The influence of plant soil moisture stress on solar-induced chlorophyll fluorescence efficiency across Africa

Solar-induced chlorophyll fluorescence (SIF) provides an indirect, physiologically linked signal associated with photosynthetic light reactions, positioning it as a powerful tool for monitoring vegetation function. However, interpreting SIF signals is challenging because SIF depends not only on absorbed photosynthetically active radiation (APAR) but also on fluorescence efficiency (ε), with changes in ε often dominating SIF variability under environmental stress. This study examines the response of ε to soil moisture availability (β) across diverse African ecosystems, addressing a knowledge gap in continental-scale SIF interpretation. Utilizing satellite-based TROPOMI SIF measurements combined with soil moisture estimates from the Joint UK Land Environment Simulator (JULES) model, we analyse how ε varies with β across multiple land-cover types. We test two core hypotheses: (i) that ε varies systematically with soil moisture availability, and (ii) that the strength of this relationship is dependent on land cover type. To achieve this, we compare a baseline SIF model with an enhanced model that explicitly incorporates soil moisture. Results show that ε tends to increase with soil moisture availability, with the strongest responses observed in croplands and grasslands, and weaker responses in evergreen forests and wetlands. These differences reflect underlying ecosystem water-use strategies and rooting depths. In semi-arid systems such as the Sahel and Southern Africa, accounting for soil moisture improves the representation of SIF dynamics, while in humid regions such as the Sudd Swamp and Okavango Delta, it adds little explanatory power or may introduce error. These findings emphasize the ecosystem-specific nature of the relationship between water availability and ε, and support the need for context-dependent interpretation of SIF across African landscapes.