The effects of tropospheric ozone on the species dynamics of calcareous grassland

Halophytic Clonal Plant Species: Important Functional Aspects for Existence in Heterogeneous Saline Habitats

Plant modularity-related traits are important ecological determinants of vegetation composition, dynamics, and resilience. While simple changes in plant biomass resulting from salt treatments are usually considered a sufficient indicator for resistance vs. susceptibility to salinity, plants with a clonal growth pattern show complex responses to changes in environmental conditions. Due to physiological integration, clonal plants often have adaptive advantages in highly heterogeneous or disturbed habitats. Although halophytes native to various heterogeneous habitats have been extensively studied, no special attention has been paid to the peculiarities of salt tolerance mechanisms of clonal halophytes. Therefore, the aim of the present review is to identify probable and possible halophytic plant species belonging to different types of clonal growth and to analyze available scientific information on responses to salinity in these species. Examples, including halophytes with different types of clonal growth, will be analyzed, such as based on differences in the degree of physiological integration, ramet persistence, rate of clonal expansion, salinity-induced clonality, etc.

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.

Effects of ozone on agriculture, forests and grasslands

The damage and injury that ground level ozone (O3) causes vegetation has become increasingly evident over the past half century with a large body of observational and experimental evidence demonstrating a variety of effects at ambient concentrations on crop, forest and grassland species and ecosystems. This paper explores the use of experimental data to develop exposure-response relationships for use in risk assessment studies. These studies have typically identified the USA mid-West, much of Europe, the Indo Gangetic Plain in South Asia and the Eastern coastal region of China as global regions where O3 is likely to threaten food supply and other ecosystems. Global risk assessment modelling estimates yield losses of staple crops between 3 to 16% causing economic losses of between US$14 to 26 billion in the year 2000. Changes in anthropogenic emissions of O3 precursors in recent decades have modified O3 concentration profiles (peaks versus background O3) and global distributions with the Northern Hemisphere seeing increases in O3 levels of between 1 and 5 ppb/decade since the 1950s and the emergence of Asia as the region with the highest O3 concentrations. In the future, O3 mitigation could focus on methane (CH4) and nitrogen oxide (NOx) emissions; these will differentially influence global and local/regional O3 concentrations and influence daily and seasonal profiles. The consequent effects on vegetation will in part depend on how these changes in O3 profile alter the exceedance of detoxification thresholds for plant damage. Adaptation options may play an important role in enhancing food supply while mitigation strategies are being implemented. An improved understanding of the mechanisms by which O3 affects plants, and how this might influence detoxification thresholds and interactions with other environmental variables such as water stress and nutrients, would help develop O3 deposition and impact models to support the development of crop, land-surface exchange and ultimately earth system models for holistic assessments of global change. This article is part of a discussion meeting issue 'Air quality, past present and future'.

Ozone affects plant, insect, and soil microbial communities: A threat to terrestrial ecosystems and biodiversity

Elevated tropospheric ozone concentrations induce adverse effects in plants. We reviewed how ozone affects (i) the composition and diversity of plant communities by affecting key physiological traits; (ii) foliar chemistry and the emission of volatiles, thereby affecting plant-plant competition, plant-insect interactions, and the composition of insect communities; and (iii) plant-soil-microbe interactions and the composition of soil communities by disrupting plant litterfall and altering root exudation, soil enzymatic activities, decomposition, and nutrient cycling. The community composition of soil microbes is consequently changed, and alpha diversity is often reduced. The effects depend on the environment and vary across space and time. We suggest that Atlantic islands in the Northern Hemisphere, the Mediterranean Basin, equatorial Africa, Ethiopia, the Indian coastline, the Himalayan region, southern Asia, and Japan have high endemic richness at high ozone risk by 2100.

Responses of a semi-natural grassland community of tropical region to elevated ozone: An assessment of soil dynamics and biomass accumulation

Despite knowing the phytotoxic effects of tropospheric ozone (O₃), which is of global concern, there is no study so far reported about its impacts on grassland community of tropical regions. Therefore, we assessed the responses of a semi-natural grassland community of Indo-Gangetic plains to elevated O₃ exposure (Ambient + 20 ppb) compared to ambient after three years of exposure using open-top chambers. Percent decreases were found in above (26%; p ≤ 0.002) and belowground (30%; p ≤ 0.003) biomass under elevated compared to ambient O₃ exposure. Percent decrements in total organic carbon (TOC; 24%; p ≤ 0.001), total nitrogen (29%; p ≤ 0.001) and available phosphorus (11%; p ≤ 0.002) in the soil were also observed under elevated O₃ exposure. Exposure at elevated O₃ reduced soil microbial biomass and activities of β-glucosidase, amylase, urease and phosphatase, while polyphenol oxidase and peroxidase showed enhancement in their activities, showing negative effects on belowground soil health. Percent reduction in root shoot ratio (10%; p ≤ 0.05) depicts that less C-allocation towards root system led to a reduction in TOC in the soil, which could affect C-sequestration under elevated O₃ condition in the semi-natural grasslands. Elevated O₃ also affected enzymes participating in N and P-cycles, causing reductions in total nitrogen and phosphorus. The study concludes that projected O₃ concentrations have serious implications for aboveground biomass as well as belowground soil health in tropical areas, identified as hotspots of O₃ in the world.

Impact of long-term nitrogen deposition on the response of dune grassland ecosystems to elevated summer ozone

Nitrogen deposition and tropospheric ozone are important drivers of vegetation damage, but their interactive effects are poorly understood. This study assessed whether long-term nitrogen deposition altered sensitivity to ozone in a semi-natural vegetation community. Mesocosms were collected from sand dune grassland in the UK along a nitrogen gradient (5-25 kg N/ha/y, including two plots from a long-term experiment), and fumigated for 2.5 months to simulate medium and high ozone exposure. Ozone damage to leaves was quantified for 20 ozone-sensitive species. Soil solution dissolved organic carbon (DOC) and soil extracellular enzymes were measured to investigate secondary effects on soil processes. Mesocosms from sites receiving the highest N deposition showed the least ozone-related leaf damage, while those from the least N-polluted sites were the most damaged by ozone. This was due to differences in community-level sensitivity, rather than species-level impacts. The N-polluted sites contained fewer ozone-sensitive forbs and sedges, and a higher proportion of comparatively ozone-resistant grasses. This difference in the vegetation composition of mesocosms in relation to N deposition conveyed differential resilience to ozone. Mesocosms in the highest ozone treatment showed elevated soil solution DOC with increasing site N deposition. This suggests that, despite showing relatively little leaf damage, the 'ozone resilient' vegetation community may still sustain physiological damage through reduced capacity to assimilate photosynthate, with its subsequent loss as DOC through the roots into the soil. We conclude that for dune grassland habitats, the regions of highest risk to ozone exposure are those that have received the lowest level of long-term nitrogen deposition. This highlights the importance of considering community- and ecosystem-scale impacts of pollutants in addition to impacts on individual species. It also underscores the need for protection of 'clean' habitats from air pollution and other environmental stressors.

Photosynthesis and biochemical responses to elevated O3 in Plantago major and Sonchus oleraceus growing in a lowland habitat of northern China

A field experiment was carried out to compare the responses to ozone (O₃) in two common herbaceous plant species, Plantago major L. and Sonchus oleraceus L., by building open-top growth chambers in situ to simulate O₃ stress (+O₃, 85±5ppb, 9hr/day for 30days) in a lowland habitat in Inner Mongolia, Northern China. Responses to O₃ of gas exchange, chlorophyll a fluorescence, leaf pigment content, antioxidant capability, soluble protein content, membrane lipid peroxidation and dark respiration (R_d) were analyzed. Results showed that elevated O₃ exposure significantly reduced the light-saturated net photosynthesis (P_Nsat), stomatal conductance (g_s) and transpiration rate (E) in both species. Although non-significant interactive effect between species and O₃ on P_Nsat was analyzed, the reduction in P_Nsat in S. oleraceus might be due primarily to the higher fraction of close PSII reaction centers and impaired activities of plant mesophyll cells as evidences by decreased maximum efficiency of PSII photochemistry after dark adapted state (F_v/F_m) and unchanged intercellular CO₂ concentration (C_i). Besides, biochemical analysis showed that S. oleraceus had lower antioxidant ability compared to P. major. As a result, S. oleraceus was damaged to the larger extent in terms of lipid peroxidation and visible O₃ injury, indicating that S. oleraceus was more sensitive to O₃ than P. major. Our results indicated that wild herbaceous plant species growing in a lowland habitat in sandy grassland were sensitive to O₃ stress and S. oleraceus can be considered as one of the bio-indicators for high O₃ concentration in semi-arid grassland of northern China.

Heterogeneous responses to ozone and nitrogen alter the species composition of Mediterranean annual pastures

Air pollution represents a threat to biodiversity throughout the world and particularly in the Mediterranean area, where high tropospheric ozone (O3) concentrations and atmospheric nitrogen (N) deposition are frequently recorded. Mediterranean annual pastures are among the most important ecosystems in southern Europe due to their high biodiversity and extension. Aiming to study the responses of these communities to the main atmospheric pollutants in the Mediterranean region, an experimental study was performed in an open-top chamber (OTC) facility. A mixture of six species representative of annual pastures was grown under field conditions inside the OTC. Plants were exposed for 39 days to four O3 treatments and three doses of N. The species responded heterogeneously to both factors. Legumes did not react to N but were very sensitive to O3: Trifolium species responded negatively, while Ornithopus responded positively, taking advantage of the greater sensitivity of clovers to O3. The grasses and the herb were more tolerant of O3 and grasses were the most responsive to N. Significant interactions between factors indicated a loss of effectiveness of N in O3-polluted atmospheres and an ability of O3 to counterbalance the damage induced by N input, but both effects were dependent on O3 and N levels. The inclusion of plant competition in the experimental design was necessary to reveal results that would otherwise be missed, such as the positive growth responses under elevated O3 levels. Surprisingly, competition within the legume family played the most important role in the overall response of the annual community to O3. Both tropospheric O3 and N deposition should be considered important drivers of the structure and biodiversity of Mediterranean annual pastures.

High tolerance of subalpine grassland to long-term ozone exposure is independent of N input and climatic drivers

In a seven-year study, we tested effects of increased N and O3 deposition and climatic conditions on biomass of subalpine grassland. Ozone risk was assessed as exposure (AOT40) and as stomatal flux (POD0,1). We hypothesized that productivity is higher under N- and lower under O3 deposition, with interactions with climatic conditions. Aboveground biomass was best correlated with growing-degree days for May (GDDMay). Nitrogen deposition increased biomass up to 60% in the highest treatment, and 30% in the lowest addition. Also belowground biomass showed a positive N-response. Ozone enrichment had no effect on biomass, and no interaction between O3 and N was observed. Growth response to N deposition was not correlated to GDDMay or precipitation, but indicated a cumulative effect over time. Productivity of subalpine grassland is tolerant to increasing ozone exposure, independent of N input and climatic drivers. N deposition rates at current critical loads, strongly increase the grassland yield.

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.

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.

Spatial heterogeneity confounded ozone-exposure experiment in semi-natural grassland

Interpretation of observations from manipulative experiments is often complicated by a multitude of uncontrolled processes operating at various spatial and temporal scales. As such processes may differ among experimental plots there is a risk that effects of experimental treatments are confounded. Here we report on a free-air ozone-exposure experiment in permanent semi-natural grassland that suggested strong ozone effects on community productivity after 5 years. We tested ozone effects and investigated the potential of confounding due to changes in nutrient management. Repeated-measure ANOVA revealed mainly negative temporal trends for frequency of abundant productive plant species. Constrained ordination additionally showed converging trajectories of species compositions for ozone and control treatments with time. Yields sampled prior to the start of the experiment and soil nitrogen concentrations revealed that spatial heterogeneity in the soil nutrient status was not accounted for by the random allocation of treatments to plots with a bias towards less productive patches in the elevated-ozone plots. Re-analysis of yield data using repeated-measure ANOVA with a covariable to account for productivity prior to the start of fumigation revealed effects on the temporal changes in total yield and yield of legumes that cannot be separated between ozone and pre-treatment nutrient status. Changes in species composition favour an ecological interpretation with spatial heterogeneity as the major cause of different yield declines. Although elevated ozone may cause subtle physiological changes with longer term implications, our new results suggest that species-rich mature grassland such as the one studied at Le Mouret may be less sensitive to elevated ozone than previously assumed. In this experiment a confounded design was hidden at the start by transitory effects of a prior change in nutrient treatments.

Effects of ozone on inter- and intra-species competition and photosynthesis in mesocosms of Lolium perenne and Trifolium repens

Trifolium repens and Lolium perenne were exposed as both monocultures and two-species mixtures to an episodic rural ozone regime in large, well-watered containers within solardomes for 12 weeks. There were reductions in biomass for T. repens, but not L. perenne, and the proportion of T. repens decreased in ozone-exposed mixtures compared to the control. In addition, leaf biomass of T. repens was maintained at the expense of biomass partitioning to the stolons. The decreased growth corresponded with decreased photosynthetic capacity for T. repens, however, by the end of the exposure there was also decreased photosynthetic capacity of L. perenne, a species previously considered insensitive to ozone. The observed decreases in photosynthetic efficiency and capacity in elevated ozone indicate that the ability of such ubiquitous vegetation to act as a sink for atmospheric carbon may be reduced in future climates.

Modelling stomatal ozone flux and deposition to grassland communities across Europe

Regional scale modelling of both ozone deposition and the risk of ozone impacts is poorly developed for grassland communities. This paper presents new predictions of stomatal ozone flux to grasslands at five different locations in Europe, using a mechanistic model of canopy development for productive grasslands to generate time series of leaf area index and soil water potential as inputs to the stomatal component of the DO(3)SE ozone deposition model. The parameterisation of both models was based on Lolium perenne, a dominant species of productive pasture in Europe. The modelled seasonal time course of stomatal ozone flux to both the whole canopy and to upper leaves showed large differences between climatic zones, which depended on the timing of the start of the growing season, the effect of soil water potential, and the frequency of hay cuts. Values of modelled accumulated flux indices and the AOT40 index showed a five-fold difference between locations, but the locations with the highest flux differed depending on the index used; the period contributing to the accumulation of AOT40 did not always coincide with the modelled period of active ozone canopy uptake. Use of a fixed seasonal profile of leaf area index in the flux model produced very different estimates of annual accumulated total canopy and leaf ozone flux when compared with the flux model linked to a simulation of canopy growth. Regional scale model estimates of both the risks of ozone impacts and of total ozone deposition will be inaccurate unless the effects of climate and management in modifying grass canopy growth are incorporated.

Factors affecting the ozone sensitivity of temperate European grasslands: an overview

This overview of experimentally induced effects of ozone aims to identify physiological and ecological principles, which can be used to classify the sensitivity to ozone of temperate grassland communities in Europe. The analysis of data from experiments with single plants, binary mixtures and multi-species communities illustrates the difficulties to relate individual responses to communities, and thus to identify grassland communities most at risk. Although there is increasing evidence that communities can be separated into broad classes of ozone sensitivity, the database from experiments under realistic conditions with representative systems is too small to draw firm conclusions. But it appears that risk assessments, based on results from individuals or immature mixtures exposed in chambers, are only applicable to intensively managed, productive grasslands, and that the risk of ozone damage for most of perennial grasslands with lower productivity tends to be less than previously expected.

Nitrogen deposition but not ozone affects productivity and community composition of subalpine grassland after 3 yr of treatment

A field experiment was established at 2000 m above sea level (asl) in the central Swiss Alps with the aim of investigating the effects of elevated ozone (O(3)) and nitrogen deposition (N), and of their combination, on above-ground productivity and species composition of subalpine grassland. One hundred and eighty monoliths were extracted from a species-rich Geo-Montani-Nardetum pasture and exposed in a free-air O(3)-fumigation system to one of three concentrations of O(3) (ambient, 1.2 x ambient, 1.6 x ambient) and five concentrations of additional N. Above-ground biomass, proportion of functional groups and normalized difference vegetation index (NDVI) were measured annually. After 3 yr of treatment, the vegetation responded to the N input with an increase in above-ground productivity and altered species composition, but without changes resulting from elevated O(3). N input > 10 kg N ha(-1) yr(-1) was sufficient to affect the composition of functional groups, with sedges benefiting over-proportionally. No interaction of O(3) x N was observed, except for NDVI; positive effects of N addition on canopy greenness were counteracted by accelerated leaf senescence in the highest O(3) treatment. The results suggest that effects of elevated O(3) on the productivity and floristic composition of subalpine grassland may develop slowly, regardless of the sensitive response to increasing N.