Monitoring Stand Level Photosynthesis from Spectral Reflectance

Global determination and monitoring of gross primary production (GPP) is a critical component of climate change research. On local scales, GPP can be assessed from measuring CO2 exchange above the plant canopy using tower-based eddy covariance (EC) systems. The limited footprint inherent to this method however, restricts observations to relatively few discrete areas making continuous predictions of global CO2 fluxes challenging. Recently, the advent of high resolution optical remote sensing devices has offered new possibilities to address some of the scaling issues related to GPP using approaches based on spectral reflectance. One key component for inferring GPP from remote sensing is the efficiency (e) with which plants can convert absorbed photosynthetically active radiation into biomass. Whilst recent years have seen progress determining e at the leaf level using the photochemical reflectance index PRI, little is known about the temporal and spatial requirements for upscaling PRI. For instance, satellite observations of canopy reflectance are subject to view and illumination geometry effects induced by the bi-directional reflectance distribution function (BRDF) of canopies that can confound the desired signal; however little is known about interactions between these effects and PRI. Further areas of research include dependencies of PRI on canopy structure, understorey and species composition. One potential way to investigate these requirements is using automated tower-based remote sensing platforms, facilitating spectral observations of the canopy with high spatial, temporal, and spectral resolution. The experimental setup presented herein features an automated spectral radiometer (AMSPEC) with a motor-driven probe allowing observations in a nearly full circle around the tower. Year round data are sampled every 5 sec., a full rotation is completed within 15 min. The spatial similarity to the flux-footprint allows direct comparisons with EC and micro-meteorological measurements, facilitating the investigation of interactions between meteorology and canopy reflectance. The wide range of illumination and viewing geometries permits comprehensive modeling of the BRDF under different physiological and atmospheric conditions. Diurnal and seasonal effects can be assessed from differences in year-round reflectance measurements and dependencies on canopy structure are derived using airborne Laser-scanning to relate PRI to canopy volume and canopy shadow fractions. Spatial dependencies are further assessed comparing EC and radiometer footprint using a meteorologically based modeling approach. Results from analytical study and reflectance observations demonstrate that PRI is useful for tracking diurnal and seasonal changes in canopy light use efficiency. Permanently established canopy reflectance measurements are vital components of ongoing research aiming at upscaling PRI based estimates of e to landscape, regional and global scales. Instruments like AMSPEC can help understanding physiological cycles of vegetation and serve as calibration tool for the broader band spectral observations available from satellite data. Ultimately, a comprehensive understanding of correlations between variations in e and PRI can help determining GPP from space.