Photosynthetic activity of a black spruce tree under cold environment and the contribution to annual CO2 budget

We have measured tower-based CO2 flux at a sub-arctic black spruce forest interior Alaska during past six years, and found interesting CO2 sink during early spring when the temperature was below zero and the ground was still frozen. There is a hot discussion on flux measurement under cold environment, that is, the self-heated open-path sensor causes apparent downward flux. To verify the downward flux during cold season, we measured CO2 exchange by attaching a leaf-chamber to a stem of black spruce. CO2 exchange by the tree was detected on DOY 67 at first, which was triggered by temperature. When the daily degree-hour of air temperature above zero (dayDH) raised over 50 degree hour, the respiration started. However, the respiration level was quite low and returned negligible when dayDH fell to zero. The CO2 accumulation, namely photosynthesis of the tree occurred on DOY 88 when the dayDH raised again over zero while daily averages of air t and soil temperature were still below zero. Spring break was DOY 121 and soil temperature at 0.1m depth was below zero by DOY 120. From the leaf-chamber measurement, we decided that the CO2 sink by the black spruce forest started after DOY 88 in 2008, which was about three weeks earlier than ordinal green-up of the forest. If we include the CO2 sink in cold season into the annual CO2 budget, the tower-based NEE increased about 10% of its sink strength.