Responses to Airborne Ozone and Soilborne Metal Pollution in Afforestation Plants with Different Life Forms

With the current increases in environmental stress, understanding species-specific responses to multiple stress agents is needed. This science is especially important for managing ecosystems that are already confronted with considerable pollution. In this study, responses to ozone (O3, ambient daily course values + 20 ppb) and mixed metal contamination in soils (MC, cadmium/copper/lead/zinc = 25/1100/2500/1600 mg kg-1), separately and in combination, were evaluated for three plant species (Picea abies, Acer pseudoplatanus, Tanacetum vulgare) with different life forms and ecological strategies. The two treatments elicited similar stress reactions, as shown by leaf functional traits, gas exchange, tannin, and nutrient markers, irrespective of the plant species and life form, whereas the reactions to the treatments differed in magnitude. Visible and microscopic injuries at the organ or cell level appeared along the penetration route of ozone and metal contamination. At the whole plant level, the MC treatment caused more severe injuries than the O3 treatment and few interactions were observed between the two stress factors. Picea trees, with a slow-return strategy, showed the highest stress tolerance in apparent relation to an enhancement of conservative traits and an exclusion of stress agents. The ruderal and more acquisitive Tanacetum forbs translocated large amounts of contaminants above ground, which may be of concern in a phytostabilisation context. The deciduous Acer trees-also with an acquisitive strategy-were most sensitive to both stress factors. Hence, species with slow-return strategies may be of particular interest for managing metal-polluted sites in the current context of multiple stressors and for safely confining soil contaminants below ground.

Novel method to determine element concentrations in foliage of poplar and willow cuttings

Measuring the uptake of the chemical elements by plants usually requires the destructive harvest of the plants. Analyzing individual leaves is unsatisfactory because their elemental concentration depends on their age and position on the branch or stem. We aimed to find an easy method to determine the elemental concentrations using a few suitable single leaves along the main shoot of poplar (Populus monviso) and willow (Salix viminalis) cuttings at the end of the first season. Using Ca, Cd, Mn, Fe, K, P, Pb, and Zn concentrations, measured in selected leaves along the main shoots of the cuttings, mathematical functions were derived, which described best their distribution. Elemental allocation patterns were independent of the soil characteristics and soil element concentrations. Based on these functions, three leaves from specific positions along the main shoot were selected, which could accurately describe the derived functions. The deviation of the calculated average concentration, based on the 3-leaves method, was ≤15% in approximately 65% of the cases compared to the measured concentration. This method could be used to calculate element concentrations and fluxes in phytomanagement, biomonitoring, or biomass productions projects using one-season poplar or willow cuttings.

Growth cessation uncouples isotopic signals in leaves and tree rings of drought-exposed oak trees

An increase in temperature along with a decrease in summer precipitation in Central Europe will result in an increased frequency of drought events and gradually lead to a change in species composition in forest ecosystems. In the present study, young oaks (Quercus robur L. and Quercus petraea (Matt.) Liebl.) were transplanted into large mesocosms and exposed for 3 years to experimental warming and a drought treatment with yearly increasing intensities. Carbon and oxygen isotopic (δ(13)C and δ(18)O) patterns were analysed in leaf tissue and tree-ring cellulose and linked to leaf physiological measures and tree-ring growth. Warming had no effect on the isotopic patterns in leaves and tree rings, while drought increased δ(18)O and δ(13)C. Under severe drought, an unexpected isotopic pattern, with a decrease in δ(18)O, was observed in tree rings but not in leaves. This decrease in δ(18)O could not be explained by concurrent physiological analyses and is not supported by current physiological knowledge. Analysis of intra-annual tree-ring growth revealed a drought-induced growth cessation that interfered with the record of isotopic signals imprinted on recently formed leaf carbohydrates. This missing record indicates isotopic uncoupling of leaves and tree rings, which may have serious implications for the interpretation of tree-ring isotopes, particularly from trees that experienced growth-limiting stresses.

Changes in the dynamics of foliar N metabolites in oak saplings by drought and air warming depend on species and soil type

Climate change poses direct or indirect influences on physiological mechanisms in plants. In particular, long living plants like trees have to cope with the predicted climate changes (i.e. drought and air warming) during their life span. The present study aimed to quantify the consequences of simulated climate change for foliar N metabolites over a drought-rewetting-drought course. Saplings of three Central European oak species (i.e. Quercus robur, Q. petraea, Q. pubescens) were tested on two different soil types (i.e. acidic and calcareous). Consecutive drought periods increased foliar amino acid-N and soluble protein-N concentrations at the expense of structural N in all three oak species. In addition, transient effects on foliar metabolite dynamics were observed over the drought-rewetting-drought course. The lowest levels of foliar soluble protein-N, amino acid-N and potassium cation with a minor response to drought and air warming were found in the oak species originating from the driest/warmest habitat (Q. pubescens) compared to Q. robur and Q. petraea. Higher foliar osmolyte-N and potassium under drought and air warming were observed in all oak species when grown on calcareous versus acidic soil. These results indicate that species-specific differences in physiological mechanisms to compensate drought and elevated temperature are modified by soil acidity.

A phenological timetable of oak growth under experimental drought and air warming

Climate change is expected to increase temperature and decrease summer precipitation in Central Europe. Little is known about how warming and drought will affect phenological patterns of oaks, which are considered to possess excellent adaptability to these climatic changes. Here, we investigated bud burst and intra-annual shoot growth of Quercus robur, Q. petraea and Q. pubescens grown on two different forest soils and exposed to air warming and drought. Phenological development was assessed over the course of three growing seasons. Warming advanced bud burst by 1-3 days °C⁻¹ and led to an earlier start of intra-annual shoot growth. Despite this phenological shift, total time span of annual growth and shoot biomass were not affected. Drought changed the frequency and intensity of intra-annual shoot growth and advanced bud burst in the subsequent spring of a severe summer drought by 1-2 days. After re-wetting, shoot growth recovered within a few days, demonstrating the superior drought tolerance of this tree genus. Our findings show that phenological patterns of oaks are modified by warming and drought but also suggest that ontogenetic factors and/or limitations of water and nutrients counteract warming effects on the biomass and the entire span of annual shoot growth.

Foliar symptoms triggered by ozone stress in irrigated holm oaks from the city of Madrid, Spain

BACKGROUND

Despite abatement programs of precursors implemented in many industrialized countries, ozone remains the principal air pollutant throughout the northern hemisphere with background concentrations increasing as a consequence of economic development in former or still emerging countries and present climate change. Some of the highest ozone concentrations are measured in regions with a Mediterranean climate but the effect on the natural vegetation is alleviated by low stomatal uptake and frequent leaf xeromorphy in response to summer drought episodes characteristic of this climate. However, there is a lack of understanding of the respective role of the foliage physiology and leaf xeromorphy on the mechanistic effects of ozone in Mediterranean species. Particularly, evidence about morphological and structural changes in evergreens in response to ozone stress is missing.

RESULTS

Our study was started after observing ozone -like injury in foliage of holm oak during the assessment of air pollution mitigation by urban trees throughout the Madrid conurbation. Our objectives were to confirm the diagnosis, investigate the extent of symptoms and analyze the ecological factors contributing to ozone injury, particularly, the site water supply. Symptoms consisted of adaxial and intercostal stippling increasing with leaf age. Underlying stippling, cells in the upper mesophyll showed HR-like reactions typical of ozone stress. The surrounding cells showed further oxidative stress markers. These morphological and micromorphological markers of ozone stress were similar to those recorded in deciduous broadleaved species. However, stippling became obvious already at an AOT40 of 21 ppm•h and was primarily found at irrigated sites. Subsequent analyses showed that irrigated trees had their stomatal conductance increased and leaf life -span reduced whereas the leaf xeromorphy remained unchanged. These findings suggest a central role of water availability versus leaf xeromorphy for ozone symptom expression by cell injury in holm oak.

Inter- and intra-specific variability in isoprene production and photosynthesis of Central European oak species

European deciduous oaks are closely related and are known for their strong emission of volatile isoprenoids. They are chemo-taxonomically diverse, but hybridise frequently. Four-year-old oak seedlings growing together in a model ecosystem facility under near-natural conditions were studied. The leaves were morphologically classified in the three oak species Quercus robur, Q. pubescens and Q. petraea (with four provenances each) and further investigated by a molecular-genetic approach. Q. robur was morphologically and genetically clearly different from Q. pubescens and Q. petraea, whereas Q. pubescens and Q. petraea individuals used in this study were morphologically and genetically more similar. There was a minor impact of among and within species variability on isoprene synthesis, isoprene emission and photosynthesis. Isoprene emission rates normalised to 25 °C leaf temperature ranged from 5.78 to 10.66 nmol m(-2)  s(-1) , whereas photosynthesis ranged from 12.8 to 17.6 μmol m(-2)  s(-1) . On cloudy days, among the provenances of each species, only net photosynthesis of the Q. robur provenance Hünenberg was reduced and isoprene synthase activity of the Q. pubescens provenance Promotogno increased. On sunny days, photosynthesis did not differ among the provenances. Over all provenances, gas exchange on cloudy days did not differ significantly from sunny days. In the combined data of cloudy and sunny days, no differences between the studied provenances and oak species were detected in isoprene emission and photosynthesis. Thus, isoprene emission and photosynthesis rates were remarkably stable among oak species and provenances. The results indicate that taxonomic differences in the studied oak species are not reflected in isoprene emission and photosynthesis, probably because of the high plasticity of gene expression resulting in high phenotypic flexibility.

Drought and air warming affects abundance and exoenzyme profiles of Cenococcum geophilum associated with Quercus robur, Q. petraea and Q. pubescens

The present study aimed to elucidate the influence of drought and elevated temperature on relative abundance and functioning of the ectomycorrhizal fungus Cenococcum geophilum on three oak species differing in adaptation to a warm and dry climate. The experiment QUERCO comprised three Quercus species (Q. robur, Q. petraea, Q. pubescens) grown for 3 years under four treatments: elevated air temperature, drought, a combination of the two, and control. Fine root samples were analysed for relative abundance and potential extracellular enzyme activities of ectomycorrhizae of C. geophilum, a fungal species known to be drought resistant. The relative abundance of C. geophilum on the roots of the oak species was significantly increased by temperature, decreased by drought, but unchanged in the combined treatment compared to the control. Although the extent of treatment effects differed among oak species, no significant influence of tree species on relative abundance of C. geophilum was detected. Exoenzyme activities of C. geophilum on Q. robur and Q. petraea (but not Q. pubescens) significantly increased in the combined treatment, but for all oak species were reduced under drought and air warming alone compared to the control. There was a significant negative correlation between abundance of C. geophilum and its leucine aminopeptidase activity. As this enzyme is not frequent among ectomycorrhizal fungi, this emphasises the functional importance of C. geophilum in the ectomycorrhizal community. Our results indicate that increased temperature and drought will influence the relative abundance and enzyme activity of C. geophilum. However, both the Quercus species and C. geophilum tolerated warming and strong drought.

Physiological and biochemical responses of Quercus pubescens to air warming and drought on acidic and calcareous soils

The drought- and thermo-tolerant Quercus pubescens, a tree species growing on both acidic and calcareous soils in the sub-Mediterranean region, was exposed to soil drought (-60% to -80% soil water content) and air warming (+1.2 °C daytime temperature), singly and in combination. The experiment was conducted on two natural forest soils with similar texture but different pH (acidic and calcareous soils). The physiological (photosynthesis) and biochemical (antioxidant system) responses of Q. pubescens were investigated. On acidic soil, Q. pubescens had a higher reactive oxygen species (ROS) content than on calcareous soil, confirming that this species is better adapted to the latter soil type. A down-regulation of ascorbate-glutathione cycle enzymes suggests that ROS were used as signalling molecules. Air warming stimulated stomatal opening, while soil drought induced stomatal closure in the late afternoon and reduced Rubisco carboxylation efficiency. Photosynthetic performance in the combined treatment was higher than under single drought stress and similar to control and air warming. Q. pubescens biochemical responses depended on soil pH. On acidic soil, Q. pubescens trees exposed to air warming used ROS as signalling molecules. On calcareous soil, these trees were able to balance both soil drought and air warming stress, avoiding ROS toxic effects by increasing antioxidant enzyme activitiy and maintaining a high enzymatic antioxidant defence. When combined, drought and air warming induced either more severe (higher oxidative pressure and impairment of the light-harvesting complex) or different responses (decline of the thermal energy dissipation capacity) relative to the single stressors. Overall, however, Q. pubescens preserved the functionality of the photosynthetic apparatus and controlled the antioxidant system response, thus confirming its drought and thermo-tolerance and therefore its potential to adapt to the ongoing climate change.

Foliage response of young central European oaks to air warming, drought and soil type

Three Central European oak species, with four provenances each, were experimentally tested in 16 large model ecosystem chambers for their response to passive air warming (AW, ambient +1-2 °C), drought (D, -43 to -60% irrigation) and their combination (AWD) for 3 years on two forest soil types of pH 4 or 7. Throughout the entire experiment, the influence of the different ambient and experimental climates on the oak trees was strong. The morphological traits of the Quercus species were affected in opposing ways in AW and D treatments, with a neutral effect in the AWD treatment. Biochemical parameters and LMA showed low relative plasticity compared to the morphological and growth parameters. The high plasticity in physiologically important parameters of the three species, such as number of intercalary veins or leaf size, indicated good drought acclimation properties. The soil type influenced leaf chlorophyll concentration, C/N and area more than drought, whereas foliage mass was more dependent on drought than on soil type. Through comparison of visible symptom development with the water deficits, a drought tolerance threshold of -1.3 MPa was determined. Although Q. pubescens had xeromorphic leaf characteristics (small leaf size, lower leaf water content, high LMA, pilosity, more chlorophyll, higher C/N) and less response to the treatments than Q. petraea and Q. robur, it suffered more leaf drought injury and shedding of leaves than Q. petraea. However, if foliage mass were used as the criterion for sustainable performance under a future climate, Q. robur would be the most appropriate species.

Water regime and growth of young oak stands subjected to air-warming and drought on two different forest soils in a model ecosystem experiment

Global climate change is expected to increase annual temperatures and decrease summer precipitation in Central Europe. Little is known of how forests respond to the interaction of these climate factors and if their responses depend on soil conditions. In a 3-year lysimeter experiment, we investigated the growth response of young mixed oak stands, on either acidic or calcareous soil, to soil water regime, air-warming and drought treatments corresponding to an intermediate climate change scenario. The air-warming and drought treatments were applied separately as well as in combination. The air-warming treatment had no effect on soil water availability, evapotranspiration or stand biomass. Decreased evapotranspiration from the drought-exposed stands led to significantly higher air and soil temperatures, which were attributed to impaired transpirational cooling. Water limitation significantly reduced the stand foliage, shoot and root biomass as droughts were severe, as shown in low leaf water potentials. Additional air warming did not enhance the drought effects on evapotranspiration and biomass, although more negative leaf water potentials were observed. After re-watering, evapotranspiration increased within a few days to pre-drought levels. Stands not subjected to the drought treatment produced significantly less biomass on the calcareous soil than on the acidic soil, probably due to P or Mn limitation. There was no difference in biomass and water regime between the two soils under drought conditions, indicating that nutrient availability was governed by water availability under these conditions. The results demonstrate that young oak stands can cope with severe drought and therefore can be considered for future forestry.

Seasonal photosynthetic responses of European oaks to drought and elevated daytime temperature

Oaks are commonly considered as drought- and heat-tolerant trees that might benefit from a warmer and drier climate. Their tolerance to drought has been frequently studied in the past, whereas studies dealing with elevated temperature or its combination with drought are very limited in number. In this study we investigated seasonal photosynthetic patterns in three European oak species (Quercus robur, Q. petraea, Q. pubescens) exposed in lysimeter-based open-top chambers (OTC) to elevated daytime temperature, drought and their combination. Stomatal and non-stomatal traits of photosynthesis were followed over an entire growing season and related to changes in daytime temperature, soil moisture and pre-dawn leaf water potential (Ψ(PD) ). Elevated daytime temperature enhanced net photosynthesis (P(N) ) in a season-dependent manner, with higher mid-summer rates than in controls exposed to ambient temperature. Drought imposed in early and mid-summer reduced the soil moisture content and caused a gradual decline in Ψ(PD) , stomatal conductance (g(S) ) and P(N) . Drought effects on Ψ(PD) and P(N) were exacerbated when drought was combined with elevated daytime temperature. In general, P(N) tended to be more affected by low soil moisture content or low Ψ(PD) in Q. robur than in Q. petraea and Q. pubescens. Non-stomatal limitations may have contributed to the drought-induced decline of P(N) in Q. robur, as indicated by a down-regulation of PSII photochemistry (F(V) /F(M) ) and decreased chlorophyll content. Taken together, our findings show that European oaks may benefit from elevated temperature, but detrimental effects can be expected when elevated temperature occurs simultaneously with drought.

Metal uptake and allocation in trees grown on contaminated land: implications for biomass production

Phytostabilization aims to reduce environmental and health risks arising from contaminated soil. To be economically attractive, plants used for phytostabilization should produce valuable biomass. This study investigated the biomass production and metal allocation to foliage and wood of willow (Salix viminalis L.), poplar (Populus monviso), birch (Betula pendula), and oak (Quercus robur) on five different soils contaminated with trace elements (TE), with varying high concentrations of Cu, Zn, Cd, and Pb as well as an uncontaminated control soil. In the treatment soils, the biomass was reduced in all species except oak. There was a significant negative correlation between biomass and foliar Cd and Zn concentrations, reaching up to 15 mg Cd kg(-1) and 2000 mg Zn kg(-1) in willow leaves. Lead was the only TE with higher wood than foliage concentrations. The highest Pb accumulation occurred in birch with up to 135 mg kg(-1) in wood and 78 mg kg(-1) in foliage. Birch could be suitable for phytostabilization of soils with high Cd and Zn but low Pb concentrations, while poplars and willows could be used to stabilise soils with high Cu and Pb and low Zn and Cd concentrations.

Nitrogen dynamics in oak model ecosystems subjected to air warming and drought on two different soils

Being tolerant to heat and drought, oaks are promising candidates for future forestry in view of climate change in Central Europe. Air warming is expected to increase, and drought decrease soil N availability and thus N supply to trees. Here, we conducted a model ecosystem experiment, in which mixed stands of young oaks (Quercus robur, Q. petraea and Q. pubescens) were grown on two different soils and subjected to four climate treatments during three growing seasons: air warming by 1-2 °C, drought periods (average precipitation reduction of 43-60%), a combination of these two treatments, and a control. In contrast to our hypotheses, neither air warming nor drought significantly affected N availability, whereas total amounts, vertical distribution and availability of soil N showed substantial differences between the two soils. While air warming had no effect on tree growth and N accumulation, the drought treatment reduced tree growth and increased, or tended to increase, N accumulation in the reduced biomass, indicating that growth was not limited by N. Furthermore, (15) N-labelling revealed that this accumulation was associated with an increased uptake of nitrate. On the basis of our results, climate change effects on N dynamics are expected to be less important in oak stands than reduced soil water availability.

Seasonal exposure to drought and air warming affects soil Collembola and mites

Global environmental changes affect not only the aboveground but also the belowground components of ecosystems. The effects of seasonal drought and air warming on the genus level richness of Collembola, and on the abundance and biomass of the community of Collembola and mites were studied in an acidic and a calcareous forest soil in a model oak-ecosystem experiment (the Querco experiment) at the Swiss Federal Research Institute WSL in Birmensdorf. The experiment included four climate treatments: control, drought with a 60% reduction in rainfall, air warming with a seasonal temperature increase of 1.4 °C, and air warming + drought. Soil water content was greatly reduced by drought. Soil surface temperature was slightly increased by both the air warming and the drought treatment. Soil mesofauna samples were taken at the end of the first experimental year. Drought was found to increase the abundance of the microarthropod fauna, but reduce the biomass of the community. The percentage of small mites (body length ≤ 0.20 mm) increased, but the percentage of large mites (body length >0.40 mm) decreased under drought. Air warming had only minor effects on the fauna. All climate treatments significantly reduced the richness of Collembola and the biomass of Collembola and mites in acidic soil, but not in calcareous soil. Drought appeared to have a negative impact on soil microarthropod fauna, but the effects of climate change on soil fauna may vary with the soil type.

Boron accumulation and toxicity in hybrid poplar (Populus nigra × euramericana)

Poplars accumulate high B concentrations and are thus used for the phytomanagement of B contaminated soils. Here, we performed pot experiments in which Populus nigra × euramericana were grown on a substrate with B concentrations ranging from 13 to 280 mg kg(-1) as H(3)BO(3). Salix viminalis, Brassica juncea, and Lupinus albus were grown under some growing conditions for comparison. Poplar growth was unaffected at soil B treatment levels up to 93 mg kg(-1). Growth was progressively reduced at levels of 168 and 280 mg kg(-1). None of the other species survived at these substrate B levels. At leaf B concentrations <900 mg kg(-1) only <10% of the poplar leaf area showed signs of toxicity. Neutron radiography revealed that chlorotic leaf tissues had B concentrations of 1000-2000 mg kg(-1), while necrotic tissues had >2000 mg kg(-1). Average B concentrations of up to 3500 mg kg(-1) were found in leaves, while spots within leaves had concentrations >7000 mg kg(-1), showing that B accumulation in leaf tissue continued even after the onset of necrosis. The B accumulation ability of P. nigra × euramericana is associated with B hypertolerance in the living tissue and storage of B in dead leaf tissue.

Provenance-specific growth responses to drought and air warming in three European oak species (Quercus robur, Q. petraea and Q. pubescens)

Provenance-specific growth responses to experimentally applied drought and air warming were studied in saplings of three European oak species: Quercus robur, Quercus petraea and Quercus pubescens. Four provenances of each species were grown in large open-top chambers and subjected to four climates: control, periodic drought, air warming or their combination in 3 subsequent years. Overall growth responses were found among species and provenances, with drought reducing shoot height growth and stem diameter growth and air warming stimulating shoot height growth but reducing stem diameter growth and root length growth. Differential growth responses in shoots, stems and roots resulted in altered allometric growth relations. Root length growth to shoot height growth increased in response to drought but decreased in response to air warming. Stem diameter growth to shoot height growth decreased in response to air warming. The growth responses in shoots and stems were highly variable among provenances indicating provenance-specific sensitivity to drought and air warming, but this response variability did not reflect local adaptation to climate conditions of provenance origin. Shoot height growth was found to be more sensitive to drought in provenances from northern latitudes than in provenances from southern latitudes, suggesting that genetic factors related to the postglacial immigration history of European oaks might have interfered with selective pressure at provenance origins.

Compartmentation of metals in foliage of Populus tremula grown on soils with mixed contamination. II. Zinc binding inside leaf cell organelles

The phytoextraction potential of plants for removing heavy metals from polluted soils is determined by their capacity to store contaminants in aboveground organs and complex them safely. In this study, the metal compartmentation, elemental composition of zinc deposits and zinc complexation within leaves from poplars grown on soil with mixed metal contamination was analysed combining several histochemical and microanalytical approaches. Zinc was the only heavy metal detected and was stored in several organelles in the form of globoid deposits showing β-metachromasy. It was associated to oxygen anions and different cations, noteworthy phosphorous. The deposit structure, elemental composition and element ratios indicated that zinc was chelated by phytic acid ligands. Maturation processes in vacuolar vs. cytoplasmic deposits were suggested by differences in size and amounts of complexed zinc. Hence, zinc complexation by phytate contributed to metal detoxification and accumulation in foliage but could not prevent toxicity reactions therein.

Compartmentation of metals in foliage of Populus tremula grown on soils with mixed contamination. I. From the tree crown to leaf cell level

In order to achieve efficient phytoextraction of heavy metals using trees, the metal allocation to aboveground tissues needs to be characterised. In his study, the distribution of heavy metals, macro- and micronutrients and the metal micro-localisation as a function of the leaf position and heavy metal treatment were analysed in poplars grown on soil with mixed metal contamination. Zinc was the most abundant contaminant in both soil and foliage and, together with cadmium, was preferentially accumulated in older foliage whereas excess copper and lead were not translocated. Changes in other element concentrations indicated an acceleration in aging as a consequence of the metal treatment. Excess zinc was irregularly accumulated inside leaf tissues, tended to saturate the veins and was more frequently stored in cell symplast than apoplast. Storage compartments including metabolically safe and sensitive subcellular sites resulted in sizable metal accumulation as well as stress reactions.

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.

Erratum to "Structural and physiological responses to ozone in Manna ash (Fraxinus ornus L.) leaves of seedlings and mature trees under controlled and ambient conditions"

Leaf-level microscopical symptom structure and physiological responses were investigated in seedlings experimentally exposed to ozone (O3) in indoor chambers (150 ppb, 8 hd(-1) per 7 weeks), and field trees of Manna ash (Fraxinus ornus) exposed to ambient O3 (max 93 ppb per one growing season). Ozone-induced leaf injury, including leaf reddening and stippling, was observed in both seedlings and mature trees, but the morphology of injury in the stipples differed, being hypersensitive-like (HR-like) in the chamber seedlings and accelerated cell senescence (ACS) in the field trees. In both exposure conditions, the main structural impact of O3 was on the mesophyll and especially the upper assimilating cell layers. The main physiological impact was on carbon assimilation and on stomatal sluggishness. These effects were not due to stomatal structural injury and were more severe in juvenile compared to mature trees because of environmental (water availability, light) and constitutional (gas exchange capacity) factors and differences in the cell physiology processes (HR-like vs. ACS) triggered by ozone stress. Given the plasticity of plant responses to ozone stress, dose/response relationships for tree seedlings in the indoor chambers cannot be extrapolated to mature trees unless ambient conditions are closely simulated.

Structural and physiological responses to ozone in Manna ash (Fraxinus ornus L.) leaves of seedlings and mature trees under controlled and ambient conditions

Leaf-level microscopical symptom structure and physiological responses were investigated in seedlings experimentally exposed to ozone (O3) in indoor chambers (150 ppb, 8 h d(-1)/7 weeks), and field trees of Manna ash (Fraxinus ornus) exposed to ambient O3 (max 93 ppb/one growing season). Ozone-induced leaf injury, including leaf reddening and stippling, was observed in both seedlings and mature trees, but the morphology of injury in the stipples differed, being hypersensitive-like (HR-like) in the chamber seedlings and accelerated cell senescence (ACS) in the field trees. In both exposure conditions, the main structural impact of O3 was on the mesophyll and especially the upper assimilating cell layers. The main physiological impact was on carbon assimilation and on stomatal sluggishness. These effects were not due to stomatal structural injury and were more severe in juvenile compared to mature trees because of environmental (water availability, light) and constitutional (gas exchange capacity) factors and differences in the cell physiology processes (HR-like vs. ACS) triggered by ozone stress. Given the plasticity of plant responses to ozone stress, dose/response relationships for tree seedlings in the indoor chambers cannot be extrapolated to mature trees unless ambient conditions are closely simulated.

Heavy metal accumulation and phytostabilisation potential of tree fine roots in a contaminated soil

Root systems of Norway spruce (Picea abies) and poplar (Populus tremula) were long-term exposed to metal-contaminated soils in open-top chambers to investigate the accumulation of the heavy metals in the fine roots and to assess the plants suitability for phytostabilisation. The heavy metals from the contaminated soil accumulated in the fine roots about 10-20 times more than in the controls. The capacity to bind heavy metals already reached its maximum after the first vegetation period. Fine roots of spruce tend to accumulate more heavy metals than poplar. Copper and Zinc were mainly detected in the cell walls with larger values in the epidermis than in the cortex. The heavy metals accumulated in the fine roots made up 0.03-0.2% of the total amount in the soils. We conclude that tree fine roots adapt well to conditions with heavy metal contamination, but their phytostabilisation capabilities seem to be very low.

Leaf responsiveness of Populus tremula and Salix viminalis to soil contaminated with heavy metals and acidic rainwater

Fast-growing trees such as Salix viminalis L. and Populus tremula L. are well suited to phytoremediate heavy metal contaminated soils. However, information on tree performance, particularly leaf function, under conditions of heavy metal contamination is scarce. We used yearly coppiced saplings of S. viminalis and P. tremula growing in model ecosytems to test four hypotheses: (1) heavy metal contamination impairs photosynthesis by injuring leaf structure; (2) the effects of heavy metal contamination are enhanced by acidified rainwater and low soil pH; (3) heavy metal contamination increases dark respiration and, thus, repair processes; and (4) heavy metal contamination is tolerated and remediated better by S. viminalis than by P. tremula. We investigated heavy metal accumulation, tissue injury and gas exchange in leaves of plants subjected to controlled soil contamination with heavy metal dust. Additional treatments included acidic and calcareous natural forest subsoils in combination with irrigation with rainwater at pH 5.5 or 3.5. In both provenances of P. tremula that were studied, but not in S. viminalis, heavy metal treatment reduced photosynthesis and transpiration by varying amounts, except in the hot and dry summer of 2003, but had no effect on dark respiration. At light saturation, net CO(2) uptake and water-use efficiency were reduced by heavy metal contamination, whereas the CO(2) concentration in the leaf intercellular air space was increased. Rainwater pH and subsoil pH only slightly modified the effects of the heavy metal treatment on P. tremula. Gas exchange responses of P. tremula to heavy metals were attributed to leaf structural and ultrastructural changes resulting from hypersensitive-response-like processes and accelerated mesophyll cell senescence and necroses in the lower epidermis, especially along the transport pathways of heavy metals in the leaf lamina. Overall, the effects of heavy metals on P. tremula corroborated Hypothesis 1, but refuted Hypotheses 2 and 3, and were inconclusive for Hypothesis 4. Both P. tremula and S. viminalis showed appreciable potential for storing heavy metals in aging foliage.

Linking stress with macroscopic and microscopic leaf response in trees: new diagnostic perspectives

Visible symptoms in tree foliage can be used for stress diagnosis once validated with microscopical analyses. This paper reviews and illustrates macroscopical and microscopical markers of stress with a biotic (bacteria, fungi, insects) or abiotic (frost, drought, mineral deficiency, heavy metal pollution in the soil, acidic deposition and ozone) origin helpful for the validation of symptoms in broadleaved and conifer trees. Differentiation of changes in the leaf or needle physiology, through ageing, senescence, accelerated cell senescence, programmed cell death and oxidative stress, provides additional clues raising diagnosis efficiency, especially in combination with information about the target of the stress agent at the tree, leaf/needle, tissue, cell and ultrastructural level. Given the increasing stress in a changing environment, this review discusses how integrated diagnostic approaches lead to better causal analysis to be applied for specific monitoring of stress factors affecting forest ecosystems.

Effects of metal-contaminated soil on the performance of young trees growing in model ecosystems under field conditions

Young Populus tremula, Salix viminalis, Betula pendula and Picea abies trees were grown together in large open-top chambers. The treatments were: without or with (Cu/Zn/Cd/Pb=640/3000/10/90 mg kg-1) metal contamination in the topsoil, irrigation pH 3.5 or 5.5, and acidic or calcareous subsoil. Growth, metal allocation to foliage and wood, as well as leaf gas exchange were measured. Biomass was reduced in P. tremula and B. pendula by the metal-contaminated topsoil relative to uncontaminated topsoil, whereas in P. tremula photosynthesis and transpiration were decreased. These effects were related to the elevated foliar Zn accumulation in P. tremula. S. viminalis showed a significant reduction in growth and an increased Zn and Cd accumulation on acidic vs. calcareous subsoil. Acidic irrigation produced only a few significant effects. P. abies showed the lowest metal uptake and no growth response to metal contamination.

Diagnosis of abiotic and biotic stress factors using the visible symptoms in foliage

Visible symptoms in the foliage of trees are recorded to monitor the effects of abiotic and biotic stress. Difficulties are reported in diagnosing the origin of stress. The present paper discusses several diagnostic criteria which are usable in different species for a better determination of the stress factor type. A new diagnosis scheme to differentiate between classes of abiotic and biotic stress factors is supplied. Abiotic stress generates gradients of symptoms. The symptom specificity is determined by the degree of interaction between the stress factor and plant defense system. Symptoms caused by abiotic stress and natural autumnal senescence can be morphologically different or undistinguishable according to the stress and plant species. With biotic stress, the class of parasitic is generally recognizable on the basis of the visible symptoms. Structurally and physiologically based explanations of the symptom morphology are still missing for many stress factors.

Diagnosis of abiotic and biotic stress factors using the visible symptoms in foliage

Visible symptoms in the foliage of trees are recorded to monitor the effects of abiotic and biotic stress. Difficulties are reported in diagnosing the origin of stress. The present paper discusses several diagnostic criteria which are usable in different species for a better determination of the stress factor type. A new diagnosis scheme to differentiate between classes of abiotic and biotic stress factors is supplied. Abiotic stress generates gradients of symptoms. The symptom specificity is determined by the degree of interaction between the stress factor and plant defense system. Symptoms caused by abiotic stress and natural autumnal senescence can be morphologically different or undistinguishable according to the stress and plant species. With biotic stress, the class of parasitic is generally recognizable on the basis of the visible symptoms. Structurally and physiologically based explanations of the symptom morphology are still missing for many stress factors.

Validation of leaf ozone symptoms in natural vegetation using microscopical methods

Ozone injury to natural vegetation is being increasingly surveyed throughout the northern hemisphere. There exists a growing list of species showing visible 'ozone-like' symptoms which needs to be validated. This study presents the results from a test survey of ozone injury to forest vegetation in the light exposed sites of five Swiss level II plots, for the new ICP-Forests protocol. With AOT40 from 14 to 28 ppm-h in 2000, ten out of 49 woody plant species displayed typical symptoms, and four showed untypical symptoms. Symptom origin was investigated in nine and validated in seven species, using morphological, histological and cellular markers of oxidative stress and ozone-induced plant response. Independent of taxonomic position, ozone effects were characterized by the induction of oxidative stress in the mesophyll resulting in discrete and light-dependent hypersensitive-like responses and in accelerated cell senescence. The presented combination of cellular and morphological markers allows differential diagnosis of visible ozone injury.

Stable isotope analysis reveals differential effects of soil nitrogen and nitrogen dioxide on the water use efficiency in hybrid poplar leaves

•  The effects of nitrogen dioxide (NO₂ ) exposure are reported on the physiology, morphology and carbon partitioning of hybrid poplar clone cuttings (Populus ×euramericana) grown under high and low soil nitrogen supply. •  Plants were exposed to filtered air or NO₂ -enriched air (80-135 nl l^-1 ) over 12 wk growth in phytotrons. Stable isotope analysis, combined with CO₂ and H₂ O gas exchange measurements, biomass analysis and morphological development, was used to assess the integrated long-term effects of NO₂ . •  NO₂ had no toxic effects. A reduced ¹⁵ N-isotope ratio indicated incorporation of NO₂ while nitrate reductase activity in leaves was stimulated. The two nitrogen sources had differential effects on water use efficiency (WUE): NO₂ exposure increased long-term WUE; soil N supply decreased WUE; a result not detectable using growth and short-term gas exchange experiments. Plants benefited from airborne NO₂ , increasing CO₂ assimilation rate and biomass; both N sources increased shoot production at the expense of root growth. NO₂ exposure induced leaf formation with reduced stomatal density and increased leaf area. •  NO₂ exposure might be beneficial although the reduced root: shoot biomass could have a detrimental effect on nutrient balance and drought resistance.

Visible and microscopic injury in leaves of five deciduous tree species related to current critical ozone levels

Because the current critical level of ozone (O(3)) for forest trees is based only on one species, the responses of five deciduous tree species were differentiated in a climate chamber experiment. The number of symptomatic leaves per tree was significantly increased, and stomatal conductance was decreased under 50% ambient+30 nl l(-1) O(3) as compared to 'normal' senescence at 50% ambient [O(3)]. Species with a high stomatal conductance did not show earlier or more leaf injury symptoms. The additional 30 nl l(-1) O(3) induced specific pectinaceous cell wall protrusions, phenolic cell wall incrustations, tonoplast vesicles, and inhomogeneous, condensed/precipitated phenolic material in the vacuoles. Due to added O(3), cell senescence was accelerated with increased electron-density of the cytoplasm, and initial chloroplast degeneration. The slow degeneration process started in mesophyll cells, and expanded into epidermal and finally guard cells. Because of the large variance in biomass between individuals and species, the current critical level is supported by the assessment of visible leaf symptoms rather than growth reduction.

Effects of elevated CO2 and soil quality on leaf gas exchange and above-ground growth in beech-spruce model ecosystems

Responses of leaf gas exchange and above-ground growth of beech (Fagus sylvatica L.) and Norway spruce (Picea abies Karst.) to atmospheric CO₂ enrichment (374 μl l^-1 vs. 590 μl l^-1 ) and increased wet deposition of N (5 vs. 50 kg N ha^-1 a^-1 ) in combination with two natural forest soil types ('acidic' and 'calcareous') were studied in large open-top chambers. Eight juvenile beech and spruce trees from different provenances, together with a ground cover composed of five understorey species, were established in each of 32 model ecosystems. Both beech and spruce showed sustained enhancement of photosynthesis in response to atmospheric CO₂ enrichment during the first 2 yr of treatment. Nevertheless, switching measurement CO₂ concentrations revealed partial downward adjustment of photosynthesis in trees grown in elevated CO₂ , beech generally showing more pronounced downward adjustment than spruce. The responsiveness of photosynthesis to CO₂ enrichment did not vary significantly among trees from different provenances. Stomatal conductance was reduced under elevated CO₂ in both tree species. In spruce, the radial growth of the main stem and the annual production of wood (shoot-wood dry mass of current-year lateral shoots), needle dry mass, and assimilation area per tree were stimulated both by CO₂ enrichment and increased N deposition, but were not significantly affected by soil type by year 2. In contrast, in beech, the radial growth of the stem and the total leaf number, foliage dry mass, and assimilation area per tree were all not significantly affected by elevated CO₂ and increased N deposition when responses of the two soil types were pooled, but were greater on calcareous than on acidic soil by year 2. However, CO₂ interacted with soil type in beech: irrespective of the N deposition rate, saplings showed growth stimulation on the calcareous soil but responded negatively to CO₂ enrichment on the acidic soil (where growth was slower). Our results suggest that complex interactions between CO₂ , species and soil quality need to be accounted for when attempting to predict forest development in a future CO₂ -rich world.

Effect of fertilization on ozone-induced changes in the metabolism of birch (Betula pendula) leaves

Cloned cuttings of Betula pendula Roth were grown in field fumigation chambers at Birmensdorf throughout one growing season in filtered air with either < 3 (control) or 90/40 nl l^-1 O₃ (day/night; ozone generated from pure oxygen). Each ozone regime was split into high and low soil nutrient regimes by watering plants with either a 0.05 % or a 0.005% solution of a fertilizer which contained macronutrients and micronutrients. Fertilization had a strong effect on plant growth, enzyme activities and the expression of ozone-induced effects at the biochemical level. The activities of PEPC and Rubisco were enhanced about threefold in the plants with high fertilization (HF). Significant effects of ozone were in most cases found only in the older leaves of the plants with low fertilization (LF), There, sucrose, glucose and fructose levels were enhanced. In both fertilization treatments, the number of starch granules along the minor veins was increased. These ozone effects point to a decreased or inhibited phloem loading. The increased PEPC activity and the enhanced malate levels in the ozone-exposed plants might be the result of a redirection of carbon flow from sucrose synthesis and translocation towards anapleurotic processes, which can feed detoxification and repair of ozone injury as indicated by enhanced respiration. These findings agree well with the observed effects of ozone in lowering the root: shoot biomass ratio. Although there was a marked reduction in the O₃ /LF plants, O₃ /HF plants showed no significant response. Inositol was decreased under ozone exposure in both fertilizer treatments, contrasting with the pattern for carbohydrates. These results demonstrate the role of fertilization as an important modifier of ozone-induced effects at the plant biochemical level. Well fertilized plants appear to cope better with the impact of ozone on metabolism.

The effects of ozone and nutrient supply on stomatal response in birch (Betula pendula) leaves as determined by digital image-analysis and X-ray microanalysis

Cuttings of Betula pendula Roth were grown in field fumigation chambers throughout one growing season in filtered air with < 3 nl l^-1 O₃ (control; C) or day/night = 90/40 nl 1^-1 O₃ (ozone fumigation: O₃ ). Plants were watered with either low (0·005 %: LF) or high-concentrated (0·05%: HF) fertilizer solution. Discs between second-order veins in the central portion of the leaves were excised and immediately cryofixed in liquid nitrogen for low-temperature scanning electron-microscopy (LTSEM) at 1000 hours and 1400 hours. Stomatal width, area and density were measured by digital image-analysis. X-ray counts of potassium (K) and calcium (Ca) ions were determined by means of energy-dispersive X-ray microanalysis in guard and subsidiary cells. Accurate and fast measurements of stomatal apertures by image analysis were possible in birch leaves, because the darkness of the stomatal pore contrasts with the brightness of the guard cells and the cuticular ledges. Regression analysis showed a close relationship between the stomatal width and the pore area (r= 0·938, P < 0·01). At all harvest times, the stomatal pores were significantly narrowed in the high fertilization control treatment (O₃ /HF vs. O₃ /LF), and in the ozone treatment at 1400 hours (O₃ /HF vs. C/LF). In addition to this fertilization effect, ozone had also narrowed the stomatal pores (O₃ /HF vs. C/HF at 1400 hours, O₃ /LF vs. C/LF at all harvest times). In addition to these functional effects, morphological effects (individual leaf area, stomatal density) were determined. Single-leaf area was increased by high fertilization, with a tendency to decrease with O₃ fumigation. The stomatal density in intercostal fields was increased by O₃ but decreased by high fertilisation. Stomatal widening was accompanied by increased K count rates in the guard cells, in contrast to constant K values in the subsidiary cells, irrespective of the fumigation or fertilization regimes. Calcium counts in the guard cells were similar to those in the subsidiary cells, and were independent of the aperture width. In samples with established ozone injury, the K/Ca ratio in collapsed guard cells increased compared with turgid guard cells irrespective of the pore aperture. Collapsed subsidiary cells only differed from turgid subsidiary cells when the guard cells had also collapsed and thus closed the pore.

Nighttime exposure to ozone reduces whole-plant production in Betula pendula

For 20 weeks during the growing season, cuttings of one birch clone (Betula pendula Roth.) were exposed in the Birmensdorf fumigation chambers to O(3)-free air (control) or 75 nl O(3) l(-1). Ozone was supplied either from 1900 until 0700 h (nighttime regime), from 0700 until 1900 h (daylight regime), or all day (24-h regime). By autumn, reductions in whole-plant biomass production, root/shoot biomass and stem weight/length ratios were evident in all three O(3) regimes. The reductions in cuttings receiving the 24-h O(3) treatment were about twofold larger than in cuttings receiving the daylight O(3) treatment. Stomata were open at night, and stomatal conductance was about 50% of its maximum daytime value. We calculated that the rate of O(3) uptake into leaves in the dark approached 4 nmol m(-2) s(-1). Whole-plant production and carbon allocation were more sensitive to O(3) during the night than during the day; however, O(3) exposure caused similar visible leaf injury in both of the 12-h regimes, although the leaves exposed to O(3) at night exhibited delayed O(3)-induced shedding. Overall, changes in production and carbon allocation were determined by the external O(3) dose rather than by the kind of O(3) exposure, indicating that, at the seasonal scale, the internal dose of ozone that was physiologically effective was a constant fraction of the external O(3) dose. We conclude that nighttime O(3) exposures should be included in the daily time period for determining critical concentrations of O(3) causing injury in trees.

Beech foliage as a bioindicator of pollution near a waste incinerator

Since 1971 unshaded leaves from the top of marked beech trees (Fagus sylvatica L.) in the vicinity of a regional waste incinerator have been sampled every year in early September. The unwashed leaf samples were analyzed for the concentration of Cl- and, in some years, for 16 other elements. The operation of the waste incinerator distinctly increased the Cl- concentration in the foliage. When the flue gas filtration did not work properly, several other elements also accumulated (without any obvious dust accumulation). There were no significant correlations between precipitation and concentration of water-soluble elements in foliage samples. This suggested that precipitation was not accelerating foliar leaching so that the bioindication of pollutant accumulation is not restricted in foliage with a well developed cuticula.

Whole-plant growth and leaf formation in ozonated hybrid poplar (Populus x euramericana)

Seasonal growth was studied in potted cuttings of hybrid poplar (one clone of Populus x euramericana) either exposed to ozone in filtered air (0 = control, 0.05, 0.10 microl litre(-1)) or in ambient air (mean = 0.03 microl litre(-1)). Only at 0.10 microl litre(-1) was biomass production reduced and related to leaf loss rather than leaf formation, since the latter was similar in all treatments. Stem length at 0.10 microl litre(-1) approached that of the control, whereas starch concentration in the green stem bark tended to be reduced, as were the ratios of stem weight/length and root/shoot biomass. The changes in carbon allocation and biomass production gradually became established during the second half of the growing season. At the altered carbon allocation at 0.10 microl litre(-1), the ratio of whole-plant production/attached foliage area resembled that of the other O(3) regimes. However, the latter ratio was strongly reduced at 0.10 microl litre(-1) when calculated on the basis of the potential foliage area, as compensated for the O(3)-induced leaf loss. Thus the carbon return/cost balance of the totally formed foliage was low, although the relative-growth rate of ozonated plants temporarily reached that of the control. The relation between leaf differentiation under ozonation (lowered stomatal density) and whole-plant production remains uncertain. The plant behavior found is discussed in terms of passive response or acclimatization to O(3) stress.