Eddy covariance was used to measure the net CO2 exchange and energy balance of a moist-tussock tundra ecosystem at Happy Valley, Alaska (69°08.54′ N, 148°50.47′ W), during the 1994–1995 growing seasons (June–August). The system operated for 75–95% of the time, and energy balance closure was within 5%, indicating good system performance.
Daily rates of evapotranspiration (ET) were on average 1.5 mm/d, while seasonal ET ranged between 100 and 150 mm. Daily ET was strongly correlated with daily fluctuations in net radiation. However, the “omega factor” (an index of the relative importance of meteorological and physiological limitations to evapotranspiration) was generally <0.5 throughout June and early July, indicating that biological limitations to ET were relatively more important than meteorological limitations during the first half of the growing season. The biological limitation to ET was presumably due to bryophyte desiccation and subsequent reductions in canopy water-vapor conductance, especially under conditions of high evaporative demand.
The moist-tussock tundra ecosystem was a net sink for atmospheric CO2 of −3.3 and −4.6 mol/m2 during the 1994 and 1995 growing seasons, respectively (negative flux depicts net CO2 accumulation). Over diel (24-h) periods, 60–90% of the variation in net CO2 exchange was explained as a hyperbolic function of photosynthetic photon flux density (PPFD), while over seasonal time scales, model estimates of the estimated quantum yield and maximum gross assimilation indicate that daily variations in net CO2uptake were driven more by the seasonal trend in ecosystem phenology than by meteorology. Approximately 70% of the variation in nighttime net CO2 exchange, an estimate of the whole-ecosystem respiration rate, was explained by variations in water-table depth and temperature. Although other environmental factors may be important, interannual differences in observed net CO2 exchange were almost completely explained by the interannual differences in estimated whole-ecosystem respiration.