Improving the CO2 storage measurements with a single profile system in a tall-dense-canopy temperate forest

The CO2 storage (Fs) measurement can contribute significantly to the estimation of the net ecosystem exchange of CO2 (NEE) especially in tall-canopy forest ecosystems. The Fs is often measured with a profile of CO2 concentration or with an eddy covariance system at the tower top, but few studies investigated potential errors in the Fs measurements. We assessed the errors in Fs relevant to the vertical distribution of sampling levels and window sizes of averaging time of CO2 mixing ratio and their effects on NEE in a temperate deciduous forest site in Northeast China using the standardized major axis method. The CO2 storage per unit height typically decreased with the height increasing, suggesting that the below-canopy layer need a higher spatial resolution. CO2 storage could be underestimated by up to one third based only on the tower-top measurement. The uncertainty (standard deviation) of the Fs decreased with the length of the averaging time window increasing. However, taking time averaging of CO2 mixing ratio caused significant underestimate of Fs, and consequently led to significant underestimates of CO2 uptake and release at a 30-min time scale. Our results highlight that appropriately combining spatial resolution and temporal resolution (response time for a whole sampling of all levels) is essential to improving the Fs estimates with the existing CO2 profile systems in forest ecosystems. Since the systematic bias and random error in Fs estimate with a single profile system are irreconcilable, there is an urgent need to develop a fast response planar-averaging profile or measure the instantaneous vertical-mean concentration to improve the accuracy of Fs measurement.