Autumn warming reduces the CO2 sink of a black spruce forest in interior Alaska based on a nine‐year eddy covariance measurement

Nine years (2003–2011) of carbon dioxide (CO₂) flux were measured at a black spruce forest in interior Alaska using the eddy covariance method. Seasonal and interannual variations in the gross primary productivity (GPP) and ecosystem respiration (RE) were associated primarily with air temperature: warmer conditions enhanced GPP and RE. Meanwhile, interannual variation in annual CO₂ balance was controlled predominantly by RE, and not GPP. During these 9 years of measurement, the annual CO₂ balance shifted from a CO₂ sink to a CO₂ source, with a 9‐year average near zero. The increase in autumn RE was associated with autumn warming and was mostly attributed to a shift in the annual CO₂ balance. The increase in autumn air temperature (0.22 °C yr^?1) during the 9 years of study was 15 times greater than the long‐term warming trend between 1905 and 2011 (0.015 °C yr^?1) due to decadal climate oscillation. This result indicates that most of the shifts in observed CO₂ fluxes were associated with decadal climate variability. Because the natural climate varies in a cycle of 10–30 years, a long‐term study covering at least one full cycle of decadal climate oscillation is important to quantify the CO₂ balance and its interaction with the climate.