On the relationship between ecosystem-scale hyperspectral reflectance and CO₂ exchange in European mountain grasslands

Abstract

Abstract. In this paper we explore the use of hyperspectral reflectance measurements and vegetation indices (VIs) derived therefrom in estimating carbon dioxide (CO2) fluxes (net ecosystem exchange – NEE; gross primary production – GPP), and some key ecophysiological variables related to NEE and GPP (light use efficiency – ε; initial quantum yield – α; and GPP at saturating light – GPPmax) for grasslands. Hyperspectral reflectance data (400–1000 nm), CO2 fluxes and biophysical parameters were measured at three grassland sites located in European mountain regions. The relationships between CO2 fluxes, ecophysiological variables and VIs derived using all two-band combinations of wavelengths available from the whole hyperspectral data space were analysed. We found that hyperspectral VIs generally explained a large fraction of the variability in the investigated dependent variables and that they generally exhibited more skill in estimating midday and daily average GPP and NEE, as well as GPPmax, than α and ε. Relationships between VIs and CO2 fluxes and ecophysiological parameters were site-specific, likely due to differences in soils, vegetation parameters and environmental conditions. Chlorophyll and water content related VIs (e.g. CI, NPCI, WI), reflecting seasonal changes in biophysical parameters controlling the photosynthesis process, explained the largest fraction of variability in most of the dependent variables. A limitation of the hyperspectral sensors is that their cost is still high and the use laborious. At the eddy covariance with a limited budget and without technical support, we suggest to use at least dual or four channels low cost sensors in the the following spectral regions: 400–420 nm; 500–530 nm; 750–770 nm; 780–800 nm and 880–900 nm. In addition, our findings have major implications for up-scaling terrestrial CO2 fluxes to larger regions and for remote and proximal sensing sampling and analysis strategies and call for more cross-site synthesis studies linking ground-based spectral reflectance with ecosystem-scale CO2 fluxes.