Soil water depletion induces discrepancies between in situ measured vegetation indices and photosynthesis in a temperate heathland

Vegetation indices (VIs) derived from optical sensors have been used as proxies for monitoring gross primary productivity (GPP). In contrast to satellite-based VIs, whose temporal resolution is typically limited, especially in cloudy areas, situ VIs may higher resolution. This fine frequency implies much larger sample sizes test the performance of by comparing with eddy covariance-based GPP estimates. Here, we tested potential measured estimate a temperate heathland ecosystem. We compared half-hourly values an covariance CO2 flux measurements several greenness-, structure- and chlorophyll-sensitive (e.g., Terrestrial Chlorophyll Index [TCI] Inverted Red-Edge [IRECI], among others) obtained multiband mounted at tower. Results showed that differed their ability capture variability during non-drought condition, all failed describing extreme drought event. After integrating indicator soil water content, precipitation/potential evapotranspiration ratio or actual/potential ratio) regression model, performances drastically improved. Among indices, IRECI TCI were most promising capturing best variation (R2 = 0.73 RMSE 1.85, R2 0.77, 1.69, respectively), provided stress was properly accounted for. Our findings implications development improvement global ecological models based on proximal remote sensing data. results also strong impact our upscale fluxes using satellite (e.g. Sentinel-2 Sentinel-3) ecosystem characterized heather vegetation which relatively resistant structural changes canopy strongly affects interpretation signals.