At the BOREAS/NOBS research site, a 30-m tall tower extends well above the canopy height of approximately 10 m. From the tower, eddy-correlation flux measurements for CO2 and H2O have been collected to directly determine the net ecosystem exchange of CO2 and the surface energy budget over diurnal, seasonal and annual time scales. These observations are coupled with a comprehensive characterization of the physical environment (PAR, soil temperature, etc.). A low-power automated array was installed to measure eddy fluxes and forest column content of CO2, sensible heat, and water vapor; soil temperatures and moisture, and incident and intercepted PAR.  The system has operated continuously since September 1993.

The eddy covariance technique (Goulden et al., 1996) is used on the four Hz data to calculate fluxes of carbon dioxide, water vapor, momentum and heat through a horizontal plane at 30 m altitude.  CO2 flux is combined with the integrated column of CO2 to determine net ecosystem exchange (NEE), a direct measure of the carbon uptake or efflux in the forest.  From the NEE and accompanying environmental measurements, photosynthesis (gross ecosystem exchange; GEE) and ecosystem respiration (R) are calculated.

Our measurements show that the forests at NOBS acts as a source of atmospheric CO2 in some years (1995-97) and a sink in others (1998-99) (Table 1).  In concert with variations in the intensity of boreal fires, these inter-annual variations of NEE represent potentially significant factors in the inter-annual variability of global atmospheric concentrations of CO2.

Table1 : Carbon Balance at NOBS; negative numbers indicate carbon assimilation by the ecosystem.  Units:  1000 kgC ha-1 y-1

Figure 1: Daily net CO2 exchange (NEE), respiration (R), and  photosynthesis (GEE).

Boreal CO2 fluxes are sensitive to the timing and seasonal cycles of their variations.  Measurable photosynthesis (GEE) begins as daytime temperatures warm, between mid-April and mid-May.  The earlier onset observed in 1998 and 1999 (Figure 1) may have contributed to the annual net uptake observed in those years.  Photosynthesis rates increase (became more negative) as nighttime temperatures rise above zero, and significant photosynthesis persists until October.  However, net uptake usually ceases in mid-summer due in part to increasing respiration rates.  This mid-summer slowdown in net carbon assimilation underscores the importance of an early start to the growing season in determining the annual net uptake or efflux.

For more information and results, see Goulden (1998) and our exhibition hall.  Data are available through our data exchange.