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

Abstract Nine years (2003–2011) of carbon dioxide ( CO 2 ) 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 CO 2 balance was controlled predominantly by RE , and not GPP . During these 9 years of measurement, the annual CO 2 balance shifted from a CO 2 sink to a CO 2 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 CO 2 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 CO 2 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 CO 2 balance and its interaction with the climate.