Optimal Physiological Thresholds of Pan-Tropical Forests Using Surface Measurements and ECHAM/MESSy Atmospheric Chemistry Numerical Modeling

Aerosols and clouds are key external factors that significantly influence the light-use efficiency of plants and their primary productivity. Since photosynthesis and transpiration are tightly linked through leaf stomata, the diffuse fertilization effect (DFE) can also impact water-use efficiency (WUE). However, surface measurements have not yet been systematically employed to enhance the ability of global models to capture these effects, particularly across pan-tropical regions. Current global models remain uncalibrated, lacking validation datasets necessary to accurately represent the annual variations in gross primary productivity (GPP) and WUE driven by the DFE. In this study, we assessed the influence of DFE on annual carbon uptake and water dynamics using traditional methods widely applied across diverse forest ecosystems globally. Our analysis utilized long-term micrometeorological data from the FLUXNET dataset in six evergreen broadleaf forest (EBF) ecosystems, located in northern South America (SA), Central Africa (AF), South Asia (AS), and Oceania (OC). Preliminary comparisons were conducted with GPP and ET values simulated using the EMAC/JSBACH numerical systems. Here, we show the optimal physiological thresholds for GPP and WUE under overcast/smoky sky conditions and the typical tipping points linked to irradiance relative (f). Our preliminary results from the FLUXNET dataset indicated GPP and WUE rates as high as 0.4-0.6 g C m^-2 h^-1 and ET 0.3-0.5 mm h^-1 for f between 1.1-0.7 (± 0.39) and solar zenith angles (SZA) ranging 0°–80°. For f ≤ 0.6, GPP and ET decreased rapidly in the studied areas, with a total breakdown of photosynthesis and evapotranspiration reaching around f 0.2 (± 0.36). The EMAC/JSBACH systems satisfactorily reproduced the behavior of the observed variables (aforementioned). However, we found systematic overestimations of temperature and solar radiation compared to FLUXNET measurements (~35%), which also can explain the GPP overestimations. Systematic calibrations in the EMAC/JSBACH are still necessary to achieve more accurate estimates of annual carbon and water losses due to DFE. Potential outcomes and benefits include: (i) improved physical representation and performance of Dynamic Global Vegetation Models (DGVMs) across different Plant Functional Types (PFTs) and landscapes; (ii) identification of the physiological optimum of forests under conditions affected by wildfires or extreme drought periods; and (iii) quantification of annual global water and carbon losses in tropical forests caused by wildfires; enhancements or photodamage caused by exposure to biomass-burning organic aerosol (BBOA), along with improved representation of global carbon cycling.