ENERGY AND CO2 FLUX DENSITIES ABOVE AND BELOW A TEMPERATE BROAD-LEAVED FOREST AND A BOREAL PINE FOREST

Fluxes of carbon dioxide, water vapor and energy were measured above a nd below a temperate broad-leaved forest and a boreal jack pine (Pinus banksiania Lamb.) forest by the eddy covariance method. The aim of th e work was to examine differences between the biological and physical processes that control the fluxes of mass and energy over these dispar ate forest stand types. Carbon and latent heat flux (LE) densities ove r the temperate broad-leaved forest were about three times larger than those observed over the boreal forest. Available energy was the key v ariable modulating LE over the temperate broad-leaved forest, whereas LE over the boreal jack pine stand was sensitive to variations in wate r vapor pressure deficits (VPDs) and available energy It was also note d that VPDs had different impacts on transpiration rates of the two fo rest stands. Increasing VPDs forced a negative feedback on jack pine t ranspiration, whereas transpiration rates of the well-watered broadlea ved forest responded favorably to increasing VPDs. Carbon dioxide flux densities over the broad-leaved forest stand were more sensitive to c hanges in absorbed photosynthetic photon flux density than those over the boreal forest. The efficiency of CO2 uptake over the jack pine sta nd was reduced, in part, because the low leaf area of the stand caused a sizable fraction of available quanta to be absorbed by nonphotosynt hetic organs, such as limbs and trunks. Over both forest stands, varia tions in photosynthetic photon flux density of photosynthetically acti ve radiation (Q(p)) explained only 50 to 60% of the variance of CO2 ex change rates. Consequently, caution should be exercised when scaling c arbon fluxes to regional scales based on unmodified, satellite-derived indices. The more open nature of the boreal jack pine forest caused w ater vapor, CO2 and heat fluxes at the forest floor to be a significan t component of whole canopy mass and energy exchange rates. About 20 t o 30% of net canopy mass and energy exchange occurred at the forest fl oor. Much smaller rates of mass and energy exchange occurred under the temperate broad-leaved forest.