Carbon dioxide and water vapor fluxes above two boreal forests

Carbon dioxide and water vapour fluxes above land surfaces are sensitive to changes in temperature and precipitation patterns and associated feedback processes. Here, we report the responses of these fluxes to interannual climate variability over the last five years in a 70-year-old aspen (Populus tremuloides Michx.) forest and a 115-year-old black spruce (Picea mariana (Mill) BSP) forest in northern Saskatchewan, Canada. The two forests are 80 km apart. The height of the aspen stand is 21-22 m, while that of the black spruce is 10-11 m. The soil in the aspen forest is a clay-loam with a thin surface organic layer and the soil in the black spruce forest is a Sphagnum peat. Fluxes above both forests were measured in 1994 and 1996, during the Boreal Ecosystem-Atmosphere Study (BOREAS). Since then, as part of the Boreal Ecosystems Research and Monitoring Sites (BERMS) program, fluxes have been measured continuously at the aspen site, while they were reinitiated at the black spruce site in spring 1999. Fluxes have been measured with a three-dimensional sonic anemometer and closed-path infrared gas analyzer using the eddy covariance technique. Net CO₂ uptake in the black spruce forest began earlier and lasted later in the year than in the aspen forest. Large CO₂ losses from the aspen forest occurred before leaf emergence and following leaf senescence. However, during the summer, daily (24-h) net CO₂ uptake in the aspen forest reached 7 g C m^-2 day^-1, which was three times higher than in the black spruce forest. This resulted in the net annual CO₂ uptake (the net ecosystem productivity) of the aspen stand far exceeding that of the black spruce forest. In the aspen forest, the primary climatic control on CO₂ uptake was spring temperature; warm springs caused early leaf emergence and significantly increased ecosystem photosynthesis, but had relatively little effect on respiration. During the summer, evaporation from the aspen forest was more than twice that from the black spruce forest. Results of using the Canadian Land Surface Scheme (CLASS) to simulate the effects of interannual climate variability on these fluxes above the two forests will also be reported. A single-layer process-based two-leaf (sunlit and shaded) model of canopy conductance and photosynthesis, based on the Farquhar approach, was incorporated into CLASS.