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We present a data set covering three months of carbon dioxide (CO2) and energy fluxes measured by the eddy covariance method over a northern boreal lake that collects waters from a surrounding catchment dominated by upland forest and wetlands. The data period comprises more than half of the open-water period of 2013. The 30-min averages of CO2 fluxes ranged from -0.02 to 0.05 mg m-2 s-1. The monthly CO2 balances varied from 20 to 30 g m-2 (emission) between July and September, and decreased in October. A small daytime uptake of CO2, probably caused by the aquatic plants growing near the measurement mast, was observed from July to September. In September, we observed a temporary enhancement of CO2 efflux, which was attributed to both high wind speed and rapid cooling of the water and subsequent water column overturn. This peak was accompanied by a period of high sensible heat flux (SHF) from the water to the atmosphere, which is known to enhance the mixing of the water. The seasonal CO2 flux during the open-water period from the shallow part of the lake was estimated to be 120 g m-2 yr-1, which corresponds to a loss of approximately 25 g m-2 yr-1 from the terrestrial part of the catchment, assuming that the observed lake CO2 emissions result from the decomposition of the imported carbon. At midday, the net energy received by the lake was used mostly to heat the water, and only a minor part of it was converted to SHF and latent heat flux (LHF), with more energy used for the latter. While the SHF showed a clear diurnal cycle with a peak early in the morning and no flux in the afternoon, the diurnal pattern of LHF was more even, with evaporation occurring throughout the day until the freezing of the lake. Our data from this northern lake highlight the importance of thermal water mixing in the air-lake CO2 flux dynamics and imply that this flux constitutes a significant part of the annual catchment-scale carbon budget. [ABSTRACT FROM AUTHOR]

The Pallas area in northern Finland has served as a meteorological monitoring site for 80 years and, more recently, as a platform for atmospheric, ecological and hydrological research. Currently, Pallas comprises one of the most important research infrastructures in Finland and in the wider circumpolar region. Moreover, it is a successful example of the benefits obtained from scientific cooperation and integration among disciplines. This paper is an introduction to a special issue that collates studies related to greenhouse gas fluxes and concentrations, atmospheric aerosols and air pollutants, which were presented at the Fourth Pallas Symposium held in 2013. We give an overview of the historical and current research activities within the Pallas area, outline the most important infrastructure projects and list the recent literature that has originated from the various scientific programs and projects. The results of these activities are illustrated in this paper with examples of long-term data sets on variations in soil, lake and river water, air quality and greenhouse gas concentrations. [ABSTRACT FROM AUTHOR]

The fluxes of carbon dioxide and energy were measured by the eddy covariance method for four contrasting ecosystems within the Pallas area in northern Finland: Kenttärova spruce forest, open Lompolojänkkä wetland, treeless top of Sammaltunturi fell, and Pallasjärvi which is a lake. Clear differences in carbon and energy exchange were found among these ecosystems, in both the instantaneous fluxes and the related longer-term balances. The available solar energy and its partitioning into sensible and latent heat fluxes differed markedly among the sites. The characteristics of the CO2 exchange at individual sites varied in terms of the maximum uptake and emission capacity and the associated responses to environmental drivers. The highest instantaneous fluxes were observed over wetland and forest. The mean annual balance showed a considerable net uptake at the wetland, while the balances of the fell top and the forest were both close to zero. The lake, on the other hand, was estimated to be a relatively large source of carbon dioxide. An upscaling exercise based on the actual land-use map of the surroundings demonstrated the importance of including all the major ecosystems in the landscape CO2 balance. [ABSTRACT FROM AUTHOR]

Four new ground-based atmospheric monitoring stations in Finland were examined for local and large-scale signals in carbon dioxide mole fraction, and the results were compared with the corresponding values obtained from WMO/GAW site Pallas, northern Finland and NOAA/ESRL marine boundary layer reference time series. The measurements were filtered with knowledge of local weather and air composition. Periods of a presumably well-mixed boundary layer and relatively pollutant-free air were close to the Pallas time series in mole fraction, particularly in the winter. Wintertime mole fractions were 5-10 ppm higher than the signal in the marine boundary layer at all stations. The seasonal amplitude was 18-24 ppm, and diurnal amplitude was 0-3 ppm in winter and 3-20 ppm in the summer months. All stations, with the possible exception of the urban site in Helsinki, showed potential for observing a large-scale carbon dioxide signal. [ABSTRACT FROM AUTHOR]

Factors that strongly affects the dynamics of ground vegetation in northern boreal forests are grazing and trampling by reindeers. Cladina stellaris, which is the main species of lichen in northern areas, is also an important food source for reindeer. The population of reindeer is high, and it has a considerable influence on the boreal forest ecosystems. In this study, we describe the reindeer herding effects on above- and below-ground vegetation and on soil carbon (C) content. We compared the changes on both sides of a fence that has excluded reindeer from one area for the last 50 years. As expected, in the grazed area there were significantly smaller amounts of lichens and other vegetation. Total above-ground biomass as well as that of trees was higher in the area where no reindeer grazing occurred. In the ungrazed area, the tree-diameter distribution was strongly skewed towards thinner trees. There was no significant effect of reindeer grazing on the soil C content. [ABSTRACT FROM AUTHOR]

The role of the world's boreal forest for our understanding of the climate system is indisputable. Due to the large area covered, the forest's biophysical (e.g. surface energy balance, albedo) and biogeochemical (e.g. bidirectional exchange of greenhouse gases or aerosol recursors) processes are known to affect today's climate, and will need to be accounted or in studies of climate feedbacks in response to anthropogenic warming. However, observations that are needed to develop and evaluate terrestrial and climate models are still relatively scarce, especially for the Siberian part of the boreal forest. Here, we present a short overview of aerosol and greenhouse gas measurements over Siberia, aiming to also survey a large fraction of the existing literature in Russian. We aim to highlight areas of least data coverage and argue that, due to the importance of Siberia in the global climate system, a coordinated research program is needed to address some of the open research questions: The Pan Siberian Experiment. [ABSTRACT FROM AUTHOR]

We compared the CO exchange and its controls in the plant communities of a strongly patterned aapa mire, the Kaamanen fen in northern Finland. Based on a systematic vegetation inventory and an ordination analysis, four plant community types were chosen for the study: Ericales-Pleurozium string tops, Betula-Sphagnum string margins, Trichophorum tussock flarks and Carex-Scorpidium wet flarks. We measured plant community CO exchange with a closed chamber technique during the growing season of 2007 and early summer of 2008. Nonlinear regression models were used for simulating the CO exchange over the measurement period for different mire components and for the whole mire. The CO exchange dynamics distinguished two functional components in the mire: an ombrotrophic component (Ericales-Pleurozium string tops) and a minerotrophic component (other plant community types). Minerotrophic plant communities responded similarly to environmental controls, the most important of these being variation in leaf area and aerobic peat volume (water level). The ombrotrophic component dynamics were more obscure; frost and possibly peat moisture played a role. The minerotrophic communities functioned as effective CO sinks in the simulation, while the net CO exchange of the ombrotrophic community was close to zero. The smaller NEE of the ombrotrophic community was due to less efficient photosynthesis per unit leaf area in combination with high ecosystem respiration resulting from a large aerobic peat volume. Our study shows that a fen/bog functional dichotomy can also exist within one mire. Wet minerotrophic communities within northern mires can act as effective CO sinks. [ABSTRACT FROM AUTHOR]

∙ In this study, we used a canopy photosynthesis model which describes changes in photosynthetic capacity with slow temperature-dependent acclimations. ∙ A flux-partitioning algorithm was applied to fit the photosynthesis model to net ecosystem exchange data for 12 evergreen coniferous forests from northern temperate and boreal regions. ∙ The model accounted for much of the variation in photosynthetic production, with modeling efficiencies (mean > 67%) similar to those of more complex models. The parameter describing the rate of acclimation was larger at the northern sites, leading to a slower acclimation of photosynthesis to temperature. The response of the rates of photosynthesis to air temperature in spring was delayed up to several days at the coldest sites. Overall photosynthesis acclimation processes were slower at colder, northern locations than at warmer, more southern, and more maritime sites. ∙ Consequently, slow changes in photosynthetic capacity were essential to explaining variations of photosynthesis for colder boreal forests (i.e. where acclimation of photosynthesis to temperature was slower), whereas the importance of these processes was minor in warmer conifer evergreen forests. [ABSTRACT FROM AUTHOR]

Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized data on the carbon dioxide (CO2) exchange obtained from eddy covariance measurements from 12 wetland sites, covering 1–7 years at each site, across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, spanning temperate to arctic climate zones. The average summertime net ecosystem exchange of CO2 (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO2. To evaluate the influence of gross primary production (GPP) and ecosystem respiration ( Reco) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and Reco. After this correction neither Reco ( P= 0.162) nor GPP ( P= 0.110) correlated significantly with NEE on an annual basis. Spatial variation in annual and summertime Reco was associated with growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. GPP showed weaker correlations with environmental variables as compared with Reco, the exception being leaf area index (LAI), which correlated with both GPP and NEE, but not with Reco. Length of growing season period was found to be the most important variable describing the spatial variation in summertime GPP and Reco; global warming will thus cause these components to increase. Annual GPP and NEE correlated significantly with LAI and pH, thus, in order to predict wetland C exchange, differences in ecosystem structure such as leaf area and biomass as well as nutritional status must be taken into account. [ABSTRACT FROM AUTHOR]

Ozone concentrations measured at the Pallas-Sodankylä Global Atmosphere Watch station, located north of the Arctic Circle on a low mountain, were analysed together with data on nitrogen oxides concentrations and cloudiness. Air parcel trajectories were used to characterise the source area of air masses. The monthly mean ozone concentrations at Pallas showed a maximum in April and a broad minimum in late summer/autumn. In summer, the highest concentrations were observed in southern air masses on average, while the Atlantic and Arctic cases dominated in the earlier and latter pint of the year, respectively. In autumn/early winter, there were large differences between the source areas. Throughout the year, ozone concentrations were depleted in the presence of clouds. In summer, the reduction was larger in unpolluted than polluted cases. It is suggested that the formation of nitrate radical and the scavenging of dinitrogen pentoxide act as an efficient ozone sink in winter, while in summer the difference between clear and cloudy conditions is due to the direct and indirect effects of aqueous chemistry. [ABSTRACT FROM AUTHOR]

Deposition velocities (vd) of molecular hydrogen were measured in the northern boreal zone in Pallas. Three separate methods were used to estimate the soil uptake rate. For soil chamber measurements, three sites (forest, wetland and above tree line) were selected according to different soil properties. The deposition velocity was calculated with radon tracer method taking advantage of nocturnal radon buildup and hydrogen decrease in the shallow mixing layer above ground. A two dimensional model was used to estimate corresponding night time values for vd and radon exhalation rate. The radon tracer and two dimensional model results were in the range of 0.18-3.52 mm s-1. All results were obtained for unfrozen soils, and there was no clear temperature dependence related to vd. Soil chamber results were in the range of 0.06-0.52 mm s-1 excluding the wetland site which had negligibly small vd due to water saturation. [ABSTRACT FROM AUTHOR]

The carbon balance of terrestrial ecosystems is particularly sensitive to climatic changes in autumn and spring, with spring and autumn temperatures over northern latitudes having risen by about 1.1 °C and 0.8 °C, respectively, over the past two decades. A simultaneous greening trend has also been observed, characterized by a longer growing season and greater photosynthetic activity. These observations have led to speculation that spring and autumn warming could enhance carbon sequestration and extend the period of net carbon uptake in the future. Here we analyse interannual variations in atmospheric carbon dioxide concentration data and ecosystem carbon dioxide fluxes. We find that atmospheric records from the past 20 years show a trend towards an earlier autumn-to-winter carbon dioxide build-up, suggesting a shorter net carbon uptake period. This trend cannot be explained by changes in atmospheric transport alone and, together with the ecosystem flux data, suggest increasing carbon losses in autumn. We use a process-based terrestrial biosphere model and satellite vegetation greenness index observations to investigate further the observed seasonal response of northern ecosystems to autumnal warming. We find that both photosynthesis and respiration increase during autumn warming, but the increase in respiration is greater. In contrast, warming increases photosynthesis more than respiration in spring. Our simulations and observations indicate that northern terrestrial ecosystems may currently lose carbon dioxide in response to autumn warming, with a sensitivity of about 0.2 PgC °C-1, offsetting 90% of the increased carbon dioxide uptake during spring. If future autumn warming occurs at a faster rate than in spring, the ability of northern ecosystems to sequester carbon may be diminished earlier than previously suggested. [ABSTRACT FROM AUTHOR]

Eddy covariance (EC) measurements of net ecosystem CO2 exchange (NEE) were conducted on a boreal sedge fen in southern Finland (61°50′N, 24°12′E) during a 1.5-yr period covering two summers in 2004–2005. The EC data were complemented by chamber measurements, which enabled the partition of the daytime NEE into respiration and photosynthesis. A special emphasis was put on the hydrometeorological responses of CO2 exchange during a drought period in July 2005. A mean CO2 efflux of 0.009 mgCO2 m−2 s−1 was observed during mid-winter (January–February), while the night-time respiration during the two Julys averaged 0.09 mgCO2 m−2 s−1. During both years the mean mid-day uptake in late July was about −0.16 mgCO2 m−2 s−1. An annual CO2 balance of −188 gCO2 m−2 was observed in 2005. A slightly higher net sink of −219 gCO2 m−2 was estimated for 2004. The drought period experienced in July 2005 caused a clear depression in the daily NEE values. From the combined analysis of EC and chamber measurements it was concluded that this was mainly due to increased respiration, but evidence was also found of suppressed photosynthesis due to a high VPD. [ABSTRACT FROM AUTHOR]

Net CO2 exchange measured under well-mixed atmospheric conditions in four different mires in Sweden and Finland were used to analyse which factors were controlling photosynthesis and respiration. The parameters of a light response function showed strong seasonal variations with similar behaviour for all mires. The half-monthly nighttime respiration rates in the central part of the growing season were about two times higher in the southernmost, warmest site, Fäje, as compared to the northernmost, coldest site, Kaamanen. However, Kaamanen had high photosynthesis rates, and this in combination with the long daylight periods in the middle of the summer caused Kaamanen to have the largest net ecosystem exchange (NEE) during the summer period. Fäje that showed the highest productivity had also the highest respiration and therefore, the lowest NEE during summer. Correlation between half-monthly components and different environmental variables showed the highest correlation between the components themselves. Thereafter came temperature except for Fäje where water table depth (WTD) explained most of the variance both for detrended and temperature-normalized components. All sites showed dependencies between WTD and the respective components during drying up periods. Temperature sensitivity was higher for productivity than for respiration indicating that CO2 uptake would increase during global warming. [ABSTRACT FROM AUTHOR]

We measured the carbon dioxide (CO2) exchange with the eddy covariance (EC) method through three winters above a cultivated peat soil. During the first winter, the soil was ploughed, while for the next two winters it was grass-covered. On a weekly timescale, the emission was controlled by the soil temperature, whereas the vegetation had no clear impact. The deeper soil temperatures better correlated with the CO2 efflux, especially in frozen soil. The correlation with the air temperature was poor. After a mid-winter snowmelt, decreased CO2 efflux rates were temporarily detected, probably resulting from a lowered diffusion of CO2 from the soil air into the atmosphere. Moderate soil-thaw CO2 pulses were observed in the springs of 2001 and 2003. CO2 emission rates measured with the EC method were found to be significantly lower as compared to those measured with the chamber method. The cumulative CO2 emission between December and mid-March ranged from 80 to 178 g m−2 during three winters, correlating positively with air and soil temperatures and the number of snow-free days during that period. The projected increase in the air temperature related to global warming would boost the wintertime CO2 efflux at our site by 30–200% (35–114 g m−2), depending on the selected emission scenario. [ABSTRACT FROM AUTHOR]

In the boreal zone, large areas of natural mires have been drained and used for agriculture, resulting in net carbon dioxide (CO2) emissions and increased nitrous oxide emissions but decreased methane emissions. However, due to structural changes in agriculture, a substantial area of cropland on organic soil has been afforested. In order to estimate the carbon balance of afforested organic-soil cropland, we measured CO2 and water vapour (H2O) fluxes during year above a Scots pine plantation (Pinus sylvestris) in the middle-boreal zone, using the micrometeorological eddy covariance method. We observed CO2 uptake by the Scots pine stand from late April to mid-October with a daily average net uptake from May to the beginning of October. However, there were also periods of daily net efflux. High ecosystem respiration rates continued throughout the winter (mean winter respiration 0.036 mg CO2 m-2 s-1). As an annual average, the 30-year-old pine stand was a small source of CO2 (+50 g m-2 a-1) to the atmosphere, showing that the CO2 sequestration into the ecosystem was able to compensate for most of the carbon that was released by heterotrophic respiration from the drained soil. [ABSTRACT FROM AUTHOR]