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The boreal biome exchanges large amounts of carbon (C) and greenhouse gases (GHGs) with the atmosphere and thus significantly affects the global climate. A managed boreal landscape consists of various sinks and sources of carbon dioxide (CO2), methane (CH4), and dissolved organic and inorganic carbon (DOC and DIC) across forests, mires, lakes, and streams. Due to the spatial heterogeneity, large uncertainties exist regarding the net landscape carbon balance (NLCB). In this study, we compiled terrestrial and aquatic fluxes of CO2, CH4, DOC, DIC, and harvested C obtained from tall‐tower eddy covariance measurements, stream monitoring, and remote sensing of biomass stocks for an entire boreal catchment (~68 km2) in Sweden to estimate the NLCB across the land–water–atmosphere continuum. Our results showed that this managed boreal forest landscape was a net C sink (NLCB = 39 g C m−2 year−1) with the landscape–atmosphere CO2 exchange being the dominant component, followed by the C export via harvest and streams. Accounting for the global warming potential of CH4, the landscape was a GHG sink of 237 g CO2‐eq m−2 year−1, thus providing a climate‐cooling effect. The CH4 flux contribution to the annual GHG budget increased from 0.6% during spring to 3.2% during winter. The aquatic C loss was most significant during spring contributing 8% to the annual NLCB. We further found that abiotic controls (e.g., air temperature and incoming radiation) regulated the temporal variability of the NLCB whereas land cover types (e.g., mire vs. forest) and management practices (e.g., clear‐cutting) determined their spatial variability. Our study advocates the need for integrating terrestrial and aquatic fluxes at the landscape scale based on tall‐tower eddy covariance measurements combined with biomass stock and stream monitoring to develop a holistic understanding of the NLCB of managed boreal forest landscapes and to better evaluate their potential for mitigating climate change. [ABSTRACT FROM AUTHOR]

Net uptake of carbon dioxide (CO 2 ) was observed during the winter when using the eddy covariance (EC) technique above a ∼90-year-old Scots pine ( Pinus sylvestris L.) stand in northern Sweden. This uptake occurred despite photosynthetic dormancy. This discrepancy led us to investigate the potential impact of decoupling of below- and above-canopy air mass flow and accompanying below-canopy horizontal advection on these measurements. We used the correlation of above- and below-canopy standard deviation of vertical wind speed (σ w ), derived from EC measurements above and below the canopy, as the main mixing criterion. We identified 0.33 m s −1 and 0.06 m s −1 as site-specific σ w thresholds for above and below canopy, respectively, to reach the fully coupled state. Decoupling was observed in 45% of all cases during the measurement period (5.11.2014–25.2.2015). After filtering out decoupled periods the above-canopy mean winter NEE shifted from −0.52 μmol m −2 s −1 to a more reasonable positive value of 0.31 μmol m −2 s −1 . None of the above-canopy data filtering criteria we tested (i.e., friction velocity threshold; horizontal wind speed threshold; single-level σ w threshold) ensured sufficient mixing. All missed critical periods that were detected only by the two-level filtering approach. Tower-surrounding topography induced a predominant below-canopy wind direction and consequent wind shear between above- and below-canopy air masses. These processes may foster decoupling and below-canopy removal of CO 2 rich air. To determine how broadly such a topographical influence might apply, we compared the topography surrounding our tower to that surrounding other forest flux sites worldwide. Medians of maximum elevation differences within 300 m and 1000 m around 110 FLUXNET forest EC towers were 24 m and 66 m, respectively, compared to 24 m and 114 m, respectively, at our site. Consequently, below-canopy flow may influence above-canopy NEE detections at many forested EC sites. Based on our findings we suggest below-canopy measurements as standard procedure at sites evaluating forest CO 2 budgets. [ABSTRACT FROM AUTHOR]