| The kinematics of subcanopy flows and their ability to transport material have received little attention in the published literature, despite growing concerns that they may be responsible for horizontally transporting the respired CO2 that is missed by eddy flux measurements above forests under calm conditions. This work outlines the design of a subcanopy study to characterize subcanopy flows, determine their dynamics, and to directly measure horizontal advection of CO2.; These measurements were made in a mixed deciduous forest in Central Massachusetts. Subcanopy flows were found to be 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 was predicted to be significant, but the simple flow reversals predicted were not observed. A speed-up of the flow at 150 m above the hill due to the Bernoulli effect of about 20–60% was observed, significantly smaller than the predicted ∼100%.; The fraction of the negative buoyancy forcing in the sum of all dynamic forces was found to be a better predictor of the occurrence of nocturnal eddy flux deficits than the commonly used friction velocity criterion, coinciding with 69% vs. 41% of the deficit nights respectively. The horizontal transport of CO2 was found to be confined to a 10 m layer above the forest floor. It was on average 0.49 ± 0.14 μmoles m−2s −1 larger on nights with flux deficits than nights without, decreasing the difference in the estimated respiration between these two classes but not fully accounting for the difference of 1.2 ± 0.5 μmoles m−2s−1. |
