Non-linear correlations exist between solar-induced chlorophyll fluorescence and canopy photosynthesis in a subtropical evergreen forest in Southwest China

Abstract

Solar-induced fluorescence (SIF) provides an exciting opportunity for quantifying terrestrial gross primary production (GPP); however, little attention has been paid to subtropical evergreen forests. Previous research has predominantly concentrated on the linear relationship between SIF and GPP, while more recent studies have gradually reported non-linear relationships and aimed to uncover the underlying mechanism. A year-long synergistic observation of tower-based fluorescence and carbon flux was conducted in the subtropical forest of Southwest China. The SIF and GPP generally exhibited comparable dynamic patterns in subtropical evergreen forests at different scales. However, SIF peaked earlier than GPP in 30-min, daily, and monthly scales. Strongly non-linear relationships existed between SIF and GPP on both 30-min (R2 = 0.42) and daily (R2 = 0.51) scales, which could also extend to different seasons and weather conditions. In particular, the SIF-GPP relationship was stronger on cloudy days than on sunny days, whereas the relationship gradually decoupled as the vapor pressure deficit (VPD) rose. These findings can help to better understand the relationship between canopy-level SIF and GPP in subtropical evergreen forests and emphasize how environmental controls affect the SIF-GPP relationship.