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.     

Meta-analysis of the relative sensitivity of semi-natural vegetation species to ozone

This study identified 83 species from existing publications suitable for inclusion in a database of sensitivity of species to ozone (OZOVEG database). An index, the relative sensitivity to ozone, was calculated for each species based on changes in biomass in order to test for species traits associated with ozone sensitivity. Meta-analysis of the ozone sensitivity data showed a wide inter-specific range in response to ozone. Some relationships in comparison to plant physiological and ecological characteristics were identified. Plants of the therophyte lifeform were particularly sensitive to ozone. Species with higher mature leaf N concentration were more sensitive to ozone than those with lower leaf N concentration. Some relationships between relative sensitivity to ozone and Ellenberg habitat requirements were also identified. In contrast, no relationships between relative sensitivity to ozone and mature leaf P concentration, Grime's CSR strategy, leaf longevity, flowering season, stomatal density and maximum altitude were found. The relative sensitivity of species and relationships with plant characteristics identified in this study could be used to predict sensitivity to ozone of untested species and communities.     

Seasonal development of ozone-induced foliar injury on tall milkweed (Asclepias exaltata) in Great Smoky Mountains National Park

The goals of this study were to document the development of ozone-induced foliar injury, on a leaf-by-leaf basis, and to develop ozone exposure relationships for leaf cohorts and individual tall milkweeds (Asclepias exaltata L.) in Great Smoky Mountains National Park. Plants were classified as either ozone-sensitive or insensitive based on the amount of foliar injury. Sensitive plants developed injury earlier in the season and to a greater extent than insensitive plants. Older leaf cohorts were more likely to belong to high injury classes by the end of each of the two growing seasons. In addition, leaf loss was more likely for older cohorts (2000) and lower leaf positions (2001) than younger cohorts and upper leaves, respectively. Most leaves abscised without prior ozone-like stippling or chlorosis. Failure to take this into account can result in underestimation of the effects of ozone on these plants.     

Gravitational infusion of ethylenediurea (EDU) into trunks protected adult European ash trees (Fraxinus excelsior L.) from foliar ozone injury

Adult ash trees (Fraxinus excelsior L.), known to be sensitive or insensitive to ozone, determined by presence or absence of foliar symptoms in previous years, were treated with ethylenediurea (EDU) at 450 ppm by gravitational trunk infusion on six occasions at 21-day intervals in summer 2005 at Turin, Italy. At the end of the season, foliar ozone injury on EDU-treated trees was not complete, but was greatly and significantly reduced when compared to results from trees infused with water. Significant symptom reduction occurred at any crown level in the treated trees suggesting that EDU protected whole crowns. Gravitational infusion of EDU resulted in protection from ozone injury for ozone-sensitive ash trees. The amount of EDU needed to provide protection is assumed to be in the range 13-26 mg m(-2) leaf.     

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.     

Growth and premature leaf fall in American aspen as bioindications for ozone

Several clones of the American aspen (Populus tremuloides Michx.) differing in ozone sensitivity were fumigated continuously for 105 days with different ozone concentrations, producing different ozone doses. The suitability of the clones as possible replacements for tobacco as bioindicators in forested areas was tested. The life span of leaves of the ozone-sensitive clones was distinctly reduced by ozone. The difference to controls was significant even in the relatively clean air at Birmensdorf. Growth was diminished, too, but the decrease in height was much less pronounced than that in weight. In sensitive clones an after-effect was detectable, since the weight of new shoots (grown from reserves) decreased with increasing ozone stress in the previous year. Since the tolerant clone grew less, it is-at first sight-of decreased economic interest from the point of view of rapid productivity. In the long run, however, it might be economically interesting because it may be able to maintain productivity (although at a lower level) due to better competitive strength.     

Simulating the growth response of aspen to elevated ozone: a mechanistic approach to scaling a leaf-level model of ozone effects on photosynthesis to a complex canopy architecture.

Predicting ozone-induced reduction of carbon sequestration of forests under elevated tropospheric ozone concentrations requires robust mechanistic leaf-level models, scaled up to whole tree and stand level. As ozone effects depend on genotype, the ability to predict these effects on forest carbon cycling via competitive response between genotypes will also be required. This study tests a process-based model that predicts the relative effects of ozone on the photosynthetic rate and growth of an ozone-sensitive aspen clone, as a first step in simulating the competitive response of genotypes to atmospheric and climate change. The resulting composite model simulated the relative above ground growth response of ozone-sensitive aspen clone 259 exposed to square wave variation in ozone concentration. This included a greater effect on stem diameter than on stem height, earlier leaf abscission, and reduced stem and leaf dry matter production at the end of the growing season. Further development of the model to reduce predictive uncertainty is discussed.     

Ozone concentrations in the Central Wasatch Mountains of Utah

In the Salt Lake Valley, a June through August SUM60 value (sum of hourly average ozone concentrations > or =60 parts per billion by volume [ppbv]) of 25,000 ppbv-hr was exceeded in 9 yr between 1978 and 1998. Ozone concentrations in the nearby Central Wasatch Mountains were monitored to determine the potential for vegetation injury. The SUM60 value of 19,000 ppbv-hr in these mountains and peak hourly concentrations >100 ppbv suggests that ozone-sensitive species may be injured. Ozone concentrations in the mountains were greatest during periods of strong upslope winds from the Salt Lake Valley. Both SUM60 values and hourly average concentrations in the Central Wasatch Mountains were strongly correlated with those in the Salt Lake Valley, suggesting that data from valley stations could be used to estimate ozone in the mountains.     

Ozone Concentrations in the Central Wasatch Mountains of Utah

Abstract

In the Salt Lake Valley, a June through August SUM60 value (sum of hourly average ozone concentrations ≥60 parts per billion by volume [ppbv]) of 25,000 ppbv-hr was exceeded in 9 yr between 1978 and 1998. Ozone concentrations in the nearby Central Wasatch Mountains were monitored to determine the potential for vegetation injury. The SUM60 value of 19,000 ppbv-hr in these mountains and peak hourly concentrations >100 ppbv suggests that ozone-sensitive species may be injured. Ozone concentrations in the mountains were greatest during periods of strong upslope winds from the Salt Lake Valley. Both SUM60 values and hourly average concentrations in the Central Wasatch Mountains were strongly correlated with those in the Salt Lake Valley, suggesting that data from valley stations could be used to estimate ozone in the mountains.     

Ozone as a Tobacco Toxicant

Ozone is very toxic to tobacco, causing serious injury in greenhouses and fields. Certain varieties, differing in sensitivity, have proved useful as indicators of the presence, and to some extent the level, of air-polluting ozone. Environmental factors alter plant response to ozone and must be taken into account. What is now recognized as typical ozone injury to tobacco was first observed in Connecticut in 1951, Maryland in 1952, eastern North Carolina in 1953, and Ontario, Canada, in 1955. Cracking of rubber strips exposed daily at six locations in 1959 in the Connecticut Valley showed ozone concentrations were highest the day before a fleck outbreak, resulting in more than a million dollar loss. Measurement with a Mast ozone recorder from July to mid-October for two seasons at Beltsville, Md., six miles northeast of the District of Columbia, indicated 2.5 pphm as the average daily maximum value, 5 pphm as the level when plant injury is expected, and 10 pphm as very high and infrequent. The neutral buffered Kl method indicated higher ozone values, but about 15 pphm was maximum with this method. Shading portions of leaves for three hours in midday prevented ozone injury. Increased injury in the different tobacco areas was related to increased culture of more susceptible varieties. Although various antiozonants applied to the leaf, or even applied to cloth shade, reduce damage, the development of resistant tobaccos has proved the better means of control. Premature senescence and related chlorosis as well as fleck were prevented by carbon-filtered air, indicating that ozone may be a more important toxicant than previously recognized. Possibly oxidants other than ozone contributed to plant injury, but, if present, they were removed by carbon-filters. Growth of an ozone-sensitive tobacco Bel-W3, as indicated by dry weight, was only half as much in ambient as in carbonfiltered air in one experiment with chambers continued for 5 weeks. Evidence of synergistic action of sulfur dioxide and ozone is reported. Subthreshold concentrations of these toxicants produced injury following exposure of tobacco to mixed gases, whereas no injury resulted from the same concentration of the individual gases.     

Difference in ozone uptake in grassland species between open-top chambers and ambient air

Exposure-response data from open-top chamber (OTC) experiments are often directly applied to ambient air (AA) conditions. Because microclimatic conditions are modified and pollutant uptake by plants may differ (i.e. 'chamber effect'), there is concern about the influence of OTCs on these relationships. In addition, AA concentrations are often measured at a height which differs from canopy height and correction for the concentration gradient (i.e. 'gradient effect') is necessary. To quantify the relative contribution of plant characteristics and microclimatic factors to these effects, ozone uptake by horizontal leaves at the top of the canopy was calculated for plants grown in OTCs or AA by using a resistance analogy model. Data from an OTC experiment in 1996/97 for six species typical of productive grasslands were used. Ozone concentration inside OTCs was set equal to the concentration measured at a height of 3 m above ground (C(z(ref))) or at canopy height (C(0)). The gradient effect resulted in a 16-27% lower average C(0) than C(z(ref)), depending on species. The main determinant of the chamber effect was a systematic difference in leaf-to-air vapour pressure deficit between OTCs and AA which affected stomatal resistance and ozone uptake. In case of monocultures both effects were species-specific. In species mixtures the gradient effect differed between mixing ratios, whereas the chamber effect was species-specific. Because of the inter-specific difference in the chamber effect on ozone uptake, it is concluded that ozone effects on species mixtures differ systematically between OTCs and AA. The data underline that extrapolation of ozone flux-response relationships from OTC experiments must be based on canopy-level ozone concentrations, and that these relationships should be applied only to single species under microclimatic conditions similar to those prevailing in the experiment.     

Influence of ozone pollution and climate variability on net primary productivity and carbon storage in China's grassland ecosystems from 1961 to 2000

Our simulations with the Dynamic Land Ecosystem Model (DLEM) indicate that the combined effect of ozone, climate, carbon dioxide and land use have caused China's grasslands to act as a weak carbon sink during 1961-2000. This combined effect on national grassland net primary productivity (NPP) and carbon storage was small, but changes in annual NPP and total carbon storage across China's grasslands showed substantial spatial variation, with the maximum total carbon uptake reduction of more than 400gm(-2) in some places of northeastern China. The grasslands in the central northeastern China were more sensitive and vulnerable to elevated ozone pollution than other regions. The combined effect excluding ozone could potentially lead to an increase of 14Tg C in annual NPP and 0.11Pg C in total carbon storage for the same time period. This implies that improvement in air quality could significantly increase productivity and carbon storage in China's grassland ecosystems.     

Consideration of Air Quality Standards for Vegetation With Respect to Ozone

Present evidence suggests that ozone is the most damaging of all air pollutants affecting vegetation. It is the principal oxidant in the photochemical smog complex. Concentrations of ozone have exceeded 0.5 part per million (ppm) in the Los Angeles area. One-tenth of this level for 8 hours is known to injure very sensitive tobacco varieties. Many plant species are visibly affected after a few hours exposure at concentrations much lower than 0.5 ppm. There is also some evidence that ozone reduces plant growth. Many factors must be taken into account when considering standards to protect vegetation from ozone damage. These include ozone concentration and methods of measurement, time of exposure, possible additive effects of other pollutants, sensitivity of plant species, their economic value, and the extent of injury which can be tolerated. The response of a species to the pollutant is conditioned by genetic factors and environmental conditions. Lack of specific routine methods for measuring ozone in ambient air is a handicap. California and Colorado established standards for oxidants at 0.15 and 0.10 ppm, respectively, for 1 hour. How these standards relate to the ozone dosage causing acute and chronic injury to various plant species is discussed.     

Impacts of atmospheric pollution on the plant communities of British acid grasslands

Air pollutants are recognised as important agents of ecosystem change but few studies consider the effects of multiple pollutants and their interactions. Here we use ordination, constrained cluster analysis and indicator value analyses to identify potential environmental controls on species composition, ecological groupings and indicator species in a gradient study of UK acid grasslands. The community composition of these grasslands is related to climate, grazing, ozone exposure and nitrogen deposition, with evidence for an interaction between the ecological impacts of base cation and nitrogen deposition. Ozone is a key agent in species compositional change but is not associated with a reduction in species richness or diversity indices, showing the subtly different drivers on these two aspects of ecosystem degradation. Our results demonstrate the effects of multiple interacting pollutants, which may collectively have a greater impact than any individual agent.     

Causes of change in nitrophytic and oligotrophic lichen species in a Mediterranean climate: impact of land cover and atmospheric pollutants

With the aim of determining the main drivers of changes in nitrophytic and oligotrophic macro-lichen communities in an industrial region with a Mediterranean climate, we considered both land-cover types and atmospheric pollutants. We determined the relation between the abundance of nitrophytic and oligotrophic species with environmental factors considering the distance of influence of land-cover types. The results showed that oligotrophic species decreased in the proximity of artificial areas, barren land and agricultural areas, associated with higher concentrations of NO₂ and Zn, and Ti, probably dust of industrial and agricultural origin. Nitrophytic species were positively related to all the mentioned land-cover types, and with higher concentrations of Fe and N. Magnesium, probably from ocean aerosols, was negatively related to oligotrophic species and positively to nitrophytic.     

Assessing plant response to ambient ozone: growth of ozone-sensitive loblolly pine seedlings treated with ethylenediurea or sodium erythorbate

One-year-old seedlings from an ozone-sensitive half-sib family of loblolly pine (Pinus taeda L.) were transplanted into replicated plots in blocks in a large forest clearing near Nacogdoches, Texas. Seedlings were either non-treated (controls) or treated bi-weekly with foliar sprays of ethylenediurea (EDU), at 150, 300 or 450 ppm or sodium erythorbate (NaE), at 515, 1030, or 1545 ppm, for three growing seasons. Results from the final third year harvest indicated that both EDU and NaE increased all growth parameters, with significant differences only for EDU at 450 ppm. Both EDU and NaE would be useful for long-term studies on assessing the effects of ambient ozone on established native plants.     

Growth response to ozone of annual species from Mediterranean pastures

Ozone (O3) phytotoxicity has been reported on a wide range of plant species. However, scarce information has been provided regarding the sensitivity of semi-natural grassland species, especially those from dehesa Mediterranean grasslands, in spite of their great biological diversity and the high O3 levels recorded in the region. A screening study was carried out in open-top chambers (OTCs) to assess the O3-sensitivity of representative therophytes of these ecosystems based on the response of selected growth-related parameters. Three O3 treatments and 3 OTCs per treatment were used. Legume species were very sensitive to O3, because 78% of the tested species showed detrimental effects on their total biomass relative growth rate (RGR) following their exposure to O3. The Trifolium genus was particularly sensitive showing O3-induced adverse effects on most of the assessed parameters. Gramineae plants were less sensitive than Leguminosae species because detrimental effects on total biomass RGR were only observed in 14% of the assessed species. No relationship was found between relative growth rates when growing in clean air and O3 susceptibility. The implications of these effects on the performance of dehesa acidic grasslands and on the definition of ozone critical levels for the protection of semi-natural vegetation are discussed.     

Predicting community sensitivity to ozone, using Ellenberg Indicator values

This paper develops a regression-based model for predicting changes in biomass of individual species exposed to ozone (RS(p)), based on their Ellenberg Indicator values. The equation (RS(p)=1.805-0.118Light-0.135 square root Salinity) underpredicts observed sensitivity but has the advantage of widespread applicability to almost 3000 European species. The model was applied to grassland communities to develop two further predictive tools. The first tool, percentage change in biomass (ORI%) was tested on data from a field-based ozone exposure experiment and predicted a 27% decrease in biomass over 5 years compared with an observed decrease of 23%. The second tool, an index of community sensitivity to ozone (CORI), was applied to 48 grassland communities and suggests that community sensitivity to ozone is primarily species-driven. A repeat-sampling routine showed that nine species were the minimum requirement to estimate CORI within 5%.     

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

Although ozone has been shown to reduce the growth of individual species and to alter the composition of simple species mixtures, there is little understanding of its long-term effects on species dynamics and composition in real communities. Intact turfs of calcareous grassland were exposed to four different ozone regimes in open-top chambers over three consecutive summers. Treatments provided a mean seasonal AOT40 ranging from approximately zero to 15 ppm h. Cumulative ozone exposure was a significant factor in compositional change, but only explained 4.6% of the variation. The dominant grass species (Festuca rubra) showed a consistent decline in cover in the high ozone treatment over time and the forb Campanula rotundifolia was lost from all three ozone treatments. The frequency of some species (Galium verum and Plantago lanceolata) increased with ozone exposure. Long-term effects of ozone on species composition in chalk grassland may be a function of both the sensitivity of individual species and the response of the dominant species.