Topographic effects on CO2 flux measurements at the Chi-Lan Mountain forest site

The multiple eddy covariance measurement approach was applied to discern topographic effects on CO2 flux measurements at the Chi-Lan Mountain (CLM) site, northern Taiwan. The results suggested that fluxes diverged between different heights above the canopy in the morning. Mean morning CO2 fluxes at 24 m in height on the main tower (T1) and 26 m in height on the second tower (T2L1) were respectively -15.3 and -14.8 μmol m-2 s -1 in summer of 2007, while the value was -11.9 μmol m -2 s-1 at 22 m in height on the second tower (T2L2). The measured fluxes of T2L2 were respectively 22 and 20% lower than those of T1 and T2L1. In addition, we propose that complex CO2 transport regimes evolve beneath the canopy during transitions of foggy/clear, day/night, and valley-wind/mountain-wind regimes. Under foggy conditions in the late afternoon and early evening, intermittent turbulence dominated and sporadically penetrated downward into the forest. Either vertical eddy flux (-1.4 μmol m -2 s-1) or storage change (0.4 μmol m-2 s-1) significantly contributed to net ecosystem exchange (NEE) of CO2 (-1.0 μmol m-2 s-1). As the fog dissipated and the atmosphere became stable in the evening, significant decreases in eddy flux plus storage change (from 3.5 to 2.3 μmol m -2 s-1) reflected that drainage flow was generated below the canopy and carried CO2 released from the soil and understory vegetation respiration downhill. The drainage effect consequently led to a 34% underestimation of the nighttime NEE. Our results revealed that topographic effects could respectively bias daytime and nighttime NEE estimations by 20∼22 and 34%, which were previously reported at 20∼80%. The topographic effects led to evident uncertainties in NEE estimates, and further research is urgently needed to develop adequate data-filtering or correction approaches.