Chlorophyll fluorescence-gross primary productivity relationships during the spring awakening of an evergreen needleleaf forest

Solar-induced chlorophyll fluorescence (SIF) has been shown as a promising approach for the estimation of gross primary productivity (GPP), but whether SIF is merely a function of canopy structure or also contains precious physiological information, is presently heavily discussed. In this study, the SIF-GPP relationship was quantified at a Pinus sylvestris forest (Mezyk, Poland) during a series of short-term cold spells throughout the spring awakening to investigate the potential of SIF as a proxy for GPP during this period characterized by cold stress. GPP was inferred from the net ecosystem CO₂ exchange measured by the eddy covariance technique. Canopy-scale SIF was measured using a high-resolution spectrometer system and retrieved via spectral fitting (SFM) algorithms. Active leaf-scale chlorophyll fluorescence measurements were conducted on seven branches using an automated field-deployable fluorometer system. Our results demonstrate a clear difference in GPP and the utilization of chlorophyll-absorbed energy between cold spell and warm days. At short, sub-daily time scales, the correlation between SIF and GPP was minor, but increased significantly when observed over extended temporal periods, when SIF exhibited a seasonal pattern that was more closely aligned with the GPP. Furthermore, the strong relationship between non-photochemical quenching (NPQ) and the photochemical reflectance index (PRI) shows good potential to better estimate GPP when integrated in the SIF-GPP model, as the integration of PRI overall increased the relation between SIF and GPP.