosettes and number of leaves after 3 and 7 weeks in agriculture, respectively. In addition, the vast majority of fumigated plants reached marketable quality faster than control plants, indicating a positive role of episodes of increased O3 concentrations during vegetation on growth and yielding. Our analysis revealed that by fumigating juvenile white cabbage plants with moderate doses of O3 the activity of catalases (CAT) and peroxidases was elevated. The activity of the examined enzymes was not affected directly after fumigation, but it increased after several weeks in the experimental field. Increased CAT activity was accompanied by changes in 2 out of the 3 CAT genes CAT1 and CAT2, where CAT2 seemed to be responsible for the induced CAT activity. The biosynthesis of low-molecular stress protectants - tocopherols and the glucosinolate (GLS) sinigrin was transiently affected by ozone. γ-Tocopherol (γ-toc) content significantly increased directly after fumigation, but after 3 weeks of vegetation in the field its concentration reached values similar to control. The biosynthesis of α-tocopherol (α-toc) and sinigrin seemed to be upregulated in fumigated plants. However, the response was delayed; no differences were registered directly after treatment, but 3 weeks after transplanting the concentration of sinigrin and α-toc was elevated.

Plant species sensitivity distributions for ozone exposure

This study derived Species Sensitivity Distributions (SSD), representing a cumulative stressor-response distribution based on single-species sensitivity data, for ozone exposure on natural vegetation. SSDs were constructed for three species groups, i.e. trees, annual grassland and perennial grassland species, using species-specific exposure-response data. The SSDs were applied in two ways. First, critical levels were calculated for each species group and compared to current critical levels for ozone exposure. Second, spatially explicit estimates of the potentially affected fraction of plant species in Northwestern Europe were calculated, based on ambient ozone concentrations. We found that the SSD-based critical levels were lower than for the current critical levels for ozone exposure, with conventional critical levels for ozone relating to 8-20% affected plant species. Our study shows that the SSD concept can be successfully applied to both derive critical ozone levels and estimate the potentially affected species fraction of plant communities along specific ozone gradients.

Photosynthetic traits of Siebold's beech and oak saplings grown under free air ozone exposure in northern Japan

We set up a free-air ozone (O(3)) exposure system for determining the photosynthetic responses of Siebold's beech (Fagus crenata) and oak (Quercus mongolica var. crispula) to O(3) under field conditions. Ten-year-old saplings of beech and oak were exposed to an elevated O(3) concentration (60 nmol mol(-1)) during daytime from 6 August to 11 November 2011. Ozone significantly reduced the net photosynthetic rate in leaves of both species in October, by 46% for beech and 15% for oak. In beech there were significant decreases in maximum rate of carboxylation, maximum rate of electron transport in photosynthesis, nitrogen content and photosynthetic nitrogen use efficiency, but not in oak. Stomatal limitation of photosynthesis was unaffected by O(3). We therefore concluded photosynthesis in beech is more sensitive to O(3) than that in oak, and the O(3)-induced reduction of photosynthetic activity in beech was due not to stomatal closure, but to biochemical limitation.

To what extent do molecular collisions arising from water vapour efflux impede stomatal O3 influx?

Pre-requisite for reliable O(3) risk assessment for plants is determination of stomatal O(3) uptake. One unaddressed uncertainty in this context relates to transpiration-induced molecular collisions impeding stomatal O(3) influx. This study quantifies, through physical modelling, the error made when estimating stomatal O(3) flux without accounting for molecular collisions arising from transpiratory mass flow of gas out of the leaf. The analysis demonstrates that the error increases with increasing leaf-to-air water vapour mole fraction difference (Δw), being zero in water vapour saturated air and 4.2% overestimation at Δw of 0.05. Overestimation is approximately twice as large in empirical studies quantifying stomatal O(3) flux from measured leaf or canopy water flux, if neglecting both water vapour-dry air collisions (causing overestimation of leaf conductance) and collisions involving O(3). Correction for transpiration-induced molecular collisions is thus relevant for both empirical research and for large-scale modelling of stomatal O(3) flux across strong spatial Δw gradients.

Assessment of the interactive effects of ambient O₃ and NPK levels on two tropical mustard varieties (Brassica campestris L.) using open-top chambers

Rising O(3) concentrations in agricultural areas have been identified as a significant threat to crop production in Asia including India. The present work reports the results of a field study conducted to assess the usefulness of higher than recommended NPK dose in modifying the physiological, growth, yield, and seed quality responses of two mustard (Brassica campestris L. var. Vardan and Aashirwad) varieties under ambient ozone level at a rural site of India, using open-top chambers. Twelve hourly mean O(3) concentrations ranged between 27.7 and 59.04 ppb during the growth period. Plants in nonfiltered chambers (NFCs) showed reductions in photosynthetic rate, stomatal conductance, and growth parameters compared to the plants in filtered chambers (FCs), but reductions were of lower magnitude at 1.5 times recommended dose of NPK (1.5 RNPK) compared to recommended (RNPK). Yield and seed quality reduced significantly in plants of NFCs compared to FCs at RNPK, but no significant differences were recorded at 1.5 RNPK. There were higher N uptake and N uptake efficiency of plants in FCs compared to NFCs. Nitrogen utilization efficiency increased in Vardan, but decreased in Aashirwad in NFCs compared to FCs suggesting higher capability of N acquisition and utilization under ambient O(3), which led to a less pronounced reduction in the yield of the former than the latter variety. The differential nitrogen utilization efficiency in these varieties may be potentially used as measure of sensitivity characteristics in breeding programs for yield improvement in mustard under the present trend of increase in O(3) concentrations.

Multivariate analysis of physiological parameters reveals a consistent O3 response pattern in leaves of adult European beech (Fagus sylvatica)

• Increasing atmospheric concentrations of phytotoxic ozone (O(3) ) can constrain growth and carbon sink strength of forest trees, potentially exacerbating global radiative forcing. Despite progress in the conceptual understanding of the impact of O(3) on plants, it is still difficult to detect response patterns at the leaf level. • Here, we employed principal component analysis (PCA) to analyse a database containing physiological leaf-level parameters of 60-yr-old Fagus sylvatica (European beech) trees. Data were collected over two climatically contrasting years under ambient and twice-ambient O(3) regimes in a free-air forest environment. • The first principal component (PC1) of the PCA was consistently responsive to O(3) and crown position within the trees over both years. Only a few of the original parameters showed an O(3) effect. PC1 was related to parameters indicative of oxidative stress signalling and changes in carbohydrate metabolism. PC1 correlated with cumulative O(3) uptake over preceding days. • PC1 represents an O(3) -responsive multivariate pattern detectable in the absence of consistently measurable O(3) effects on individual leaf-level parameters. An underlying effect of O(3) on physiological processes is indicated, providing experimental confirmation of theoretical O(3) response patterns suggested previously.

Ground-level ozone differentially affects nitrogen acquisition and allocation in mature European beech (Fagus sylvatica) and Norway spruce (Picea abies) trees

Impacts of elevated ground-level ozone (O(3)) on nitrogen (N) uptake and allocation were studied on mature European beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.) in a forest stand, hypothesizing that: (i) chronically elevated O(3) limits nutrient uptake, and (ii) beech responds more sensitively to elevated O(3) than spruce, as previously found for juvenile trees. Tree canopies were exposed to twice-ambient O(3) concentrations (2 × O(3)) by a free-air fumigation system, with trees under ambient O(3) serving as control. After 5 years of O(3) fumigation, (15)NH(4)(15)NO(3) was applied to soil, and concentrations of newly acquired N (N(labelled)) and total N (N(total)) in plant compartments and soil measured. Under 2 × O(3), N(labelled) and N(total) were increased in the bulk soil and tended to be lower in fine and coarse roots of both species across the soil horizons, supporting hypothesis (i). N(labelled) was reduced in beech foliage by up to 60%, and by up to 50% in buds under 2 × O(3). Similarly, N(labelled) in stem bark and phloem was reduced. No such reduction was observed in spruce, reflecting a stronger effect on N acquisition in beech in accordance with hypothesis (ii). In spruce, 2 × O(3) tended to favour allocation of new N to foliage. N(labelled) in beech foliage correlated with cumulative seasonal transpiration, indicating impaired N acquisition was probably caused by reduced stomatal conductance and, hence, water transport under elevated O(3). Stimulated fine root growth under 2 × O(3) with a possible increase of below-ground N sink strength may also have accounted for lowered N allocation to above-ground organs. Reduced N uptake and altered allocation may enhance the use of stored N for growth, possibly affecting long-term stand nutrition.

How closely does stem growth of adult beech (Fagus sylvatica) relate to net carbon gain under experimentally enhanced ozone stress?

The hypothesis was tested that O(3)-induced changes in leaf-level photosynthetic parameters have the capacity of limiting the seasonal photosynthetic carbon gain of adult beech trees. To this end, canopy-level photosynthetic carbon gain and respiratory carbon loss were assessed in European beech (Fagus sylvatica) by using a physiologically based model, integrating environmental and photosynthetic parameters. The latter were derived from leaves at various canopy positions under the ambient O(3) regime, as prevailing at the forest site (control), or under an experimental twice-ambient O(3) regime (elevated O(3)), as released through a free-air canopy O(3) fumigation system. Gross carbon gain at the canopy-level declined by 1.7%, while respiratory carbon loss increased by 4.6% under elevated O(3). As this outcome only partly accounts for the decline in stem growth, O(3)-induced changes in allocation are referred to and discussed as crucial in quantitatively linking carbon gain with stem growth.

Ozone uptake by adult urban trees based on sap flow measurement

The O(3) uptake in 17 adult trees of six urban species was evaluated by the sap flow-based approach under free atmospheric conditions. The results showed very large species differences in ground area scaled whole-tree ozone uptake (F(o)₃), with estimates ranging from 0.61 ± 0.07 nmol m(-2) s(-1) in Robinia pseudoacacia to 4.80 ± 1.04 nmol m(-2) s(-1) in Magnolia liliiflora. However, average F(o)₃by deciduous foliages was not significantly higher than that by evergreen ones (3.13 vs 2.21 nmol m(-2) s(-1), p = 0.160). Species of high canopy conductance for O(3) (G(o)₃) took up more O(3) than those of low G(o)₃, but that their sensitivity to vapour pressure deficit (D) were also higher, and their F(o)₃decreased faster with increasing D, regardless of species. The responses of F(o)₃to D and total radiation led to the relative high flux of O(3) uptake, indicating high ozone risk for urban tree species.