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The regional variability in tundra and boreal carbon dioxide (CO 2 ) fluxes can be high, complicating efforts to quantify sink-source patterns across the entire region. Statistical models are increasingly used to predict (i.e., upscale) CO 2 fluxes across large spatial domains, but the reliability of different modeling techniques, each with different specifications and assumptions, has not been assessed in detail. Here, we compile eddy covariance and chamber measurements of annual and growing season CO 2 fluxes of gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem exchange (NEE) during 1990-2015 from 148 terrestrial high-latitude (i.e., tundra and boreal) sites to analyze the spatial patterns and drivers of CO 2 fluxes and test the accuracy and uncertainty of different statistical models. CO 2 fluxes were upscaled at relatively high spatial resolution (1 km 2 ) across the high-latitude region using five commonly used statistical models and their ensemble, that is, the median of all five models, using climatic, vegetation, and soil predictors. We found the performance of machine learning and ensemble predictions to outperform traditional regression methods. We also found the predictive performance of NEE-focused models to be low, relative to models predicting GPP and ER. Our data compilation and ensemble predictions showed that CO 2 sink strength was larger in the boreal biome (observed and predicted average annual NEE -46 and -29 g C m -2  yr -1 , respectively) compared to tundra (average annual NEE +10 and -2 g C m -2  yr -1 ). This pattern was associated with large spatial variability, reflecting local heterogeneity in soil organic carbon stocks, climate, and vegetation productivity. The terrestrial ecosystem CO 2 budget, estimated using the annual NEE ensemble prediction, suggests the high-latitude region was on average an annual CO 2 sink during 1990-2015, although uncertainty remains high., (© 2021 John Wiley & Sons Ltd.)

Sphagnum mosses account for most accumulated dead organic matter in peatlands. Therefore, understanding their responses to increasing atmospheric CO 2 is needed for estimating peatland C balances under climate change. A key process is photorespiration: a major determinant of net photosynthetic C assimilation that depends on the CO 2 to O 2 ratio. We used climate chambers to investigate photorespiratory responses of Sphagnum fuscum hummocks to recent increases in atmospheric CO 2 (from 280 to 400 ppm) under different water table, temperature, and light intensity levels. We tested the photorespiratory variability using a novel method based on deuterium isotopomers (D6 S /D6 R ratio) of photosynthetic glucose. The effect of elevated CO 2 on photorespiration was highly dependent on water table. At low water table (-20 cm), elevated CO 2 suppressed photorespiration relative to C assimilation, thus substantially increasing the net primary production potential. In contrast, a high water table (~0 cm) favored photorespiration and abolished this CO 2 effect. The response was further tested for Sphagnum majus lawns at typical water table levels (~0 and -7 cm), revealing no effect of CO 2 under those conditions. Our results indicate that hummocks, which typically experience low water table levels, benefit from the 20th century's increase in atmospheric CO 2 ., (© 2021 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.)

The net ecosystem CO 2 exchange (NEE) drives the carbon (C) sink-source strength of northern peatlands. Since NEE represents a balance between various production and respiration fluxes, accurate predictions of its response to global changes require an in depth understanding of these underlying processes. Currently, however, detailed information of the temporal dynamics as well as the separate biotic and abiotic controls of the NEE component fluxes is lacking in peatland ecosystems. In this study, we address this knowledge gap by using an automated chamber system established across natural and trenching/vegetation removal plots to partition NEE into its production (i.e., gross and net primary production; GPP and NPP) and respiration (i.e., ecosystem, heterotrophic and autotrophic respiration; ER, Rh and Ra) fluxes in a boreal peatland in northern Sweden. Our results showed that daily NEE patterns were driven by GPP while variations in ER were governed by Ra rather than Rh. Moreover, we observed pronounced seasonal shifts in the Ra/Rh and above/belowground NPP ratios throughout the main phenological phases. Generalized linear model analysis revealed that the greenness index derived from digital images (as a proxy for plant phenology) was the strongest control of NEE, GPP and NPP while explaining considerable fractions also in the variations of ER and Ra. In addition, our data exposed greater temperature sensitivity of NPP compared to Rh resulting in enhanced C sequestration with increasing temperature. Overall, our study suggests that the temporal patterns in NEE and its component fluxes are tightly coupled to vegetation dynamics in boreal peatlands and thus challenges previous studies that commonly identify abiotic factors as key drivers. These findings further emphasize the need for integrating detailed information on plant phenology into process-based models to improve predictions of global change impacts on the peatland C cycle., (© 2018 John Wiley & Sons Ltd.)

The stability of northern peatland's carbon (C) store under changing climate is of major concern for the global C cycle. The aquatic export of C from boreal peatlands is recognized as both a critical pathway for the remobilization of peat C stocks as well as a major component of the net ecosystem C balance (NECB). Here, we present a full year characterization of radiocarbon content ( 14 C) of dissolved organic carbon (DOC), carbon dioxide (CO 2 ), and methane (CH 4 ) exported from a boreal peatland catchment coupled with 14 C characterization of the catchment's peat profile of the same C species. The age of aquatic C in runoff varied little throughout the year and appeared to be sustained by recently fixed C from the atmosphere (<60 years), despite stream DOC, CO 2 , and CH 4 primarily being sourced from deep peat horizons (2-4 m) near the mire's outlet. In fact, the 14 C content of DOC, CO 2 , and CH 4 across the entire peat profile was considerably enriched with postbomb C compared with the solid peat material. Overall, our results demonstrate little to no mobilization of ancient C stocks from this boreal peatland and a relatively large resilience of the source of aquatic C export to forecasted hydroclimatic changes., (© 2017 The Authors Global Change Biology Published by John Wiley & Sons Ltd.)

At high latitudes, winter climate change alters snow cover and, consequently, may cause a sustained change in soil frost dynamics. Altered winter soil conditions could influence the ecosystem exchange of carbon dioxide (CO 2 ) and, in turn, provide feedbacks to ongoing climate change. To investigate the mechanisms that modify the peatland CO 2 exchange in response to altered winter soil frost, we conducted a snow exclusion experiment to enhance winter soil frost and to evaluate its short-term (1-3 years) and long-term (11 years) effects on CO 2 fluxes during subsequent growing seasons in a boreal peatland. In the first 3 years after initiating the treatment, no significant effects were observed on either gross primary production (GPP) or ecosystem respiration (ER). However, after 11 years, the temperature sensitivity of ER was reduced in the treatment plots relative to the control, resulting in an overall lower ER in the former. Furthermore, early growing season GPP was also lower in the treatment plots than in the controls during periods with photosynthetic photon flux density (PPFD) ≥800 μmol m -2  s -1 , corresponding to lower sedge leaf biomass in the treatment plots during the same period. During the peak growing season, a higher GPP was observed in the treatment plots under the low light condition (i.e. PPFD 400 μmol m -2  s -1 ) compared to the control. As Sphagnum moss maximizes photosynthesis at low light levels, this GPP difference between the plots may have been due to greater moss photosynthesis, as indicated by greater moss biomass production, in the treatment plots relative to the controls. Our study highlights the different responses to enhanced winter soil frost among plant functional types which regulate CO 2 fluxes, suggesting that winter climate change could considerably alter the growing season CO 2 exchange in boreal peatlands through its effect on vegetation development., (© 2017 John Wiley & Sons Ltd.)

In high-latitude regions, carbon dioxide (CO 2 ) emissions during the winter represent an important component of the annual ecosystem carbon budget; however, the mechanisms that control the winter CO 2 emissions are currently not well understood. It has been suggested that substrate availability from soil labile carbon pools is a main driver of winter CO 2 emissions. In ecosystems that are dominated by annual herbaceous plants, much of the biomass produced during the summer is likely to contribute to the soil labile carbon pool through litter fall and root senescence in the autumn. Thus, the summer carbon uptake in the ecosystem may have a significant influence on the subsequent winter CO 2 emissions. To test this hypothesis, we conducted a plot-scale shading experiment in a boreal peatland to reduce the gross primary production (GPP) during the growing season. At the growing season peak, vascular plant biomass in the shaded plots was half that in the control plots. During the subsequent winter, the mean CO 2 emission rates were 21% lower in the shaded plots than in the control plots. In addition, long-term (2001-2012) eddy covariance data from the same site showed a strong correlation between the GPP (particularly the late summer and autumn GPP) and the subsequent winter net ecosystem CO 2 exchange (NEE). In contrast, abiotic factors during the winter could not explain the interannual variation in the cumulative winter NEE. Our study demonstrates the presence of a cross-seasonal link between the growing season biotic processes and winter CO 2 emissions, which has important implications for predicting winter CO 2 emission dynamics in response to future climate change., (© 2016 John Wiley & Sons Ltd.)

Terrestrial vegetation currently absorbs approximately a third of anthropogenic CO2 emissions, mitigating the rise of atmospheric CO2. However, terrestrial net primary production is highly sensitive to atmospheric CO2 levels and associated climatic changes. In C3 plants, which dominate terrestrial vegetation, net photosynthesis depends on the ratio between photorespiration and gross photosynthesis. This metabolic flux ratio depends strongly on CO2 levels, but changes in this ratio over the past CO2 rise have not been analyzed experimentally. Combining CO2 manipulation experiments and deuterium NMR, we first establish that the intramolecular deuterium distribution (deuterium isotopomers) of photosynthetic C3 glucose contains a signal of the photorespiration/photosynthesis ratio. By tracing this isotopomer signal in herbarium samples of natural C3 vascular plant species, crops, and a Sphagnum moss species, we detect a consistent reduction in the photorespiration/photosynthesis ratio in response to the ∼100-ppm CO2 increase between ∼1900 and 2013. No difference was detected in the isotopomer trends between beet sugar samples covering the 20th century and CO2 manipulation experiments, suggesting that photosynthetic metabolism in sugar beet has not acclimated to increasing CO2 over >100 y. This provides observational evidence that the reduction of the photorespiration/photosynthesis ratio was ca. 25%. The Sphagnum results are consistent with the observed positive correlations between peat accumulation rates and photosynthetic rates over the Northern Hemisphere. Our results establish that isotopomers of plant archives contain metabolic information covering centuries. Our data provide direct quantitative information on the "CO2 fertilization" effect over decades, thus addressing a major uncertainty in Earth system models.

Boreal peatlands store large amounts of carbon, reflecting their important role in the global carbon cycle. The short-term exchange and the long-term storage of atmospheric carbon dioxide (CO2 ) in these ecosystems are closely associated with the permanently wet surface conditions and are susceptible to drought. Especially, the single most important peat forming plant genus, Sphagnum, depends heavily on surface wetness for its primary production. Changes in rainfall patterns are expected to affect surface wetness, but how this transient rewetting affects net ecosystem exchange of CO2 (NEE) remains unknown. This study explores how the timing and characteristics of rain events during photosynthetic active periods, that is daytime, affect peatland NEE and whether rain event associated changes in environmental conditions modify this response (e.g. water table, radiation, vapour pressure deficit, temperature). We analysed an 11-year time series of half-hourly eddy covariance and meteorological measurements from Degerö Stormyr, a boreal peatland in northern Sweden. Our results show that daytime rain events systematically decreased the sink strength of peatlands for atmospheric CO2 . The decrease was best explained by rain associated reduction in light, rather than by rain characteristics or drought length. An average daytime growing season rain event reduced net ecosystem CO2 uptake by 0.23-0.54 gC m(-2) . On an annual basis, this reduction of net CO2 uptake corresponds to 24% of the annual net CO2 uptake (NEE) of the study site, equivalent to a 4.4% reduction of gross primary production (GPP) during the growing season. We conclude that reduced light availability associated with rain events is more important in explaining the NEE response to rain events than rain characteristics and changes in water availability. This suggests that peatland CO2 uptake is highly sensitive to changes in cloud cover formation and to altered rainfall regimes, a process hitherto largely ignored., (© 2015 John Wiley & Sons Ltd.)

The global soil carbon pool is approximately three times larger than the contemporary atmospheric pool, therefore even minor changes to its integrity may have major implications for atmospheric CO2 concentrations. While theory predicts that the chemical composition of organic matter should constitute a master control on the temperature response of its decomposition, this relationship has not yet been fully demonstrated. We used laboratory incubations of forest soil organic matter (SOM) and fresh litter material together with NMR spectroscopy to make this connection between organic chemical composition and temperature sensitivity of decomposition. Temperature response of decomposition in both fresh litter and SOM was directly related to the chemical composition of the constituent organic matter, explaining 90% and 70% of the variance in Q10 in litter and SOM, respectively. The Q10 of litter decreased with increasing proportions of aromatic and O-aromatic compounds, and increased with increased contents of alkyl- and O-alkyl carbons. In contrast, in SOM, decomposition was affected only by carbonyl compounds. To reveal why a certain group of organic chemical compounds affected the temperature sensitivity of organic matter decomposition in litter and SOM, a more detailed characterization of the (13) C aromatic region using Heteronuclear Single Quantum Coherence (HSQC) was conducted. The results revealed considerable differences in the aromatic region between litter and SOM. This suggests that the correlation between chemical composition of organic matter and the temperature response of decomposition differed between litter and SOM. The temperature response of soil decomposition processes can thus be described by the chemical composition of its constituent organic matter, this paves the way for improved ecosystem modeling of biosphere feedbacks under a changing climate., (© 2013 John Wiley & Sons Ltd.)

While wetlands are the largest natural source of methane (CH4 ) to the atmosphere, they represent a large source of uncertainty in the global CH4 budget due to the complex biogeochemical controls on CH4 dynamics. Here we present, to our knowledge, the first multi-site synthesis of how predictors of CH4 fluxes (FCH4) in freshwater wetlands vary across wetland types at diel, multiday (synoptic), and seasonal time scales. We used several statistical approaches (correlation analysis, generalized additive modeling, mutual information, and random forests) in a wavelet-based multi-resolution framework to assess the importance of environmental predictors, nonlinearities and lags on FCH4 across 23 eddy covariance sites. Seasonally, soil and air temperature were dominant predictors of FCH4 at sites with smaller seasonal variation in water table depth (WTD). In contrast, WTD was the dominant predictor for wetlands with smaller variations in temperature (e.g., seasonal tropical/subtropical wetlands). Changes in seasonal FCH4 lagged fluctuations in WTD by ~17 ± 11 days, and lagged air and soil temperature by median values of 8 ± 16 and 5 ± 15 days, respectively. Temperature and WTD were also dominant predictors at the multiday scale. Atmospheric pressure (PA) was another important multiday scale predictor for peat-dominated sites, with drops in PA coinciding with synchronous releases of CH4 . At the diel scale, synchronous relationships with latent heat flux and vapor pressure deficit suggest that physical processes controlling evaporation and boundary layer mixing exert similar controls on CH4 volatilization, and suggest the influence of pressurized ventilation in aerenchymatous vegetation. In addition, 1- to 4-h lagged relationships with ecosystem photosynthesis indicate recent carbon substrates, such as root exudates, may also control FCH4. By addressing issues of scale, asynchrony, and nonlinearity, this work improves understanding of the predictors and timing of wetland FCH4 that can inform future studies and models, and help constrain wetland CH4 emissions.

Globally, soils store two to three times as much carbon as currently resides in the atmosphere, and it is critical to understand how soil greenhouse gas (GHG) emissions and uptake will respond to ongoing climate change. In particular, the soil-to-atmosphere CO2 flux, commonly though imprecisely termed soil respiration (RS ), is one of the largest carbon fluxes in the Earth system. An increasing number of high-frequency RS measurements (typically, from an automated system with hourly sampling) have been made over the last two decades; an increasing number of methane measurements are being made with such systems as well. Such high frequency data are an invaluable resource for understanding GHG fluxes, but lack a central database or repository. Here we describe the lightweight, open-source COSORE (COntinuous SOil REspiration) database and software, that focuses on automated, continuous and long-term GHG flux datasets, and is intended to serve as a community resource for earth sciences, climate change syntheses and model evaluation. Contributed datasets are mapped to a single, consistent standard, with metadata on contributors, geographic location, measurement conditions and ancillary data. The design emphasizes the importance of reproducibility, scientific transparency and open access to data. While being oriented towards continuously measured RS , the database design accommodates other soil-atmosphere measurements (e.g. ecosystem respiration, chamber-measured net ecosystem exchange, methane fluxes) as well as experimental treatments (heterotrophic only, etc.). We give brief examples of the types of analyses possible using this new community resource and describe its accompanying R software package.

To understand patterns in CO2 partial pressure (PCO2) over time in wetlands' surface water and porewater, we examined the relationship between PCO2 and land–atmosphere flux of CO2 at the ecosystem scale at 22 Northern Hemisphere wetland sites synthesized through an open call. Sites spanned 6 major wetland types (tidal, alpine, fen, bog, marsh, and prairie pothole/karst), 7 Köppen climates, and 16 different years. Ecosystem respiration (Reco) and gross primary production (GPP), components of vertical CO2 flux, were compared to PCO2, a component of lateral CO2 flux, to determine if photosynthetic rates and soil respiration consistently influence wetland surface and porewater CO2 concentrations across wetlands. Similar to drivers of primary productivity at the ecosystem scale, PCO2 was strongly positively correlated with air temperature (Tair) at most sites. Monthly average PCO2 tended to peak towards the middle of the year and was more strongly related to Reco than GPP. Our results suggest Reco may be related to biologically driven PCO2 in wetlands, but the relationship is site-specific and could be an artifact of differently timed seasonal cycles or other factors. Higher levels of discharge do not consistently alter the relationship between Reco and temperature normalized PCO2. This work synthesizes relevant data and identifies key knowledge gaps in drivers of wetland respiration. [ABSTRACT FROM AUTHOR]

We measured CO 2 and CH 4 fluxes using chambers and eddy covariance (only CO 2) from a moist moss tundra in Svalbard. The average net ecosystem exchange (NEE) during the summer (9 June–31 August) was negative (sink), with - 0.139 ± 0.032 µ mol m -2 s -1 corresponding to - 11.8 g C m -2 for the whole summer. The cumulated NEE over the whole growing season (day no. 160 to 284) was - 2.5 g C m -2. The CH 4 flux during the summer period showed a large spatial and temporal variability. The mean value of all 214 samples was 0.000511 ± 0.000315 µ mol m -2 s -1 , which corresponds to a growing season estimate of 0.04 to 0.16 g CH 4 m -2. Thus, we find that this moss tundra ecosystem is closely in balance with the atmosphere during the growing season when regarding exchanges of CO 2 and CH 4. The sink of CO 2 and the source of CH 4 are small in comparison with other tundra ecosystems in the high Arctic. Air temperature, soil moisture and the greenness index contributed significantly to explaining the variation in ecosystem respiration (Reco), while active layer depth, soil moisture and the greenness index were the variables that best explained CH 4 emissions. An estimate of temperature sensitivity of Reco and gross primary productivity (GPP) showed that the sensitivity is slightly higher for GPP than for Reco in the interval 0–4.5 ∘ C; thereafter, the difference is small up to about 6 ∘ C and then begins to rise rapidly for Reco. The consequence of this, for a small increase in air temperature of 1 ∘ (all other variables assumed unchanged), was that the respiration increased more than photosynthesis turning the small sink into a small source (4.5 g C m -2) during the growing season. Thus, we cannot rule out that the reason why the moss tundra is close to balance today is an effect of the warming that has already taken place in Svalbard. [ABSTRACT FROM AUTHOR]

Ditch cleaning (DC) is increasingly applied to facilitate forest regeneration following clear-cutting in Fennoscandinavia. However, its impact on the ecosystem carbon and greenhouse gas (GHG) balances is poorly understood. We conducted chamber measurements to assess the initial DC effects on carbon dioxide (CO2) and methane (CH4) fluxes in a recent forest clear-cut on wet mineral soil in boreal Sweden. Measurements were conducted in two adjacent areas over two pre-treatments (2018/19) and two years (2020/21) after conducting DC in one area. We further assessed the spatial variation of fluxes at three distances (4, 20, 40 m) from ditches. We found that DC lowered the water table level by 12 ± 2 cm (mean ± standard error) and topsoil moisture by 0.12 ± 0.01 m3 m−3. DC had a limited initial effect on the net CO2 exchange and its component fluxes. CH4 emissions were low during the dry pre-treatment years but increased particularly in the control area during the wet years of 2020/21. Distance to ditch had no consistent effects on CO2 and CH4 fluxes. Model extrapolations suggest that annual carbon emissions decreased over the four years from 6.7 ± 1.4 to 1.6 ± 1.6 t-C ha−1 year−1, without treatment differences. Annual CH4 emissions contributed <2.5% to the carbon balance but constituted 39% of the GHG balance in the control area during 2021. Overall, our study suggests that DC modified the internal carbon cycling but without significant impact on the carbon and GHG balances. [ABSTRACT FROM AUTHOR]

We measured CO2 and CH4 fluxes using chambers and eddy covariance (only CO2) from a moist moss tundra in Svalbard. The average net ecosystem exchange (NEE) during the summer (JuneAugust) was -0.40 g C m-2 day-1or -37 g C m-2 for the whole summer. Including spring and autumn periods the NEE was reduced to -6.8 g C m-2 and the annual NEE became positive, 24.7 g C m-2 due to the losses during the winter. The CH4 flux during the summer period showed a large spatial and temporal variability. The mean value of all 214 samples was 0.000511±0.000315 μmol m-2s-1 which corresponds to a growing season estimate of 0.04 to 0.16 g CH4 m-2. We find that this moss tundra emits about 94-100 g CO2-equivalents m-2 yr-1 of which CH4 is responsible for 3.5-9.3% using GWP100 of 27.9 respectively GWP20. Air temperature, soil moisture and greenness index contributed significantly to explain the variation in ecosystem respiration (Reco) while active layer depth, soil moisture and greenness index were the variables that best explained CH[sub 4 ]emissions. Estimate of temperature sensitivity of Reco and gross primary productivity showed that a modest increase in air temperature of 1 degree did not significantly change the NEE during the growing season but that the annual NEE would be even more positive adding another 8.5 g C m[sup -2] to the atmosphere. We tentatively suggest that the warming of the Arctic that has already taken place is partly responsible for the fact that the moist moss tundra now is a source of CO[sub 2 ]to the atmosphere. [ABSTRACT FROM AUTHOR]

• A managed boreal landscape in northern Sweden was a net sink for atmospheric CO 2. • A managed boreal landscape was a weaker CO 2 sink compared to a mature forest stand. • Earlier spring and sunnier autumn enhanced net CO 2 uptake in the boreal landscape. • Tall tower EC together with footprint model provide accurate landscape CO 2 estimates. Boreal forests exchange large amounts of carbon dioxide (CO 2) with the atmosphere. A managed boreal landscape usually comprises various potential CO 2 sinks and sources across forest stands of varying age classes, clear-cut areas, mires, and lakes. Due to this heterogeneity and complexity, large uncertainties exist regarding the net CO 2 balance at the landscape scale. In this study, we present the first estimate of the net CO 2 exchange over a managed boreal landscape (∼68 km2) in northern Sweden, based on tall tower eddy covariance measurements. Our results suggest that from March 1, 2016 to February 28, 2018, the heterogeneous landscape was a net CO 2 sink with a 2-year mean uptake of −87 ± 6 g C m−2 yr−1. Due to an earlier and warmer spring and sunnier autumn, the landscape was a stronger CO 2 sink during the first year (−122 ± 8 g C m−2) compared to the second year (−52 ± 9 g C m−2). Footprint analysis shows that 87% of the CO 2 flux measurements originated from forests, whereas mires, clear-cuts, lakes, and grassland contributed 11%, 1%, 0.7%, and 0.2%, respectively. Altogether, the CO 2 sink strength of the heterogeneous landscape was up to 38% lower compared to the sink strength of a mature stand surrounding the tower. Overall, this study suggests that the managed boreal landscape acted as a CO 2 sink and advocates tall tower eddy covariance measurements to improve regional carbon budget estimates. [ABSTRACT FROM AUTHOR]

Abstract: The net ecosystem CO2 exchange (NEE) drives the carbon (C) sink–source strength of northern peatlands. Since NEE represents a balance between various production and respiration fluxes, accurate predictions of its response to global changes require an in depth understanding of these underlying processes. Currently, however, detailed information of the temporal dynamics as well as the separate biotic and abiotic controls of the NEE component fluxes is lacking in peatland ecosystems. In this study, we address this knowledge gap by using an automated chamber system established across natural and trenching/vegetation removal plots to partition NEE into its production (i.e., gross and net primary production; GPP and NPP) and respiration (i.e., ecosystem, heterotrophic and autotrophic respiration; ER, Rh and Ra) fluxes in a boreal peatland in northern Sweden. Our results showed that daily NEE patterns were driven by GPP while variations in ER were governed by Ra rather than Rh. Moreover, we observed pronounced seasonal shifts in the Ra/Rh and above/belowground NPP ratios throughout the main phenological phases. Generalized linear model analysis revealed that the greenness index derived from digital images (as a proxy for plant phenology) was the strongest control of NEE, GPP and NPP while explaining considerable fractions also in the variations of ER and Ra. In addition, our data exposed greater temperature sensitivity of NPP compared to Rh resulting in enhanced C sequestration with increasing temperature. Overall, our study suggests that the temporal patterns in NEE and its component fluxes are tightly coupled to vegetation dynamics in boreal peatlands and thus challenges previous studies that commonly identify abiotic factors as key drivers. These findings further emphasize the need for integrating detailed information on plant phenology into process‐based models to improve predictions of global change impacts on the peatland C cycle. [ABSTRACT FROM AUTHOR]

In contrast to previous peatland carbon dioxide (CO2/ model sensitivity analyses, which usually focussed on only one or a few processes, this study investigates interactions between various biotic and abiotic processes and their parameters by comparing CoupModel v5 results with multiple observation variables. Many interactions were found not only within but also between various process categories simulating plant growth, decomposition, radiation interception, soil temperature, aerodynamic resistance, transpiration, soil hydrology and snow. Each measurement variable was sensitive to up to 10 (out of 54) parameters, from up to 7 different process categories. The constrained parameter ranges varied, depending on the variable and performance index chosen as criteria, and on other calibrated parameters (equifinalities). Therefore, transferring parameter ranges between models needs to be done with caution, especially if such ranges were achieved by only considering a few processes. The identified interactions and constrained parameters will be of great interest to use for comparisons with model results and data from similar ecosystems. All of the available measurement variables (net ecosystem exchange, leaf area index, sensible and latent heat fluxes, net radiation, soil temperatures, water table depth and snow depth) improved the model constraint. If hydraulic properties or water content were measured, further parameters could be constrained, resolving several equifinalities and reducing model uncertainty. The presented results highlight the importance of considering biotic and abiotic processes together and can help modellers and experimentalists to design and calibrate models as well as to direct experimental set-ups in peatland ecosystems towards modelling needs. [ABSTRACT FROM AUTHOR]

• We studied ditch cleaning (DC) effects on GHG fluxes in peatland forest clear-cuts. • Delayed herbaceous plant growth after DC reduced total production and respiration (R). • Impacts on nitrous oxide and methane fluxes after DC were minor. • Total GHG emissions from soil and ditches decreased by 44% after DC. • Reduced emissions were due to lower heterotrophic R likely caused by drought stress. Ditch cleaning (DC) is a well-established forestry practice across Fennoscandia to lower water table levels (WTL) and thereby facilitate the establishment of tree seedlings following clear-cutting. However, the implications from these activities for ecosystem-atmosphere greenhouse gas (GHG) exchanges are poorly understood at present. We assessed the initial DC effects on the GHG fluxes in a forest clear-cut on a drained fertile peatland in hemiboreal Sweden, by comparing chamber measurements of carbon dioxide (CO 2), methane (CH 4) and nitrous oxide (N 2 O) fluxes from soil and ditches in DC and uncleaned (UC) areas over the first two post-harvest years. We also evaluated spatial effects by comparing fluxes at 4 m and 40 m from ditches. We found that 2 years after DC, mean (±standard error) WTL of −65 ± 2 cm was significantly lower in the DC area compared to −56 ± 2 cm in the UC area. We further observed lower gross primary production and ecosystem respiration in the first year after DC which coincided with delayed development of herbaceous ground vegetation. We also found higher CH 4 uptake but no difference in N 2 O fluxes after DC. Greater CH 4 uptake occurred at 4 m compared to 40 m away from both cleaned and uncleaned ditches. Model extrapolation suggests that total annual GHG emissions in the second year were reduced from 49.4 ± 17.0 t-CO 2 -eq-ha−1-year−1 in the UC area to 27.8 ± 10.3 t-CO 2 -eq-ha−1-year−1 in the DC area. A flux partitioning approach suggested that this was likely caused by decreased heterotrophic respiration, possibly because of enhanced soil dryness following DC during the dry meteorological conditions. CH 4 and N 2 O fluxes from clear-cut areas contributed <2 % to the total (soil, ditches) GHG budget. Similarly the area-weighted contributions by CO 2 and CH 4 emissions from both cleaned and uncleaned ditches were <2 %. Thus, our study highlights that DC may considerably alter the post-harvest GHG fluxes of drained peatland forests. However, long-term observations under various site conditions and forest rotation stages are warranted to better understand DC effects on the forest GHG balance. [ABSTRACT FROM AUTHOR]

• Forest-floor is an annual net CO 2 source, which increases with stand age. • Understory production is higher in pine than in spruce stands and declines with age. • Total forest-floor respiration is primarily regulated by its autotrophic components. • Heterotrophic soil respiration remains stable across the managed boreal landscape. • Tree biomass drives the landscape-scale variability of forest-floor CO 2 fluxes. The forest-floor represents an important interface for various carbon dioxide (CO 2) fluxes, however, our knowledge of their variability and drivers across a managed boreal forest landscape is limited. Here, we used a three-year (2016−2018) data set of biometric- and chamber-based flux measurements to investigate the net forest-floor CO 2 exchange (NE ff) and its component fluxes across 50 forest stands spanning different soil types, tree species, and age classes within a 68 km2 boreal catchment in Sweden. We found that the forest-floor acted as a net CO 2 source with the 10th–90th percentile (used hereafter for describing reported variations) ranging from 149 to 399 g C m−2 yr−1. Among the key landscape attributes, stand age strongly affected most NE ff component fluxes, whereas tree species and soil type effects were weak and absent, respectively. Specifically, forest-floor net CO 2 emissions increased with stand age due to declining understory gross and net primary production, ranging between 77–275 and 49–163 g C m−2 yr−1, respectively. Furthermore, we observed higher understory production rates in pine than in spruce stands. Across the 50 stands, the total forest-floor respiration ranged between 340 and 549 g C m−2 yr−1 and its spatial variation was primarily regulated by its autotrophic components, i.e., understory and tree root respiration, which displayed divergent increasing and decreasing age-related trends, respectively. Furthermore, heterotrophic soil respiration remained within a relatively narrow range (154–290 g C m−2 yr−1), possibly owing to compensating gradients in forest-floor properties. We further identified tree biomass as the major driver of the landscape-scale variations of CO 2 fluxes, likely attributable to modulating effects on forest-floor resource availability and growing conditions. This implies that tree growth responses to forest management and global change will be particularly important for regulating magnitudes and spatial variations of forest-floor CO 2 fluxes in boreal forests. [ABSTRACT FROM AUTHOR]

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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]

Soil processes in high-latitude regions during winter are important contributors to global carbon circulation, but our understanding of the mechanisms controlling these processes is poor and observed temperature response coefficients of CO2 production in frozen soils deviate markedly from thermodynamically predicted responses (sometimes by several orders of magnitude). We investigated the temperature response of CO2 production in 23 unfrozen and frozen surface soil samples from various types of boreal forests and peatland ecosystems and also measured changes in water content in them after freezing. We demonstrate that deviations in temperature responses at subzero temperatures primarily emanates from water deficiency caused by freezing of the soil water, and that the amount of unfrozen water is mainly determined by the quality of the soil organic matter, which is linked to the vegetation cover. Factoring out the contribution of water limitation to the CO2 temperature responses yields response coefficients that agree well with expectations based on thermodynamic theory concerning biochemical temperature responses. This partitioning between a pure temperature response and the effect of water availability on the response of soil CO2 production at low temperatures is crucial for a thorough understanding of low-temperature soil processes and for accurate predictions of C-balances in northern terrestrial ecosystems. [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]

Peatlands play an important role in the global carbon cycle as they contain a large soil carbon stock. However, current climate change could potentially shift peatlands from being carbon sinks to carbon sources. Remote sensing methods provide an opportunity to monitor carbon dioxide (CO2) exchange in peatland ecosystems at large scales under these changing conditions. In this study, we developed empirical models of the CO2 balance (net ecosystem exchange, NEE), gross primary production (GPP), and ecosystem respiration (ER) that could be used for upscaling CO2 fluxes with remotely sensed data. Two to three years of eddy covariance (EC) data from five peatlands in Sweden and Finland were compared to modelled NEE, GPP and ER based on vegetation indices from 10 m resolution Sentinel-2 MSI and land surface temperature from 1 km resolution MODIS data. To ensure a precise match between the EC data and the Sentinel-2 observations, a footprint model was applied to derive footprint-weighted daily means of the vegetation indices. Average model parameters for all sites were acquired with a leave-one-out-cross-validation procedure. Both the GPP and the ER models gave high agreement with the EC-derived fluxes (R2 = 0.70 and 0.56, NRMSE = 14% and 15%, respectively). The performance of the NEE model was weaker (average R2 = 0.36 and NRMSE = 13%). Our findings demonstrate that using optical and thermal satellite sensor data is a feasible method for upscaling the GPP and ER of northern boreal peatlands, although further studies are needed to investigate the sources of the unexplained spatial and temporal variation of the CO2 fluxes. [ABSTRACT FROM AUTHOR]

Drainage and peat harvesting may induce considerable changes in the fluxes to the atmosphere of the greenhouse gases CH4 and CO2 in peatlands. In this study, fluxes of CH4 and CO2 were measured with the closed chamber method in 6Swedish peatlands that are being mined. Fluxes of CH4 were much higher from ditches than from mining 'strips', i.e. emissions from ditches dominated in most peatlands. The total CH4 emission during the growing season (0.41 to 4.5 g CH4 m-2 yr-1) was similar to emissions from virgin peatlands. Emissions from ditches can probably be kept low by keeping the ditches clear from vegetation. Like CH4, CO2 wasreleased to the atmosphere from both ditches and strips in most peatlands. The total emission during the growing season (0.23 to 1.0 kg CO2 m-2 yr-1) was strongly dominated by the strips. Compared to the total peat yield during mining these CO2 emission rates imply that on average ca 6% of the peat carbon is lost by microbial decomposition in the mire. [ABSTRACT FROM AUTHOR]

Temperature profoundly affects saprotrophic respiration rates, and carbon quality theory predicts that the rates' temperature sensitivity should increase as the quality of the carbon source declines. However, reported relationships between saprotrophic respiration responses to temperature and carbon quality vary widely. Some of this variability may arise from confounding effects related to both substrate quality and substrate availability. The importance of these variables, as well as substrate diffusion and uptake rates, for the temperature sensitivity of saprotrophic respiration has been validated theoretically, but not empirically demonstrated. Thus, we tested effects of varying substrate uptake rates on the temperature sensitivity of organic carbon degradation. For this purpose we created a model system using the organic layer (O-horizon), of a boreal forest soil, specifically to test effects of varying monomer uptake and release rates. The addition of both monomers and polymers generally increased the temperature sensitivity of saprotrophic respiration. In response to added monomers, there was a linear increase in the temperature sensitivity of both substrate-induced respiration and the specific growth rate with increasing rate of substrate uptake as indicated by the CO2 production at 14 °C. Both of these responses diverge from those predicted by the carbon quality theory, but they provide the first empirical evidence consistent with model predictions demonstrating increased temperature sensitivity with increased uptake rate of carbon monomers over the cell membrane. These results may explain why organic material of higher carbon quality induces higher temperature responses than lower carbon quality compounds, without contradicting carbon quality theory. [ABSTRACT FROM AUTHOR]

Abstract: Studies using satellite sensor-derived data as input to models for CO2 exchange show promising results for closed forest stands. There is a need for extending this approach to other land cover types, in order to carry out large-scale monitoring of CO2 exchange. In this study, three years of eddy covariance data from two peatlands in Sweden were averaged for 16-day composite periods and related to data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and modeled photosynthetic photon flux density (PPFD). Noise in the time series of MODIS 250m vegetation indices was reduced by using double logistic curve fits. Smoothed normalized difference vegetation index (NDVI) showed saturation during summertime, and the enhanced vegetation index (EVI) generally gave better results in explaining gross primary productivity (GPP). The strong linear relationships found between GPP and the product of EVI and modeled PPFD (R 2 =0.85 and 0.76) were only slightly stronger than for the product of EVI and MODIS daytime 1km land surface temperature (LST) (R 2 =0.84 and 0.71). One probable reason for these results is that several controls on GPP were related to both modeled PPFD and daytime LST. Since ecosystem respiration (ER) was largely explained by diurnal LST in exponential relationships (R 2 =0.89 and 0.83), net ecosystem exchange (NEE) was directly related to diurnal LST in combination with the product of EVI and modeled PPFD in multiple exponential regressions (R 2 =0.81 and 0.73). Even though the R 2 values were somewhat weaker for NEE, compared to GPP and ER, the RMSE values were much lower than if NEE would have been estimated as the sum of GPP and ER. The overall conclusion of this study is that regression models driven by satellite sensor-derived data and modeled PPFD can be used to estimate CO2 fluxes in peatlands. [Copyright &y& Elsevier]

Abstract: Several recent studies have highlighted the importance of soil organic matter (SOM) mineralization at high latitudes during winter for ecosystem carbon (C) balances, and the ability of the soil to retain unfrozen water at sub-zero temperatures has been shown to be a major determinant of C mineralization rates. Further, SOM is believed to strongly influence the liquid water contents in frozen surface layers of boreal forest soils and tundra, but the mechanisms and specific factors involved are currently unknown. Here we evaluate the effects of the chemical composition of SOM on the amount of unfrozen water, the pore size equivalents in which unfrozen water can exist, and the microbial heterotrophic activity at sub-zero temperatures in boreal forest soils. To do this, we have characterized the chemical composition of SOM in forest soil samples (surface O-horizons) using solid state CP-MAS (cross polarization magic angle spinning) NMR spectroscopy. The acquired information was then used to elucidate the extent to which different fractions of SOM can explain the observed variations in unfrozen water content, pore size equivalents, and biogenic CO2 production rates in the examined soil samples under frozen conditions (−4°C). The data evaluation was done by the use of principal component analysis (PCA) and projections to latent structures by means of partial least square (PLS). We conclude that aromatic, O-aromatic, methoxy/N-alkyl and alkyl C are the major SOM components affecting frozen boreal forest soil’s ability to retain unfrozen water and sustain heterotrophic activity (95% confidence level). Our results reveal that solid carbohydrates have a significant negative impact (95% confidence level) on CO2 production in frozen boreal spruce forest soils, in contrast to the positive effects of carbohydrate polymers during unfrozen conditions. We conclude that the hierarchy of environmental factors controlling SOM mineralization changes as soils freeze. The effect of SOM composition on pore size distribution and unfrozen water content has a superior influence on SOM mineralization and hence on heterotrophic CO2 production of frozen soils. [Copyright &y& Elsevier]

• Below-canopy cospectra models were developed to improve EC flux measurements. • Boreal forest floor acted as an annual net CO 2 source to the atmosphere. • Forest floor contributed to 82 ± 8% of R eco and 12 ± 3% of GPP. • Forest floor fluxes drive differences in C balances of contrasting boreal stands. • Forest floor CO 2 effluxes increased by summer drought and warm non-growing season. The forest floor provides an important interface of soil-atmosphere CO 2 exchanges but their controls and contributions to the ecosystem-scale carbon budget are uncertain due to measurement limitations. In this study, we deployed eddy covariance systems below- and above-canopy to measure the spatially integrated net forest floor CO 2 exchange (NFFE) and the entire net ecosystem CO 2 exchange (NEE) at two mature contrasting stands located in close vicinity in boreal Sweden. We first developed an improved cospectra model to correct below-canopy flux data. Our empirical below-canopy cospectra models revealed a greater contribution of large- and small-scale eddies in the trunk space compared to their distribution in the above-canopy turbulence cospectra. We found that applying the above-canopy cospectra model did not affect the below-canopy annual CO 2 fluxes at the sparse pine forest but significantly underestimated fluxes at the dense mixed spruce-pine stand. At the mixed spruce-pine stand, forest floor respiration (R ff) was higher and photosynthesis (GPP ff) was lower, leading to a 1.4 times stronger net CO 2 source compared to the pine stand. We further found that drought enhanced R ff more than GPP ff , leading to increased NFFE. Averaged across the six site-years, forest floor fluxes contributed 82% to ecosystem-scale respiration (R eco) and 12% to gross primary production (GPP). Since the annual GPP was similar between both stands, the considerable difference in their annual NEE was due to contrasting R eco , the latter being primarily driven by the variations in NFFE. This implies that NFFE acted as the driver for the differences in NEE between these two contrasting stands. This study therefore highlights the important role of forest floor CO 2 fluxes in regulating the boreal forest carbon balance. It further calls for extended efforts in acquiring high spatiotemporal resolution data of forest floor fluxes to improve predictions of global change impacts on the forest carbon cycle. [ABSTRACT FROM AUTHOR]