Assessing the Effects of Non-photochemical Quenching and Structural Effects on the Seasonal Cycle of Solar-induced Fluorescence in a Coniferous Forest with a Land Surface Model

Recent advances in remote quantification of photosynthesis related sun-induced chlorophyll fluorescence (SIF) have brought new possibilities to explore vegetation functioning from space. Chlorophyll fluorescence (ChlF) is the process taking place in green plant leaves when part of absorbed incoming radiation is re-emitted. The relationship between photosynthesis and chlorophyll fluorescence in leaves is however partly controlled by non-photochemical quenching (NPQ), which dissipates excess energy as heat. Better understanding of the NPQ mechanisms will help in understanding the remotely sensed observations of SIF and further widen their applicability. Coniferous evergreen forests living in climatic regions with harsh winters have developed different protection mechanisms against challenging environmental conditions. These mechanisms include sustained NPQ taking place in the needles. The sustained NPQ changes slowly compared to the reversible NPQ and is largest during the winter period with gradual changes during the shoulder seasons. Our aim is to use modelling to assess the role of sustained non-photochemical quenching from the structural effects that also influence the observed SIF signal. Our study site is a subalpine coniferous forest at Niwot Ridge, U.S, with in-situ top of canopy observations with Photospec that we use as a reference to different model simulations. We use a state-of-the-art land surface model QUantifying Interactions between terrestrial Nutrient CYcles and the climate system (QUINCY). We have implemented a ChlF model inside QUINCY as well as three different radiative transfer models of the SIF signal. These three different radiative transfer models have differing complexities and model the structural effects somewhat differently. The most complex solution is based on the mSCOPE and Fluspect models, that explicitly calculates signal transfer. The intermediate solution is based on a two-stream flux approach and the most simple is using a simple fraction for the escape ratio of SIF. Already the simple approach accounted for the structural effects during the shoulder seasons robustly and improved the seasonal behavior of simulated SIF against the observed signal in comparison to a simple upscaling of the leaf level simulated SIF signal.