There is increasing concern in the FLUXNET community about how well 1-dimensional eddy covariance flux measurements over forests in complex terrain can describe the complete carbon exchange picture. Although speculation abounds regarding the role of understory flows in generating significant advection, we are still hampered by an insufficient understanding of their nature, primarily because of the lack of detailed field data.

Observations were made at Harvard Forest (Massachusetts) and Borden (Ontario), two ecologically similar yet topographically different AmeriFlux sites, to obtain a better understanding of the physics of understory flows. In addition to the standard suite of continuous tower measurements of eddy covariances and profiles, arrays of sonic anemometers were deployed near ground level to study the horizontal characteristics of the understory flow fields. At Harvard Forest, the associated understory CO2 field was also mapped.

At Harvard Forest, flows in the understory are aligned with those aloft only 33% of the time, and are more likely to come from the north, i.e. the direction of significant up-slopes. However, only 32% of the nights when the eddy covariance appears to miss some of the respired CO2 are associated with well-defined drainage flows. On the other hand, on 35% of the nights with drainage flow there are no indications of missed fluxes. Clearly the picture is more complicated than generally assumed. Even at Borden, we observe striking differences between overstory and understory flows despite the apparently simple topography.

Our measurements of the horizontal understory CO2 and wind fields, along with vertical profiles, allow the estimation of advective terms in the CO2 budget. The array of anemometers at ground level show significant localized differences even on a scale of ~100m, which makes it necessary to composite over long periods of time to obtain convergence of the terms to useful noise levels. This paper will describe the characteristics of flows in the understory, our analysis of the advective terms, as well as relationships between these terms, drainage flows and eddy fluxes.