Linking the Seasonal Variation of Vegetation Indices to Tower Flux Measurements

Across the globe, there are now over 200 FLUXNET sites sampling tower fluxes over many vegetation types. However the spatial resolution of the tower sites is limited and additional information is needed to provide the Global Change Research community with an accurate way to identify and quantify carbon sources and sinks on regional, continental and global scales. Remote sensing is a major tool capable of providing information about the dynamics of the terrestrial biosphere with continuous spatial and temporal coverage on a global scale. For this purpose vegetation indices (VI's) are chosen specifically to enhance the contribution of vegetation properties to surface reflectances. Remote sensing products generally produce information on GPP (or net primary productivity, NPP), in terms of a light use efficiency (ε) and the amount of absorbed visible sunlight (fPAR). Linking remote sensing with FLUXNET sites is crucial in providing reliable estimates of the magnitude and dynamics of the terrestrial carbon budget. An important issue is the spatial mismatch between the NASA Moderate Resolution Imaging Spectrometer (MODIS) and the footprint of tower observations. In this study we have conducted seasonal observations of VI's using three different ground based sensors: a high resolution spectrometer on a weekly basis, and a spectrally-selective light emitting diode spectrometer and a broadband radiometer providing continuous measurements for a highly dynamic oak-savanna ecosystem. The lowed cost LED based systems to measure VI's have great promise to expand ground based measurements within the footprint of flux towers. For other FLUXNET sites we have focused on the use of MODIS data subsets provided by the ORNL DAAC. We investigate seasonal changes in ε, drought induced changes in carbon uptake (NEE) and their link to different VI's like the Normalized Differential Vegetation Index (NDVI), Enhanced Vegetation Index (EVI), the Photochemical Reaction Index (PRI), and the Land Surface Water Index (LSWI). Certain indices have shown promise for the observation and modeling ecosystem carbon exchange. In this paper we expand the analysis across Mediterranean climate type ecosystems along the American West Coast that experience similar summer drought patterns to investigate the detection of soil and plant water status, evaporation and carbon exchange through the use of remotely sensed VI's under drought conditions.