In this study, we investigated the relationship between meteorological-based broadband simple ratio (SR) and canopy photosynthetic capacity (maximum carboxylation rate normalized to 25°C, Vc,25) in three Fagus crenata stands in the cold-temperate zone of Japan. Broadband SR was calculated from recorded up- and down-looking photosynthetically active radiation (PAR) and global radiation (GR) data. The study reveals that broadband SR seasonal courses closely follow Vc,25 seasonal trajectories, with a statistically significant linear regression relationship between the two. Linear regression models show that R2 ranges from 0.59 (for 550 m site) to 0.91 (for 1500 m site), but eventually drops to 0.37 when all data pool together. Despite variations in R2 for the different sites, the relationship remains statistically significant (P < 0.000). Though spatially limited, broadband SR can serve as an easy but robust indicator of seasonal variations in Vc,25 required for accurate carbon fixation simulations in gas exchange models.