Field Spectroscopy versus GPP. A test case using JB Devices at flux sites.

Field spectroscopy is a powerful tool for understanding plant carbon uptake and for providing a link between local flux measurements and global satellite remote sensing. In fact, optical remote sensing techniques can capture valuable information on both phenology and physiology. Even if the concept is of interest for the scientific community, the integration of field spectroscopy techniques in flux networks is still challenging and largely unresolved. Further, mainly due to the lack of available instruments, obtained relationships between remote sensing measurements and gross primary productivity (GPP) are often site specific and so far poorly exploited.

JB devices (FloX and RoX) are field spectrometer systems acquiring in situ radiometric measurements with standardized routines. They have been installed over the last five years across several ecosystems (e.g. croplands, forests, grasslands), often at eddy covariance stations. Recently, an effort was made to standardize the data processing and to align it to flux networks. The development of an open source processing chain was made and the definition of all the metadata needed to correctly interpret the measurements, including devices specification and setup, is ongoing.

With this contribution, we aim to advance understanding on the relationship between optical proximal sensing information and GPP using a selection of eight diverse sites equipped with JB devices and belonging to flux networks (ICOS or FLUXNET). Selected sites represent different vegetation types, including broadleaf forest, needle leaf forests, croplands, and grasslands. The time series length varies from one season to 4 years, depending on the time the instrument was installed. From the hyperspectral data, a subset of vegetation indices (e.g. indices related to biomass, chlorophyll content, carotenoids, and sun induced chlorophyll fluorescence (SIF, NIRv), and was computed where available (i.e. only on six of the equipped FloX sites). The data output was qualitatively checked and aggregated to the same temporal resolution as for the flux data (30 mins). Several models were tested to investigate this relationship, exploited both on half-hourly interval and on daily aggregation. Overall, good results were found. In general, SIF and NIRv were found to be the best predictor for GPP at half-hourly scales (r2 =0.75 over croplands and broadleaf forests). When the analysis was computed on daily aggregation, the indices related to chlorophyll content showed the best agreement with GPP. Significant differences were found according to vegetation types, where needle leaf forests were giving the poorest results. Our analysis demonstrates the valuable information carried by field spectroscopy data in the context of understanding GPP dynamics, supporting the hypothesis that the link between optical sensing and fluxes can be better interpreted with a growing number of field spectrometer data available at flux sites.