Observing Subcanopy CO2 Advection

Underestimation of nocturnal CO₂ respiration under calm conditions remains an unsolved problem at many forest flux stations. In this paper, the hypothesis is tested that horizontal mean transport of CO₂, not previously measured, may account for the missing CO₂. A systematic methodology was developed that comprises characterizing the subcanopy motions, determining the appropriate size of the subcanopy network required to make the measurements, developing a method of integrating the measurements in the vertical, and determining the required averaging time. Measurements were performed at the Harvard Forest (Petersham, MA), over 4 years. Subcanopy flows were decoupled from the flows aloft 75% of the time. While stress divergence contributed 38% of the daytime forcing of subcanopy flows, negative buoyancy dominated nocturnal forcing 58% of the time, generating drainage flows 51% of the time. The drainage flow direction was dictated by the length of the slope, not its angle. Pressure gradient forcing across the lee side of the hills was predicted to be significant, but the simple flow reversals expected were not observed. The appropriate horizontal size of the network of wind and CO₂ sensors was shown to be on the order of 100 m, ensuring that sensors were generally observing coherent processes on this scale or larger and thus displaying some correlation. Horizontal transport of CO₂ at Harvard Forest was found to be restricted to the bottom ~10 m of the forest. The fraction of the negative buoyancy force in the sum of dynamic driving forces described nights with missing flux problems ("deficit nights") significantly better than the commonly used friction velocity criterion. Including the measured horizontal transport terms did not on average fully account for the observed difference in NEE of 1.2 +/- 0.3 μ moles m^-2s^-1 between deficit and non-deficit nights, but decreased the difference to 0.7 +/- 0.5 μ moles m^-2s^-1.