Seasonal Variation Of Carbon Dioxide, Water Vapor, And Energy Exchanges Of A Boreal Black Spruce Forest

Measurements of the fluxes of latent heat λE, sensible heat H, and CO₂ were made by eddy covariance in a boreal black spruce forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS) for 120 days through the growing season in 1994. BOREAS is a multiscale study in which satellite, airborne, stand-scale, and leaf-scale observations were made in relation to the major vegetation types [Sellers et al., 1995]. The eddy covariance system comprised a sonic anemometer mounted 27 m above the forest, a system for transferring air rapidly and coherently to a closed path, infrared gas analyzer and a computer with the Edinburgh EdiSol software. Over the measurement period, closure of the energy balance on a 24 hour basis was good: (H + λE)/(R_n – G – B – S) = 0.97. The midday Bowen ratio was typically in the range 1.0–2.5, with an average value of ∼1.9 in the first Intensive Field Campaign (IFCl) and 1.3–1.4 in IFC2 and IFC3. Daily ecosystem evapotranspiration from moss, understory, and trees followed daily net radiation. Mean half-hourly net ecosystem flux followed photosynthetic photon flux density (PPFD) closely, reaching −9 μmol m⁻² s⁻¹ in June and August. The mean respiratory efflux on nights during which the atmosphere was well mixed (u*>0.4 m s⁻¹) reached 6 μmol m⁻²s⁻¹. The PPFD-saturated biotic CO₂ assimilation reached 20 μmol; m⁻² s⁻¹ and showed little response to air temperature or vapor pressure deficit (VPD). Storage of CO₂ in the air column at night did not account adequately for respiration on stable nights, so nighttime efflux was modeled for periods when u*<0.4 m s⁻¹. There was a net gain of CO₂ on most of the 120 days, but on 31 days of high temperature or low PPFD there was a net carbon loss. High PPFD promoted influx of CO₂ by the foliage, whereas high temperatures reduced net CO₂ influx through high respiration rates by the roots and soil microorganisms, leading to lower net uptake at high PPFD. Over the 120 day period, 95 g m⁻² of C were stored (an average of 0.8 g m⁻² d⁻¹), and 237 mm of water evaporated (an average of 2 mm d⁻¹).