A B S T R A C TThe northern wetlands are one of the major sources of methane into the atmosphere. We measured annual methane emission from a boreal minerotrophic fen, Siikaneva, by the eddy covariance method. The average wintertime emissions were below 1 mg m −2 h −1 , and the summertime emissions about 3.5 mg m −2 h −1 . The water table depth did have any clear effect on methane emissions. During most of the year the emission depended on the temperature of peat below the water table. However, during the high and late summer the emission was independent on peat temperature as well. No diurnal cycle of methane flux was found. The total annual emission from the Siikaneva site was 12.6 g m −2 . The emissions of the snow free period contributed 91% to the annual emission. The emission pulse during the snow melting period was clearly detectable but of minor importance adding only less than 3% to the annual emission. Over 20% of the carbon assimilated during the year as carbon dioxide was emitted as methane. Thus methane emission is an important component of the carbon balance of the Siikaneva fen. This indicates need of taking methane into account when studying carbon balances of northern fen ecosystems.

A B S T R A C TLakes and other inland waters contribute significantly to regional and global carbon budgets. Emissions from lakes are often computed as the product of a gas transfer coefficient, k 600 , and the difference in concentration across the diffusive boundary layer at the airÁwater interface. Eddy covariance (EC) techniques are increasingly being used in lacustrine gas flux studies and tend to report higher values for derived k 600 than other approaches. Using results from an EC study of a small, boreal lake, we modelled k 600 using a boundarylayer approach that included wind shear and cooling. During stratification, fluxes estimated by EC occasionally were higher than those obtained by our models. The high fluxes co-occurred with winds strong enough to induce deflections of the thermocline. We attribute the higher measured fluxes to upwelling-induced spatial variability in surface concentrations of CO 2 within the EC footprint. We modelled the increased gas concentrations due to the upwelling and corrected our k 600 values using these higher CO 2 concentrations. This approach led to greater congruence between measured and modelled k values during the stratified period. k 600 has a well-resolved and Âcubic relationship with wind speed when the water column is unstratified and the dissolved gases well mixed. During stratification and using the corrected k 600 , the same pattern is evident at higher winds, but k 600 has a median value of Â7 cm h(1 when winds are less than 6 m s (1 , similar to observations in recent oceanographic studies. Our models for k 600 provide estimates of gas evasion at least 200% higher than earlier wind-based models. Our improved k 600 estimates emphasize the need for integrating within lake physics into models of greenhouse gas evasion.

Dynamics of carbon dioxide and energy exchange over a small boreal lake were investigated.Flux measurements have been carried out by the eddy covariance technique during two open-water periods (June-October) at Lake Kuivajärvi in Finland. Sensible heat (H) flux peaked in the early morning, and upward sensible heat flux at night results in unstable stratification over the lake. Minimum H was measured in the late afternoon, often resulting in adiabatic conditions or slightly stable stratification over the lake. The latent heat flux (LE) showed a different pattern, peaking in the afternoon and having a minimum at night. High correlation (r 2 = 0.75) between H and water-air temperature difference multiplied by wind speed (U) was found, while LE strongly correlated with the water vapor pressure deficit multiplied by U (r 2 = 0.78). Monthly average values of energy balance closure ranged between 70 and 99%. The lake acted as net source of carbon dioxide, and the measured flux (F CO2 ) averaged over the two open-water periods (0.7 μmol m À2 s À1) was up to 3 times higher than those reported in other studies. Furthermore, it was found that during period of high wind speed (>3 m s À1) shear-induced water turbulence controls the water-air gas transfer efficiency. However, under calm nighttime conditions, F CO2 was poorly correlated with the difference between the water and the equilibrium CO 2 concentrations multiplied by U. Nighttime cooling of surface water enhances the gas transfer efficiency through buoyancy-driven turbulent mixing, and simple wind speed-based transfer velocity models strongly underestimate F CO2 .

A B S T R A C TThe aim of this study was to asses how the variability in carbon gas exchange at the plant community scale affected the C gas exchange estimates at the ecosystem scale in a fen that was homogeneous in a micrometeorological sense, that is, had an even surface topography and plant cover. CO 2 and CH 4 exchange was measured at the plant community scale with chambers and at the ecosystem scale with the eddy covariance (EC) technique. Community-scale measurements were upscaled to the ecosystem scale by weighting the community-specific estimates by the area of the community. All communities were net CO 2 sinks and CH 4 sources during the growing season, but net ecosystem production (NEP) and CH 4 emissions ranged from 21 to 190 g CO 2 -C m −2 and from 4.3 to 13 g CH 4 -C m −2 , respectively, between the communities. The seasonal estimates of NEP and CH 4 , upscaled to the 200 m radius from the EC tower, were 82 and 7.9 g CH 4 -C m −2 , which agreed well with the EC measurements. As the communities differed markedly in their C gas dynamics, their proportions controlled the ecosystem scale estimates. Successful upscaling required detailed knowledge on the proportions and leaf area of the communities.

We have analyzed decade‐long methane flux data set from a boreal fen, Siikaneva, together with data on environmental parameters and carbon dioxide exchange. The methane flux showed seasonal cycle but no systematic diel cycle. The highest fluxes were observed in July–August with average value of 73 nmol m−2 s−1. Wintertime fluxes were small but positive, with January–March average of 6.7 nmol m−2 s−1. Daily average methane emission correlated best with peat temperatures at 20–35 cm depths. The second highest correlation was with gross primary production (GPP). The best correspondence between emission algorithm and measured fluxes was found for a variable‐slope generalized linear model (r2 = 0.89) with peat temperature at 35 cm depth and GPP as explanatory variables, slopes varying between years. The homogeneity of slope approach indicated that seasonal variation explained 79% of the sum of squares variation of daily average methane emission, the interannual variation in explanatory factors 7.0%, functional change 5.3%, and random variation 9.1%. Significant correlation between interannual variability of growing season methane emission and that of GPP indicates that on interannual time scales GPP controls methane emission variability, crucially for development of process‐based methane emission models. Annual methane emission ranged from 6.0 to 14 gC m−2 and was 2.7 ± 0.4% of annual GPP. Over 10‐year period methane emission was 18% of net ecosystem exchange as carbon. The weak relation of methane emission to water table position indicates that space‐to‐time analogy, used to extrapolate spatial chamber data in time, may not be applicable in seasonal time scales.

Abstract. Emissions of various C 2 -C 10 hydrocarbons (VOCs) and halogenated hydrocarbons (VHOCs) from a boreal wetland and a Scots pine forest floor in south-western Finland were measured by the static chamber technique. Isoprene was the main non-methane hydrocarbon emitted by the wetland, but small emissions of ethene, propane, propene, 1-butene, 2-methylpropene, butane, pentane and hexane were also detected. The isoprene emission from the wetland was observed to follow the commonly-used isoprene emission algorithm. The mean emission potential of isoprene was 224 µg m −2 h −1 for the whole season. This is lower than the emission potentials published earlier; that is probably at least partly due to the cold and cloudy weather during the measurements. No emissions were detected of monoterpenes or halogenated hydrocarbons from the wetland. The highest hydrocarbon emissions from the Scots pine forest floor were measured in spring and autumn. However, only a few measurements were conducted during summer. The main compounds emitted were monoterpenes. Isoprene emissions were negligible. The total monoterpene emission rates varied from zero to 373 µg m −2 h −1 . The results indicated that decaying plant litter may be the source for these emissions. Small emissions of chloroform (100-800 ng m −2 h −1 ), ethene, propane, propene, 2-methylpropene, cis-2-butene, pentane, hexane and heptane were detected. Comparison with Scots pine emissions showed that the forest floor may be an important monoterpene source, especially in spring.

A B S T R A C T Eddy covariance (EC) measurements of net ecosystem CO 2 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 CO 2 exchange during a drought period in July 2005. A mean CO 2 efflux of 0.009 mgCO 2 m −2 s −1 was observed during mid-winter (January-February), while the night-time respiration during the two Julys averaged 0.09 mgCO 2 m −2 s −1 . During both years the mean midday uptake in late July was about −0.16 mgCO 2 m −2 s −1 . An annual CO 2 balance of −188 gCO 2 m −2 was observed in 2005. A slightly higher net sink of −219 gCO 2 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.

Cities account for most anthropogenic greenhouse‐gas emissions, CO2 being most important. We evaluate the net urban contribution to CO2emissions by performing a meta‐analysis of all available 14 annual CO2budget studies. The studies are based on direct flux measurements using the eddy‐covariance technique which excludes all strong point sources. We show that the fraction of natural area is the strongest predictor of urban CO2 budgets, and this fraction can be used as a robust proxy for net urban CO2emissions. Up‐scaling, based on that proxy and satellite mapping of the fraction of natural area, identifies urban hotspots of CO2emissions; and extraction of 56 individual cities corroborates their inventory‐based estimates. Furthermore, cities are estimated as carbon‐neutral when the natural fraction is about 80%. This fresh view on the importance of cities in climate change treats cities as urban ecosystems: incorporating natural areas like vegetation.