On the relationship between ecosystem-scale hyperspectral reflectance and CO.sub.2 exchange in European mountain grasslands

• Published in: Biogeosciences Vol. 12; no. 10; pp. 3089 - 6177
• Main Authors: Balzarolo, M, Vescovo, L, Hammerle, A, Gianelle, D, Papale, D, Tomelleri, E, Wohlfahrt, G
• Format: Journal Article
• Language: English
• Published: Copernicus GmbH 28.05.2015
• Subjects: Analysis; Ecosystems; Grasslands
• ISSN: 1726-4170; 1726-4189

In this paper we explore the skill of hyperspectral reflectance measurements and vegetation indices (VIs) derived from these in estimating carbon dioxide (CO.sub.2) fluxes of grasslands. Hyperspectral reflectance data, CO.sub.2 fluxes and biophysical parameters were measured at three grassland sites located in European mountain regions using standardized protocols. The relationships between CO.sub.2 fluxes, ecophysiological variables, traditional VIs and VIs derived using all two-band combinations of wavelengths available from the whole hyperspectral data space were analysed. We found that VIs derived from hyperspectral data generally explained a large fraction of the variability in the investigated dependent variables but differed in their ability to estimate midday and daily average CO.sub.2 fluxes and various derived ecophysiological parameters. Relationships between VIs and CO.sub.2 fluxes and ecophysiological parameters were site-specific, likely due to differences in soils, vegetation parameters and environmental conditions. Chlorophyll and water-content-related VIs explained the largest fraction of variability in most of the dependent variables. Band selection based on a combination of a genetic algorithm with random forests (GA-rF) confirmed that it is difficult to select a universal band region suitable across the investigated ecosystems. Our findings have major implications for upscaling terrestrial CO.sub.2 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 CO.sub.2 fluxes.