Proximal sensing of drought stress with sun-induced chlorophyll fluorescence: A tale of scaling up and down

Sun-induced chlorophyll fluorescence (SIF) is used to infer canopy photosynthesis, yet its ability to isolate physiological from canopy structural signals remains uncertain. We aimed at testing whether SIF can disentangle the onset and progression of physiological drought stress amidst concurrent canopy biochemical/physical changes, and to evaluate links between leaf- and canopy-scale fluorescence. To this end, a mesocosm experiment manipulating water availability (control vs. drought) in two herbaceous canopies with contrasting leaf angle distributions (planophile vs. erectophile) was conducted. We monitored environmental conditions, chlorophyll fluorescence at leaf- and canopy-scale using an active and passive approach, canopy biochemical/physical properties, and used the SCOPE model to upscale from leaf- to canopy-scale and analyse drivers. Drought progressively reduced soil water content, depressed stomatal conductance, and upregulated non-photochemical quenching, yielding lower fluorescence yields at leaf and canopy scales. TOC SIF showed a significant response to drought; however, neither TOC SIF nor down-scaled SIF detected stress earlier than NIRv. Modelling and observations indicated strong non-physiological influences on TOC SIF, especially as drought altered biochemical/physical canopy attributes. Leaf-scale TOC fluorescence yields were higher and poorly correlated with canopy-scale yields, whereas SCOPE-based upscaling improved agreement. We conclude that SIF robustly captures drought onset, but offers limited early-detection advantage over greenness indices when concurrent canopy structural/biochemical changes are substantial.