The FETCH4-Osmo Model: Accounting for Salt Stress Effects on Mangrove Carbon and Water Fluxes

Mangroves are halophytes that thrive across salinity gradients along tropical and subtropical coastlines. Salinity affects photosynthesis and plant water use, influencing carbon and energy fluxes and ultimately blue carbon storage. However, many plant hydrodynamic models do not account for osmotic potential or osmoregulation, limiting our ability to represent mangrove responses to salinity. Here, we present a modification to the advanced plant hydrodynamic model FETCH4 that incorporates soil osmotic potential, xylem osmoregulation, and salinity-dependent plant traits. The updated model, FETCH4-Osmo, couples plant hydraulics with the Farquhar photosynthesis model, allowing plant water potential to regulate stomatal conductance and photosynthesis. The model was parameterized using species-specific hydraulic and photosynthetic traits of black and white mangroves (Avicennia germinans and A. marina) and evaluated with sap flux and photosynthesis measurements from two field sites: a low to moderate salinity site in Louisiana, USA, and a hypersaline site near Exmouth, Australia. FETCH4-Osmo reproduced transpiration well (R² = 0.80, normalized mean absolute error (NMAE) = 0.19) and improved simulations compared to the original FETCH4 model (R² = 0.77, NMAE = 0.37). Under hypersaline conditions, inclusion of osmoregulation substantially improved performance (NMAE = 0.20 vs. 3.78). Comparisons with leaf-level photosynthesis and stomatal conductance suggest that the model captures salinity effects on carbon fluxes. FETCH4-Osmo improves our ability to simulate salinity driven controls on water and carbon fluxes in coastal mangrove ecosystems.