Long-term ozone effects on vegetation, microbial community and methane dynamics of boreal peatland microcosms in open-field conditions

To study the effects of elevated ozone concentration on methane dynamics and a sedge species, Eriophorum vaginatum, we exposed peatland microcosms, isolated by coring from an oligotrophic pine fen, to double ambient ozone concentration in an open‐air ozone exposure field for four growing seasons. The field consists of eight circular plots of which four were fumigated with elevated ozone concentration and four were ambient controls. At the latter part of the first growing season (week 33, 2003), the methane emission was 159±14 mg CH₄ m^?2 day^?1 (mean±SE) in the ozone treatment and 214±8 mg CH₄ m^?2 day^?1 under the ambient control. However, towards the end of the experiment the ozone treatment slightly, but consistently, enhanced the methane emission. At the end of the third growing season (2005), microbial biomass (estimated by phospholipid fatty acid biomarkers) was higher in peat exposed to ozone (1975±108 nmol g^?1 dw) than in peat of the control microcosms (1589±115 nmol g^?1 dw). The concentrations of organic acids in peat pore water showed a similar trend. Elevated ozone did not affect the shoot length or the structure of the sedge E. vaginatum leaves but it slightly increased the total number of sedge leaves towards the end of the experiment. Our results indicate that elevated ozone concentration enhances the general growth conditions of microbes in peat by increasing their substrate availability. However, the methane production did not reflect the increase in the concentration of organic acids, probably because hydrogenotrophic methane production dominated in the peat studied. Although, we used isolated peatland microcosms with limited size as study material, we did not find experimental factors that could have hampered the basic conclusions on the effects of ozone.     

Growth and reproduction of aphids and levels of free amino acids in Scots pine and Norway spruce in an open-air fumigation with ozone

To estimate the susceptibility of conifer seedlings to aphids under future tropospheric ozone levels, Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) seedlings were exposed to ambient and elevated ozone levels in an open-air exposure system. Growth and reproduction of the aphids Schizolachnus pineti and Cinara pinea on Scots pine and Cinara pilicornis on Norway spruce were monitored. Levels of free amino acids in foliage and young shoots were used as indicators of host plant quality. In elevated treatment plots the ozone doses were between 1.2 and 1.7 times the dose in ambient plots in 1990–93. Half of the seedling material in 1992–93 was subjected to nitrogen fertilization treatment to evaluate the effects of increased N deposition. In 1990, population density of S. pineti on pine did not differ between ambient and elevated ozone treatments during growing season, but remained higher in the elevated ozone plot than in the ambient plot at the end of the growing season. This was associated with elevated levels of glutamic acid in foliage. In August 1992, the numbers of S. pineti were consistent between the two ambient ozone plots, but deviated highly between the two ozone-fumigated plots. Glycine concentration in pine foliage was elevated by ozone, but free amino acid concentrations were not related to aphid performance. In 1993, ozone and nitrogen did not significantly affect the relative growth rate (RGR) of S. pineti or C. pinea nymphs on Scots pine, but glutamic acid concentration in foliage was increased by nitrogen fertilization. On Norway spruce, fecundity of C. pilicornis females was higher in elevated ozone treatment, but RGR of nymphs was not affected in 1992. In 1993, RGR of C. pilicornis nymphs was increased by nitrogen fertilization in June, but not affected by ozone. Nitrogen fertilization increased the levels of total free amino acids, aspartic acid, glutamic acid and proline in elongating shoots of Norway spruce, and ozone reduced the concentrations of valine and γ-butyric acid. Our results suggest that availability of nitrogen from soil has a stronger impact on the concentrations of free amino acids in conifer seedlings than ozone. Some episodes of high ozone concentration may increase free amino acids in foliage. Aphid response to ozone was extremely variable, in agreement with previous laboratory experiments. The expected 20–70% increase in ambient concentrations of tropospheric ozone may in some occasions enhance aphid performance on Scots pine and Norway spruce seedlings, but in most cases the ozone effect on the susceptibility of conifer seedlings to sucking insect pests is not important.     

Severe photobiont injuries of lichens are strongly associated with air pollution

The photobiont ultrastructure of the epiphytic lichens Bryoria fuscescens and Bryoria fremontii was studied along the pollution gradient from two Cu-Ni smelters in Nikel and Monchegorsk in northern Finland and north-western Russia. The relationship between ultrastructural characteristics of B. fuscescens and environmental factors (i.e. climate, atmospheric SO₂ and bark element concentrations) was studied by using a principal component analysis (PCA) aiming to assess the air quality in a northern environment. Based on PCA, increased plasmolysis and mitochondrial changes in the Trebouxia photobiont were significantly correlated with elevated pollutant concentrations. Degenerated cells, showing altered chloroplasts and electron-translucent pyrenoglobuli, occurred in lichens growing 35–50 km from the Monchegorsk smelter. Cell wall and cytoplasmic lipid volumes, and size of pyrenoglobuli, positively correlated with the distance from the Monchegorsk smelter. Vacuoles and electron-opaque vacuolar deposits were significantly increased at the Finnish site in the vicinity of a pulp mill. Swelling of mitochondrial cristae and thylakoids showed little correlation with environmental factors, but indicated of initial stage of injuries and were observed at several slightly polluted sites in northern Finland and north-western Russia. The results suggest that the severe photobiont injuries of lichens are strongly associated with poor air quality.     

Plant-mediated effects of elevated ultraviolet-B radiation on peat microbial communities of a subarctic mire

Elevated ultraviolet‐B (UVB) radiation has been reported to have few effects on plants but to alter the soil microbial community composition. However, the effects on soil microorganisms have to be mediated via plants, because direct radiation effects are only plausible on the uppermost millimeters of soil. Here, we assessed secondary effects of UVB on soil microbes. The responses in the dominant plant Eriophorum russeolum, peat pore water and microbial communities in the peat were recorded at a subarctic mire in the middle of the third growing season under field exposure simulating 20% depletion in the ozone layer. The UVB treatment significantly reduced the sucrose and the total soluble sugar (sucrose+glucose+fructose) concentration of the plant leaves while increasing the sucrose concentration in the belowground storage organ rhizome. The starch concentration of the leaves was also slightly reduced by elevated UVB. In the plant roots, carbohydrate concentrations remained unaffected but the total phenolics concentration increased under elevated UVB. We suggest that the simultaneously observed decrease in bacterial growth rate and the altered bacterial community composition are due to UVB‐induced changes in the plant photosynthate allocation and potential changes in root exudation. There were no effects of elevated UVB on microbial biomass, peat pore water or nutrient concentrations in the peat. The observed responses are in line with the previously reported lower ecosystem dark respiration under elevated UVB, and they signify that the changed plant tissue quality and lower bacterial activity are likely to reduce decomposition.     

Do elevated atmospheric CO2 and O3 affect food quality and performance of folivorous insects on silver birch?

The individual and combined effects of elevated CO₂ and O₃ on the foliar chemistry of silver birch (Betula pendula Roth) and on the performance of five potential birch‐defoliating insect herbivore species (two geometrid moths, one lymantrid moth and two weevils) were examined. Elevated CO₂ decreased the water concentration in both short‐ and long‐shoot leaves, but the effect of CO₂ on the concentration of nitrogen and individual phenolic compounds was mediated by O₃ treatment, tree genotype and leaf type. Elevated O₃ increased the total carbon concentration only in short‐shoot leaves. Bioassays showed that elevated CO₂ increased the food consumption rate of juvenile Epirrita autumnata and Rheumaptera hastata larvae fed with short‐ and long‐shoot leaves in spring and mid‐summer, respectively, but had no effect on the growth of larvae. The contribution of leaf quality variables to the observed CO₂ effects indicate that insect compensatory consumption may be related to leaf age. Elevated CO₂ increased the food preference of only two tested species: Phyllobius argentatus (CO₂ alone) and R. hastata (CO₂ combined with O₃). The observed stimulus was dependent on tree genotype and the measured leaf quality variables explained only a portion of the stimulus. Elevated O₃ decreased the growth of flush‐feeding young E. autumnata larvae, irrespective of CO₂ concentration, apparently via reductions in general food quality. Therefore, the increasing tropospheric O₃ concentration could pose a health risk for juvenile early‐season birch folivores in future. In conclusion, the effects of elevated O₃ were found to be detrimental to the performance of early‐season insect herbivores in birch whereas elevated CO₂ had only minor effects on insect performance despite changes in food quality related foliar chemistry.