Risk assessments for forest trees: the performance of the ozone flux versus the AOT concepts

Published ozone exposure-response relationships from experimental studies with young trees performed at different sites across Europe were re-analysed in order to test the performance of ozone exposure indices based on AOTX (Accumulated exposure Over a Threshold of X nmol mol(-1)) and AF(st)Y (Accumulated Stomatal Flux above a threshold of Y nmol m(-2) s(-1)). AF(st)1.6 was superior, as compared to AOT40, for explaining biomass reductions, when ozone sensitive species with differing leaf morphology were included in the analysis, while this was not the case for less sensitive species. A re-analysis of data with young black cherry trees, subject to different irrigation regimes, indicated that leaf visible injuries were more strongly related to the estimated stomatal ozone uptake, as compared to the ozone concentration in the air. Experimental data with different clones of silver birch indicated that leaf thickness was also an important factor influencing the development of ozone induced leaf visible injury.     

Comparison of calculated and measured foliar O3 flux in crop and forest species

We designed a new gas exchange system that concurrently measures foliar H2O, O3, and CO2 flux (HOC flux system) while delivering known O3 concentrations. Stomatal responses of three species were tested: snapbean, and seedlings of California black oak (deciduous broadleaf) and blue oak (evergreen broadleaf). Acute O3 exposure (120-250 ppb over an hour) was applied under moderate light and low vapor pressure deficits during near steady state conditions. The rate of stomatal closure was measured when the whole plant was placed in the dark. An adjacent leaf on each plant was also concurrently measured in an O3-free cuvette. Under some conditions, direct measurements and calculated foliar O3 flux were within the same order of magnitude; however, endogenously low gs or O3 exposure-induced depression of gs resulted in an overestimation of calculated O3 fluxes compared with measured O3 fluxes. Sluggish stomata in response to light extinction with concurrent O3 exposure, and incomplete stomatal closure likewise underestimated measured O3 flux.     

Measurement of the dry deposition flux of NH3 on to coniferous forest

The dry deposition flux of NH3 to coniferous forest was determined by the micrometeorological gradient method using a 36 m high tower. Aerodynamic characteristics of the site were studied, using a second tower erected in the forest 100 m from the first. Fluxes and gradients of heat and momentum measured on both towers indicated a fairly homogeneous turbulent flow field over the studied area of the forest. Site specific flux profile functions for heat were derived from continuous measurements of turbulent fluxes and gradients. These functions were used to derive fluxes from the observed gradients of NH3. In total, eighty 90-min NH3 flux runs were performed. The results indicate a strong nonstomatal uptake of NH3 by the forest. A representative dry deposition velocity for NH3 of 3.6 cm(-1) s was derived. The annual average flux was roughly estimated to be equivalent to 50 kg N ha(-1) yr, significantly higher than the critical load for coniferous forest.     

The challenge of making ozone risk assessment for forest trees more mechanistic

Upcoming decades will experience increasing atmospheric CO₂ and likely enhanced O₃ exposure which represents a risk for the carbon sink strength of forests, so that the need for cause-effect related O₃ risk assessment increases. Although assessment will gain in reliability on an O₃ uptake basis, risk is co-determined by the effective dose, i.e. the plant's sensitivity per O₃ uptake. Recent progress in research on the molecular and metabolic control of the effective O₃ dose is reported along with advances in empirically assessing O₃ uptake at the whole-tree and stand level. Knowledge on both O₃ uptake and effective dose (measures of stress avoidance and tolerance, respectively) needs to be understood mechanistically and linked as a pre-requisite before practical use of process-based O₃ risk assessment can be implemented. To this end, perspectives are derived for validating and promoting new O₃ flux-based modelling tools.     

Modeling speciated terpenoid emissions from the European boreal forest

We present the first estimates of speciated monoterpene emissions from the North European coniferous forests. Measured emission factors and emission profiles of boreal tree species (Picea abies, Pinus sylvestris, Betula pendula, Salix phylicifolia, Populus tremula, and Alnus incana) were used together with detailed satellite land cover information and meteorological data in an emission model based on the Guenther emission algorithms. The variation of the coniferous biomass within the boreal region (60°N to 70°N) was obtained from forest inventory data, and the seasonal variability of the deciduous biomass was taken into account through simple boreal climatology parameterisation. The annual biogenic emissions in the boreal zone are dominated by coniferous species, but in the summer months, the deciduous contribution to the monoterpene and isoprene emissions is considerable. Norway spruce (Picea abies) is the most important isoprene emitter in the north European boreal forests. The biogenic emission fluxes in the South boreal zone are approximately twice as high as fluxes in the North boreal zone. α- and β-pinene, carene, and cineole are the most abundant emitted terpenes, with a strong contribution of isoprene and linalool during the summer months.     

Validation of the stomatal flux approach for the assessment of ozone visible injury in young forest trees. Results from the TOP (transboundary ozone pollution) experiment at Curno, Italy

This paper summarises some of the main results of a two-year experiment carried out in an Open-Top Chambers facility in Northern Italy. Seedlings of Populus nigra, Fagus sylvatica, Quercus robur and Fraxinus excelsior have been subjected to different ozone treatments (charcoal-filtered and non-filtered air) and soil moisture regimes (irrigated and non-irrigated plots). Stomatal conductance models were applied and parameterised under South Alpine environmental conditions and stomatal ozone fluxes have been calculated.The flux-based approach provided a better performance than AOT40 in predicting the onset of foliar visible injuries. Critical flux levels, related to visible leaf injury, are proposed for P. nigra and F. sylvatica (ranging between 30 and 33 mmol O₃ m⁻²). Soil water stress delayed visible injury appearance and development by limiting ozone uptake. Data from charcoal-filtered treatments suggest the existence of an hourly flux threshold, below which may occur a complete ozone detoxification.     

Using plant biomonitors and flux modelling to develop O3 dose-response relationships in Catalonia

We used tobacco Bel-W3 biomonitoring data and ozone flux modelling (WINDEP model) with the aim of developing the absorbed dose-response relationship, and comparing this approach with the most commonly used AOT40 (the sum of hourly ozone concentrations above a cut-off of 40 ppb during daylight hours, when global radiation exceeds 50 W m(-2)) in the estimation of exposure-damage curves. Leaf damage values were more related to OAD(15 days, potential) (potential ozone absorbed dose calculated over 15 consecutive days) than to AOT40 in all the studied stations. An OAD(15 days, potential) of 180 mg m(-2) was found to be the threshold for damage to the most sensitive species in this region under well watered conditions. The results show the applicability of the flux approach for risk assessment at the local scale, the improvement of the ozone damage estimation when the potential absorbed dose is modelled and used instead of just the ozone exposure, and finally, the possibilities opened by the use of biomonitoring networks.     

Assessing the risk caused by ground level ozone to European forest trees: a case study in pine, beech and oak across different climate regions

Two different indices have been proposed for estimation of the risk caused to forest trees across Europe by ground-level ozone, (i) the concentration based AOT40 index (Accumulated Over a Threshold of 40 ppb) and (ii) the recently developed flux based AFstY index (Accumulated stomatal Flux above a flux threshold Y). This paper compares the AOT40 and AFstY indices for three forest trees species at different locations in Europe. The AFstY index is estimated using the DO(3)SE (Deposition of Ozone and Stomatal Exchange) model parameterized for Scots pine (Pinus sylvestris), beech (Fagus sylvatica) and holm oak (Quercus ilex). The results show a large difference in the perceived O(3) risk when using AOT40 and AFstY indices both between species and regions. The AOT40 index shows a strong north-south gradient across Europe, whereas there is little difference between regions in the modelled values of AFstY. There are significant differences in modelled AFstY between species, which are predominantly determined by differences in the timing and length of the growing season, the periods during which soil moisture deficit limits stomatal conductance, and adaptation to soil moisture stress. This emphasizes the importance of defining species-specific flux response variables to obtain a more accurate quantification of O(3) risk.     

Energy performance of European countries by considering the role of forest

Environmental Science and Pollution Research - This paper applies the modified undesirable dynamic data envelopment analysis (DEA) model by considering the role of forest carbon sinks to evaluate...     

Ozone risk assessment for agricultural crops in Europe: Further development of stomatal flux and flux–response relationships for European wheat and potato

Applications of a parameterised Jarvis-type multiplicative stomatal conductance model with data collated from open-top chamber experiments on field grown wheat and potato were used to derive relationships between relative yield and stomatal ozone uptake. The relationships were based on thirteen experiments from four European countries for wheat and seven experiments from four European countries for potato. The parameterisation of the conductance model was based both on an extensive literature review and primary data. Application of the stomatal conductance models to the open-top chamber experiments resulted in improved linear regressions between relative yield and ozone uptake compared to earlier stomatal conductance models, both for wheat (r²=0.83) and potato (r²=0.76). The improvement was largest for potato. The relationships with the highest correlation were obtained using a stomatal ozone flux threshold. For both wheat and potato the best performing exposure index was AF_st6 (accumulated stomatal flux of ozone above a flux rate threshold of 6 nmol ozone m⁻² projected sunlit leaf area, based on hourly values of ozone flux). The results demonstrate that flux-based models are now sufficiently well calibrated to be used with confidence to predict the effects of ozone on yield loss of major arable crops across Europe. Further studies, using innovations in stomatal conductance modelling and plant exposure experimentation, are needed if these models are to be further improved.     

Environmental control of stomatal conductance in forest trees of the Great Smoky Mountains National Park

To determine if stomatal conductance (g(s)) of forest trees could be predicted from measures of leaf microclimate, diurnal variability in in situ g(s) was measured in black cherry (Prunus serotina), red maple (Acer rubrum), and northern red oak (Quercus rubra). Relative to overstory trees, understory saplings exhibited little diurnal variability in g(s) and ozone uptake. Depending on species and site, up to 30% of diurnal and seasonal variation in g(s )of overstory trees was explained by photosynthetically active radiation and vapor pressure deficit. Daily maximum g(s) was significantly related to soil moisture in overstory northern red oak and black cherry (R(2) ranged from 33 to 65%). Although g(s) was not fully predicted using instantaneous measures of leaf microclimate, ozone uptake of large forest trees was reduced by low soil moisture.     

Respiration controls the unexpected seasonal pattern of carbon flux in an Asian tropical rain forest

Tropical rain forests play important roles in the global carbon cycle. We report a six-year eddy covariance carbon flux campaign in a primary tropical seasonal rain forest in southwest China. An unexpected seasonal pattern of net ecosystem carbon exchange was detected, with carbon lost during the rainy season and stored in the dry season. Strong seasonality of ecosystem respiration was suggested to primarily account for this seasonal pattern. The annual net uptake of CO₂ by the forest varied from 0.98 to 2.35 metric tons of carbon per hectare between 2003 and 2008. 6-year averaged sink strength was 1.68 metric tons of carbon per hectare.     

Numerical study on adjusting and controlling effect of forest cover on PM10 and O3

A new-coupled air quality numerical modeling system has been developed and applied to the study on the adjusting and controlling effect of forest cover on air quality. The modeling system is composed of Plant Canopy Layer Model (PCLM), Urban Scale Meteorological Model (USMM), and High-Resolution Chemical Model (HRCM). The system was applied to the study on the ecological adjusting and controlling effects on PM₁₀ and O₃ in Jinan City, China. The results show that the forest cover can adjust and control PM₁₀ and O₃ significantly by reducing the concentrations of PM₁₀ while increasing the concentrations of O₃ with the increase of forest cover.     

Mapping of exceedances of ozone critical levels for crops and forest trees in The Netherlands: Preliminary results

Within the framework of the UN-ECE Convention on Long-Range Transboundary Air Pollution (CLRTAP) critical levels of ozone for forest trees and crops have been determined. In this paper procedures are presented for constructing maps of The Netherlands showing measured ozone concentrations for comparison with the critical levels. The critical ozone levels for forest trees and crops are exceeded widely and frequently in The Netherlands. The frequency and size of exceedances depend on meteorological conditions: during a hot, sunny summer like 1989 or 1990 exceedances of the critical levels by a factor of two or three were observed in the whole of The Netherlands; in a cold, rainy summer like 1991 or 1993 only minor exceedances were observed in some parts (mainly in the south) of the country.     

Soil contamination by crude oil: impact on the mycorrhizosphere and on the revegetation potential of forest trees

In vitro and greenhouse biotests were carried out to study the effects of various concentrations of crude oil on the mycorrhizosphere and the ability of ectomycorrhizal fungi to colonise Norway spruce and poplar seedlings grown on contaminated soil. Ectomycorrhizal fungi grown in pure cultures showed a variety of reactions to crude oil, ranging from growth stimulation to total inhibition of growth, depending on the species of fungi. Germination of poplar and spruce seeds was not significantly affected. The growth of spruce seedlings was not affected by crude oil, whereas that of poplar seedlings was significantly reduced at high concentrations. None of the concentrations had any effect on the degree of ectomycorrhizal and endomycorrhizal colonisation of poplar. With spruce, however, the ectomycorrhizal fungi showed species-specific reactions to increasing concentrations, in accordance with the results of the pure culture test. The length of time between soil contamination and seeding affects both seedling growth and the mycorrhizal infection potential of the soil. The results confirm the importance of mycorrhizal fungi in the bioremediation of soils contaminated by crude oil.     

Stomatal characteristics and infection biology of Pyrenopeziza betulicola in Betula pendula trees grown under elevated CO2 and O3

Two silver birch clones were exposed to ambient and elevated concentrations of CO₂ and O₃, and their combination for 3 years, using open-top chambers. We evaluated the effects of elevated CO₂ and O₃ on stomatal conductance (g_s), density (SD) and index (SI), length of the guard cells, and epidermal cell size and number, with respect to crown position and leaf type. The relationship between the infection biology of the fungus (Pyrenopeziza betulicola) causing leaf spot disease and stomatal characteristics was also studied. Leaf type was an important determinant of O₃ response in silver birch, while crown position and clone played only a minor role. Elevated CO₂ reduced the g_s, but had otherwise no significant effect on the parameters studied. No significant interactions between elevated CO₂ and O₃ were found. The infection biology of P. betulicola was not correlated with SD or g_s, but it did occasionally correlate positively with the length of the guard cells.     

Direct damage to vegetation caused by acid rain and polluted cloud: definition of critical levels for forest trees

The concept of critical levels was developed in order to define short-term and long-term average concentrations of gaseous pollutants above which plants may be damaged. Although the usual way in which pollutants in precipitation (wet deposition) influence vegetation is by affecting soil processes, plant foliage exposed to fog and cloud, which often contain much greater concentrations of pollutant ions than rain, may be damaged directly. The idea of a critical level has been extended to define concentrations of pollutants in wet deposition above which direct damage to plants is likely. Concentrations of acidity and sulphate measured in mountain and coastal cloud are summarised. Vegetation at risk of injury is identified as montane forest growing close to the cloud base, where ion concentrations are highest. The direct effects of acidic precipitation on trees are reviewed, based on experimental exposure of plants to simulated acidic rain, fog or mist. Although most experiments have reported results in terms of pH (H(+) concentration), the accompanying anion is important, with sulphate being more damaging than nitrate. Both conifers and broadleaved tree seedlings showing subtle changes in the structural characteristics of leaf surfaces after exposure to mist or rain at or about pH 3.5, or sulphate concentration of 150 micromol litre(-1). Visible lesions on leaf surfaces occur at around pH 3 (500 micromol litre(-1) sulphate), broadleaved species tending to be more sensitive than conifers. Effects on photosynthesis and water relations, and interactions with other stresses (e.g. frost), have usually been observed only for treatments which have also caused visible injury to the leaf surface. Few experiments on the direct effects of polluted cloud have been conducted under field conditions with mature trees, which unlike seedlings in controlled conditions, may suffer a growth reduction in the absence of visible injury. Although leaching of cations (Ca(2+), Mg(2+), K(+)) is stimulated by acidic precipitation, amounts leached are small compared with root uptake, unless soils have been impoverished. This aspect of the potential effects of acidic precipitation is best considered in terms of the long-term critical-load of pollutants to the soil. Given the practical difficulties in monitoring cloud water composition, a method for defining critical levels is proposed, which uses climatological average data to identify the duration and frequency of hill cloud, and combines this information with measured or modelled concentrations of particulate sulphate in the atmosphere, to derive cloud water concentrations as a function of cloud liquid water content. For forests within 100 m of the cloud base the critical levels of particulate sulphate, corresponding to solution concentrations in the range 150-500 micromol litre(-1), are in the range 1-3.3 microg S m(-3). These concentrations are observed over much of central Europe, suggesting that many montane forests are at risk of direct effects of fossil-fuel-derived pollutants in cloud.     

Variations of CO and O3 in the troposphere: Evidence of O3 photochemistry

The possibility of a long term evolution for tropospheric ozone, connected to a rise of anthropogenic releases, raises the question of a human climatic action. In order to bring an experimental confirmation to photochemical theories, continuous CO and O₃ measurements were started in November at the Pic du Midi observatory (3000 m altitude in the Pyrenean range). The results obtained confirm the photochemical hypothesis about the ozone origin. An ozone maximum is found in summer, corresponding to a CO minimum, which agrees with the increase of the u.v. flux. The seasonal variation does not exceed 19% of the value integrated over the year, which limits to 20% the maximal stratospheric contribution. The standard diurnal variations show the existence of a variable lag (0–4 h) between the O₃ minimum and the CO maximum, which is interpreted as showing a photochemical O₃ formation in the afternoon in the air coming from the lower layers.     

Contents of free amino acids in needles of Norway Spruce trees in relation to novel forest decline. Studies on trees from a site in the northern Black Forest

The free amino acid content in needles of Norway spruce trees (about 45-year-old) was determined by means of HPLC. The studied trees have been growing at a forestry site in the Black Forest which is characterized by a high impact of ozone and magnesium deficiency. Measurements were carried out on visibly healthy and on damaged trees on several dates during two vegetation periods and during the course of a day. The amino acids occurring at the highest concentrations were glutamate, arginine, aspartate, and glutamine. The typical seasonal changes in the content of free amino acids, with their minimum during summer, were disturbed in the needles of the damaged trees. Particularly in summer and autumn the damaged trees showed increased levels of total amino acids and of most single amino acids. During the course of a day, the needles of the damaged trees showed a reduced amplitude in diurnal variations with the absolute level almost always higher in comparison to that of the undamaged trees. The results suggest that a shift in protein turnover towards enhanced degradation has taken place.     

Constructing 3D magnetic flower-like Fe3O4@SiO2@Co3O4@BiOCl heterojunction photocatalyst for degrading rhodamine B

Environmental Science and Pollution Research - In this work, the 3D magnetic flower-like Fe3O4@SiO2@Co3O4@BiOCl heterojunction photocatalyst was successfully prepared. The combination of BiOCl with...