Relating CO2 fluxes to spectral vegetation indices in tundra landscapes: importance of footprint definition

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Abstract
Carbon flux measurements made at an elevated point are associated with an effective upwind area or ‘footprint.’ Since Arctic tundra landscapes can exhibit substantial heterogeneity within the footprint of an eddy correlation tower, it may be necessary to determine the relative point source contributions to the observed flux if landscape properties are to be related to the flux. This study evaluates the potential importance of representing footprint source contributions in relationships that are developed between tower observations of net ecosystem exchange of carbon dioxide (NEE) and a remotely sensed spectral vegetation index. Satellite data collected over the foothills region of the North Slope of Alaska are used to determine spatial patterns of a spectral vegetation index in the calculated footprints of 30 randomly selected tower locations. A previously developed relationship between NEE and the vegetation index is used to calculate NEE at each tower location using two techniques, one that explicitly considers the footprint pattern of relative contributions to tower fluxes and another that ignores these patterns. The results indicate that if carbon fluxes measured at a tower are to be related to remotely sensed spectral vegetation indices, then it is necessary to consider the relative flux contributions from within the tower footprints for sites on the North Slope of Alaska.