[1] The purpose of this paper is to examine the mechanism that controls the variation of surface energy partitioning between latent and sensible heat fluxes at a temperate deciduous forest site in central Missouri, USA. Taking advantage of multiple micrometeorological and ecophysiological measurements and a prolonged drought in the middle of the 2005 growing season at this site, we studied how soil moisture, atmospheric vapor pressure deficit (VPD), and net radiation affected surface energy partitioning. We stratified these factors to minimize potential confounding effects of correlation among them. We found that all three factors had direct effects on surface energy partitioning, but more important, all three factors also had crucial indirect effects. The direct effect of soil moisture was characterized by a rapid decrease in Bowen ratio with increasing soil moisture when the soil was dry and by insensitivity of Bowen ratio to variations in soil moisture when the soil was wet. However, the rate of decrease in Bowen ratio when the soil was dry and the level of soil moisture above which Bowen ratio became insensitive to changes in soil moisture depended on atmospheric conditions. The direct effect of increased net radiation was to increase Bowen ratio. The direct effect of VPD was very nonlinear: Increased VPD decreased Bowen ratio at low VPD but increased Bowen ratio at high VPD. The indirect effects were much more complicated. Reduced soil moisture weakened the influence of VPD but enhanced the influence of net radiation on surface energy partitioning. Soil moisture also controlled how net radiation influenced the relationship between surface energy partitioning and VPD and how VPD affected the relationship between surface energy partitioning and net radiation. Furthermore, both increased VPD and increased net radiation enhanced the sensitivity of Bowen ratio to changes in soil moisture and the effect of drought on surface energy partitioning. The direct and indirect effects of atmospheric conditions and soil moisture on surface energy partitioning identified in this paper provide a target for testing atmospheric general circulation models in their representation of land-atmosphere coupling.

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[1] CO 2 storage in a 30-min period in a tall forest canopy often makes significant contributions to net ecosystem exchange (NEE) in the early morning and at night. When CO 2 storage is properly measured and taken into account, underestimations of NEE on calm nights can be greatly reduced. Using CO 2 data from a 12-level profile at the Missouri Ozark flux site (an oak-hickory forest in central Missouri, USA), we demonstrate that the lower canopy layer (below the thermal inversion) is a disproportionately large contributor to the total CO 2 storage. This is because time derivative of CO 2 density (Dc/Dt) generally shows increasing magnitude of mean and standard deviation with decreasing heights at night and from sunrise to 1000 h in both growing and dormant seasons. Effects of resolution and configuration in a profiling system on the accuracy of CO 2 storage estimation are evaluated by comparing subset profiles to the 12-level benchmark profile. It is demonstrated that the effectiveness of a profiling system in estimating CO 2 storage is not only determined by its number of sampling levels but, more importantly, by its vertical configuration. To optimize a profile, one needs to balance the influence of two factors, Dc/Dt and layer thickness, among all vertical sections within a forest. As a key contributor to the total CO 2 storage, the lower canopy requires a higher resolution in a profile system than the layers above. However, if the upper canopy is oversparsely sampled relative to the lower canopy, the performance of a profile system might be degraded since, in such a situation, the influence of layer thickness dominates over that of Dc/Dt. We also find that because of different level of complexity in canopy structure, more sampling levels are necessary at our site in order to achieve the same level of accuracy as at a boreal aspen site. These results suggest that in order to achieve an adequate accuracy in CO 2 storage measurements, the number of sampling levels in a profile and its design should be subject to the site properties, e.g., canopy architecture and the resulted thermodynamic and flow structures. If CO 2 density from a single profile is averaged in time and then used in assessing CO 2 storage to reduce random errors, biases associated with this averaging procedure become inevitable. Generally, larger window sizes used in averaging CO 2 density generate poorer estimates of CO 2 storage. If absolute errors are concerned, it appears that the more significant the CO 2 storage is during a period, the larger effects the averaging procedure has.

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