Near-surface remote sensing techniques provide unmatched spatiotemporal information on ecosystem photosynthesis, i.e., gross primary productivity (GPP). Yet, our understanding of the relationship between remotely sensed proxies and GPP - and how this relationship changes with space and time scale, biophysical constraint, vegetation type, etc. - remains limited. This knowledge gap is especially apparent for dryland ecosystems, which have high spatial and temporal variability and are under-represented by long-term, continuous field measurements. Here, for two eddy covariance flux tower sites – the Kendall grassland (US-WKG) and Santa Rita Mesquite savanna (US-SRM) sites – we evaluate site-level observations of the normalized difference vegetation index (NDVI), the near infrared reflectance index (NIRv), the photochemical reflectivity index (PRI), and sun-induced chlorophyll fluorescence (SIF) as proxies for GPP. To better understand diurnal to seasonal sensitivity to drought stress, we divided our observations into pre-monsoon, monsoon, and post-monsoon periods, and examined a range of temporal scales.
Results/Conclusions
Preliminary results suggest that PRI and SIF are able to capture diurnal GPP dynamics at these sites, including midday suppressions of GPP during the pre-monsoon period. At the seasonal scale, NDVI, NIRv, and SIF all appear to roughly track GPP. However, SIF appears to be most sensitive to the response of GPP to drought stress during key periods. This may be due to decoupling between NDVI/NIRv and GPP – which is especially apparent at the savanna (US-SRM) sites – during periods of low soil moisture and/or high vapor pressure deficit. Analyses related to how these responses change through space and time are ongoing. Our initial findings indicate that the integration of these independent vegetation proxies could yield synergistic improvements in remote sensing-based GPP estimates.