Annual cycles of water vapour and carbon dioxide fluxes in and above a boreal aspen forest

Abstract

Water vapour and CO 2 fluxes were measured using the eddy correlation method 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 Solent, respectively) and closed‐path gas analysers (LI‐COR 6262) with 6‐m and 4.7‐m long heated sampling tubing, respectively. Continuous measurements were made from early October to mid‐November 1993 and from early February to late‐September 1994. Soil CO 2 flux (respiration) was measured using a LI‐COR 6000‐09 soil chamber and soil evaporation was measured using Iysimetry. The leaf area index of the aspen and hazelnut understorey reached 1.8 and 3.3, respectively. The maximum daily evapotranspiration (E) rate was 5–6 mm d−1. Following leaf‐out the hazelnut and soil accounted for 22% of the forest E. The estimated total E was 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 CO 2 from the atmosphere into the forest reached 1.2 mg CO 2 m−2 s−1 (33 μmol C m−2 s−1) during the daytime. Downward 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 CO 2 above the aspen and hazelnut, corrected for canopy storage, increased exponentially with soil temperature at the 2‐cm depth. Estimates of daytime respiration rates using these 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 (GEP = 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.