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During the growing season, nighttime ecosystem respiration emits 30–100% of the daytime net photosynthetic uptake of carbon, and therefore measurements of rates and understanding of its control by the environment are important for understanding net ecosystem exchange. Ecosystem respiration can be measured at night by eddy covariance methods, but the data may not be reliable because of low turbulence or other methodological problems. We used relationships between woody tissue, foliage, and soil respiration rates and temperature, with temperature records collected on site to estimate ecosystem respiration rates at six coniferous BOREAS sites at half-hour or 1-hour intervals, and then compared these estimates to nocturnal measurements of CO2 exchange by eddy covariance. Soil surface respiration was the largest source of CO2 at all sites (48–71%), and foliar respiration made a large contribution to ecosystem respiration at all sites (25–43%). Woody tissue respiration contributed only 5–15% to ecosystem respiration. We estimated error for the scaled chamber predictions of ecosystem respiration by using the uncertainty associated with each respiration parameter and respiring biomass value. There was substantial uncertainty in estimates of foliar and soil respiration because of the spatial variability of specific respiration rates. In addition, more attention needs to be paid to estimating foliar respiration during the early part of the growing season, when new foliage is growing, and to determining seasonal trends of soil surface respiration. Nocturnal eddy covariance measurements were poorly correlated to scaled chamber estimates of ecosystem respiration (r 2=0.06–0.27) and were consistently lower than scaled chamber predictions (by 27% on average for the six sites). The bias in eddy covariance estimates of ecosystem respiration will alter estimates of gross assimilation in the light and of net ecosystem exchange rates over extended periods.

Measurements of net ecosystem CO2 exchange by eddy correlation, incident photosynthetically active photon flux density (PPFD), soil temperature, air temperature, and air humidity were made in a black spruce (Picea mariana) boreal woodland near Schefferville, Quebec, Canada, from June through August 1990. Nighttime respiration was between 0.5 and 1.5 kg C ha−1 h−1, increasing with temperature. Net uptake of carbon during the day peaked at 3 kg C ha−1 h−1, and the daily net uptake over the experiment was 12 kg C ha−1 day−1. Photosynthesis dropped substantially at leaf-to-air vapor pressure deficit (VPD) greater than 7 mb, presumably as a result of stomatal closure. The response of ecosystem photosynthesis to incident PPFD was markedly non-linear, with an abrupt saturation at 600 μmol m−2 s−1. This sharp saturation reflected the geometry of the spruce canopy (isolated conical crowns), the frequently overcast conditions, and an increase in VPD coincident with high radiation. The ecosystem light-use efficiency increased markedly during overcast periods as a result of a more even distribution of light across the forest surface. A mechanistic model of forest photosynthesis, parameterized with observations of leaf density and nitrogen content from a nearby stand, provided accurate predictions of forest photosynthesis. The observations and model results indicated that ecosystem carbon balance at the site is highly sensitive to temperature, and relatively insensitive to cloudiness.

Exchanges of methane and carbon dioxide between the atmosphere and the Arctic tundra were measured continuously near Bethel, Alaska (61°05.41'N 162°00.92'W), for 5 weeks during July and August 1988. Fluxes were obtained directly using eddy correlation at 12-m altitude, and concentrations were measured sequentially at eight altitudes between 0 and 12 m. A prototype differential infrared absorption methane instrument based on a Zeeman-split HeNe laser was used for determination of methane and a flame ionization detector for total hydrocarbons (THC). Methane was found to account for nearly all the THC concentrations and fluxes. Methane fluxes at the tower site were apportioned to various methane-producing habitats, using a satellite image to classify surface vegetation at 20 × 20 m resolution. The “footprint” of the tower was computed using a Gaussian plume model for dispersion in the surface layer. Grid cells classified as dry tundra (water level 5 cm below surface) emitted methane at an average rate of 11±3 (standard error) mgCH4/m2/d, and wet meadow tundra (water level near surface) emitted 29±3 mgCH4/m2/d. Fluxes from lakes depended on wind speed, averaging 57±6 mgCH4/m2/d at the site, where the mean wind speed was 5ms−1. The mean emission rate for tundra (including lakes) around the tower was 25±1 mgCH4/m2/d, notably smaller than adopted for boreal wetlands in recent inventories of global methane sources. Emissions from major habitats derived from the tower measurements were in reasonable agreement with data from chamber studies. Errors of a factor of ∼2 accrued in scaling up the chamber data, representing 1 m2 plots, to the footprint of the tower measurements (103 m), because the satellite could not fully resolve heterogeneous methane-producing habitats. Fluxes obtained at the tower site were in harmony with fluxes from aircraft overflights. The aircraft data represent mainly afternoon periods with good flying weather, conditions associated with maximum CH4 fluxes in the tower time series. Mean fluxes from the aircraft are consequently ∼2 × higher than seasonal means from the region. Solar irradiance provided the primary control on the net ecosystem exchange (NEE) of carbon dioxide. The mean maximum uptake near the local solar noon was 1.4±0.2 gC/m2/d, and nocturnal respiration averaged 0.73±0.18 gC/m2/d. Net uptake of carbon dioxide averaged 0.30 gC/m2/d (0.1 tons C/hectare) during the period of the Arctic Boundary Layer Experiment (ABLE 3A). About 6% of the seasonal net uptake was returned to the atmosphere as methane., Engineering and Applied Sciences, Version of Record