Effect of clearcuts on footprints and flux measurements above a forest canopy

A two-equation model based on transport equations of turbulent kinetic energy (E) and specific dissipation (ω) (E-ω model) is used to investigate both the footprint and the effect of surface heterogeneities on flux measurements. The present study examines the influence of a model bare soil patch located upwind of the eddy-covariance tower on fluxes in a managed forest plantation. Using as a testbed measured flow statistics in the managed pine plantation of the Florida AmeriFlux site, scalar fluxes from a clearcut-forest transect with swaths of logged land of dimensions varying with wind direction were modeled. In sharp contrast with momentum fluxes, the magnitude of CO2 and scalar fluxes is found to be sensitive to clearcut widths. The adjustment to new underlying scalar flux values as a function of distance from the forest leading edge appears to be far greater for scalar fluxes than for momentum fluxes. This result is consistent for all modeled clearcut swaths-forest canopy interfaces, suggesting that CO2 flux measurements using the eddy-covariance technique require a larger fetch for forest flux towers than previously thought. The present footprint analysis indicates flux contributions from the clearcut, forest floor, and forest canopy to the tower flux hundreds of meters downwind of the clearcut-forest interface and highlights the need for caution in the interpretation of data away from the leading forest edge (up to 30 canopy heights). This is specially true when the strength of both surface and in-canopy sources are of comparable magnitude. Furthermore, depending on the geometry of the surface heterogeneity, results from this study suggest that aerodynamic parameters characterizing vegetation, such as displacement height and roughness length, can be a function of both distance from the clearcut and the clearcut's dimensions with respect to wind direction. Such parameters, when derived from flow information collected at the flux tower downwind of a bare soil patch-forest interface, cannot be used as specific parameters for the vegetation stand studied until the flux has reached equilibrium with the underlying surface. This study also suggests that, to improve our current assessment of net carbon uptake, attention should be given to the importance of careful tower location selection in a landscape characterized by a mosaic of surface properties as observed in a managed forest plantation or in most natural ecosystems.