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Abstract: Forest harvest and subsequent stand development can have major effects on the carbon cycle of boreal stands. Carbon dioxide (CO2) fluxes of a three-point black spruce harvest chronosequence located in the boreal forest of eastern North America were measured over a one-year period at the ecosystem scale with the eddy covariance technique and CO2 efflux from soils was measured with a portable infrared gas analyzer. The three sites (pre-harvest, recently harvested, and juvenile) were 105-, 8- and 33-years old, respectively. On an annual basis, the pre-harvest site (EOBS) was a weak carbon sink (6±4gCm−2 yr−1), the recently harvested site (HBS00) a source (−87±3gCm−2 yr−1) and the juvenile site (HBS75) a moderate to strong sink (143±35gCm−2 yr−1). Annual gross ecosystem production (GEP) at the pre-harvest site was only 28% greater than at the recently harvested site (646±6 versus 504±5gCm−2 yr−1), while GEP at the juvenile site (1107±32gCm−2 yr−1) was more than double that at the recently harvested site, suggesting significant physiological constraints to photosynthesis at the pre-harvest site. Annual ecosystem respiration (R e) followed the same pattern, but intersite differences were somewhat less (640±8 to 591±6 to 964±50gCm−2 yr−1). Annual soil respiration (R s) decreased following harvest from 593 to 500gCm−2 yr−1 and increased with further stand development to 644gCm−2 yr−1, although the changes were less than for GEP and R e. Q 10 and R 10 of R s for the snow-free period varied between sites, were lowest for the recently harvested site, and appeared to be related to GEP via substrate supply. The annual ratio of R s to R e was lower for the juvenile site (67%) than for the pre-harvest and recently harvested sites (93 and 85%, respectively). These results quantify how some of the major physiological processes that influence the carbon cycle of boreal black spruce stands evolve following harvest and should be useful for better incorporating stand-age effects into regional and global scale models. [Copyright &y& Elsevier]

Abstract: Carbon (C) cycling in the boreal forest is driven by both natural and human disturbances, but there is little information on the impact of forest management practices on ecosystem-level C balance. We evaluated the C balance of a recently harvested boreal site in eastern Canada by measuring CO2 fluxes with the eddy covariance technique for 1 year before the application of a scarification treatment (mechanical site preparation) and for 1 year after the treatment was applied to approximately 40% of the study area. Net annual exchange indicated a source of 111gCm−2 year−1 before scarification that increased to 175gCm−2 year−1 after treatment. Annual gross ecosystem productivity (GEP) and net C flux varied between years, but there was no significant difference for ecosystem respiration. Since the differences in climate between years did not explain the changes in the site C balance and daytime net C sequestration under non-limiting environmental conditions was generally lower after the treatment, the large difference in C emissions was most likely due to decreased GEP resulting from the destruction of approximately 60% of the living aboveground vegetation within the scarified areas. Although daily NEP was almost always negative throughout the year, a net daily C sink was observed during a 2-week period in late summer 2003 when air temperatures were approximately 8°C cooler than preceding weeks. An analysis of the residuals from light–response regressions showed that soil water content and vapor pressure deficit were the second most important variables explaining morning and afternoon net C flux in 2003 and 2004, respectively. [Copyright &y& Elsevier]

Abstract: A boreal bog located in the James Bay lowlands, Canada, was instrumented with an open-path gas analyzer to monitor the turbulent fluxes of methane throughout the summer of 2012. The mostly continuous eddy covariance measurements permitted the study of methane dynamics at the hourly, daily and seasonal scales. To exclude data segments for which the biological methane fluxes were underestimated due to inefficient atmospheric transport under stable stratification, we applied a novel approach based on both the atmospheric stability parameter ζ = z/L and the friction velocity u ∗, where z is the measurement height and L the Obukhov length. The field measurements revealed the existence of at least one sustained ebullition event, triggered by low barometric pressures, a declining water table and increasing mechanical turbulence – suggesting that large-scale release of methane bubbles can be an important transport mechanism of methane in boreal bogs. The validity of similarity scaling for atmospheric methane under convective conditions was also assessed and the normalized standard deviations of methane concentrations did not scale well with ζ, highlighting the heterogeneity in natural methane production and release across the bog. Overall the hourly emissions ranged between −2.0 and 32.1 mg CH4 m−2 h−1, with a summertime mean of 2.4 mg CH4 m−2 h−1. At the daily scale, the two main controls on methane emissions were found to be the water table position and the peat temperature at 0.3 m under the surface. Contrary to other studies, seasonal methane emissions peaked when the water table was at its maximum distance from the surface, around mid-August. No clear diurnal pattern could be found in methane emissions, indicating that methane was produced quite deep within the peat. The seasonal emissions were estimated at 4.4 g CH4 m−2, and compared well with other observations over similar landscapes using different measurement techniques. Given that methane releases and transport are greatly affected by local characteristics such as climate and vegetative cover, this study emphasizes the need for further in situ continuous measurements of methane fluxes across northern peatlands. [ABSTRACT FROM AUTHOR]