Secrets of the Canopy: Unexpected Impacts of Nighttime Canopy Flow Events on Carbon Flux Exchange at the Tonzi AmeriFlux Site

Stable thermal stratification of the nocturnal lower boundary layer inhibits convective turbulence, such that turbulent transfer of ecosystem carbon dioxide (CO2), water vapor (H2O) and energy is driven by mechanically forced turbulence, either from frictional forces near the ground or top of a plant canopy, or from flows aloft. Studying the influence of above-canopy flow on turbulence flux exchange is difficult, as most eddy covariance (EC) flux towers do not extend very far above the height of the canopy; however, remote sensing devices can be deployed to measure flows aloft. This presentation investigates a novel application of atmospheric laser detection and ranging (lidar) at the Tonzi AmeriFlux site, an open oak savannah in the lower Sierra foothills of California. While the local fetch is relatively flat the terrain becomes increasingly complex in the easterly direction towards the Sierra Nevada, creating unique topographic flow events such as katabatic/anabatic winds. Lidar measurements of wind speed, direction and turbulence were collected for 100 days coinciding with eddy covariance measurements from an overstory and understory system. These measurements detected katabatic flows on over 40% of the campaign nights. During these events we observed a wind maximum 10-40 m above the oak canopy and strong momentum fluxes (e.g., ustar) at the overstory EC system, yet gradients of [CO2] within the canopy showed poor vertical mixing which led to reduced transport of ecosystem CO2 flux to the overstory station and an underestimation of nighttime respiration. On these nights the ratio of mean flow to a modified turbulence parameter showed better agreement with predicting canopy mixing than from relying on the traditional friction velocity (u*) correction method. Deploying lidar at a greater number of EC flux towers would provide more definitive answers for how important above-canopy atmospheric phenomena are for interpreting nighttime CO2 fluxes.