A Canopy-Scale Fluorescence Approach for Evaluating Seasonal Photochemical Dynamics Across Different U.S. Forests

Terrestrial ecosystems regulate atmospheric carbon exchange through photosynthesis while providing ecosystem services relevant to forest health, land management, and climate adaptation. Forests are particularly important because of their central role within global carbon cycling. Solar-induced fluorescence (SIF), which originates from chlorophyll fluorescence associated with photosystem II (PSII), has become a valuable remote-sensing signal for monitoring vegetation activity across ecosystems. While laboratory fluorescence measurements are widely used to evaluate PSII function, efforts to translate comparable fluorescence-based concepts to satellite observations remain limited, particularly across different forests, phenology, and climatic conditions. Here, we analyze evergreen needleleaf, mixed, and deciduous forests across the United States using satellite-derived SIF observations developed from Orbiting Carbon Observatory-2 (OCO-2) retrievals. We derive a canopy-scale analog of PSII efficiency (ΦCPSII) from fluorescence yield (ΦSIF) using observed minimum and maximum bounds within each observation period, following established relationships between steady-state fluorescence yield and PSII operating efficiency. We further estimate a canopy-scale electron transport rate (ETRC) by integrating ΦCPSII with absorbed photosynthetically active radiation (APAR). Seasonal analyses showed clear temporal behaviors between and across forest types. While ΦSIF exhibited consistent seasonal trajectories with relatively weak sensitivity to short-term atmospheric variability, ΦCPSII captured stronger seasonal shifts in relative photochemical behavior. Both ΦSIF and ΦCPSII showed weak or inconsistent relationships with gross primary productivity (GPP), indicating that both metrics alone do not fully represent canopy-scale carbon assimilation. APAR and derived ETRC were positively related to GPP at all four sites. However, ETRC explained more variation than APAR only at the mixed and deciduous sites, whereas APAR performed better at the evergreen needleleaf sites. These findings indicate that much of the ETRC–GPP relationship was attributable to absorbed radiation, while the additional contribution of PSII was site-dependent. Thus, combining absorbed radiation with derived canopy-scale PSII efficiency may provide additional information about photosynthetic activity under some canopy conditions, but it does not consistently improve upon APAR alone.