Energy and mass fluxes over a boreal forest in northern Manitoba: Net ecosystem exchange and energy balances for the BOREAS NSA young jack pine and fen sites

Climatological measurements, including radiation, energy, and carbon dioxide flux densities, were made from April to September in 1994 and from April to November in 1996 at a young jack pine (Pinus banksiana Lamb.) forest and wetland fen near Thompson, Manitoba, Canada, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Simultaneous measurements at these sites and the concurrent analysis and discussion of the results allowed a unique opportunity to investigate the similarities and differences of these typical boreal land surfaces at two moisture extremes. An economical eddy covariance system was constructed for the measurement of carbon dioxide exchanges, and an analysis guide was developed for the correction of sensible heat, latent heat/water vapour, and CO₂ fluxes measured with single-axis eddy covariance measurements.; The solar albedo at the young jack pine site YJP remained steady at 0.13 in both years but the solar albedo at the fen site (FEN) responded to the phenology of the fen vegetation. At both sites the albedo for photosynthetic photon flux density (PPFD), 0.05, showed no response to vegetation phenological changes.; Water vapour fluxes at FEN were similar between the two years and were symmetrical about early-afternoon maximums. In contrast, water vapor flux densities at YJP often peaked at midmorning and slowly decreased throughout the afternoon. Overall, closure of the energy balance was 82–83% at FEN and 91–95% at YJP.; The range in CO₂ flux densities at YJP was the same in both years. Average diurnal patterns of CO₂ exchange were characterized by maximum uptake in the morning and a gradual decrease through the afternoon. At FEN, the period of CO₂ uptake was similar for both years. Warmer air temperatures and an earlier snow melt in the spring of 1994, and warmer and drier conditions in the fall of 1994 promoted CO₂ production and efflux at times when the vascular vegetation was not photosynthesizing. As a result the fen was a net sink for CO₂-carbon. If a phonological approach to period calculations is used, the fen in 1994 appeared to be a net source of CO₂-carbon to the atmosphere. Results from the nonphenological approach, would also indicate that the fen surface was a net source of carbon to the atmosphere in 1994 if extrapolations were made for as little as 6 days beyond the period of study. Given the immense store of carbon in boreal peatlands, this response to earlier spring warming and drier conditions is an important finding in increasing our understanding of the impact of climate change on the carbon balance for northern ecosystems. (Abstract shortened by UMI.)...