Broadband simple ratio closely traced seasonal trajectory of canopy photosynthetic capacity

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, V c,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 V c,25 seasonal trajectories, with a statistically significant linear regression relationship between the two. Linear regression models show that R 2 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 R 2 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 V c,25 required for accurate carbon fixation simulations in gas exchange models.