Evolution of atmospheric carbon dioxide concentration at different temporal scales recorded in a tall forest

Understanding the temporal evolution of CO ₂ over specific ecosystems can provide additional insight into atmospheric CO ₂ variability. From 2003 to 2010, CO ₂ ambient concentrations in an old-growth temperate forest were measured continuously by using an infrared analyzer. The results show that daily cycle and seasonal variation of CO ₂ were mostly dominated by vegetation activity, whereas the weak turbulent mixing at night contributed frequently to CO ₂ build-up below the canopy. As a result, CO ₂ concentrations changed considerably during daytime and nighttime in the forest stand. The monthly means exhibited a seasonal variation characterized by a primary peak in December and a minimum during July to August, with an average seasonal amplitude (the difference of minimum and maximum monthly means) of 60.5 ppmv. By fitting a harmonic model to the monthly means, linear increase trends of 1.7 ppmv yr ^-1 in the annual mean CO ₂ concentrations and 0.58 ppmv yr ^-1 in the amplitude of the seasonal cycle were estimated. The upward trend of annual mean CO ₂ concentration may be attributed to the dramatic global increase in CO ₂ emissions during the study period. This study also found that the stratification of CO ₂ concentrations below the canopy during nighttime was easily disrupted after sunrise, which suggests that forest floor plants are not, as previous assumed, exposed to significantly greater CO ₂ concentrations than the normal atmospheric concentrations.