Seeing through the canopy: COS-constrained SCOPE modelling to investigate plant drought responses

Understanding plant biophysical and biochemical responses to drought is essential for predicting ecosystem carbon–water exchange and gross primary productivity (GPP) under a changing climate. Here, we investigate stomatal and non-stomatal responses to natural drought events by integrating carbonyl sulfide (COS) fluxes as a novel observational constraint into the Soil–Canopy Observation of Photosynthesis and Energy fluxes (SCOPE) model. Because plants take up COS in parallel with CO₂ via practically similar pathways but do not re-emit COS, it can be used as promising proxy for GPP and stomatal conductance. SCOPE is a physically based land-surface model that links plant processes to spectrally resolved within-canopy radiative transfer; adding COS provides an independent constraint on canopy conductance and carbon uptake. We leverage multiple years of concurrent COS and CO₂ flux as well as hyperspectral reflectance measurements from a Scots pine dominated montane forest in Austria, including naturally occurring drought periods, to refine model representations of stomatal regulation, internal conductance, and water-stress responses at the ecosystem scale. This model–data integration framework improves detection and prediction of drought impacts on canopy function and enhances constraints on ecosystem carbon–water dynamics.