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Canopy color indices for estimating gross primary production: insights from a savanna ecosystem in the Dry-Hot Valley of Southwest China
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DOI：10.3389/ffgc.2026.1890392.png)
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Abstract
En 中文
IntroductionThe deployment of digital cameras on flux towers offers a practical approach to exploring how seasonal canopy photosynthesis is related to canopy color indices (CCIs). However; the pronounced spatial and temporal heterogeneity of savanna ecosystems remains a major challenge.MethodsWe compared EC-derived gross primary production (GPP) with six LUE model variants; including two direct-FPAR models; three CCI-proxy models; and one MODIS NDVI-proxy model. This framework allowed us to distinguish direct FPAR inputs from vegetation-index proxies for the FPAR term in LUE-based GPP estimation.ResultsDuring the extreme high-temperature drought of 2019; the peak values of Sgreen; Hue; and GEI decreased by approximately 50%. Annual GPP declined from 850 to 567 g C m−2; leaf area index (LAI) decreased from 0.989 to 0.670 m2 m−2; and LUE decreased from 0.40 to 0.25 g C MJ−1. The seasonal pattern of GPP remained broadly consistent; although the length of the growing season (LOS) varied substantially; ranging from 213 to 111 d in 2016 and 2019; respectively. During the growing season (GS); the CCIs were closely synchronized with daily GPP and LAI; during the non-growing season (NGS); a lag of approximately 20 days was observed. Among the six LUE variants; the MODIS NDVI-proxy model performed best (R2 = 0.77; slope = 1.19); whereas the MODIS FPAR model showed moderate performance and underestimated GPP (R2 = 0.61; slope = 0.66). The tower-derived FPAR model overestimated GPP; and the CCI-proxy models performed moderately; especially with lower accuracy during the non-growing season.DiscussionOverall; digital camera-derived CCIs can be used as empirical FPAR-like proxies for estimating seasonal GPP dynamics in savanna ecosystems; but they should not be interpreted as physical FPAR measurements. The comparison with tower-derived FPAR and MODIS FPAR highlights the importance of explicitly distinguishing direct FPAR inputs from vegetation-index proxies in LUE-based GPP modeling.
Keywords:
gross primary production
canopy color indices
digital camera
LUE models
savanna ecosystem
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