ANNUAL CYCLES OF WATER-VAPOR AND CARBON-DIOXIDE FLUXES IN AND ABOVE ABOREAL ASPEN FOREST

Water vapour and CO2 fluxes were measured using the eddy correlation m ethod above and below the overstorey of a 21-m tall aspen stand in the boreal forest of central Saskatchewan as part of the Boreal Ecosystem -Atmosphere Study (BOREAS). Measurements were made at the 39.5-m and 4 -m heights using 3-dimensional sonic anemometers (Kaijo-Denki and Sole nt, respectively) and closed-path gas analysers (LI-COR 6262) with 6-m and 4.7-m long heated sampling tubing, respectively. Continuous measu rements were made from early October to mid-November 1993 and from ear ly February to late-September 1994. Soil CO2 flux (respiration) was me asured using a LI-COR 6000-09 soil chamber and soil evaporation was me asured using lysimetry. The leaf area index of the aspen and hazelnut understorey reached 1.8 and 3.3, respectively. The maximum daily evapo transpiration (E) rate was 5-6 mm d(-1). Following leaf-out the hazeln ut and soil accounted for 22% of the forest E. The estimated total E w as 403 mm for 1994. About 88% of the precipitation in 1994 was lost as evapotranspiration. During the growing season, the magnitude of half- hourly eddy fluxes of CO2 from the atmosphere into the forest reached 1.2 mg CO2 m(-2) s(-1) (33 mu mol C m(-2) s(-1)) during the daytime. D ownward eddy fluxes at the 4-m height were observed when the hazelnut was growing rapidly in June and July. Under well-ventilated night-time conditions, the eddy fluxes of CO2 above the aspen and hazelnut, corr ected for canopy storage, increased exponentially with soil temperatur e at the 2-cm depth, Estimates of daytime respiration rates using thes e relationships agreed well with soil chamber measurements. During the 1994 growing season, the cumulative net ecosystem exchange (NEE) was -3.5 t C ha(-1) y(-1) (a net gain by the system). For 1994, cumulative NEE, ecosystem respiration (R) and gross ecosystem photosynthesis (GE P = R - NEE) were estimated to be -1.3, 8.9 and 10.2 t C ha(-1) y(-1), respectively. Gross photosynthesis of the hazelnut was 32% of GEP.