Stomatal uptake of O3 in aspen and aspen-birch forests under free-air CO2 and O3 enrichment

Vertical gradient of needle ozone uptake within the canopy of Cryptomeria japonica

Leaf ozone uptake through the stomata is an important index for the ozone risk assessments on trees. Stomatal conductance (gs) and ozone concentration ([O3]), determinants of the leaf ozone uptake, are known to show vertical gradients within a tree canopy. However, less is known about the within-canopy vertical gradient of leaf ozone uptake. This study was aimed to elucidate how the vertical gradient of [O3] and gs affect needle ozone uptake within a canopy of mature Cryptomeria japonica trees in a suburban forest at Tokyo, Japan. For this purpose, a multilayer gas exchange model was applied to estimate the vertical gradient of needle gs and the accumulated ozone uptake during the study period (POD1, Phytotoxic Ozone Dose above a threshold of 1 nmol m-2 s-1). In addition, we also tested several scenarios of vertical gradient of [O3] within the canopy for sensitivity analysis. The POD1 was declined from the top to the bottom of the canopy. This tendency strongly depended on the vertical gradient of gs and was hardly affected by the changes in simulated vertical reductions of the [O3]. We further assessed the photosynthesis of sunlit needles (needles absorbing both direct and diffuse light) and shaded needles (needles only absorbing diffuse light). The photosynthesis of shaded needles in the upper half of the canopy made a great contribution to the entire canopy photosynthesis. In addition, given that their POD1 was lower than that of sunlit needles, ozone may affect sunlit and shaded needles differently. We concluded that these considerations should be incorporated into modeling of the calculation of ozone uptake for mature trees to make accurate predictions of the ozone effects on trees at the canopy scale.

Reduced ascorbate pool and its maintenance are important determinants of O3 damage to net photosynthetic rate in Fagus crenata under elevated CO2 and soil N supply

Many studies have reported modification in the degree of O3 damage to photosynthesis by elevated CO2 and soil N supply. However, the mechanism underlying the modification is unclear. To clarify the important determinants in the degree of O3 damage to net photosynthetic rate (A) in the leaves of Fagus crenata (Siebold's beech) under elevated CO2 and with different soil N supply, F. crenata seedlings were grown for two growing seasons under combinations of two O3 levels (low concentration at approximately 4 nmol mol-1 and two times the ambient concentration), two CO2 levels (ambient and 700 μmol mol-1), and three levels of soil N supply (0, 50 and 100 kg N ha-1 year-1). During the second growing season, we determined A, stomatal conductance for calculating phytotoxic O3 dose (POD), antioxidant concentrations, and antioxidative enzyme activities in the leaves for evaluating O3 detoxification capacity. We calculated the O3-induced reduction in mean A (ΔAmean) during the second growing season using the data reported in our previous study and plotted it against mean daily POD without flux threshold (POD0). There was no significant linear nor non-linear relationship, suggesting that not only POD0 but also O3 detoxification capacity are important determinants of ΔAmean under elevated CO2 and N supply. We found significant negative linear relationships of ΔAmean per unit POD0 (ΔAmean/POD0) with reduced ascorbate concentration in the low O3 treatment, and with percentage of O3-induced change in activity of monodehydroascorbate reductase (MDAR). In addition, the ΔAmean/POD0 was positively and significantly correlated with the activity ratio of ascorbate peroxidase to MDAR. These results suggest that reduced ascorbate pool and its maintenance through the action of MDAR could be important determinants in the degree of O3 damage to net photosynthesis under elevated CO2 and soil N supply.

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.

Ozone critical levels for (semi-)natural vegetation dominated by perennial grassland species

New critical levels for ozone based on accumulated flux through stomata (phytotoxic ozone dose, POD), for temperate perennial grassland (semi-)natural vegetation, have been agreed for use within the Convention on Long-Range Transboundary Air Pollution. These were based on data from several experiments conducted under naturally fluctuating environmental conditions that were combined and analysed to give linear dose-response relationships. Dose-response functions and flux-based critical levels were derived based on biomass and flower number. These parameters showed a statistically significant decline with increasing accumulated stomatal ozone flux. The functions and critical levels derived are based on sensitive species and can be used for risk assessments of the damaging effect of ozone on temperate vegetation communities dominated by perennial grassland species. The critical level based on flower number was lower than that for biomass, representing the greater sensitivity of flower number to ozone pollution.

Stomatal response drives between-species difference in predicted leaf water-use efficiency under elevated ozone

Ozone-induced changes in the relationship between photosynthesis (A_n) and stomatal conductance (g_s) vary among species, leading to inconsistent water use efficiency (WUE) responses to elevated ozone (O₃). Thus, few vegetation models can accurately simulate the effects of O₃ on WUE. Here, we conducted an experiment exposing two differently O₃-sensitive species (Cotinus coggygria and Magnolia denudata) to five O₃ concentrations and investigated the impact of O₃ exposure on predicted WUE using a coupled A_n-g_s model. We found that increases in stomatal O₃ uptake caused linear reductions in the maximum rates of Rubisco carboxylation (V_cmax) and electron transport (J_max) in both species. In addition, a negative linear correlation between O₃-induced changes in the minimal g_s of the stomatal model (g₀) derived from the theory of optimal stomatal behavior and light-saturated photosynthesis was found in the O₃-sensitive M. denudata. When the O₃ dose-based responses of V_cmax and J_max were included in a coupled A_n-g_s model, simulated A_n under elevated O₃ were in good agreement with observations in both species. For M. denudata, incorporating the O₃ response of g₀ into the coupled model further improved the accuracy of the simulated g_s and WUE. In conclusion, the modified V_cmax, J_max and g₀ method presented here provides a foundation for improving the prediction for O₃-induced changes in A_n, g_s and WUE.

Effects of Elevated Temperature and Ozone in Brassica juncea L.: Growth, Physiology, and ROS Accumulation

Global warming and ozone (O3) pose serious threats to crop yield and ecosystem health. Although neither of these factors will act individually in reality, most studies have focused on the responses of plants to air pollution or climate change. Interactive effects of these remain poorly studied. Therefore, this study was conducted to assess the effects of optimal (22/20 °C day/night) and elevated temperature (27/25 °C) and/or ambient (10 ± 10 nL L−1) and elevated O3 concentrations (100 ± 10 nL L−1) on the growth, physiology, and reactive oxygen species (ROS) accumulation of leaf mustard (Brassica juncea L.). The aim was to examine whether elevated temperature increase the O3 damage due to increasing stomatal conductance, and thus, O3 flux into the leaf. Significant reductions in photosynthetic rates occurred under O (elevated O3 with optimal temperatures) and OT (elevated O3 and temperature) conditions compared to C (controls). Stomatal conductance was significantly higher under T than in the C at 7 DAE. Under OT conditions, O3 flux significantly increased compared to that in O conditions at 7 days after exposure (DAE). Significant reductions in total fresh and dry weight were observed under OT conditions compared to those under O. Furthermore, significant reductions in levels of carotenoids and ascorbic acid were observed under OT conditions compared to O. Lipid peroxidation and accumulation of ROS such as hydroxyl radical, hydrogen peroxide, and superoxide radical were higher under O and OT conditions than in C conditions at 7 and 14 DAE. As a result of O3 stress, the results of the present study indicated that the plant injury index significantly increased under OT compared to O conditions. This result suggested that elevated temperature (+5 °C) may enhance O3 damage to B. juncea by increasing stomatal conductance and O3 flux into leaves.

Quantification of ozone exposure- and stomatal uptake-yield response relationships for soybean in Northeast China

High ground-level O3 is a new threat to agricultural production in Northeast China with the increasing ambient O3 concentration. Little is known about its impacts on soybean production in this key agricultural region. Accumulated O3 exposure-response and stomatal O3 flux-response relationships were developed during two continuous growing seasons to evaluate O3-induced yield reduction of four typical soybean cultivars in Northeast China. Results showed that critical levels of AOT40 (accumulated hourly O3 concentrations over a threshold of 40nmol·mol-1), SUM06 (sum of all hourly average O3 concentrations over 0.06μmol·mol-1) and W126 (sum of O3 concentrations weighted by a sigmoidal function) in relation to 5% reduction in relative seed yield were 4.2, 7.6 and 6.8μmol·mol-1·h, respectively. The effect of O3 on plants was influenced by leaf position in canopy. An improved Jarvis stomatal conductance model including leaf (node) position fitted well with field measurements. The best linear relationship between stomatal O3 flux and relative soybean yield was obtained when phytotoxic ozone dose was integrated over a threshold of 9.6nmol·m-2·s-1 (POD9.6) to represent the detoxification capacity of soybean. POD9.6 and the commonly used POD6 in relation to 5% reduction in relative seed yield of soybean were 0.9mmol·m-2 and 1.8mmol·m-2, respectively. O3 concentrations above ~38nmol·mol-1 contributed to POD9.6 and caused seed yield loss in soybean. Current annual yield loss of soybean at ambient O3 was estimated to range between 23.4% and 30.2%. The O3 dose-response relationships and corresponding thresholds obtained here will benefit regional O3 risk assessment on soybean production in Northeast China.

Stomatal uptake of O3 in a Schima superba plantation in subtropical China derived from sap flow measurements

Canopy stomatal ozone (O3) flux (Fst,O3) in a plantation of Schima superba, an ecologically and economically important evergreen pioneer tree species in subtropical China, was quantified based on sap flow measurements during a 2-year period. Mean Fst,O3 and accumulated Fst,O3 (AFst0) were significantly higher in wet seasons from April to September (4.62 nmol m(-2) s(-1) and 35.37 mmol m(-2), respectively) than in dry seasons from October to March (3.90 nmol m(-2) s(-1) and 24.15 mmol m(-1), respectively), yet comparable between the 2 years of the experiment, being 4.23 nmol m(-2) s(-1) and 58.23 mmol m(-2) in April 2013-March 2014 and 4.29 nmol m(-2) s(-1) and 60.80 mmol m(-2) in April 2014-March 2015, respectively. At the diurnal scale, Fst,O3 generally peaked in the early to middle afternoon hours (13:00-15:00), while the maximum stomatal conductance (Gst,O3) typically occurred in the middle to late morning hours (09:00-11:00). Monthly integrated AFst0 reached the maximum in July, although accumulated O3 exposure (SUM0) was highest in October. Seasonally or yearly, the accumulated O3 doses, either exposure-based or flux-based, notably exceeded the currently adopted critical thresholds for the protection of forest trees. These results, on the one hand, demonstrated the decoupling between the stomatal uptake of O3 and its environmental exposure level; on the other hand, indicated the potential O3 risk for S. superba in the experimental site. Therefore, the present study endorses the use of sap flow measurements as a feasible tool for estimating Fst,O3, and the transition from the exposure-based toward flux-based metrics for assessing O3 risk for forest trees. Further studies are urgently needed to relate stomatal O3 uptake doses with tree growth reductions for an improved understanding of O3 effects on trees under natural conditions.

Ozone impacts on vegetation in a nitrogen enriched and changing climate

This paper provides a process-oriented perspective on the combined effects of ozone (O3), climate change and/or nitrogen (N) on vegetation. Whereas increasing CO2 in controlled environments or open-top chambers often ameliorates effects of O3 on leaf physiology, growth and C allocation, this is less likely in the field. Combined responses to elevated temperature and O3 have rarely been studied even though some critical growth stages such as seed initiation are sensitive to both. Under O3 exposure, many species have smaller roots, thereby enhancing drought sensitivity. Of the 68 species assessed for stomatal responses to ozone, 22.5% were unaffected, 33.5% had sluggish or increased opening and 44% stomatal closure. The beneficial effect of N on root development was lost at higher O3 treatments whilst the effects of increasing O3 on root biomass became more pronounced as N increased. Both responses to gradual changes in pollutants and climate and those under extreme weather events require further study.

Concentration- and flux-based ozone dose-response relationships for five poplar clones grown in North China

Concentration- and flux-based O3 dose-response relationships were developed for poplars in China. Stomatal conductance (gs) of five poplar clones was measured to parameterize a Jarvis-type multiplicative gs model. The maximum gs and other model parameters varied between clones. The strongest relationship between stomatal O3 flux and total biomass was obtained when phytotoxic ozone dose (POD) was integrated using an uptake rate threshold of 7 nmol m(-2) s(-1). The R(2) value was similar between flux-based and concentration-based dose-response relationships. Ozone concentrations above 28-36 nmol mol(-1) contributed to reducing the biomass production of poplar. Critical levels of AOT40 (accumulated O3 exposure over 40 nmol mol(-1)) and POD7 in relation to 5% reduction in total biomass for poplar were 12 μmol mol(-1) h and 3.8 mmol m(-2), respectively.

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.

Growth losses in Swiss forests caused by ozone: epidemiological data analysis of stem increment of Fagus sylvatica L. and Picea abies Karst

The estimate of growth losses by ozone exposure of forest trees is a significant part in current C sequestration calculations and will also be important in future modeling. It is therefore important to know if the relationship between ozone flux and growth reduction of young trees, used to derive a Critical Level for ozone, is also valid for mature trees. Epidemiological analysis of stem increment data from Fagus sylvatica L. and Picea abies Karst. observed in Swiss forest plots was used to test this hypothesis. The results confirm the validity of the flux-response relationship at least for beech and therefore enable estimating forest growth losses by ozone on a country-wide scale. For Switzerland, these estimates amount to 19.5% growth reduction for deciduous forests, 6.6% for coniferous forests and 11.0% for all forested areas based on annual ozone stomatal uptake during the time period 1991-2011.

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.

The effects of tropospheric ozone on net primary productivity and implications for climate change

Tropospheric ozone (O(3)) is a global air pollutant that causes billions of dollars in lost plant productivity annually. It is an important anthropogenic greenhouse gas, and as a secondary air pollutant, it is present at high concentrations in rural areas far from industrial sources. It also reduces plant productivity by entering leaves through the stomata, generating other reactive oxygen species and causing oxidative stress, which in turn decreases photosynthesis, plant growth, and biomass accumulation. The deposition of O(3) into vegetation through stomata is an important sink for tropospheric O(3), but this sink is modified by other aspects of environmental change, including rising atmospheric carbon dioxide concentrations, rising temperature, altered precipitation, and nitrogen availability. We review the atmospheric chemistry governing tropospheric O(3) mass balance, the effects of O(3) on stomatal conductance and net primary productivity, and implications for agriculture, carbon sequestration, and climate change.

Exposure to moderate concentrations of tropospheric ozone impairs tree stomatal response to carbon dioxide

With rising concentrations of both atmospheric carbon dioxide (CO(2)) and tropospheric ozone (O(3)), it is important to better understand the interacting effects of these two trace gases on plant physiology affecting land-atmosphere gas exchange. We investigated the effect of growth under elevated CO(2) and O(3), singly and in combination, on the primary short-term stomatal response to CO(2) concentration in paper birch at the Aspen FACE experiment. Leaves from trees grown in elevated CO(2) and/or O(3) exhibited weaker short-term responses of stomatal conductance to both an increase and a decrease in CO(2) concentration from current ambient level. The impairement of the stomatal CO(2) response by O(3) most likely developed progressively over the growing season as assessed by sap flux measurements. Our results suggest that expectations of plant water-savings and reduced stomatal air pollution uptake under rising atmospheric CO(2) may not hold for northern hardwood forests under concurrently rising tropospheric O(3).

Advances of air pollution science: from forest decline to multiple-stress effects on forest ecosystem services

Over the past 20 years, the focus of forest science on air pollution has moved from forest decline to a holistic framework of forest health, and from the effects on forest production to the ecosystem services provided by forest ecosystems. Hence, future research should focus on the interacting factorial impacts and resulting antagonistic and synergistic responses of forest trees and ecosystems. The synergistic effects of air pollution and climatic changes, in particular elevated ozone, altered nitrogen, carbon and water availability, must be key issues for research. Present evidence suggests air pollution will become increasingly harmful to forests under climate change, which requires integration amongst various stressors (abiotic and biotic factors, including competition, parasites and fire), effects on forest services (production, biodiversity protection, soil protection, sustained water balance, socio-economical relevance) and assessment approaches (research, monitoring, modeling) to be fostered.