Recent advances in satellite remote sensing of solar-induced fluorescence (SIF) are promising for estimating gross primary production (GPP). However, the empirical link between GPP and SIF has mostly been observed at coarse spatial and temporal scales and lacks a direct mechanistic explanation. Here, we compare high-resolution SIF with GPP and needle-scale physiology at a winter-dormant coniferous forest. Needles retained chlorophyll year-round despite cold temperatures and high light in winter; meanwhile SIF tracks GPP at hourly to weekly timescales. We show that both GPP and SIF are regulated by seasonal changes in photoprotective pigments and that SIF is directly related to needle physiology. This highlights the unique ability of SIF compared with traditional remote-sensing methods to capture GPP seasonality.