Controlling factors on the interannual CO2 budget at a subarctic black spruce forest in interior Alaska

The eddy covariance method was applied to measure net ecosystem CO2 exchange (NEE) at a subarctic black spruce forest in interior Alaska during 2003 and 2004. To clarify the budget of CO2, we divided photosynthesis and respiration by applying the Carbon Budget Analysis Tool, in which the potential photosynthetic rate, the light use efficiency, and the suppression factors on photosynthesis were evaluated. The potential photosynthetic rate and the light use efficiency were related with the understory leaf area index. The determined optimal temperature of photosynthesis was higher than 15 ◦C, which was higher than that of other boreal forests. Drought in midsummer of 2004 suppressed both photosynthesis and respiration, but the suppression was more effective in photosynthesis, resulting in a slightly decreased NEE. Cumulative respiration and photosynthesis were 2.29 and −2.50 kg CO2 m−2 in 2003, and 2.37 and −2.44 kg CO2 m−2 in 2004, resulting in calculated annual CO2 sink budgets of −210 and −70 g CO2 m−2 in 2003 and 2004, respectively.