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Reporting the greenhouse gas (GHG) emissions from the LULUCF sector in the GHG inventory requires sound methods for estimating both the inputs and outputs of carbon (C) in managed ecosystems. Soil CO2 balance of forests consists of the CO2 released from decomposing soil organic matter (SOM) and the C entering the soil through aboveground and belowground plant litter input. Peatlands drained for forestry release soil C as CO2 because the drainage deepens the oxic peat layer prone to SOM decomposition. IPCC Guidelines provide default CO2 emission factors for different climatic zones and the defaults or locally adapted static emission factors are commonly in use in GHG inventory reporting for drained peatlands. In this paper, we describe a new dynamic method to estimate the CO2 balance of drained peatland forest soils in Finland. Contrary to static emission factors, the annual CO2 release from soil is in our method estimated using empirical regression models driven by time series of tree basal area (BA), derived from the national forest inventories in Finland, time series of air temperature and the drained peatland forest site type. Aboveground and belowground litter input is also estimated using empirical models with newly acquired turnover rates for tree fine roots and BA as a dynamic driver. All major components of litter input from ground vegetation and live, harvested and naturally died trees are included. Our method produces an increasing trend of emissions from 1.4 to 7.9 Mt CO2 for drained peatland forest soils in Finland for the period 1990–2021, with a statistically significant difference between years 1990 and 2021. Across the period 1990–2021, annual emissions are on average 3.4 Mt and −0.3 Mt in southern and northern parts of Finland, respectively. When combined with data of the CO2 sink created by trees, it appears that in 2021 drained peatland forest ecosystems were a source of 2.3 Mt CO2 in southern Finland and a sink of 2.5 Mt CO2 in northern Finland. We compare the emissions produced by the new method with those produced by the old GHGI method of Finland and discuss the strengths and vulnerabilities of our method in comparison to static emission factors.

Estimates of regional and global freshwater N2O emissions have remained inaccurate due to scarce data and complexity of the multiple processes driving N2O fluxes the focus predominantly being on summer time measurements from emission hot spots, agricultural streams. Here, we present four‐season data of N2O concentrations in the water columns of randomly selected boreal lakes covering a large variation in latitude, lake type, area, depth, water chemistry, and land use cover. Nitrate was the key driver for N2O dynamics, explaining as much as 78% of the variation of the seasonal mean N2O concentrations across all lakes. Nitrate concentrations varied among seasons being highest in winter and lowest in summer. Of the surface water samples, 71% were oversaturated with N2O relative to the atmosphere. Largest oversaturation was measured in winter and lowest in summer stressing the importance to include full year N2O measurements in annual emission estimates. Including winter data resulted in fourfold annual N2O emission estimates compared to summer only measurements. Nutrient‐rich calcareous and large humic lakes had the highest annual N2O emissions. Our emission estimates for Finnish and boreal lakes are 0.6 and 29 Gg N2O‐N/year, respectively. The global warming potential of N2O from lakes cannot be neglected in the boreal landscape, being 35% of that of diffusive CH4 emission in Finnish lakes., Up‐scaling of freshwater N2O emissions at regional to global scales has remained challenging due to sparse data based on summer measurements. We collected seasonal data on N2O concentrations from 112 randomly selected boreal lakes in Finland and determined a representative set of possible drivers. Our data underline the key role of nitrate in regulating seasonal and spatial N2O concentrations. Nitrate explained 78% of the variation in N2O across all lakes. The Global Warming Potential of N2O in our data was 35% of that of diffusive CH4 emission underlining the importance to include N2O in landscape GHG evasion estimates.

Human-made reservoirs are now recognized as potentially significant sources of greenhouse gases, comparable to other anthropogenic sources, yet efforts to estimate these reservoir emissions have been hampered by the complexity of the underlying processes and a lack of coherent budgeting approaches. Here we present a unique modelling framework, the G-res Tool, which was explicitly designed to estimate the net C footprint of reservoirs across the globe. The framework involves the development of statistically robust empirical models describing the four major emission pathways for carbon-based greenhouse gases (GHG) from reservoirs: diffusive CO2 and CH4 emissions, bubbling CH4 emissions from the reservoir surface, and CH4 emissions due to degassing downstream the reservoir, based on an extensive meta-analysis of published data from the past three decades. These empirical models allow the prediction of reservoir-specific emissions, how they may shift over time and account for naturally occurring GHG generating pathways in aquatic networks.

Freshwater reservoirs are a known source of greenhouse gas (GHG) to the atmosphere, but their quantitative significance is still only loosely constrained. Although part of this uncertainty can be attributed to the difficulties in measuring highly variable fluxes, it is also the result of a lack of a clear accounting methodology, particularly about what constitutes new emissions and potential new sinks. In this paper, we review the main processes involved in the generation of GHG in reservoir systems and propose a simple approach to quantify the reservoir GHG footprint in terms of the net changes in GHG fluxes to the atmosphere induced by damming, that is, ´€˜what the atmosphere sees’. The approach takes into account the pre-impoundment GHG balance of the landscape, the temporal evolution of reservoir GHG emission profile as well as the natural emissions that are displaced to or away from the reservoir site resulting from hydrological and other changes. It also clarifies the portion of the reservoir carbon burial that can potentially be considered an offset to GHG emissions.

We studied CO2 release dynamics between a stockpile and the atmosphere and estimated dry matter losses of forest chips made from small-diameter Scots pine, either whole trees or stemwood. A stockpile established in early August in northern Finland was monitored for six months. Based on changes in the basic density of the chip samples, a dry matter loss of 2.6–2.8% occurred during the storage period, but there were no statistically significant differences between the two chip assortments. The dry matter loss derived from the CO2 effluxes was 1.3–1.5%. Even when capturing only about half of the carbon loss deduced from dry mass measurements, the CO2 measurements provided valuable information about decomposition dynamics in stockpiles. The CO2 flux rates were not static, but could differ by an order of magnitude, depending on the spatial location of flux measurement and the evolution of temperature conditions within the pile. Whole-tree chips generated more heat during the first two months after pile construction. The highest emissions and largest mass flows were measured at that time near the top of the pile, where also the humidity had condensed, suggesting that the stockpile structure and temperature gradient probably channeled the gas flows.

This study explores the variability in concentrations of dissolved CH4 and annual flux estimates in the pelagic zone in a statistically defined sample of 207 lakes in Finland. The lakes were situated in the boreal zone, in an area where the mean annual air temperature ranges from −2.8 to 5.9°C. We examined how lake CH4 dynamics related to regional lake types assessed according to the EU water framework directive. Ten lake types were defined on the basis of water chemistry, color, and size. Lakes were sampled for dissolved CH4 concentrations four times per year, at four different depths at the deepest point of each lake. We found that CH4 concentrations and fluxes to the atmosphere tended to be high in nutrient rich calcareous lakes, and that the shallow lakes had the greatest surface water concentrations. Methane concentration in the hypolimnion was related to oxygen and nutrient concentrations, and to lake depth or lake area. The surface water CH4 concentration was related to the depth or area of lake. Methane concentration close to the bottom can be viewed as proxy of lake status in terms of frequency of anoxia and nutrient levels. The mean pelagic CH4 release from randomly selected lakes was 49 mmol m−2 a−1. The sum CH4 flux (storage and diffusion) correlated with lake depth, area and nutrient content, and CH4 release was greatest from the shallow nutrient rich and humic lakes. Our results support earlier lake studies regarding the regulating factors and also the magnitude of global emission estimate. These results propose that in boreal region small lakes have higher CH4 fluxes per unit area than larger lakes, and that the small lakes have a disproportionate significance regarding to the CH4 release.

Question Measurements of ecosystem carbon (C) exchange are usually labour-intensive and expensive. In peatlands, these temporally and spatially limited measurements are often up-scaled over comparable ecosystems, such as ombrotrophic bogs, to provide an estimate for ecosystem level carbon dioxide (CO2) fluxes. Peatlands typically have moisture variations reflected in the presence of microforms (e.g. hummocks and hollows), each with characteristic plant life forms. However, so far the applicability of peatland type (e.g. bog, fen) or microforms for up-scaling has not been assessed. Does the vegetation composition or function of associated species differ so greatly between the same types of peatland that up-scaling is impossible? Location Five ombrotrophic bogs in Northern Europe; in Ireland, Finland and western Russia. Methods We described the variation in vegetation of microforms within and between ombrotrophic bogs using multivariate analyses. Thereafter, we measured CO2 exchange at different microforms and evaluated the relationship between vegetation structure and the light response of photosynthesis. Results Our results show that the community composition of hummocks, lawns and hollows was rather uniform at the plant life-form level. The photosynthetic capacity per leaf area unit was quite similar within microform classes over the different bogs. The observed differences between sites in capacity were mainly related to variation in leaf area. Conclusions A reliable estimate of ecosystem-level photosynthesis requires knowledge of the proportion of different microforms in an area and the leaf area characteristics for each microform in the year(s) in question. Assessments of ecosystem-level photosynthesis are important with regard to current and future changes in climate, as the most dramatic changes in peatlands involve water level drawdown, which in turn is likely to lead to changes in the relative proportions of microforms within peatlands.

Temporal and spatial variation in CH

This special issue comprising 14 articles emerged from the PAGES supported meeting: Holocene Circum Arctic Peatland Carbon Dynamics Community Wide Data Synthesis and Modeling Initiatives which took place from the 12 16 October 2013 in Bethlehem Pennsylvania. It is a precursor product of PAGES' C PEAT Working Group. ABSTRACT: Here we present results from the most comprehensive compilation of Holocene peat soil properties with associated carbon and nitrogen accumulation rates for northern peatlands. Our database consists of 268 peat cores from 215 sites located north of 45°N. It encompasses regions within which peat carbon data have only recently become available such as the West Siberia Lowlands the Hudson Bay Lowlands Kamchatka in Far East Russia and the Tibetan Plateau. For all northern peatlands carbon content in organic matter was estimated at 42 ± 3 (standard deviation) for Sphagnum peat 51 ± 2 for non Sphagnum peat and at 49 ± 2 overall. Dry bulk density averaged 0.12 ± 0.07 g/cm3 organic matter bulk density averaged 0.11 ± 0.05 g/cm3 and total carbon content in peat averaged 47 ± 6. In general large differences were found between Sphagnum and non Sphagnum peat types in terms of peat properties. Time weighted peat carbon accumulation rates averaged 23 ± 2 (standard error of mean) g C/m2/yr during the Holocene on the basis of 151 peat cores from 127 sites with the highest rates of carbon accumulation (25–28 g C/m2/yr) recorded during the early Holocene when the climate was warmer than the present. Furthermore we estimate the northern peatland carbon and nitrogen pools at 436 and 10 gigatons respectively. The database is publicly available at https://peatlands.lehigh.edu.

Peatlands are a major terrestrial carbon store and a persistent natural carbon sink during the Holocene, but there is considerable uncertainty over the fate of peatland carbon in a changing climate. It is generally assumed that higher temperatures will increase peat decay, causing a positive feedback to climate warming and contributing to the global positive carbon cycle feedback. Here we use a new extensive database of peat profiles across northern high latitudes to examine spatial and temporal patterns of carbon accumulation over the past millennium. Opposite to expectations, our results indicate a small negative carbon cycle feedback from past changes in the long-term accumulation rates of northern peatlands. Total carbon accumulated over the last 1000 yr is linearly related to contemporary growing season length and photosynthetically active radiation, suggesting that variability in net primary productivity is more important than decomposition in determining long-term carbon accumulation. Furthermore, northern peatland carbon sequestration rate declined over the climate transition from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA), probably because of lower LIA temperatures combined with increased cloudiness suppressing net primary productivity. Other factors including changing moisture status, peatland distribution, fire, nitrogen deposition, permafrost thaw and methane emissions will also influence future peatland carbon cycle feedbacks, but our data suggest that the carbon sequestration rate could increase over many areas of northern peatlands in a warmer future.

Greenhouse gas emissions from managed peatlands are annually reported to the UNFCCC. For the estimation of greenhouse gas (GHG) balances on a country-wide basis, it is necessary to know how soil–atmosphere fluxes are associated with variables that are available for spatial upscaling. We measured momentary soil–atmosphere CO 2 (heterotrophic and total soil respiration), CH 4 and N 2 O fluxes at 68 forestry-drained peatland sites in Finland over two growing seasons. We estimated annual CO 2 effluxes for the sites using site-specific temperature regressions and simulations in half-hourly time steps. Annual CH 4 and N 2 O fluxes were interpolated from the measurements. We then tested how well climate and site variables derived from forest inventory results and weather statistics could be used to explain between-site variation in the annual fluxes. The estimated annual CO 2 effluxes ranged from 1165 to 4437 g m −2 year −1 (total soil respiration) and from 534 to 2455 g m −2 year −1 (heterotrophic soil respiration). Means of 95% confidence intervals were ±12% of total and ±22% of heterotrophic soil respiration. Estimated annual CO 2 efflux was strongly correlated with soil respiration at the reference temperature (10 °C) and with summer mean air temperature. Temperature sensitivity had little effect on the estimated annual fluxes. Models with tree stand stem volume, site type and summer mean air temperature as independent variables explained 56% of total and 57% of heterotrophic annual CO 2 effluxes. Adding summer mean water table depth to the models raised the explanatory power to 66% and 64% respectively. Most of the sites were small CH 4 sinks and N 2 O sources. The interpolated annual CH 4 flux (range: −0.97 to 12.50 g m −2 year −1 ) was best explained by summer mean water table depth ( r 2 = 64%) and rather weakly by tree stand stem volume ( r 2 = 22%) and mire vegetation cover ( r 2 = 15%). N 2 O flux (range: −0.03 to 0.92 g m −2 year −1 ) was best explained by peat CN ratio ( r 2 = 35%). Site type explained 13% of annual N 2 O flux. We suggest that water table depth should be measured in national land-use inventories for improving the estimation of country-level GHG fluxes for peatlands.

Hot spots of CH4 emissions are a typical feature of pristine peatlands at the microsite and landscape scale. To determine whether rewetting and lake construction in a cutaway peatland would result in the re-creation of hot spots, we first measured CH4 fluxes over a 2-year period with static chambers and estimated annual emissions. Second, to assess whether rewetting and lake creation would produce hot spots at the landscape level, we hypothesized a number of alternative land use scenarios for the peatland following the cessation of peat extraction. Using the results from this study and other studies from literature, we calculated the global warming potential (GWP) of each scenario and the respective contribution of CH4. The results showed that hot spots of CH4 fluxes were observed as a consequence of microsite-specific differences in water table (WT) position and plant productivity. CH4 fluxes were closely related to peat temperature at 10 cm depth and WT position. Annual emissions ranged from 4.3 to 38.8 g CH4 m−2 yr−1 in 2002 and 3.2 to 28.8 g CH4 m−2 yr−1 in 2003. The scenario results suggest that lake creation is likely to result in the re-creation of a hot spot at the landscape level. However, the transition from cutaway to wetland ecosystem may lead to a reduction in the GWP of the peatland.

Peatland buffer zones with sedimentation ponds are established with the intention of capturing solids and nutrients liberated in drained forestry catchments. As noted in earlier fertilization experiments, added nitrogen (N) immediately increases nitrous oxide (N2O) emissions in such buffers, and we expected the same to happen after disturbances in the catchment caused by clear-cutting, soil preparation, and ditch cleaning. We measured N2O fluxes, water table dynamics, and vegetation cover from a wetland one year before and two years after the clear-cut and buffer establishment. The low pre-harvest emissions did not increase, but N2O emissions from the sedimentation pond exceeded those from humic lakes with a high N load. In the soil profile, N2O concentrations were high, indicating a potential to produce N2O in the buffer. In one sub-site the soil N2O concentration was below the atmospheric level, which was in accordance with the high concentrations of carbon dioxide (CO2) and methane (CH4). The change in vegetation along the overland flow paths could be explained by a shift in the species thriving in wet conditions but not in those requiring higher nutrient levels. In spite of the apparent potential of soil to produce N2O, the fluxes to the atmosphere remained low. Transformation of N2O to unobserved N2 may explain some of the low N emissions, together with the low concentrations entering the buffer.

Forest clear-cutting followed by soil preparation means disturbance for soil microorganisms and disruption of N and C cycles. We measured fluxes of N2O and dissolved organic carbon (DOC) in upland soil (podzol) and adjacent peat within a clear-cut forest catchment. Both soil types behaved in a similar way, showing net uptake of N2O in the first year after the clear-cutting, and turning to net release in the second. The N2O flux dynamics were similar to those of N content in logging residues, as reported from a nearby site. As organic matter is used in the food web of the decomposers, we attempted to explain the dynamics of N2O uptake and release by measuring the concurrent dynamics of the low molecular weight (LMW) fraction and the aromaticity of DOC in a soil solution. The labile and most readily available LMW fractions of DOC were nearly absent in the year following the clear-cutting, but rose after two years. The more refractory high molecular weight (HMW) fraction of DOC decreased two years after the clear-cutting. The first year’s net uptake of N2O could be accounted for by the growth of decomposer biomass in the logging residues and detritus from the degenerating ground vegetation, resulting in immobilization of nitrogen. Simultaneously, the labile, LMW fraction of DOC became almost completely exhausted. The low availability of the LMW fraction could retard the growth and cause the accumulated decomposer biomass to collapse. During the following winter and summer the fraction of LMW clearly increased, followed by increased N2O emissions. The presence of LMW DOC fractions, not the concentration of DOC, seems to be an important controller for N2O liberation after a major disturbance such as clear-cutting and site preparation. The complex connection between DOC characteristics, nitrification or denitrification merits further studies.

A high-resolution plant macrofossil analysis was applied to investigate wetness dynamics in a southern Finnish boreal bog, Kontolanrahka, during the last 5000 years. The chronological control and the age—depth model were based on 40 AMS radiocarbon dates. Pollen analysis provided information on regional-scale vegetation changes. Macrofossil analysis revealed prominent changes in vegetation assemblages during the late Holocene, indicating fluctuations in water-table. The reconstruction suggests that at the coring point, which nowadays is a relatively wet lawn, habitat type has repeatedly varied between transient communities similar to those currently represented in dry hummocks, very wet lawns and even hollows. In order to quantify historical changes in water-table, Generalized Additive Models (GAM) were used to investigate the current relationships between surface plant species and water-table depth. Modern water-table measurements and a survey of associated plant communities along moisture gradients provided data for GAM-analyses. The plant species showed unimodal distributions with apparent optima and narrow tolerances along the water-table gradient. A transfer function for water-table reconstruction was created by calibrating plant macrofossil records against the modern vegetation/water-table relationship using the weighted averaging partial least squares (WA-PLS) regression method. The quantitative water-table reconstruction for the late Holocene showed that the water-table depth had varied between 38 and 2.5 cm, the root mean square error of prediction being 3 cm. The detected historical wet and dry shifts were compared with other similar data from Finland, Sweden and Estonia, and from Western Europe. Despite some similarities, especially during the last c. 1000 years, noticeable differences in timing and duration occur, suggesting they may not have been driven only by climate, but also by local factors, including surface fires.

Closed (non-steady state) chambers are widely used for quantifying carbon dioxide (CO2) fluxes between soils or low-stature canopies and the atmosphere. It is well recognised that covering a soil or vegetation by a closed chamber inherently disturbs the natural CO2 fluxes by altering the concentration gradients between the soil, the vegetation and the overlying air. Thus, the driving factors of CO2 fluxes are not constant during the closed chamber experiment, and no linear increase or decrease of CO2 concentration over time within the chamber headspace can be expected. Nevertheless, linear regression has been applied for calculating CO2 fluxes in many recent, partly influential, studies. This approach has been justified by keeping the closure time short and assuming the concentration change over time to be in the linear range. Here, we test if the application of linear regression is really appropriate for estimating CO2 fluxes using closed chambers over short closure times and if the application of nonlinear regression is necessary. We developed a nonlinear exponential regression model from diffusion and photosynthesis theory. This exponential model was tested with four different datasets of CO2 flux measurements (total number: 1764) conducted at three peatlands sites in Finland and a tundra site in Siberia. Thorough analyses of residuals demonstrated that linear regression was frequently not appropriate for the determination of CO2 fluxes by closed-chamber methods, even if closure times were kept short. The developed exponential model was well suited for nonlinear regression of the concentration over time c(t) evolution in the chamber headspace and estimation of the initial CO2 fluxes at closure time for the majority of experiments. However, a rather large percentage of the exponential regression functions showed curvatures not consistent with the theoretical model which is considered to be caused by violations of the underlying model assumptions. Especially the effects of turbulence and pressure disturbances by the chamber deployment are suspected to have caused unexplainable curvatures. CO2 flux estimates by linear regression can be as low as 40% of the flux estimates of exponential regression for closure times of only two minutes. The degree of underestimation increased with increasing CO2 flux strength and was dependent on soil and vegetation conditions which can disturb not only the quantitative but also the qualitative evaluation of CO2 flux dynamics. The underestimation effect by linear regression was observed to be different for CO2 uptake and release situations which can lead to stronger bias in the daily, seasonal and annual CO2 balances than in the individual fluxes. To avoid serious bias of CO2 flux estimates based on closed chamber experiments, we suggest further tests using published datasets and recommend the use of nonlinear regression models for future closed chamber studies.

The restoration of cutaway peatlands provides an opportunity to return the carbon (C) sink function and to examine the influence of climate on peat formation and C accumulation. We studied CO2 exchange dynamics in 2002 and 2003 at a rewetted cutaway peatland located within the temperate maritime climatic zone. Gross photosynthesis (PG), ecosystem respiration (RTOT), and net ecosystem exchange (NEE) were observed in a range of microsites representing a hydroseral succession gradient: Typha latifolia – Phalaris arundinacea, Eriophorum angustifolium – Carex rostrata, and Juncus effusus – Holcus lanatus vegetation communities and areas of bare (unvegetated) peat. Annual rainfall was 26% higher in 2002 and 4% lower in 2003 than the long-term average and influenced water table position at all microsites. Observed instantaneous CO2 fluxes varied temporally and spatially at all microsites. Modelled PG was strongly dependent on irradiation (Photosynthetically Active Radiation) and the Vascular Green Area ...

Proxy climate data can be obtained from reconstructions of hydrological changes on ombrotrophic (rain-fed) peatlands using biological indicators, such as testate amoebae. Reconstruc- tions are based on transfer functions, relating modern assemblage composition to water table and moisture content, applied to fossil sequences. Existing transfer functions in Europe and elsewhere are limited geographically and there are often problems with missing or poor analogues. This paper presents a new palaeohydrological transfer function based on sampling raised mires from across Europe. Relationships between assemblages and hydrological variables are described using ordination analyses. Transfer functions are developed for depth to water table (n ¼ 119) and moisture content (n ¼ 132) with root mean squared errors (RMSEP) of 5.6 cm and 2.7% respectively. Both transfer functions have an r 2 of 0.71, based on 'leave one out' cross-validation. Comparisons with an existing transfer function for Britain show that the European transfer function performs well in inferring measured water tables in Britain but that the British data cannot be used to infer water tables for other European sites with confidence. Several of the key missing and poor analogue taxa problems encountered in previous transfer functions are solved. The new transfer function will be an important tool in developing peat-based palaeoclimatic reconstructions for European sites. Copyright 2006 John Wiley & Sons, Ltd.

Summary 1. We studied the patterns of litter decomposition in lake littoral habitats and investigated whether decay rates, as an integrating proxy for environmental conditions in the sediment, would co-vary with net carbon dioxide (CO2) exchange and methane (CH4) efflux. These gas fluxes are known to be sensitive to environmental conditions. Losses in the mass of cellulose, root, rhizome and moss litter were measured during 2 years in boreal littoral wetlands in Finland and compared with published data on concurrently measured gas fluxes. Four study sites covered a range of sediment types and hydrological conditions. 2. Decomposition was not linearly related to the duration of flooding but depended on sediment type. Readily decomposable litter fractions, such as cellulose and rhizome litter, lost mass at a faster rate in marshes with a longer period of flooding but wide water level fluctuations that hinder establishment of a Sphagnum cover, than in peat-forming fens. In marshes, the mean first-year mass losses were 83–99% and 19–62% for cellulose and rhizomes, respectively. In fens, the respective losses were 40–53% and 33%. In the first year, the loss in the mass of the more recalcitrant root litter did not differ between sites (mean 19–30%) and moss litter lost no mass. 3. The estimated first-year carbon loss from belowground litter was about 0.1–0.3 times ecosystem respiration and roughly similar to net carbon gas (CO2, CH4) efflux, suggesting that vascular plants and recent plant residues contribute substantially to ecosystem release of carbon gases. On the other hand, at least 40% of the mass of the belowground litter remained on a littoral site after the first 2 years of decomposition. Slow decomposition may indicate the accumulation of organic-rich sediments. The accumulated carbon could explain the excess CO2 release found in most littoral sites. In continuously inundated sites decomposition rates were similar to those in periodically flooded sites, but ecosystem-atmosphere CO2 exchange fell to close to zero. This discrepancy implies that the released CO2 is dissolved in water and may be exported into the pelagic zone of the lake.

We studied vegetation dynamics at peatlands, differing in their climate, land use management history and vegetation community in Ireland and Finland over a two-year period. Our aim was to develop a species-specific method to be used to (1) describe the seasonal dynamics of green (photosynthetic) area (GA) of the vegetation and (2) incorporate these changes into CO2 exchange models. The extent of temporal and spatial variation between and within communities indicated the need for a two-step calculation approach for each community. Firstly, at biweekly to monthly intervals, GA of all species within a range of vascular plant communities was estimated by non-destructive field measurements. Gaussian or log-normal models were fitted to describe the seasonal dynamics of each species. Secondly, an estimate of community vascular green area (VGA) was obtained by summing the modelled daily GA of all species within the community. The highest values of VGA (2.1–6.0 m2 m−2) occurred within the reed communities at the rewetted cutaway peatland in Ireland and the lowest at the ombrotrophic lawn communities in Finland (0.5–1.0 m2 m−2). The relationship between light saturated gross photosynthesis (P G) and VGA was either linear or hyperbolic depending on the degree of self-shading that occurred within each community. The addition of the VGA term into P G models improved the explaining power of the model by 57.6, 24.5 and 23% within the Typha latifolia, Phalaris arundinacea and Eriophorum angustifolium/Carex rostrata communities, respectively. VGA proved useful in recording the seasonal development of a wide range of peatland vascular plant communities over geographically and climatically different regions.

We show that sediment respiration is one of the key factors contributing to the high CO2 supersaturation in and evasion from Finnish lakes, and evidently also over large areas in the boreal landscape, where the majority of the lakes are small and shallow. A subpopulation of 177 randomly selected lakes (o100km 2 ) and 32 lakes with the highest total phosphorus (Ptot) concentrations in the Nordic Lake Survey (NLS) data base were sampled during four seasons and at four depths. Patterns of CO2 concentrations plotted against depth and time demonstrate strong CO2 accumulation in hypolimnetic waters during the stratification periods. The relationship between O2 departure from the saturation and CO2 departure from the saturation was strong in the entire data set (r 2 50.79, n 52 740, Po0.0001). CO2 concentrations were positively associated with lake trophic state and the proportion of agricultural land in the catchment. In contrast, CO2 concentrations negatively correlated with the peatland percentage indicating that either input of easily degraded organic matter and/or nutrient load from agricultural land enhance degradation. The average lake-area-weighted annual CO2 evasion based on our 177 randomly selected lakes and all Finnish lakes 4100km 2 (Rantakari & Kortelainen, 2005) was 42gCm � 2 LA (lake area), approximately 20% of the average annual C accumulation in Finnish forest soils and tree biomass (covering 51% of the total area of Finland) in the 1990s. Extrapolating our estimate from Finland to all lakes of the boreal region suggests a total annual CO2 evasion of about 50TgC, a value upto 40% of current estimates for lakes of the entire globe, emphasizing the role of small boreal lakes as conduits for transferring terrestrially fixed C into the atmosphere.

Hydrology controls the physical, chemical and biological processes in peatlands and hence could be the most important process regulating their function, development and characteristic biogeochemistry. Models describing hydrological processes and soil heat exchange phenomena are an important tool in understanding the peatland biogeochemical cycles of C and N. Presented in this paper is a peat soil climate model that uses weather data, mire site and peat characteristics as input data. The model is related to the heterogeneous peat characteristics (e.g. bulk density, degree of humification, remains of Sphagnum , Carex , Eriophorum and woody tissues), which in turn vary with depth. Evapotranspiration is related to weather parameters and tree stand characteristics. The model simulations were validated against the observed data collected during 1993 and 1994 growing seasons at a fen and bog sites at the Lakkasuo mire complex located in central Finland. The observed and simulated day-to-day variations in water table levels and soil temperatures during both seasons appeared overall to be in phase. As the model lacks the data needed to characterize the peat matrix hysteresis, the model was found lacking in response to wetting and drying cycles. The mean differences between simulated and observed water table levels during 1994 were −0.3 ± 1.3 and −0.5 ± 2.3 cm for the fen and bog sites, respectively. The model was found to be sensitive to mire surface characteristics and evapotranspiration, particularly for the bog site with an unsaturated zone deeper than at the fen. The absolute mean differences between the simulated and measured peat temperatures from 5 to 150 cm were less than 1.0 °C with a maximal standard deviation of 1.6 °C. The model deviations for the upper layers showed larger variations compared to deeper layers, implying a greater accuracy in defining the lower boundary of the thermal regime within the peat column.

Summary 1 In order to study the dynamics of primary production and decomposition in the lake littoral, an interface zone between the pelagial, the catchment and the atmosphere, we measured ecosystem/atmosphere carbon dioxide (CO2) exchange in the littoral zone of an eutrophic boreal lake in Finland during two open water periods (1998–1999). We reconstructed the seasonal net CO2 exchange and identified the key factors controlling CO2 dynamics. The seasonal net ecosystem exchange (NEE) was related to the amount of carbon accumulated in plant biomass. 2 In the continuously inundated zones, spatial and temporal variation in the density of aerial shoots controlled CO2 fluxes, but seasonal net exchange was in most cases close to zero. The lower flooded zone had a net CO2 uptake of 1.8–6.2 mol m−2 per open water period, but the upper flooded zone with the highest photosynthetic capacity and above-ground plant biomass, had a net CO2 loss of 1.1–7.1 mol m−2 per open water period as a result of the high respiration rate. The excess of respiration can be explained by decomposition of organic matter produced on site in previous years or leached from the catchment. 3 Our results from the two study years suggest that changes in phenology and water level were the prime cause of the large interannual difference in NEE in the littoral zone. Thus, the littoral is a dynamic buffer and source for the load of allochthonous and autochthonous carbon to small lakes.

Lake littoral zones have a transitional nature and dynamic conditions, which are reflected in their CH4 emissions. Thus, detailed studies are needed to assess the littoral CH4 emissions in a regional scale. In this study, CH4 fluxes were followed during the ice-free seasons in 1998 and 1999 by using the static chamber method in the littoral zone of two lakes in Finland. An exceptionally high water level in 1998 caused an unusually long inundation in otherwise ephemerally flooded zone. The flooding was normal in year 1999. The factors controlling CH4 emissions were examined and statistical response functions were constructed. Further, the effect of extended flooding on the littoral CH4 budged was estimated. The methane flux was primarily regulated by the water level in grass and sedge dominated eulittoral zone, but not in infralittoral reed and water lily stands. Methane emissions in the sedge dominated zone decreased significantly, when the flood was high enough to submerge the venting structures of the plants. Besides water level, sediment temperature determined CH4 emission. The cumulative CH4 emissions from the whole littoral wetlands in wet year were 1.1 times (L. Kevaton), or 0.61 and 0.79 times (L. Mekrijarvi) those in dry year. The crucial factor was the discrepancy between the exceptional and the average water level. The extension of inundated area does not necessarily increase CH4 emissions if the flood reaches infrequently inundated areas, which apparently have low CH4 production potential. This is the case especially, if the emissions in lower zones simultaneously decrease due to high water level. Our study analyses these complex responses between CH4 emissions and water level.

1. The effects of increasing CO 2 and nitrogen loading and of a change in water table and temperature on littoral CH4, N 2 O and CO 2 fluxes were studied in a glasshouse experiment with intact sediment cores including vegetation (mainly sedges), taken from a boreal eutrophic lake in Finland. Sediments with the water table held at a level of 0 or at -15 cm were incubated in an atmosphere of 360 or 720 p.p.m. CO 2 for 18 weeks. The experiment included fertilisation with NO - 3 and NH + 4 (to a total 3 g N m -2 ). 2. Changes in the water table and temperature strongly regulated sediment CH 4 and cCO 2 fluxes (community CO 2 release), but did not affect N 2 O emissions. Increase in the water table increased CH 4 emissions but reduced cCO 2 release, while increase in temperature increased emissions of both CO 2 and CH 4 . 3. The raised CO 2 increased carbon turnover in the sediments, such that cCO 2 release was increased by 16-26%. However, CH 4 fluxes were not significantly affected by raised CO 2 , although CH 4 production potential (at 22 °C) of the sediments incubated at high CO 2 was increased. In the boreal region, littoral CH 4 production is more likely to be limited by temperature than by the availability of carbon. Raised CO 2 did not affect N 2 O production by denitrification, indicating that this process was not carbon limited. 4. A low availability of NO - 3 did severely limit N 2 O production. The NO - 3 addition caused up to a 100-fold increase in the fluxes of N 2 O. The NH + 4 addition did not increase N 2 O fluxes, indicating low nitrification capacity in the sediments.

The springtime methane (CH4) emission from a small, eutrophied boreal lake was assessed during the winter ice-cover by measurement of gas ebullition and CH4 accumulation in the water column in association with the development of oxygen depletion after ice formation. The winter CH4 production was estimated to result in a loss of 3.6-7.9 g CH4 m(-2) from the lake to the atmosphere during the short period of ice melt. This could account for 22-48% of the annual CH4 emission from the pelagic zone of the lake. The contribution of winter to the annual CH4 release can be similar or even higher in seasonally ice-covered northern aquatic ecosystems than in northern terrestrial wetlands, thus winter must be considered in any studies into the aquatic CH4 emissions. The trophic state and wintertime oxygen conditions, linked to the changes in land-use in the catchments and climate, are important factors controlling the springtime lake CH4 emissions.

We studied the response of methane production and oxidation potentials in a minerotrophic and an ombrotrophic mire to water table fluctuations. In profiles where water table had not varied, the water-saturated layers showed significant potentials while the unsaturated layers did not. The production potentials in the saturated layers below water level ranged from 0.1 to 2.4 m gC H4 h ˇ1 (g d.w.) ˇ 1 and oxidation potentials (first order reaction rate constants) betweenˇ0.010 andˇ0.120 h ˇ1 (g d.w.) ˇ 1 . In profiles with constant water level, the maximal production potential occurred 20 cm and maximal oxidation potential 10 cm below water level. When water table varied only a little, production potentials slightly increased towards the autumn. After a water level draw-down, in the profiles from the dry microsites, production and oxidation potentials were detected in layers that had been unsaturated up to 6 weeks. The maximal oxidation zone was shifted downwards during low water periods. In a wet microsite, a 2 week period of unsaturation eliminated the production potentials and decreased the oxidation potentials. After a rise in the water level, the potentials were reactivated more rapidly in the wet than in the dry microsites. # 1999 Elsevier Science Ltd. All rights reserved.

CO2 and CH4 fluxes during the winter were measured at natural and drained bog and fen sites in eastern Finland using both the closed chamber method and calculations of gas diffusion along a concentration gradient through the snowpack. The snow diffusion results were compared with those obtained by chamber, but the winter flux estimates were derived from chamber data only. CH4 emissions from a poor bog were lower than those from an oligotrophic fen, while both CO2 and CH4 fluxes were higher in the Carex rostrata-occupied marginal (lagg) area of the fen than in the slightly less fertile centre. Average estimated winter CO2-C losses from virgin and drained forested peatlands were 41 and 68 g CO2-C m- 2, respectively, accounting for 23 and 21% of the annual total CO2 release from the peat. The mean release of CH4-C was 1.0 g in natural bogs and 3.4 g m-2 in fens, giving rise to winter emissions averaging to 22% of the annual emission from the bogs and 10% of that from the fens. These wintertime carbon gas losses in Finnish natural peatlands were even greater than reported average long-term annual C accumulation values (less than 25 g C m-2). The narrow range of 10–30% of the proportion of winter CO2 and CH4 emissions from annual emissions found in Finnish peatlands suggest that a wider generalization in the boreal zone is possible. Drained forested bogs emitted 0.3 g CH4-C m-2 on the average, while the effectively drained fens consumed an average of 0.01 g CH4-C m- 2. Reason for the low CH4 efflux or net oxidation in drained peatlands probably lies in low substrate supply and thus low CH4 production in the anoxic deep peat layers. N2O release from a fertilized grassland site in November–May was 0.7 g N2O m-2, accounting for 38% of the total annual emission, while a forested bog released none and two efficiently drained forested fens 0.09 (28% of annual release) and 0.04 g N2O m- 2 (27%) during the winter, respectively.

Northern peatlands are important terrestrial carbon stores, and they show large spatial and temporal variation in the atmospheric exchange of CO2 and CH4. Thus, annual carbon balance must be studied in detail in order to predict the climatic responses of these ecosystems. Closed-chamber methods were used to study CO2 and CH4 in hollow, Sphagnum angustifolium lawn, S. fuscum lawn, and hummock microsites within an om- brotrophic S. fuscum bog. Micrometeorological tower measurements were used as a ref- erence for the CH4 efflux from the bog. Low precipitation during May-August in 1994 (84 mm below the long-term average for the same period) and a warm July-August period caused the water table to drop by more than 15 cm below the peat surface in the hollows and to 48 cm below the surface in high hummocks. Increased annual total respiration exceeded gross production and resulted in a net C loss of 4-157 g/m2 in the different microsites. Drought probably caused irreversible desiccation in some lawns of S. angus- tifolium and S. balticum and in S. fuscum in the hummocks, while S. balticum growing in hollows retained its moisture and even increased its photosynthetic capacity during the July-August period. Seasonal (12 May-4 October) CH4 emissions ranged from 2 g CH4- C/M2 in drier S. fuscum hummocks and lawns to 7 and 14 g/m2 in wetter S. angustifolium- S. balticum lawns and hollows, respectively. Aerodynamic gradient measurements at the tower showed slightly higher CH4 flux rates than the average estimates for the whole bog obtained by closed-chamber methods. Winter C efflux comprised 30 g C02-C/m2 and 1 g CH4-C/m2 out of a total loss of 90 g C/M2 on average in the bog, and there was an estimated annual loss of 7 g C/M2 by leaching. This study shows how delicately the boreal bog's C balance in different microsites depends on climatic variations, especially the distribution of precipitation. It also confirms that severe C losses can occur in boreal bogs during extended summer droughts, even in years with annual temperatures close to the long-term average and with precipitation clearly greater than the long-term average.

1 In a glasshouse experiment we studied the effect of raised CO2 concentration (720 p.p.m.) on CH4 emission at natural boreal peat temperatures using intact cores of boreal peat with living vascular plants and Sphagnum mosses. After the end of the growing season half of the cores were kept unnaturally warm (17–20 °C). The potential for CH4 production and oxidation was measured at the end of the emission experiment. 2 The vascular cores (‘Sedge’) consisted of a moss layer with sedges, and the moss cores (‘Sphagnum’) of Sphagnum mosses (some sedge seedlings were removed by cutting). Methane efflux was 6–12 times higher from the Sedge cores than from the Sphagnum cores. The release of CH4 from Sedge cores increased with increasing temperature of the peat and decreased with decreasing temperature. Methane efflux from Sphagnum cores was quite stable independent of the peat temperatures. 3 In both Sedge and Sphagnum samples, CO2 treatment doubled the potential CH4 production but had no effect on the potential CH4 oxidation. A raised concentration of CO2 increased CH4 efflux weakly and only at the highest peat temperatures (17–20 °C). 4 The results suggest that in cool regions, such as boreal wetlands, temperature would restrict decomposition of the extra substrates probably derived from enhanced primary production of mire vegetation under raised CO2 concentrations, and would thus retard any consequent increase in CH4 emission.

Methane (CH4) fluxes were measured at 17 peatland sites with different nutritional and hydrological characteristics in the southern and middle boreal zones in Finland by a static chamber technique. Many of the natural peatlands also had counterparts drained for forestry 30–50 years ago. The mean emissions from May to September were 8.0 g CH4 m−2 for natural ombrogenous bogs and 19.0 g CH4 m−2 for natural minerogenous fens thus being higher than the 2 g CH4 m−2 yr−1 estimated for the Canadian peatlands. Change in the mean water table level had greater effect on CH4 fluxes on natural fens than on natural bogs. The mean CH4 emissions on drained bogs and fens were 3.9 g CH4 m−2 and 0.3 g CH4m−2, respectively. Some drained fens even had CH4 uptake from the atmosphere. The change in the mean water table had the lowest effect on CH4 fluxes on drained peatlands. The CH4 fluxes on peatlands (natural fens and bogs and drained peatlands) are associated with peat aeration, nutrient level, vegetation cover, peat compaction, peat temperature, and finally with microbial processes responsible for the net release of CH4. We could explain 67% of the variation in mean CH4 fluxes on Finnish peatlands by measuring the mean water table, peat bulk density, and peat pH. The present results can be used to predict the possible changes in CH4 emissions on peatlands if the climate is drying in north. For example, lowering of the present water table by 10 cm would induce a 70% reduction in the CH4 emissions from fens and a 45% reduction from bogs.

Carbon dioxide (CO2) exchange was studied at flark (minerotrophic hollow), lawn and hummock microsites in an oligotrophic boreal pine fen. Statistical response functions were constructed for the microsites in order to reconstruct the annual CO2 exchange balance from climate data. Carbon accumulation was estimated from the annual net CO2 exchange, methane (CH4) emissions and leaching of carbon. Due to high water tables in the year 1993, the average carbon accumulation at the flark, Eriophorum lawn, Carex lawn and hummock microsites was high, 2.91, 6.08, 2.83 and 2.66 mol C m–2, respectively, and for the whole peatland it was 5.66 mol m–2 year–1. During the maximum primary production period in midsummer, hummocks with low water tables emitted less methane than predicted from the average net ecosystem exchange (NEE), while the Carex lawns emitted slightly more. CH4 release during that period corresponded to 16% of the contemporary NEE. Annual C accumulation rate did not correlate with annual CH4 release in the microsites studied, but the total community CO2 release seemed to be related to CH4 emissions in the wet microsites, again excluding the hummocks. The dependence of CO2 exchange dynamics on weather events suggests that daily balances in C accumulation are labile and can change from net carbon uptake to net release, primarily in high hummocks on fens under warmer, drier climatic conditions.

Temporal and spatial variation in CH4 emissions was studied at hummock, Eriophorum lawn, flark and Carex lawn microsites in an oligotrophic pine fen over the growing season using a static chamber method, and CH4 production and oxidation potentials in peat profiles from hummock and flark were determined in laboratory incubation experiments. Emissions were lowest in the hummocks, and decreased with increasing hummock height, while in the lawns and flarks they increased with increasing sedge cover. Statistical response functions with water table and peat temperature as independent variables were calculated in order to reconstruct seasonal CH4 emissions by reference to the time series for peat temperature and water table specific to each microsite type. Mean CH4 emissions in the whole area in the snow-free period of 1993, weighted in terms of the proportions of the microsites, were 1.7 mol CH4 m-2. Potential CH4 production and oxidation rates were very low in the hummocks rising above the groundwater table, but were relatively similar when expressed per dry weight of peat both in the hummocks and flarks below the water table. The CH4 production potential increased in autumn at both microsites and CH4 oxidation potential seemed to decrease. The decrease in temperature in autumn certainly reduced in situ decomposition processes, possibly leaving unused substrates in the peat, which would explain the increase in CH4 production potential.

The effects of temperature, water table, and precipitation on the methane fluxes from a boreal low-sedge Sphagnum papillosum pine fen were analyzed with statistical cross correlations of daily data. The six measurement sites represented different vegetation surfaces of the mire (hummocks, lawns, and flarks) with increasing moisture. The dynamics were analyzed separately for the early summer (May-July) and the late summer (August-October) periods in addition to the whole summer (May-October) period. Methane emissions increased with increasing peat temperature. During the late summer period, changes in peat temperatures at depths of 20 and 50 cm were reflected in methane emissions within 2 days. The persistently high water tables during the measurement period probably did not reveal the dynamics between water table fluctuations and methane emissions very clearly. Methane emission levels correlated negatively with depths of the water tables, that is, high methane emissions were associated with low water tables and vice versa. The suppression of methane emissions by filling the unsaturated gas space during precipitation and the increased release rate caused by a declining water table could explain the result. Methane emissions correlated positively with changes in water tables, that is, a rise in water table increased methane emissions during the early and whole summer periods. Precipitation increased emissions with a lag from zero to several days throughout the summer. Generally, the estimated responses of methane fluxes to precipitation and changes in water table indicated similar time lags. Methane flux from the flark surfaces seemed to respond rapidly to rainfall and changes in water table with a lag of zero or 1 day. In the lawn-low hummock, the lawn and one hummock site, methane flux showed a slow response with several days lag. This study strongly indicates that temperature, water table, and precipitation affect methane emissions with complex interactions.

The CO2 released in soil respiration is formed from organic matter which differs in age and stability, ranging from soluble root exudates to more persistent plant remains. The contribution of roots, a relatively fast component of soil cycling, was studied in three experiments. (1) Willows were grown in a greenhouse and CO2 fluxes from the substrate soil (milled peat) and from control peat were measured. (2) CO2 fluxes from various peatland sites were measured at control points and points where the roots were severed from the plants. (3) CO2 fluxes in cultivated grassland established on peatland were measured in grassy subsites and in subsites where the growth of grass was prevented by regular tilling. The root-derived respiration followed the typical annual phenology of the vegetation, being at its maximum in the middle and late summer. All the experiments gave similar results, root-derived respiration accounting for 35–45% of total soil respiration in the middle and late summer at sites with an abundant vegetation. The root-derived respiration from the virgin peatland sites correlated well with the tree biomass, and also partly with the understorey vegetation, but in the drained sites the root effect was greater, even in the presence of less understorey vegetation than at virgin subsites.

We determined the landscape variation of forest floor (FF) CO2 uptake (photosynthesis, P), FF CO2 emission (respiration, R) in relation to net ecosystem CO2 exchange (NEE) and environmental factors along a forest-mire ecotone in Finland. The 450 m long ecotone extended from xeric, upland pine dominated habitats, through spruce and transitional spruce-pine-birch forest, to sedge peatlands downslope. The CO2 fluxes were measured at nine stations during 2005 using chamber and IR techniques. Instantaneous P and R measurements for each station were interpolated by fitting their response to continuous records of light (mean R2= 0.66) and temperature (mean R2= 0.77) recorded nearby to give annual estimates. Stand biomass increment was used to estimate the annual CO2 exchange contribution of the trees. Annual P values from −307 to −1632 gCO2m-2yr-1 were inversely correlated with FF light (r =−0.96), FF above-ground biomass (r =−0.92) and canopy openness (r =−0.95). Annual R values from 1263 to 2813 gCO2 m-2 yr-1 were correlated with tree stand foliar biomass (r = 0.77). Estimated NEE values varied from 546 to −1679 gCO2m-2/yr-1, with P contributing from −307 to −1632 gCO2m-2yr-1 (4–90%) to gross ecosystem photosynthetic production (GPP), and R from 1263 to 2813 gCO2m-2yr-1 (70–98%) to gross ecosystem respiration (GR).DOI: 10.1111/j.1600-0889.2007.00328.x

[1] Diurnal variation in methane fluxes can cause systematic errors in flux estimates, particularly at places with aquatic vegetation. Closed chamber measurements were made during the ice-free period in central Finland. Methane fluxes were quantified at an interval of a few hours for several boreal lakeshore communities consisting of Phragmites australis (Cav.) Trin. ex Steud. and of wetland sedges, grasses, and herbs. The automated measurement system was operated for 4 months in a P. australis stand and for 2 months in a site with mixed wetland vegetation. Additional manual measurements lasted for a few days. In P. australis stands the CH4 fluxes were typically highest around noon. On the average, efflux of CH4 measured during office hours (0800–1600 local time (LT)) should be corrected using a factor 0.68 to obtain a more reliable estimate of daily CH4 release. The fluctuation magnitude varied between different locations and over the study period, being the greatest in the most productive parts of the stands in August. A harmonic regression model, scaled with temperature and stand biomass, was successfully applied to reconstruct CH4 flux in P. australis stand. In the sedge, grass and herb stands the diurnal fluctuation in CH4 flux was generally small. Commonly, daily maximum in efflux occurred during evening or night and large (threefold) diurnal differences were occasionally found. Daytime measurements alone can result in a slight or moderate underestimate of the total flux for the wetland communities. These results recommend that spatial and seasonal differences in diurnal CH4 flux dynamics should be considered when planning measuring or modeling CH4 fluxes.

[1] Littoral zones at the interface of the lake and the catchment are intensive sites for mineralization of organic matter, but the contribution of vegetated littoral zone to winter fluxes of carbon dioxide (CO2) and methane (CH4) from lake ecosystems into the atmosphere is poorly known. We studied littoral carbon gas fluxes and their spatial controls at five boreal lakes of varying trophic state during three consequent winters with contrasting snow conditions and flooding regimes. Lake-wide estimates including littoral winter release and potential pelagic spring pulse of gases were calculated for three lakes. Large interannual and spatial differences in carbon gas fluxes were controlled by the interaction of climatic factors, ice and snow cover, on-site hydrology, and apparently substrate supply from biomass production of the previous growing season. Littoral CO2 fluxes ranged from 0.9 to 7.5 mol m−2 winter−1, and the CH4 fluxes ranged from 0.04 to 0.38 mol m−2 winter−1, the latter being highest in eutrophic lakes. The vegetated littoral contributed the most (66–78%) to winter CH4 emissions from two lakes; in the smallest and most productive lake, pelagic accumulation exceeded littoral release. The large variation in littoral CO2 release could contribute to between-winter differences of 82% in lake-wide carbon gas emissions. The water level of the preceding summer and precipitation during early winter were found to be useful predictors for littoral carbon gas fluxes in winter. This suggests that the carbon gas exchange of a shallow boreal lake can be highly sensitive to changes in snowfall and subsequent flooding.

We have examined how some major catchment disturbances may affect the aquatic greenhouse gas fluxes in the boreal zone, using gas flux data from studies made in 1994-1999 in the pelagic regions of seven lakes and two reservoirs in Finland. The highest pelagic seasonal average methane (CH(4)) emissions were up to 12 mmol x m(-2) x d(-1) from eutrophied lakes with agricultural catchments. Nutrient loading increases autochthonous primary production in lakes, promoting oxygen consumption and anaerobic decomposition in the sediments and this can lead to increased CH(4) release from lakes to the atmosphere. The carbon dioxide (CO(2)) fluxes were higher from reservoirs and lakes whose catchment areas were rich in peatlands or managed forests, and from eutrophied lakes in comparison to oligotrophic and mesotrophic sites. However, all these sites were net sources of CO(2) to the atmosphere. The pelagic CH(4) emissions were generally lower than those from the littoral zone. The fluxes of nitrous oxide (N(2)O) were negligible in the pelagic regions, apparently due to low nitrate inputs and/or low nitrification activity. However, the littoral zone, acting as a buffer for leached nitrogen, did release N(2)O. Anthropogenic disturbances of boreal lakes, such as increasing eutrophication, can change the aquatic greenhouse gas balance, but also the gas exchange in the littoral zone should be included in any assessment of the overall effect. It seems that autochthonous and allochthonous carbon sources, which contribute to the CH(4) and CO(2) production in lakes, also have importance in the greenhouse gas emissions from reservoirs.

Northern peatlands accumulate atmospheric CO2 thus counteracting climate warming. However, CH4 which is more efficient as a greenhouse gas than CO2, is produced in the anaerobic decomposition processes in peat. When peatlands are taken for forestry their water table is lowered by ditching. We studied long-term effects of lowered water table on the development of vegetation and the annual emissions of CO2, CH4 and N2O in an ombrotrophic bog and in a minerotrophic fen in Finland. Reclamation of the peat sites for forestry had changed the composition and coverage of the field and ground layer species, and increased highly the growth of tree stand at the drained fen. In general, drainage increased the annual CO2 emissions but the emissions were also affected by the natural fluctuations of water table. In contrast to CO2, drainage had decreased the emissions of CH4, the drained fen even consumed atmospheric CH4. CO2 and CH4 emissions were higher in the virgin fen than in the virgin bog. There were no N2O emissions from neither type of virgin sites. Drainage had, however, highly increased the N2O emissions from the fen. The results suggest that post-drainage changes in gas fluxes depend on the trophy of the original mires.

We determined the landscape variation of forest floor (FF) CO 2 uptake (photosynthesis, P), FF CO 2 emission (respiration, R) in relation to net ecosystem CO 2 exchange (NEE) and environmental factors along a forest-mire ecotone in Finland. The 450 m long ecotone extended from xeric, upland pine dominated habitats, through spruce and transitional spruce-pine-birch forest, to sedge peatlands downslope. The CO 2 fluxes were measured at nine stations during 2005 using chamber and IR techniques. Instantaneous P and R measurements for each station were interpolated by fitting their response to continuous records of light (mean R 2 = 0.66) and temperature (mean R 2 = 0.77) recorded nearby to give annual estimates. Stand biomass increment was used to estimate the annual CO 2 exchange contribution of the trees. Annual P values from −307 to −1632 gCO 2 m -2 yr -1 were inversely correlated with FF light (r =−0.96), FF above-ground biomass (r =−0.92) and canopy openness (r =−0.95). Annual R values from 1263 to 2813 gCO 2 m -2 yr -1 were correlated with tree stand foliar biomass (r = 0.77). Estimated NEE values varied from 546 to −1679 gCO 2 m -2 /yr -1 , with P contributing from −307 to −1632 gCO 2 m -2 yr -1 (4–90%) to gross ecosystem photosynthetic production (GPP), and R from 1263 to 2813 gCO 2 m -2 yr -1 (70–98%) to gross ecosystem respiration (GR). DOI: 10.1111/j.1600-0889.2007.00328.x