Abstract
Abstract. This study investigates the performances of different optical indices to estimate gross primary production (GPP) of herbaceous stratum in a Mediterranean savanna with different nitrogen (N) and phosphorous (P) availability. Sun-induced chlorophyll fluorescence yield computed at 760 nm (Fy760), scaled photochemical reflectance index (sPRI), MERIS terrestrial-chlorophyll index (MTCI) and normalized difference vegetation index (NDVI) were computed from near-surface field spectroscopy measurements collected using high spectral resolution spectrometers covering the visible near-infrared regions. GPP was measured using canopy chambers on the same locations sampled by the spectrometers. We tested whether light-use efficiency (LUE) models driven by remote-sensing quantities (RSMs) can better track changes in GPP caused by nutrient supplies compared to those driven exclusively by meteorological data (MM). Particularly, we compared the performances of different RSM formulations – relying on the use of Fy760 or sPRI as a proxy for LUE and NDVI or MTCI as a fraction of absorbed photosynthetically active radiation (fAPAR) – with those of classical MM. Results showed higher GPP in the N-fertilized experimental plots during the growing period. These differences in GPP disappeared in the drying period when senescence effects masked out potential differences due to plant N content. Consequently, although MTCI was closely related to the mean of plant N content across treatments (r2 = 0.86, p < 0.01), it was poorly related to GPP (r2 = 0.45, p < 0.05). On the contrary sPRI and Fy760 correlated well with GPP during the whole measurement period. Results revealed that the relationship between GPP and Fy760 is not unique across treatments, but it is affected by N availability. Results from a cross-validation analysis showed that MM (AICcv = 127, MEcv = 0.879) outperformed RSM (AICcv =140, MEcv = 0.8737) when soil moisture was used to constrain the seasonal dynamic of LUE. However, residual analyses demonstrated that GPP predictions with MM are inaccurate whenever no climatic variable explicitly reveals nutrient-related changes in the LUE parameter. These results suggest that RSM is a valuable means to diagnose nutrient-induced effects on the photosynthetic activity.
Highlights
Human-induced nutrient imbalances are affecting essential processes that lead to important changes in ecosystem structure and functioning (Peñuelas et al, 2013)
MERIS terrestrial-chlorophyll index (MTCI) was closely related to the mean of plant N content across treatments (r2 = 0.86, p < 0.01), it was poorly related to gross primary production (GPP) (r2 = 0.45, p < 0.05)
Results revealed that the relationship between GPP and fluorescence yield computed at 760 nm (Fy760) is not unique across treatments, but it is affected by N availability
Summary
Human-induced nutrient imbalances are affecting essential processes that lead to important changes in ecosystem structure and functioning (Peñuelas et al, 2013). Assuming a correlation between leaf chlorophyll pigments and leaf N content, noting that N atoms are basic components of the chlorophyll molecular structure, several studies have demonstrated that leaf N content can be estimated through chlorophyll-related hyperspectral vegetation indices (Baret et al, 2007; Schlemmer et al, 2013). Among these indices, the MERIS (Medium Resolution Imaging Spectrometer) terrestrial-chlorophyll index (MTCI; Dash and Curran, 2004) has been used as a proxy for f APAR (Rossini et al, 2010; Wang et al, 2012). Recent literature has called for better physiological descriptors of the dynamic behavior of LUE (Guanter et al, 2014)
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