Examining the Continental-Level Relationship Between Solar-Induced Fluorescence and Gross Primary Productivity at High Latitudes

The seasonality of photosynthesis in high-latitude terrestrial ecosystems is undergoing significant changes with global warming. Accurately evaluating these changes is critical to projecting the future role of these ecosystems in the global carbon cycle. However, measuring photosynthetic productivity is challenging due to sparse in situ observational data, both spatially and temporally. While remote sensing techniques can provide continuous spatial coverage, typical vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), have limited skill to explain physiological changes in evergreen needle leaf forests, a common biome type at high latitudes. The physiological link between Solar-Induced Fluorescence (SIF) and Gross Primary Productivity (GPP) of terrestrial ecosystems shows the potential of SIF observation from space. Global SIF products can help to monitor photosynthetic seasonality across space and time. Yet, current studies have shown substantially different relationships between the seasonal trajectories of SIF and GPP globally. Applying a universal linear scaling scheme to estimate GPP using SIF at high latitudes at the continental level is not favorable due to diverse biome types, canopy structures, and meteorological gradients. Here, we examine the spatial patterns of linear correlation between SIF (TROPOMI) and GPP (Fluxcom-RS) seasonal trajectories at high latitudes in terms of slope, correlation coefficients, and goodness of fits. There are clear differences across biome types and thus canopy structures, pointing to variations in the relationship between GPP and SIF at high latitudes. To analyze the drivers of these variations, we employ flux tower measurements to derive GPP, relevant vegetation indices and canopy radiative transfer simulations to evaluate the impact of canopy structure and viewing geometries on SIF, vegetation indices and GPP.