Diurnal and Seasonal Variations in Vegetation Function at Leaf and Canopy Scales Using Field and Satellite Data

Recent advancements in the development of field instruments enable monitoring vegetation function at leaf and canopy levels at dense temporal and spatial scales relevant to the dynamics of plant photosynthesis. Optical measurements were collected on rain-fed maize (Zea mays L.) under 0 and 100% of optimal nitrogen (N) treatments during the summer of 2019 at the USDA OPE3 site in Maryland, USA. The field-based measurements include: 1) leaf-level fluorescence and photosynthetic efficiency collected using the pulse amplitude modulated (PAM) approach; 2) canopy reflectance and solar induced fluorescence (both SIFB in the atmospheric O2B and SIFA in the atmospheric O2A bands; mW/m2/nm/sr) collected using a dual spectrometer FLoX; and 3) CO2 flux partitioned into gross primary productivity (GPP) and ecosystem respiration, measured by a 10 m eddy-covariance flux tower. These ground measurements were combined with imagery from the Vegetation and Environment monitoring on a New MicroSatellite (VENµS) with 12 visible and near infrared spectral bands. Relationships between the diurnal leaf observations of photosynthetic efficiency, canopy SIF, and GPP were established. Overall, SIFB and SIFA were higher and reached their maxima later in the growing season for the N-treated maize than for untreated maize. The chlorophyll red-edge VIs and the photochemical Reflectance Index (PRI) of N-treated maize were consistently higher than those of untreated maize through the growing season. Mid-day FLoX and VENµS chlorophyll red-edge VIs and Normalized Difference Vegetation Index (NDVI) were strongly correlated (p<0.001, r2=0.95/0.99). Our study contributes to improving the ability to scale and relate the commonly measured leaf-level parameters to the canopy-level SIF measurements. The analysis facilitates the development of algorithm prototypes characterizing photosynthetic function, as anticipated with space-based instruments having full visible-shortwave infrared spectrometers, such as the forthcoming NASA Surface Biology and Geology (SBG) and ESA Fluorescence EXplorer (FLEX) missions.