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European mountain grasslands are increasingly affected by land-use changes and climate, which have been suggested to exert important controls on grassland carbon (C) and nitrogen (N) pools. However, so far there has been no synthetic study on whether and how land-use changes and climate interactively affect the partitioning of these pools amongst the different grassland compartments. We analyzed the partitioning of C and N pools of 36 European mountain grasslands differing in land-use and climate with respect to above- and belowground phytomass, litter and topsoil (top 23 cm). We found that a reduction of management intensity and the abandonment of hay meadows and pastures increased above-ground phytomass, root mass and litter as well as their respective C and N pools, concurrently decreasing the fractional contribution of the topsoil to the total organic carbon pool. These changes were strongly driven by the cessation of cutting and grazing, a shift in plant functional groups and a related reduction in litter quality. Across all grasslands studied, variation in the impact of land management on the topsoil N pool and C/N-ratio were mainly explained by soil clay content combined with pH. Across the grasslands, below-ground phytomass as well as phytomass- and litter C concentrations were inversely related to the mean annual temperature; furthermore, C/N- ratios of phytomass and litter increased with decreasing mean annual precipitation. Within the topsoil compartment, C concentrations decreased from colder to warmer sites, and increased with increasing precipitation. Climate generally influenced effects of land use on C and N pools mainly through mean annual temperature and less through mean an- nual precipitation. We conclude that site-specific conditions need to be considered for understanding the effects of land use and of current and future climate changes on grassland C and N pools.

Forage systems are the major land use, and provide essential resources for animal feeding. Assessing the influence of forage species on net ecosystem CO2 exchange (NEE) is key to develop management strategies that can help to mitigate climate change, while optimizing productivity of these systems. However, little is known about the effect of forage species on CO2 exchange fluxes and net biome production (NBP), considering: species ecophysiological responses; growth and fallow periods separately; and the management associated with the particular sown species. Our study assesses the influence of cereal monocultures vs. cereal legume mixtures on (1) ecosystem scale CO2 fluxes, for the whole crop season and separately for the two periods of growth and fallow; (2) potential sensitivities of CO2 exchange related to short-term variations in light, temperature and soil water content; and (3) NBP during the growth period; this being the first long term (seven years) ecosystem scale CO2 fluxes dataset of an intensively managed forage system in the Pyrenees region. Our results provide strong evidence that cereal-legume mixtures lead to higher net CO2 uptake than cereal monocultures, as a result of higher gross CO2 uptake, while respiratory fluxes did not significantly increase. Also, management associated with cereal legume mixtures favoured vegetation voluntary regrowth during the fallow period, which was decisive for the cumulative net CO2 uptake of the entire crop season. All cereal legume mixtures and some cereal monocultures had a negative NBP (net gain of C) during the growth period, indicating C input to the system, besides the yield. Overall, cereal legume mixtures enhanced net CO2 sink capacity of the forage system, while ensuring productivity and forage quality.

In a transdisciplinary and pan-region programme like PEEX, building a strong communications platform with an efficient reach across countries is vital to foster collaborations, announce local findings or events to a wider audience, and build a sense of international community. In addition to its website, PEEX offers a quarterly e-newsletter and online Blog to its community.The PEEX e-newsletter is sent to ~650 international subscribers, where the majority of readers come from Russia, China, Northern Europe and the USA. It is particularly important given that China and Russia have their own national social media channels like Weebo, WeChat and VK, which are popular alternatives to Twitter, Facebook or Instagram. Thus, having an online platform that integrates readers from Russia and China is vital for information exchange.In addition to having an international readership, your article contribution to the PEEX Blog and e-newsletter facilitates your own efforts of dissemination. All articles have their unique web link, hosted in the PEEX blog, and hence can be embedded in your own research group’s website, news, social media accounts, etc.The PEEX blog and newsletter welcomes articles from early career scientists. Great examples of these include field work stories that communicate the research aim but also the infrastructure or instrumentation available across the PEEX domain. Additionally, it provides a training opportunity for students to write popular scientific articles.UArctic and FutureEarth are both partner programmes of PEEX. As the PEEX newsletter evolves, the aim is to integrate more overlapping relevant news and opportunities across these partners. Presently, UArctic shares the PEEX newsletter among its channels, as part of the thematic network the Arctic Boreal Hub.Expanding the offer of channels for dissemination within the PEEX community and overall public will allow us to discuss science in a more collaborative, open and inclusive manner.

The Pan-Eurasian Experiment (PEEX) program was initiated as a bottom-up approach by the researchers coming from Finland and Russia in October 2012. The PEEX China kick off meeting was held in November 2013. During its five years in operation, the program has established a governance structure and delivered a science plan for the Northern Eurasian region. PEEX has also introduced a concept design for a modelling platform and ground-based in situ observation systems for detecting land-atmosphere and ocean-atmosphere interactions. Today, PEEX has an extensive researcher’s network representing research communities coming from the Nordic countries, Russia and China. PEEX is currently carrying out its research activities on a project basis, but is looking for more coordinated funding bases, especially in Russia and in China. The near-future challenge in implementing the PEEX research agenda is to achieve a successful integration and identification of the methodological approaches of the socio-economic research to environmental sciences. Here we give insight into these issues and provide an overview on the main tasks for the upcoming years. The Pan-Eurasian Experiment (PEEX) program was initiated as a bottom-up approach by the researchers coming from Finland and Russia in October 2012. The PEEX China kick off meeting was held in November 2013. During its five years in operation, the program has established a governance structure and delivered a science plan for the Northern Eurasian region. PEEX has also introduced a concept design for a modelling platform and ground-based in situ observation systems for detecting land-atmosphere and ocean-atmosphere interactions. Today, PEEX has an extensive researcher’s network representing research communities coming from the Nordic countries, Russia and China. PEEX is currently carrying out its research activities on a project basis, but is looking for more coordinated funding bases, especially in Russia and in China. The near-future challenge in implementing the PEEX research agenda is to achieve a successful integration and identification of the methodological approaches of the socio-economic research to environmental sciences. Here we give insight into these issues and provide an overview on the main tasks for the upcoming years. The Pan-Eurasian Experiment (PEEX) program was initiated as a bottom-up approach by the researchers coming from Finland and Russia in October 2012. The PEEX China kick off meeting was held in November 2013. During its five years in operation, the program has established a governance structure and delivered a science plan for the Northern Eurasian region. PEEX has also introduced a concept design for a modelling platform and ground-based in situ observation systems for detecting land-atmosphere and ocean-atmosphere interactions. Today, PEEX has an extensive researcher’s network representing research communities coming from the Nordic countries, Russia and China. PEEX is currently carrying out its research activities on a project basis, but is looking for more coordinated funding bases, especially in Russia and in China. The near-future challenge in implementing the PEEX research agenda is to achieve a successful integration and identification of the methodological approaches of the socio-economic research to environmental sciences. Here we give insight into these issues and provide an overview on the main tasks for the upcoming years.

Forage systems are the major land use and provide essential resources for animal feeding. Assessing the influence of annual forage crop species on net ecosystem CO2 exchange (NEE) is key to develop management strategies that can help to mitigate climate change, while optimizing productivity of these systems. However, little is known about the effect of annual forage crop species on CO2 exchange fluxes, considering separately net CO2 balances during the growth and fallow periods, and species ecophysiological responses. Our study assesses the influence of cereal monocultures vs. cereal‑legume mixtures on (1) ecosystem‑scale CO2 fluxes, for the whole crop season and separately for the two periods of growth and fallow; and (2) potential sensitivities of CO2 exchange related to short-term variations in light, temperature, and soil water content. This is the first mid‑term (seven years) ecosystem‑scale CO2 fluxes assessment of an intensively managed forage system in the Pyrenees region. Our results provide strong evidence that cereal‑legume mixtures lead to higher net CO2 uptake than cereal monocultures, as a result of higher gross CO2 uptake, while respiratory fluxes did not significantly increase. Also, cereal-legume mixtures favoured a higher voluntary regrowth during the fallow period, which was decisive for the cumulative net CO2 uptake of the entire crop season. Overall, cereal‑legume mixtures enhanced net CO2 sink capacity of the forage crop, while ensuring productivity and forage quality., Field Crops Research, 272, ISSN:0378-4290, ISSN:1872-6852

The Integrated Carbon Observation System is a Pan-European distributed research infrastructure that has as its main goal to monitor the greenhouse gas balance of Europe. The ecosystem component of Integrated Carbon Observation System consists of a multitude of stations where the net greenhouse gas exchange is monitored continuously by eddy covariance measurements while, in addition many other measurements are carried out that are a key to an understanding of the greenhouse gas balance. Amongst them are the continuous meteorological measurements and a set of non-continuous measurements related to vegetation. The latter include Green Area Index, aboveground biomass and litter biomass. The standardized methodology that is used at the Integrated Carbon Observation System ecosystem stations to monitor these vegetation related variables differs between the ecosystem types that are represented within the network, whereby in this paper we focus on forests, grasslands, croplands and mires. For each of the variables and ecosystems a spatial and temporal sampling design was developed so that the variables can be monitored in a consistent way within the ICOS network. The standardisation of the methodology to collect Green Area Index, above ground biomass and litter biomass and the methods to evaluate the quality of the collected data ensures that all stations within the ICOS ecosystem network produce data sets with small and similar errors, which allows for inter-comparison comparisons across the Integrated Carbon Observation System ecosystem network.

Research infrastructures play a key role in launching a new generation of integrated long-term, geographically distributed observation programmes designed to monitor climate change, better understand its impacts on global ecosystems, and evaluate possible mitigation and adaptation strategies. The pan-European Integrated Carbon Observation System combines carbon and greenhouse gas (GHG; CO2, CH4, N2O, H2O) observations within the atmosphere, terrestrial ecosystems and oceans. High-precision measurements are obtained using standardised methodologies, are centrally processed and openly available in a traceable and verifiable fashion in combination with detailed metadata. The Integrated Carbon Observation System ecosystem station network aims to sample climate and land-cover variability across Europe. In addition to GHG flux measurements, a large set of complementary data (including management practices, vegetation and soil characteristics) is collected to support the interpretation, spatial upscaling and modelling of observed ecosystem carbon and GHG dynamics. The applied sampling design was developed and formulated in protocols by the scientific community, representing a trade-off between an ideal dataset and practical feasibility. The use of open-access, high-quality and multi-level data products by different user communities is crucial for the Integrated Carbon Observation System in order to achieve its scientific potential and societal value.

Chamber measurements of trace gas fluxes between the land surface and the atmosphere have been conducted for almost a century. Different chamber techniques, including static and dynamic, have been used with varying degrees of success in estimating greenhouse gases (CO2, CH4, N2O) fluxes. However, all of these have certain disadvantages which have either prevented them from providing an adequate estimate of greenhouse gas exchange or restricted them to be used under limited conditions. Generally, chamber methods are relatively low in cost and simple to operate. In combination with the appropriate sample allocations, chamber methods are adaptable for a wide variety of studies from local to global spatial scales, and they are particularly well suited for in situ and laboratory-based studies. Consequently, chamber measurements will play an important role in the portfolio of the Pan-European long-term research infrastructure Integrated Carbon Observation System. The respective working group of the Integrated Carbon Observation System Ecosystem Monitoring Station Assembly has decided to ascertain standards and quality checks for automated and manual chamber systems instead of defining one or several standard systems provided by commercial manufacturers in order to define minimum requirements for chamber measurements. The defined requirements and recommendations related to chamber measurements are described here.

There are many factors that influence ecosystem scale carbon, nitrogen and greenhouse gas dynamics, including the inherent heterogeneity of soils and vegetation, anthropogenic management interventions, and biotic and abiotic disturbance events. It is important therefore, to document the characteristics of the soils and vegetation and to accurately report all management activities, and disturbance events to aid the interpretation of collected data, and to determine whether the ecosystem either amplifies or mitigates climate change. This paper outlines the importance of assessing both the spatial and temporal variability of soils and vegetation and to report all management events, the import or export of C or N from the ecosystem, and the occurrence of biotic/abiotic disturbances at ecosystem stations of the Integrated Carbon Observation System, a pan-European research infrastructure.

Graphical abstract, Measurements of greenhouse gas (GHG) fluxes, particularly methane (CH4) and nitrous oxide (N2O) in mountain ecosystems are scarce due to the complexity and unpredictable behavior of these gases, in addition to the remoteness of these ecosystems. In this context, we measured CO2, CH4, and N2O fluxes in four semi-natural pastures in the Pyrenees to investigate their magnitude and range of variability. Our interest was to study GHG phenomena at the patch-level, therefore we chose to measure the gas-exchange using a combination of a gas analyzer and manual chambers. The analyzer used is a photoacoustic field gas-monitor that allows multi-gas instantaneous measurements. After implementing quality control and corrections, data was of variable quality. We tackled this by categorizing data as to providing quantitative or only qualitative information: • 50% and 59% of all CH4 and N2O data, respectively, provided quantitative information above the detection limit. • We chose not to discard data providing only qualitative information, because they identify highest- and lowest-flux peak periods and indicate the variability of the fluxes, along different altitudes and under different climatic conditions. • We chose not to give fluxes below detection limit a quantitative value but to acknowledge them as values identifying periods with low fluxes.

Biogenic volatile organic compounds (BVOCs) produced by plants have a major role in atmospheric chemistry. The different physicochemical properties of BVOCs affect their transport within and out of the plant as well as their reactions along the way. Some of these compounds may accumulate in or on the waxy surface layer of conifer needles and participate in chemical reactions on or near the foliage surface. The aim of this work was to determine whether terpenes, a key category of BVOCs produced by trees, can be found on the epicuticles of Scots pine (Pinus sylvestris L.) and, if so, how they compare with the terpenes found in shoot emissions of the same tree. We measured shoot-level emissions of pine seedlings at a remote outdoor location in Central Finland and subsequently analysed the needle surface waxes for the same compounds. Both emissions and wax extracts were clearly dominated by monoterpenes, but the proportion of sesquiterpenes was higher in the wax extracts. There were also differences in the terpene spectra of the emissions and the wax extracts. The results, therefore, support the existence of BVOC associated to the epicuticular waxes. We briefly discuss the different pathways for terpenes to reach the needle surfaces and the implications for air chemistry.

This study scrutinizes a decade-long series of ozone deposition measurements in a boreal forest in search for the signature and relevance of the different deposition processes. The canopy-level ozone flux measurements were analysed for deposition characteristics and partitioning into stomatal and non-stomatal fractions, with the main focus on growing season day-time data. Ten years of measurements enabled the analysis of ozone deposition variation at different time-scales, including daily to inter-annual variation as well as the dependence on environmental variables and concentration of biogenic volatile organic compounds (BVOC-s). Stomatal deposition was estimated by using multi-layer canopy dispersion and optimal stomatal control modelling from simultaneous carbon dioxide and water vapour flux measurements, non-stomatal was inferred as residual. Also, utilising the big-leaf assumption stomatal conductance was inferred from water vapour fluxes for dry canopy conditions. The total ozone deposition was highest during the peak growing season (4 mm s−1) and lowest during winter dormancy (1 mm s−1). During the course of the growing season the fraction of the non-stomatal deposition of ozone was determined to vary from 26 to 44% during day time, increasing from the start of the season until the end of the growing season. By using multi-variate analysis it was determined that day-time total ozone deposition was mainly driven by photosynthetic capacity of the canopy, vapour pressure deficit (VPD), photosynthetically active radiation and monoterpene concentration. The multi-variate linear model explained the high portion of ozone deposition variance on daily average level (R2 = 0.79). The explanatory power of the multi-variate model for ozone non-stomatal deposition was much lower (R2 = 0.38). The set of common environmental variables and terpene concentrations used in multivariate analysis were able to predict the observed average seasonal variation in total and non-stomatal deposition but failed to explain the inter-annual differences, suggesting that some still unknown mechanisms might be involved in determining the inter-annual variability. Model calculation was performed to evaluate the potential sink strength of the chemical reactions of ozone with sesquiterpenes in the canopy air space, which revealed that sesquiterpenes in typical amounts at the site were unlikely to cause significant ozone loss in canopy air space. The results clearly showed the importance of several non-stomatal removal mechanisms. Unknown chemical compounds or processes correlating with monoterpene concentrations, including potentially reactions at the surfaces, contribute to non-stomatal sink term.

The calculation of ozone uptake to vegetation requires the determination of both the stomatal uptake and the non-stomatal sinks. This paper analyses the relative contribution of stomatal and non-stomatal sinks at the shoot level based on direct measurements of ozone flux that included both components. Ozone deposition was estimated for Scots pine shoots enclosed in chambers, connected to a gas-exchange system, at the SMEAR II field station, in southern Finland. For the analysis of stomatal conductance, we utilised the formulation within the dry deposition module of the photochemical EMEP model, for which we revised the parameterisation for Scots pine. In addition, we introduced modifications in the treatment of spring phenology and parameter estimation. Provided these modifications, the parameter values estimated from our data fell close to the default values, although presenting a less sensitive VPD response during the summer. A major proportion of the measured removal was due to the non-stomatal component. We introduced a non-stomatal term dependent on the ambient relative humidity, which significantly improved the agreement between the calculated and measured values of total shoot conductance. The non-stomatal contribution to total conductance was in the order of 50%.

The carbon dioxide and water vapour fluxes were measured by the eddy covariance (EC) technique from July to September 2000 at two closely located sites in southern Finland: over a 38-year-old pine forest and over a 5-year-old forest clearing. The night-time respiration was of the same magnitude at both sites. At day-time the pine forest was a strong sink but the clearing close to CO2 balance, indicating that CO2 uptake of ground vegetation over the clearing balanced the release from the soil. The shoot scale gas exchange measurements in combination with process-based modelling were used to evaluate the measured CO2 exchange of the forest ecosystem. The forest CO2 exchange was explained by soil respiration and photosynthesis of forest canopy, while the contribution of understory and ground vegetation CO2 exchange could be neglected. During the study period the forest was a net sink of CO2 and the clearing a source. The daily average uptake of CO2 by the forest was −2.4 and − 1.7 gm −2 per day in July–August and September periods, respectively; and average release by the clearing 4.0 and 2.5 g m −2 per day during the same periods. This shows that carbon losses 5 years after clear-cutting are substantial. The evapotranspiration (ET) was higher over the forest compared to clearing as a result of transpiration from the forest canopy. The difference in ET was small during the July–August period when precipitation frequently occurred. © 2002 Elsevier Science B.V. All rights reserved.

Values of ozone (O 3 ) flux measured on the shoot scale by enclosure techniques can provide detailed information about the partition and seasonality of the removal pathways on the foliage. However, O 3 reactivity makes such measurements potentially inaccurate, particularly in field conditions where ambient concentration is low. Special consideration should be given to the losses on the chamber wall material, which can rival in magnitude and dynamics the measure of shoot removal. We propose a way of taking into account this dynamic behaviour in the context of field monitoring with simple chambers based on dynamic principles. We accommodated an existing field monitoring gas exchange system for CO 2 and water vapour to measure simultaneous,but independent, O 3 fluxes. We report here the system's performance based on the analysis of 19-month data. The background noise of the measurements is mainly imposed by the analyser and is around 2 μg m −3 . The resolution of the system is in the order of 10 −2 μg m −2 s −1 . The disturbance due to wall-losses was corrected thoroughly but it was mostly needed when the shoot activity was low and the mass flow signals produced by the shoots and the walls came close to each other. In spite of the disfavouring combination of chamber material, O 3 reactivity and low concentrations, we have successfully followed diurnal and seasonal trends of O 3 removal in Scots pine foliage.

Measurements of rapid changes in concentrations and fluxes of gaseous compounds relating to photosynthetic gas exchange are commonly performed using flow-through cuvettes in connection with infrared gas analysers. The accuracy and repeatability of these measurements relies ultimately upon the design of the system as a whole, rather than upon each of its components, and therefore the calibration and testing of the system should be performed keeping this in mind. We present here a simple and efficient method for the calibration of such a measurement system using a precisely determined CO2 flow. This method gives us the opportunity to take into account any disturbing effects caused by undesired properties of the chamber or tubing materials. With the proposed calibration method, the accuracy of the CO2 flux measurement is improved from 8% up to the level of 2%, determined mainly by the accuracy of the control gas used for calibration of the CO2 analyser.

Cuvette measurements are a tool to analyse CO2 exchange, transipiration and deposition/emission of different trace gases by plants. To verify these experimental methods and to use them efficiently we have developed a numerical model with atmospheric chemical reactions. The model includes reactions between 54 different chemical species in the gas phase. Using the model we are able to determine optimal size/flow rate ratios and cuvette cycles (closure times) from an experimental point of view. Using the cuvette model with atmospheric chemistry more accurate estimates for emissions/deposition rates of different species can be found. Some chemical reactions are significant, e.g. for NO and terpenes, as regards the analysis and interpretation of measured concentrations. With slower flow rates through a cuvette the significance of reactions is more pronounced. However, there are some species like ozone, where stomatal deposition is a dominant phenomenon and chemistry plays a minor role.

This paper analyses the existence and relative strength of ozone (O 3 ) sinks at the leaf level, in particular the implications for the partition of the O 3 flux amongst the several physically, chemically and physiologically differing inner and outer surfaces of a leaf. We used a single-stomatal scale theoretical model to simulate the O 3 transfer into leaves and estimate the flux partition. The theoretical scenarios were compared with experimental values from shoot-scale measurements of O 3 flux onto Scots pine. The conditions where external sinks would prevent the stomatal diffusion involved the existence of very strong sinks at the external surfaces but yielded unrealistically high flux values. Only the possibility of a strong sink localized in the antechamber and/or pore could be of significance. Results also showed that in most instances a significant proportion of the total flux was generated by the external surfaces. For scenarios that consider strong scavenging in the mesophyll and weaker removal in the exterior, the proportion was about 60–80% for small stomatal apertures (∼0.5 μ m), and 10–40% for larger apertures (∼1.5 μ m). In these cases, however big the proportion of total flux is due to the external surfaces, the existence of sinks at the external surfaces does not prevent the diffusion through open and unoccluded stomata. DOI: 10.1111/j.1600-0889.2008.00344.x

Thum, T., et al. (2011), 'Soil carbon model alternatives for ECHAM5/JSBACH climate model: Evaluation and impacts on global carbon cycle estimates', Journal of Geophysical Research, Vol. 116, G02028, published 29 June 2011. The version of record is available at doi:10.1029/2010JG001612 © 2011 American Geophysical Union.

The compensation point of bidirectional NO x fluxes of plants is often determined in chamber measurements by varying the NO x concentration and searching for the point where the net flux is zero. We hypothesized that the compensation point can vary with changing conditions so much that one constant value is not sufficiently accurate. The compensation point for NO x fluxes of Scots pine was analysed with a model describing the NO x consumption and production processes. Consumption was assumed to occur via the stomata and on needle surfaces, with the rate depending on ambient concentration and degree of stomatal opening. Production was assumed to occur on the needle surfaces depending on solar UV-A irradiance. We used NO y flux data measured with two gas-exchange chambers in southern Finland to parameterize the model. The estimated compensation points increased with increasing UV-A irradiance and decreased with increasing stomatal conductance. With open stomata, it varied several ppb even under field conditions.

Thirty-three snowpack models of varying complexity and purpose were evaluated across a wide range of hydrometeorological and forest canopy conditions at five Northern Hemisphere locations, for up to two winter snow seasons. Modeled estimates of snow water equivalent (SWE) or depth were compared to observations at forest and open sites at each location. Precipitation phase and duration of above-freezing air temperatures are shown to be major influences on divergence and convergence of modeled estimates of the subcanopy snowpack. When models are considered collectively at all locations, comparisons with observations show that it is harder to model SWE at forested sites than open sites. There is no universal "best'' model for all sites or locations, but comparison of the consistency of individual model performances relative to one another at different sites shows that there is less consistency at forest sites than open sites, and even less consistency between forest and open sites in the same year. A good performance by a model at a forest site is therefore unlikely to mean a good model performance by the same model at an open site (and vice versa). Calibration of models at forest sites provides lower errors than uncalibrated models at three out of four locations. However, benefits of calibration do not translate to subsequent years, and benefits gained by models calibrated for forest snow processes are not translated to open conditions.

Ecosystems and the atmosphere: This review describes the state of understanding the processes involved in the exchange of trace gases and aerosols between the earth's surface and the atmosphere. The gases covered include NO, NO2, HONO, HNO3, NH3, SO2, DMS, Biogenic VOC, O-3, CH4, N2O and particles in the size range 1 nm-10 mu m including organic and inorganic chemical species. The main focus of the review is on the exchange between terrestrial ecosystems, both managed and natural and the atmosphere, although some new developments in ocean-atmosphere exchange are included. The material presented is biased towards the last decade, but includes earlier work, where more recent developments are limited or absent. New methodologies and instrumentation have enabled, if not driven technical advances in measurement. These developments have advanced the process understanding and upscaling of fluxes, especially for particles, VOC and NH3. Examples of these applications include mass spectrometric methods, such as Aerosol Mass Spectrometry (AMS) adapted for field measurement of atmosphere-surface fluxes using micrometeorological methods for chemically resolved aerosols. Also briefly described are some advances in theory and techniques in micrometeorology. For some of the compounds there have been paradigm shifts in approach and application of both techniques and assessment. These include flux measurements over marine surfaces and urban areas using micrometeorological methods and the up-scaling of flux measurements using aircraft and satellite remote sensing. The application of a flux-based approach in assessment of O-3 effects on vegetation at regional scales is an important policy linked development secured through improved quantification of fluxes. The coupling of monitoring, modelling and intensive flux measurement at a continental scale within the NitroEurope network represents a quantum development in the application of research teams to address the underpinning science of reactive nitrogen in the cycling between ecosystems and the atmosphere in Europe. Some important developments of the science have been applied to assist in addressing policy questions, which have been the main driver of the research agenda, while other developments in understanding have not been applied to their wider field especially in chemistry-transport models through deficiencies in obtaining appropriate data to enable application or inertia within the modelling community. The paper identifies applications, gaps and research questions that have remained intractable at least since 2000 within the specialized sections of the paper, and where possible these have been focussed on research questions for the coming decade. (C) 2009 Published by Elsevier Ltd.

Ozone (O3) is a reactive gas present in the troposphere in the range of parts per billion (ppb), i.e. molecules of O3 in 109 molecules of air. Its strong oxidative capacity makes it a key element in tropospheric chemistry and a threat to the integrity of materials, including living organisms. Knowledge and control of O3 levels are an issue in relation to indoor air quality, building material endurance, respiratory human disorders, and plant performance. Ozone is also a greenhouse gas and its abundance is relevant to global warming. The interaction of the lower troposphere with vegetated landscapes results in O3 being removed from the atmosphere by reactions that lead to the oxidation of plant-related components. Details on the rate and pattern of removal on different landscapes as well as the ultimate mechanisms by which this occurs are not fully resolved. This thesis analysed the controlling processes of the transfer of ozone at the air-plant interface. Improvement in the knowledge of these processes benefits the prediction of both atmospheric removal of O3 and its impact on vegetation. This study was based on the measurement and analysis of multi-year field measurements of O3 flux to Scots pine (Pinus sylvestris L.) foliage with a shoot-scale gas-exchange enclosure system. In addition, the analyses made use of simultaneous CO2 and H2O exchange, canopy-scale O3, CO2 and H2O exchange, foliage surface wetness, and environmental variables. All data was gathered at the SMEAR measuring station (southern Finland). Enclosure gas-exchange techniques such as those commonly used for the measure of CO2 and water vapour can be applied to the measure of ozone gas-exchange in the field. Through analysis of the system dynamics the occurring disturbances and noise can be identified. In the system used in this study, the possible artefacts arising from the ozone reactivity towards the system materials in combination with low background concentrations need to be taken into account. The main artefact was the loss of ozone towards the chamber walls, which was found to be very variable. The level of wall-loss was obtained from simultaneous and continuous measurements, and was included in the formulation of the mass balance of O3 concentration inside the chamber. The analysis of the field measurements in this study show that the flux of ozone to the Scots pine foliage is generated in about equal proportions by stomatal and non-stomatal controlled processes. Deposition towards foliage and forest is sustained also during night and winter when stomatal gas-exchange is low or absent. The non-stomatal portion of the flux was analysed further. The pattern of flux in time was found to be an overlap of the patterns of biological activity and presence of wetness in the environment. This was seen to occur both at the shoot and canopy scale. The presence of wetness enhanced the flux not only in the presence of liquid droplets but also during existence of a moisture film on the plant surfaces. The existence of these films and their relation to the ozone sinks was determined by simultaneous measurements of leaf surface wetness and ozone flux. The results seem to suggest ozone would be reacting at the foliage surface and the reaction rate would be mediated by the presence of surface wetness. Alternative mechanisms were discussed, including nocturnal stomatal aperture and emission of reactive volatile compounds. The prediction of the total flux could thus be based on a combination of a model of stomatal behaviour and a model of water absorption on the foliage surfaces. The concepts behind the division of stomatal and non-stomatal sinks were reconsidered. This study showed that it is theoretically possible that a sink located before or near the stomatal aperture prevents or diminishes the diffusion of ozone towards the intercellular air space of the mesophyll. This obstacle to stomatal diffusion happens only under certain conditions, which include a very low presence of reaction sites in the mesophyll, an extremely strong sink located on the outer surfaces or stomatal pore. The relevance, or existence, of this process in natural conditions would need to be assessed further. Potentially strong reactions were considered, including dissolved sulphate, volatile organic compounds, and apoplastic ascorbic acid. Information on the location and the relative abundance of these compounds would be valuable. The highest total flux towards the foliage and forest happens when both the plant activity and ambient moisture are high. The highest uptake into the interior of the foliage happens at large stomatal apertures, provided that scavenging reactions located near the stomatal pore are weak or non-existent. The discussion covers the methodological developments of this study, the relevance of the different controlling factors of ozone flux, the partition amongst its component, and the possible mechanisms of non-stomatal uptake. Otsoni on kaasu jonka pitoisuudet ympärillämme ovat pieniä, mutta ihmistoiminnan seurauksena kohoamassa maailmanlaajuisesti. Koska otsoni on erittäin hapettava aine, pienetkin pitoisuuden nousut ovat merkittäviä. Otsoni reagoi kaikkien materiaalien kanssa -seinien maalipinnoista keuhkorakkuloiden sisäkerroksiin, ja myös kasvien lehtisolukoiden kanssa. Tästä on usein seurauksena muutoksia jotka muistuttavat ennenaikaista vanhenemista, ja aiheuttavat kliinisiä vaivoja ihmisille ja sadon menetyksiä viljelykasveille. Siksi on tärkeää tietää, millaisia otsonipitoisuuksia eläinten ja kasvien hengitysilmassa on, ja miten paljon vahinkoa nämä pitoisuudet saavat aikaan. Tässä väitöskirjassa tarkastellaan, mitä kasvien läheisyydessä olevalle otsonille tapahtuu; kulkeutuuko se lehtien sisään, ja miten mahdollinen kulkeutuminen tapahtuu. Tämän tapahtumaketjun tunteminen auttaa arvioimaan otsonin haitallisia vaikutuksia kasvien elintoiminnoille. Viime kädessä tätä tietoa voidaan käyttää säätelemään ihmistoiminnasta, esim. liikenteestä ja teollisuuspäästöistä aiheutuvaa saastumista. Yksi tapa saada tietoa kasvin ja ilman ainesosasten vuorovaikutuksista on oksan sulkeminen kammioon jossa oleven kaasujen pitoisuusmuutoksia voidaan mitata. Näin saaduilla kaasuvuon arvoilla voidaan luonnehtia kasvin ja ilmakehän välisiä kaasujenvaihtoilmiöitä. Vuomittaukset voidaan kytkeä samanaikaisiin muutoksiin esimerkiksi lämpötilassa tai kosteudessa, ja myös kasvin fysiologiseen tilaan. Tämän tutkimuksen aineisto kerättiin Helsingin yliopiston Metsäekologian ja Fysikaalisten tieteiden laitosten yhteisellä SMEAR II mittausasemalla Hyytiälässä. Pitkälle automatisoitu mittausasema sijaitsee keskellä mäntymetsää, ja siellä mitataan jatkuvatoimisesti monia ilmakehän, puiden ja maaperän toimintaa kuvaavia tekijöitä. Näitä mittauksia yhdistelemällä analysoimme otsonin vaihtoa mäntymetsässä. Ensimmäisenä kehitettiin menetelmä, jolla kasvien otsoninvaihtoa voidaan mitata. Samankaltainen menetelmä on laajalti käytossä mitattaessa kasvien yhteytystä ja hengitystä hiilidioksidinvaihdon avulla. Otsonin jatkuvasti vaihtelevat reaktiot kammion pintamateriaalien kanssa häiritsivät kuitenkin mittauksia, joten näitä häiriötä oli tarpeen seurata jatkuvasti ja ottaa ne huomioon vuolaskelmissa. Jotta otsoni aiheuttaisi kasville haittaa, sen täytyy kulkeutua lehden sisäosiin lehden pinnalla olevien aukkojen, ilmarakojen kautta. Mittauksissamme havaittiin, että vain noin puolet siitä otsonista joka ilman ja lehden välisissä reaktioissa on osallisena, päätyy lehden sisälle. Loppuosa reagoi ja katoaa ennen pääsyään lehteen. Tämä voi tapahtua monella tavalla, mutta tässä tutkimuksessa selvitimme että lehden pintakosteudella oli otsonin katoamisessa tärkeä osa. Se puolestaan merkitsee, että kosteissa oloissa, kuten boreaalisissa havumetsissä, mitattu otsonivuo on aina suurempi kuin kasvin kannalta haitallinen otsonivuo. Tämä pitäisi ottaa huomioon ennusteissa. Väitöskirjassa pohditaan otsonivuon mittauksen metodologiaa, eri tekijöiden merkitystä otsonivuon suuruuden määräytymisessä, vuon jakautumista eri komponentteihin ja mahdollisia mekanismeja joilla otsoni tuhoutuu ennen joutumistaan ilmarakoon ja solukkoon. Julkaistu painettuna: Helsinki: The Finnish Society of Forest Science, 2006. Dissertationes Forestales, 1795-7389

This paper addresses the potential role of surface wetness in ozone deposition to plant foliage. We studied Scots pine foliage in field conditions at the SMEARII field measurement station in Finland. We used a combination of data from flux measurement at the shoot (enclosure) and canopy scale (eddy covariance), information from foliage surface wetness sensors, and a broad array of ancillary measurements such as radiation, precipitation, temperature, and relative humidity. Environmental conditions were defined as moist during rain or high relative humidity and during the subsequent twelve hours from such events, circumstances that were frequent at this boreal site. From the measured fluxes we estimated the ozone conductance using it as the expression of the strength of ozone removal surface sink or total deposition. Further, we estimated the stomatal contribution and the remaining deposition was interpreted and analysed as the non-stomatal sink. The combined time series of measurements showed that both shoot and canopy-scale ozone total deposition were enhanced when moist conditions occurred. On average, the estimated stomatal deposition accounted for half of the measured removal at the shoot scale and one third at the canopy scale. However, during dry conditions the estimated stomatal uptake predicted the behaviour of the measured deposition, but during moist conditions there was disagreement. The estimated non-stomatal sink was analysed against several environmental factors and the clearest connection was found with ambient relative humidity. The relationship disappeared under 70% relative humidity, a threshold that coincides with the value at which surface moisture gathers at the foliage surface according to the leaf surface wetness measurements. This suggests the non-stomatal ozone sink on the foliage to be modulated by the surface films. We attempted to extract such potential modulation with the estimated film formation via the theoretical expression of adsorption. Whereas this procedure could predict the behaviour of the non-stomatal sink, it implied a chemical sink that was not accountable as simple ozone decomposition. We discuss the existence of other mechanisms whose relevance in the removal of ozone needs to be clarified, in particular: a significant nocturnal stomatal aperture neglected in the estimations, and a potentially large chemical sink offered by reactive biogenic organic volatile compounds.

[1] Thinning is a routine forest management operation that changes tree spacing, number, and size distribution and affects the material flows between vegetation and the atmosphere. Here, using direct micrometeorological ecosystem-scale measurements, we show that in a boreal pine forest, thinning decreases the deposition velocities of fine particles as expected but does not reduce the carbon sink, water vapor flux, or ozone deposition. The thinning decreased the all-sided leaf area index from 8 to 6, and we suggest that the redistribution of sources and sinks within the ecosystem compensated for this reduction in foliage area. In the case of water vapor and O 3 , changes in light penetration and among-tree competition seem to increase individual transpiration rates and lead to larger stomatal apertures, thus enhancing also O 3 deposition. In the case of CO 2 , increased ground vegetation assimilation and decreased autotrophic respiration seem to cancel out opposite changes in canopy assimilation and heterotrophic respiration. Current soil-vegetation-atmosphere transfer models should be able to reproduce these observations.

The possibility of NOx (NO + NO2) emissions from plants and the underlying mechanisms are still under discussion. Excess NO created possibly as a result of nitrite accumulation in plant leaves has been suggested to result in emissions. Such emission has been observed in laboratory conditions due to nitrate fertilization. In this study, we tested whether nitrate fertilization of Scots pine seedlings growing outdoors leads to accumulation of NO3- or NO2- in the needles and subsequent NOx emission. The experiment was done at the SMEAR II station in Southern Finland. The seedlings received nitrate or ammonium fertilizer or neither. Shoot NOx emissions were measured with dynamic chambers. Total dissolved nitrogen, inorganic nitrogen, NO3-, NO2- and NH4+ concentrations in the soil and needles were determined. NOx fluxes from the shoots were, on average, deposition. There was no indication of fertilization-induced NO or NOx emissions. The highest NO fluxes were observed at night and were humidity-related. It seems unlikely that additional nitrate in the soil could cause significant NOx emission from boreal Scots pine forests in field conditions, possibly because of soil chemistry.

Understanding the mechanisms underlying net ecosystem CO2 exchange (NEE) in mountain grasslands is important to quantify their relevance in the global carbon budget. However, complex interactions between environmental variables and vegetation on NEE remain unclear; and there is a lack of empirical data, especially from the high elevations and the Mediterranean region. A chamber-based survey of CO2 exchange measurements was carried out in two climatically contrasted grasslands (montane v. subalpine) of the Pyrenees; assessing the relative contribution of phenology and environmental variables on CO2 exchange at the seasonal scale, and the influence of plant functional type dominance (grasses, forbs and legumes) on the NEE light response. Results show that phenology plays a crucial role as a CO2 exchange driver, suggesting a differential behaviour of the vegetation community depending on the environment. The subalpine grassland had a more delayed phenology compared to the montane, being more temperature than water constrained. However, temperature increased net CO2 uptake at a higher rate in the subalpine than in the montane grassland. During the peak biomass, productivity (+74%) and net CO2 uptake (NEE +48%) were higher in the subalpine grassland than in the montane grassland. The delayed phenology at the subalpine grassland reduced vegetation's sensitivity to summer dryness, and CO2 exchange fluxes were less constrained by low soil water content. The NEE light response suggested that legume dominated plots had higher net CO2 uptake per unit of biomass than grasses. Detailed information on phenology and vegetation composition is essential to understand elevation and climatic differences in CO2 exchange. The current study was developed within the project CAPACITI supported by a Marie Curie Intra European Fellowship within the 7th European Community Framework for Nuria Altimir (PIEF-GA-2010-275855) and the project BIOGEI (CGL2013-49142-C21-R) supported by a FPI fellowship for Mercedes Ibáñez (BES-2014-069243) funded by the Spanish Science Foundation (FECYT).