Turbulence kinetic energy budget and spatial heterogeneity of surface fluxes above and beneath an open pine canopy

We examine the average contributions from shear generation, buoyancy production and vertical turbulent transport to the turbulence kinetic energy (TKE) tendency for two vertical layers of a tall open pine forest: a lower layer in the subcanopy space in contact with the understory and the surface, and an upper layer extending from the top half of the open subcanopy trunk space upwards to about twice the canopy height. The subcanopy TKE is strongly related to the TKE above the canopy, with approximately one-tenth the magnitude. In the upper layer, shear generation of turbulence dominates, while in the lower layer the vertical turbulent transfer of TKE is the largest of the three terms. The vertical turbulent transport in the lower layer is of similar magnitude but opposite sign to that in the upper layer. The observations suggest that a significant fraction of the subcanopy turbulence is driven by shear generation of turbulence above the canopy that is subsequently transported downward by turbulence into the subcanopy. In addition, we evaluate the spatial heterogeneity of the TKE, the three TKE tendency terms, the CO2 flux, moisture flux, heat flux and the friction velocity in the subcanopy. In terms of fluxes, the heterogeneity is by far the largest for the CO2 flux. Based on the observed weak heterogeneity of the turbulence and the strong heterogeneity of the CO2 flux, we conclude that significant variations in the CO2 source terms occur on scales smaller than our network, where the distance between measurement sites is 87 m. We also observe large systematic differences in the long-term average subcanopy moisture and especially CO2 flux depending on location within the network of measurement sites despite the lack of any clear differences in soils, exposure, vegetation, elevation or terrain slope.