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

Abstract

AbstTactWater vapour and CO2 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 lateSeptember 1994. Soil CO2 flux (respiration) was measured using a LI-COR 6000-09 soil chamber and soil evaporation was measured using lysimetry.The leaf area index of the aspen and hazelnut understorey reached 1.8 and 3.3, respectively. The maximum daily evapotranspiration (£) rate was 5-6 mm d^^ Following leaf-out the hazelnut and soil accounted for 22% of the forest £. The estimated total £ 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 CO2 from the atmosphere into the forest reached 1.2 mg CO2 m'^ s^* (33 |imol C m~^ s"^) 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 CO2 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 (N££) was -3.5 t C ha"^ y"^ (a net gain by the system). For 1994, cumulative NEE, ecosystem respiration (K) and gross ecosystem photosynthesis (GEP = R-NEE) were estimated to be -1.3, 8.9 and 10.2 t C ha"^ y~^, respectively. Gross photosynthesis of the hazelnut was 32% of GEP.

Responses of net ecosystem exchanges of carbon dioxide to changes in cloudiness: Results from two North American deciduous forests

“…From the raw data, half-hourly fluxes were calculated. At both sites the ecosystem carbon dioxide fluxes were measured using an eddy covariance system deployed above the canopy [Black et al, 1996;; Lee et al, 1999]. To examine the environmental controls on the forest NEE, microclimate measurements above and within the forests were made at both sites, utilizing identical measurement systems.…”

Carbon dioxide efflux from the floor of a boreal aspen forest. II. Evaluation of methods — verification by infra-red analysis of a dynamic closed chamber

“…Continuous measurements were made from February through September 1994 with 3-D sonic anemometers and infra-red gas analyzers. Details of this research are given in Black et al (1996). An attempt to measure soil Fc continuously at a height of 0.4 m (i.e.…”

“…Estimates of soil CO 2 efflux (soil Fc), often referred to as soil respiration, are required from terrestrial ecosystems to quantify gross ecosystem photosynthesis (Black et al 1996;Russell and Voroney 1998) and to investigate its relationship to climate and the global exchange of C upon climate change (Raich and Schlesinger 1992;Kirschbaum 1995). Furthermore, measurements of soil Fc can be used to characterize soil biological activity and the response of soil biota to environmental variables (Nadelhoffer 1990;Anderson and Domsch 1993;Howard and Howard 1993).…”

Long‐term observation of the atmospheric exchange of CO2 with a temperate deciduous forest in southern Ontario, Canada

“…A limited number of long-term (>1 year) studies have reported CO2 flux data for tropical [Grace et al, 1995a;Malhi et al, 1998], temperate [Wofsy et al, 1993;Greco and Baldocchi, 1996], and boreal forests [Goulden et al, 1997;Black et al, 1996;Lindroth et al, 1998]. More studies are needed to expand the coverage by the tower flux network of forest types and climate gradients in order to allow cross-biome and within-biome syntheses.…”