Decoupling of greenness and photosynthesis regulates phenological shifts across Australian ecosystems

The complex coupling between structural greenness and functional photosynthesis represents a critical interaction underlying uncertainties in the terrestrial carbon cycle. Greenness-based indicators, such as the leaf area index (LAI) and reflectance-derived indices, represent structural signals of canopy state but often fail to capture the spatiotemporal variations of photosynthetic function. In recent years, solar-induced chlorophyll fluorescence (SIF), as a direct remote-sensing proxy of photosynthesis, has provided a new avenue for exploring vegetation structure–function relationships. In this study, we treated SIF as a functional signal and greenness proxies as structural signals and examined when and where these signals were coupled (i.e., varied synchronously) or decoupled (i.e., differed in timing or magnitude) across Australia using multi-source remote sensing data. We then quantified the spatial variability of SIF–greenness coupling, assessed its associations with environmental and biological factors, and evaluated how variations in coupling accounted for discrepancies between structure- and function-based phenology. Our results indicated that the coupling between vegetation structure and function was primarily associated with vegetation composition and environmental aridity. In herbaceous ecosystems, low soil moisture (SMC) and high vapor pressure deficit (VPD) were linked to stronger coupling between greenness and photosynthesis. With increasing woody fraction (f_woody), the importance of SMC over VPD in influencing greenness–photosynthesis coupling became more pronounced. The relative roles of hydroclimatic and biological factors in greenness–photosynthesis coupling varied with the physical nature of each greenness indicator. Increasing decoupling between vegetation structure and function led to later peaks and shorter growing seasons in greenness-based phenology compared with SIF-based phenology, and these differences intensified with stronger decoupling. These findings advance the understanding of how environmental and biological factors are associated with vegetation structure–function coupling and its implications for phenology and carbon cycle modeling.