Carbon dioxide flux within and above a boreal aspen forest
Creators: name: Yang, Paul Chenggang
Publisher: name: University of British Columbia
Year: 2009

description: 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 (OA) 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). Due to the relative openness of the aspen canopy, 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, especially for CO₂ concentration. On average there was no significant enhanced CO₂ transport above that estimated using MOS theory both above the forest and in the trunk space. The rate of change in CO₂ storage in the air column (ΔS[sub a]/Δt) beneath the above-canopy eddy covariance system could be well estimated with concentrations measured at one height above the forest and at one height (2.3 m) in the trunk space. ΔS[sub a]/Δt was significantly large near sunrise (6-9 CST) and sunset (18-22 CST). Within the trunk 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 concentration 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[sub s]/Δt) was estimated as ΔS[sub s]/Δt = (l-M)R[sub sha] where R[sub sha] (the forest respiration) is a function of the soil temperature and M is a function of the friction velocity. Long-term carbon sequestration was estimated by summing the eddy covariance CO₂ fluxes (F[sub c]) because changes in storage average to zero over periods of a week or more. 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[sub 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. The model explained 73% of the variance in half-hourly P[sub e] obtained at the OA site during the 1996 growing season. In 1994, the OA forest photosynthesized about 1140 g C m⁻², of which 83% was accounted for by the aspen overstory. Total forest respiration was about 920 g C m⁻², of which 53% was estimated to be soil respiration. 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 F[sub c]. Assuming that half of the soil respiration was heterotrophic, net primary productivity in 1994 was estimated to be 450 g C m⁻².