REGIONAL-SCALE SURFACE FLUX OBSERVATIONS ACROSS THE BOREAL FOREST DURING BOREAS

A major role of the National Center for Atmospheric Research (NCAR) El ectra aircraft during the Boreal Ecosystem-Atmosphere Study (BOREAS) w as to measure fluxes of momentum, sensible and latent heat, carbon dio xide, and ozone on a transect that crossed the entire boreal forest bi ome. The observations spanned the growing season (late May to mid-Sept ember 1994) and extended the fluxes obtained in two intensive study ar eas to larger spatial scales to help provide a data set that is useful for comparison with and validation of large-scale models and satellit e retrievals. We found the deciduous forests to be more photosynthetic ally active than nearby coniferous forests. Coniferous forest fluxes a cross the transect from the BOREAS southern to northern study areas sh ow no apparent spatial trend, though smaller-scale variability is larg e. The fluxes make a smooth transition from the BOREAS northern study area to the subarctic tundra. Typical midsummer, midday, large-scale n et ecosystem exchanges of carbon dioxide were about -10 mu mol m(-2) s (-1) for primarily deciduous forests, about -6 mu mol m(-2) s(-1) for the primarily coniferous regions between and including the two BOREAS study areas, and about -2 mu mol m(-2) s(-1) for the subarctic tundra. The first two values are similar to those observed by flux towers in the region. Throughout the boreal forest the fluxes are influenced by the presence of lakes. Lake fraction is found to be a dominant source of variability in the fluxes observed along the transect. Lakes are al so found to be large sinks of available radiant energy. Regional groun d storage of heat is estimated to be about 30% of the net radiation ov er the forest, and 40% over the subarctic tundra, largely due to the p resence of lakes.