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To evaluate the effects on CO2 exchange of clearcutting a mixed forest and replacing it with a plantation, 4.5 years of continuous eddy covariance measurements of CO2 fluxes and soil respiration measurements were conducted in a conifer‐broadleaf mixed forest in Hokkaido, Japan. The mixed forest was a weak carbon sink (net ecosystem exchange, −44 g C m−2 yr−1), and it became a large carbon source (569 g C m−2 yr−1) after clearcutting. However, the large emission in the harvest year rapidly decreased in the following 2 years (495 and 153 g C m−2 yr−1, respectively) as the gross primary production (GPP) increased, while the total ecosystem respiration (RE) remained relatively stable. The rapid increase in GPP was attributed to an increase in biomass and photosynthetic activity of Sasa dwarf bamboo, an understory species. Soil respiration increased in the 3 years following clearcutting, in the first year mainly owing to the change in the gap ratio of the forest, and in the following years because of increased root respiration by the bamboo. The ratio of soil respiration to RE increased from 44% in the forest to nearly 100% after clearcutting, and aboveground parts of the vegetation contributed little to the RE although the respiration chamber measurements showed heterogeneous soil condition after clearcutting.

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Summary1. Eddy covariance was used to measure the net CO 2 flux of Alaskan moist-tussock and wet-sedge tundra ecosystems between 1 June and 31 August 1995. The sites were located within 2·5 km of each other and, depending on wind speed and thermal stability, the eddy-covariance measurements integrated surface fluxes over 0·8-26·5 ha (mean 3·8 ha) at the moist-tussock site and 0·1-4·2 ha (mean 0·6) at the wet-sedge site. 2. Both sites were net sinks for atmospheric CO 2 during the 92-day measurement period but wet-sedge tundra accumulated 1·5 times more CO 2 than moist-tussock tundra. Wet-sedge tundra was a net CO 2 sink of -6·4 mol m -2 and moist-tussock tundra was a net sink of -4·6 mol m -2 over the June-August measurement period. 3. Estimates of whole-ecosystem respiration ( R ) were made using multiple non-linear regression by quantifying the response of measured nocturnal CO 2 efflux ( F n ) to fluctuations in air temperature and water-table depth ( r 2 = 0·68). Partial regression analysis revealed that water-table depth explained relatively more of the variance in F n (45%) than temperature (11%). R was estimated to be 14·9 mol m -2 for moist-tussock tundra and 5·4 mol m -2 for wet-sedge tundra over the 92-day measurement period. The large difference in R was apparently owing to a substantially higher water table in wet-sedge tundra. 4. Estimated rates of gross primary production (GPP), calculated from measured net CO 2 flux and estimated R , were 40% lower for wet-sedge tundra over the 92-day measurement period. Rates of GPP integrated over diel (24 h) periods were substantially larger for moist-tussock tundra ecosystems throughout June and July but during the month of August, total diel rates of GPP for moist-tussock and wet-sedge tundra were comparable. 5. Our results demonstrate that R and GPP of moist-tussock and wet-sedge tundra ecosystems can be reliably estimated from eddy-covariance measurements using functional relationships developed from plot-scale studies. Given the large spatial differences in hydrology, the larger sink observed for wet-sedge tundra was probably the result of relatively lower rates of R .