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Abstract: Urban environments can modify the local climate and are net CO2 emitters, both of which can affect the life of an increasing proportion of the world’s population that lives in and around cities. Observational information on the seasonal and diurnal variations of CO2 fluxes as well as on the contribution from the different CO2 sources and sinks is needed at fine spatial resolution to help develop and validate weather prediction and atmospheric dispersion models. This study reports on measured using the eddy covariance technique for a two-year period over a suburban and an urban residential area as well as a reference agricultural site located in the Montreal region, which experiences cold winters and warm summers. The seasonal and diurnal variability of and its response to incoming light and to air temperature was analysed. Typical weekday and weekend vehicular traffic CO2 emissions (EVT) were estimated from inventory data for both residential sites. The urban site was a net source of CO2 throughout the entire measurement period on the order of 200 t CO2 ha−1 year−1. The suburban site was a winter source and a summer daytime sink of CO2 resulting in a net source of 50 t CO2 ha−1 y−1. Lower emissions at the suburban site are attributed to the large biological uptake in summer and to its low anthropogenic emissions. Higher emissions at the urban site are partly associated with its greater population and building density, promoting higher emissions from vehicular traffic and heating fuel combustion. The cold climate induced increased heating fuel combustion as compared to cities under a warmer climate. [Copyright &y& Elsevier]

We measured respiratory fluxes of carbon dioxide by aboveground tree components and soil respiration with chambers in 2005 and scaled up these measurements over space and time to estimate annual ecosystem respiration (Re) at a mature black spruce (Picea mariana (Mill.) B.S.P.) ecosystem in Quebec, Canada. We estimated periodic annual net primary production (NPP) for this ecosystem also. Re was estimated at 10.32 Mg C ha-1 year-1; heterotrophic respiration (Rh) accounted for 52% of Re and autotrophic respiration (Ra) accounted for the remainder. We estimated NPP at 3.02 Mg C ha-1 year-1, including production of bryophyte biomass but not including shrub NPP. We used these estimates of carbon fluxes to calculate a carbon use efficiency [CUE = NPP/(NPP + Ra)] of 0.38. This estimate of CUE is similar to those reported for other boreal forest ecosystems and it is lower than the value frequently used in global studies. Based on the estimate of Rh being greater than the estimate of NPP, the ecosystem was determined to emit approximately 2.38 Mg C ha-1 year-1 to the atmosphere in 2005. Estimates of gross primary production (GPP = NPP + Ra) and Re differed substantially from estimates of these fluxes derived from eddy covariance measurements during 2005 at this site. The ecological estimates of GPP and Re were substantially greater than those estimated for eddy covariance measurements. Applying a correction for lack of energy balance closure to eddy covariance estimates reduces differences with ecological estimates.We reviewed possible sources of systematic error in ecological estimates and discuss other possible explanations for these discrepancies. [ABSTRACT FROM AUTHOR]

Although mature black spruce forests are a dominant cover type in the boreal forest of North America, it is not clear how their carbon (C) budgets vary across the continent. The installation of an eddy covariance flux tower on an Old Black Spruce (OBS) site in eastern Canada (EOBS, Québec) provided a first opportunity to compare and contrast its annual (2004) and seasonal C exchange with two other pre-existing OBS flux sites from different climatic regions located in Saskatchewan [Southern OBS (SOBS)] and Manitoba [Northern OBS (NOBS)]. Although there was a relatively uniform seasonal pattern of net ecosystem productivity (NEP) among sites, EOBS had a lower total annual NEP than the other two sites. This was primarily because warmer soil under a thicker snowpack at EOBS appeared to increase winter C losses and low light suppressed both NEP and gross ecosystem productivity (GEP) in June. Across sites, greater total annual GEP and ecosystem respiration ( R) were associated with greater mean annual air temperatures and an earlier beginning of the growing season. Also, GEP at all three sites showed a stronger relationship with air temperature in spring and early summer compared with later in the growing season, highlighting the importance of springtime conditions to the C budget of these boreal ecosystems. The three sites had different parameter estimates describing the responses of R and GEP at the half hour time scale to near surface temperature and light, respectively. On the other hand, the responses of both R and GEP to temperature at the monthly scale did not differ among sites. These results suggest that a general parameterization could be sufficient at coarse time resolutions to model the response of C exchange to environmental factors of mature black spruce forests from different climatic regions. [ABSTRACT FROM AUTHOR]