Carbon dioxide flux within and above a boreal aspen forest

Carbon dioxide, water vapour, sensible heat and momentum fluxes were continuously measured using the eddy covariance technique above and below the overstory in a 70-year old aspen stand in northern Saskatchewan from October to November 1993 and from February to September 1994, and above the overstory from April to December 1996 as a part of the Boreal Ecosystem-Atmosphere Study (BOREAS).; The air within the forest was usually stably stratified at night and unstable during the daytime. The relationships of the variances of the vertical velocity and scalars (air temperature, CO₂ concentration and specific humidity) to the stability parameter above the forest followed the Monin-Obukhov similarity (MOS) relationships, while the applicability of MOS theory in the trunk space was poor.; The rate of change in CO₂ storage in the air column (Δ S_a/Δt) beneath the above-canopy eddy covariance system could be well estimated with concentrations measured at one height above the form and at one height (2.3 m) in the trunk space. Within the hunk space, eddy covariance sensible and latent heat flux measurements at one position were representative of an area extending for at least two tree heights. The same was the case for CO₂ flux and during the daytime. At night, however, they exhibited significant horizontal variability but were representative of the above area when averaged over several days.; Evidence supporting the hypothesis that the low nighttime CO₂ fluxes resulted from the short-term changes in CO₂ storage in the air-filled pores of soil/snow was presented. The rate of change of this storage (ΔS_a/Δt) was estimated as ΔS_s/Δt = (1 − M)R_sha where R_sha (forest respiration) is a function of soil temperature and M is a function of the friction velocity.; Photosynthetic rates (P) were modelled as a product of P₁, P₂ and P₃. P₁ is a rectangular hyperbolic function of the absorbed photosynthetic photon flux density (PPFD), and P₂ and P₃ are second order polynomial functions of saturation deficit and air temperature, respectively. This empirical model explained about 80%, 76% and 26% of the variance in the measured half-hourly photosynthesis of the forest (P_e), aspen overstory and hazelnut understory, respectively, in 1994. The corresponding percentage of the variances explained by absorbed PPFD were 74%, 68% and 25%, respectively.; In 1994, the forest photosynthesized about 1140 g C m−2 . Total forest respiration was about 920 g C m−2. Thus, carbon sequestration by the forest was about 220 g C m−2 , which is slightly higher than the value (200 g C m−2 obtained by directly summing the eddy covariance CO₂ fluxes. (Abstract shortened by UMI.)...